TMRC 2016 August 17 – 19, 2016 | Stanford, California

Sponsored by: IEEE Magnetics Society

Co-sponsored by: Center for Magnetic Nanotechnology (Stanford University) Computer Mechanics Lab (UC Berkeley) Center for Memory and Recording Research (UC San Diego) Center for Materials for Information Technology (U of Alabama) Center for Micromagnetics & Information Technologies (U of Minnesota) Data Storage Systems Center (Carnegie Mellon University)

Corporate sponsors: Western Digital Corporation Seagate Technology Headway Technologies AVP Technology Stanford University presents TMRC 2016 August 17-19, 2016 │ Stanford, California

TMRC 2016 will focus on “Enhanced future recording technologies for hard disk drives beyond 10 TByte capacity”, spin transfer torque random access memory (STT-RAM), and other related topics. Approximately 36 invited papers of the highest quality will be presented orally at the conference and will later be published in the IEEE Transactions on Magnetics. Poster sessions will also be held consecutive to the oral sessions and will feature posters from the invited speakers and a limited number of contributed posters. The contributed posters will be eligible for publication in the IEEE Transactions on Magnetics after peer review.

Topics to be presented include: Perpendicular Magnetic Recording at More Than 1Tbit/in2 (Readers, Writers, Tribology, Signal Processing) Two-Dimensional Magnetic Recording Heat Assisted Magnetic Recording MRAM

TMRC 2016 Organization

Conference Chair Poster Co-Chairs Jinshan Li, Western Digital Shafa Dahandeh, Western Digital Baoxi Xu, DSI Program Co-Chairs Yoichiro Tanaka, Toshiba Publicity Co-Chairs Fatih Erden, Seagate Technology Jan-Ulrich Thiele, Seagate Technology Hans Richter, Western Digital Qing Dai, HGST Publication Co-Chairs Treasurer Michael Alex, Western Digital Chris Rea, Seagate Technology Ganping Ju, Seagate Technology Local Chair Shan X. Wang, Stanford University

1 Copyright © the Institute of Electrical and Electronics Engineers, Inc. For copying, reprint or republication, write to:

Manager, Rights and Permissions IEEE Services Center PO Box 1331 445 Hoes Lane Piscataway, NJ 08855-1331 723-562-3966 2 TMRC 2016│Chairman’s Welcome

Conference chair The insatiable demand for data storage puts ever increasing pressure on storage Jinshan Li density and recording system improvements from many perspectives. TMRC, as the Western Digital premier conference for all aspects of magnetic recording and storage technologies, presents a set of invited talks from renowned worldwide leaders working on various Local chair research and development fronts in data storage. We have selected presentations Shan X. Wang which offer broad coverage from exploratory recording fields, to discoveries in magnetics, to novel development of existing recording technologies and emerging Stanford University applications of magnetism. In addition, we are very excited to announce that for the first time in TMRC history, TMRC 2016 poster session papers will also be published in the IEEE Transactions on Magnetics! Program co-chairs Yoichiro Tanaka Furthermore, we are honored to have keynote speaker Mr. Luis Carbonell, CEO of Toshiba MagArray, share with you the exciting field of utilizing magnetic sensors for cancer Fatih Erden diagnostics. This field carries great business potential, significant healthcare implications, and the thrust to propel vital technological breakthroughs in Seagate healthcare/molecular diagnostics. We hope this keynote brings a fresh perspective Hans Richter on the great opportunity of extending magnetics into new fields. Western Digital We are also very fortunate to host this conference at Stanford University in Silicon Valley, the epicenter of many of the world’s technical innovations. With its Publication co-chairs proximity to leading storage companies, a uniquely high concentration of scientific Michael Alex institutions and universities, and full support from local chair professor Shan Wang, Western Digital we anticipate this conference will be one of the most successful TMRCs. Ganping Ju I wish to use this opportunity to express my deep gratitude to the all-volunteer Seagate members of the organizing committee who have worked tirelessly, professionally and enthusiastically to bring this conference to you. Our Program and Poster Chairs have put together an excellent technical program; the ‘bierstube’ during the poster Poster co-chairs sessions is a venue to foster open and lively discussions. Our local chair professor Shafa Dahandeh Wang has arranged the state of the art Stanford Huang Engineering Center to host Western Digital us. The Publication Chairs have supported our authors with timely and precise guidance. The strong registration numbers speak for themselves about the work of BaoXi Xu our Publicity Chairs, and our very experienced treasurer will bring you the best DSI conference experience within his capabilities.

I would also like to point out this conference would not have been made possible Treasurer without the generous support of our corporate sponsors. To them, and on behalf of Chris Rea the entire TMRC community, I offer our sincere gratitude. Seagate I’d like to cordially invite you to attend the conference and contribute to the continued success of this conference series, which will grow and evolve together Publicity co-chairs with the data storage industry. Jan-Ulrich Thiele Seagate Thank you for your support and participation; we hope you enjoy this year’s TMRC. Qing Dai HGST Jinshan Li TMRC 2016 Conference Chair

3 TMRC 2016│Corporate Sponsors

TMRC 2016 would not have been possible without the generous support of the following sponsors:

Gold level sponsors: Western Digital Seagate Technology Headway Technology

Silver level sponsor: AVP Technology

4 TMRC 2016 |Local information The 27th Magnetic Recording Conference

Conference venue Jen-Hsun Huang Engineering Center NVIDIA Auditorium, Basement Floor August 17 - 19, 2016 Directions to Event Parking From US 101: Take the Embarcadero Road exit West toward Stanford. Continue on Embarcadero for 2.2 miles. You will pass Middlefield Road and El Camino Real. As Embarcadero crosses El Camino Real and enters the Stanford campus, the street becomes Galvez. Continue on Galvez for 0.8 miles, staying in the left lane. The Galvez Lot entrance will be on your right, between Nelson Road and Campus Drive.

From Interstate 280: Take the Page Mill Road exit and go East, continuing to the right as you leave the exit ramp. Continue on Page Mill Road for 2.7 miles. You will pass Deer Creek Road, Foothill Expressway, and Ramos Street. Turn left on El Camino Real; continue for 0.7 miles. Turn left on Campus Drive East, then right turn onto Galvez. The Galvez Lot entrance will be on your left, between Nelson Road and Campus Drive.

Event Parking: Using event code 4810, the daily parking fee in the Galvez Lot is $8. Pay machines accept credit cards or cash.

Directions to the Huang Building

Via Marguerite Shuttle – Line Y: You may catch the free Marguerite Shuttle from the Galvez Lot to the Huang Building. Take the Line Y shuttle at the Arrillaga Alumni Center (across from Galvez lot). The Huang building is accessible from the Y2E2/Via Ortega stop. We are adjacent to the Y2E2 building on Via Ortega. You can access Huang by walking down the corridor from Y2E2 to Huang.

Via Marguerite Shuttle – SLAC: From the Galvez Lot, walk across Campus Drive. Turn right onto Galvez St., follow for approximately two blocks. Turn right on Serra Mall and walk towards the water fountains. Take the SLAC shuttle at Memorial Auditorium to the Y2E2/Via Ortega stop. You can access Huang by walking down the corridor from Y2E2 to Huang.

Walk to Meeting Site: From the Galvez Lot, walk across Campus Drive. Turn right onto Galvez St., and follow for approximately two blocks. Turn right on Serra Mall (0.5 mile) and turn left on North-South Axis. Follow the pathway onto the Science and Engineering Quad straight ahead. Huang is the octagon shaped building on your right.

Limited Visitor Parking at Via Ortega Garage: For those who may require parking close to the meeting site, limited visitor parking is available on the 1st floor and lower level of the Via Ortega Garage at Panama St. and Via Ortega. Pay machines accept credit cards and cash. Cost is $2.00 - $2.50 per hour. 5 TMRC 2016 | Local information The 27th Magnetic Recording Conference Campus map and parking Event Parking $8 daily parking fee with event code, 4810. Pay machines accept credit cards and cash.

Visitor Parking Conference st Limited visitor parking on 1 floor and lower level. Pay machines accept credit cards and Site cash ($2.00 - $2.50 per hour).

6 TMRC 2016 | Local information The 27th Magnetic Recording Conference

Line Y Weekday Service (Monday - Friday) Serves additional stops along route (check sign at stop/ask driver)

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TMRC 2016 | Local information The 27th Magnetic Recording Conference

Line Y Weekday Service (Monday - Friday), cont. Serves additional stops along route (check sign at stop/ask driver)

8 TMRC 2016 | Local information The 27th Magnetic Recording Conference

Line Y route map Event Parking

Conference Site

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TMRC 2016 | Local information The 27th Magnetic Recording Conference

SLAC Weekday Service (Monday - Friday) Serves additional stops along route (check sign at stop/ask driver)

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TMRC 2016 | Local information The 27th Magnetic Recording Conference SLAC route map

Event Parking

Conference Site

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12 TMRC 2016│Schedule at a glance

Wed. August 17 Thu. August 18 Fri. August 19

8:00 AM Coffee/Pastries 8:00 AM Coffee/Pastries 7:30 AM Coffee/Pastries On-site registration On-site registration On-site registration 8:50 AM: Opening Address SessionR A Session C Session F MAMR, BPMR, Advanced HAMR-I HAMR-HDI Readers 9:00 AM - 11:45 AM 9:00 AM - 10:30 AM 8:30 AM - 11:45 AM Coffee Break 10:30AM - 10:45AM Coffee Break 10:30AM - 10:45AM Coffee Break 10:00 AM - 10:15AM

Session D MRAM-I 10:45 AM - 11:45 AM

Lunch Lunch Lunch 11:45 AM - 1:15 PM 11:45 AM - 1:15 PM 11:45 AM – 1:15 PM Session B Session E Session G HAMR-II MRAM-II PMR and TDMR 1:15 PM - 4:30 PM 1:15 PM - 4:30 PM 1:15 PM - 5:00 PM Coffee Break 2:45 - 3:00 Coffee Break 2:45 - 3:00 Coffee Break 2:45 - 3:00 Poster session H Poster session I Closing Remarks Sessions A, B, C, D & Sessions E, F, G & 5:00 PM Contributed Contributed Bierstube Bierstube 4:45 PM - 6:15 PM 4:45 PM - 6:15 PM Banquet 6:15 PM - 7:45 PM Keynote Speech 7:45 PM - 8:30 PM

13 TMRC 2016│Keynote speaker

Luis Carbonell CEO, MagArray, Inc.

As the CEO of MagArray for the past six years, Luis Carbonell has been at the forefront of developing novel, practical uses for magnetic sensor technology that originated from the magnetic recording head industry. With a background in the pharmaceutical and diagnostics industries, he has guided MagArray through the challenges of using a novel technology to develop innovative diagnostic tests for the early detection of cancer and other diseases. Prior to MagArray, Mr. Carbonell led operations for Oncotech, a clinical laboratory in Southern California that specialized in drug resistance testing for cancer patients. In his role as Chairman of the Diagnostic Steering Committee, he oversaw the validation and launch of all new diagnostic products, including tests for colon and lung cancer. Earlier in his career, while working at Eisai, a major Japanese pharmaceutical manufacturer, he managed the establishment of pharmaceutical manufacturing, R&D, and sales & marketing subsidiaries in Research Triangle Park, North Carolina and Suzhou, China which steadily grew to produce over $2 billion of pharmaceuticals per year. In addition to Luis’ leadership roles in many successful entrepreneurships, he has also accumulated vast experience in establishing partnerships, especially in international settings and between large and small companies. He has negotiated deals with pharmaceutical companies, set up joint ventures, transitioned through acquisitions, and managed technology transfers in the US, Europe and Asia. Luis will share his insights into the challenges of developing successful life science products as well as the opportunities available in this exciting field.

14 TMRC 2016 INVITED PRESENTATIONS

Wednesday AM, August 17th Wednesday PM, August 17th

9:00 AM - 11:45 AM 1:15 PM - 4:30 PM Session A: HAMR-I Session B: HAMR-II Co-chairs: Michael Grobis, Western Digital Co-chairs: Timothy Klemmer, Seagate Technology Douglas Saunders, Seagate Technology Hans-Juergen Richter, Western Digital

1. 9:00 AM: High Track Pitch Capability for 1. 1:15 PM: Thermal Gradient Measurements for HAMR Recording, Chris REA, Pradeep SUBEDI, Heat Assisted Magnetic Recording: Method and Applications, Douglas SAUNDERS, Julius HOHLFELD, A Hua ZHOU, Douglas SAUNDERS, Michael B CORDLE, Pu-Ling LU, Peter J CZOSCHKE, Xuan ZHENG, Tim RAUSCH, Chris REA, Seagate Stephanie HERNANDEZ, Minjie MA, Radek Technology, USA. LOPUSNIK, Yingguo PENG, Jan-Ulrich THIELE, Alexander Q. WU, Ganping JU, Tim RAUSCH, 2. 1:45 PM: Optimizing the Optical and Mike SEIGLER, Edward GAGE, Seagate Thermal Design of Heat Assisted Technology, USA. B Magnetic Recording Media, Pierre-Olivier JUBERT, Michael K. GROBIS, HGST, a 2. 9:30 AM: Areal Density Impact of Western Digital Company, USA. Transition Curvature in Heat Assisted 3. 2:15 PM: NFT Material Q: Assumptions and A Magnetic Recording, Jian-Gang (Jimmy) ZHU, Hai LI, Carnegie Mellon University, USA. Reality, M. STAFFARONI, T. MALETZKY, TDK B Headway Technologies, USA. 3. 10:00 AM: Imaging of the Thermal Spot in Heat Assisted Magnetic Recording, Robert 2:45 - 3:00 PM: Break A EATON, Andreas MOSER, Daniel WOLF, Western Digital, USA. 4. 3:00 PM: Designs and Requirements for Heat Assisted Recording for 10Tb/in2, Dieter SUESS, C. 10:30 - 10:45 AM: Break B VOGLER, University of Vienna.

4. 10:45 AM: Using Ensemble Waveform Signal- 5. 3:30 PM: Heated Dot Magnetic Recording to-Noise Analysis to Compare Heat Assisted Media – the Ultimate Recording Density, A Magnetic Recording Characteristics of B David S. KUO1, Kim Y. LEE1, Xiaomin YANG1, Modeled and Experimental Signals, Stephanie Shuaigang XIAO1, Yautzong HSU1, Zhaoning HERNANDEZ, Pu-Ling LU, Pavol KRIVOSIK, Pin- YU1, Michael FELDBAUM1, Tim KLEMMER1, Wei HUANG, Walter EPPLER, Tim RAUSCH and Yukiko KUBOTA1, Jan-Ulrich THIELE1, Philip Edward GAGE, Seagate Technology, USA. STEINER1, Koichi WAGO1, Stefano DALLORTO2,3, and Deirdre OLYNICK2, 1) Seagate Media Research, Fremont, California, USA, 2) 5. 11:15 AM: Writing Process Modelling and Molecular Foundry, Lawrence Berkeley Identification for Heat-Assisted Magnetic National Lab, Berkeley, California, USA, 3) A Recording, Kun MA1,2, Wai Ee WONG2, Jianyi Oxford Instruments, Concord, Massachusetts, WANG2, Guoxiao GUO2, and Youyi WANG1, 1) USA. Nanyang Technological University, Singapore, 2) Western Digital, USA. 6. 4:00 PM: FePt Damping: Physical Origins and Recording Impact on HAMR and STT- B RAM, Randall H. VICTORA, Tao QU, University of Minnesota, USA.

15 TMRC 2016 INVITED PRESENTATIONS

Thursday AM, August 18th Thursday PM, August 18th 9:00 AM - 10:30 AM 1:15 PM - 4:30 PM Session C: HAMR-HDI Session E: MRAM-II Co-chairs: Erhard Schreck, HGST Co-chairs: Pr. Yu Shiratsuchi, Osaka University Huan H. Tang, Seagate Technology Pr. Jianping Wang, University of Minnesota 1. 1:15 PM: Spin Torque MRAM, Daniel C. 1. 9:00 AM: Write-Induced Head WORLEDGE, Guohan HU, Junghyuk LEE, Janusz J. Contamination In HAMR, James D. KIELY, E NOWAK, Jonathan Z. SUN, Anthony ANNUNZIATA, C Paul M. JONES, Y. YANG, John L. BRAND, Manuel Stephen BROWN, Younghyun KIM, Chandrasekharan ANAYA-DUFRESNE, Patrick C. FLETCHER, KOTHANDARAMAN, Gen LAUER, Nathan Florin ZAVALICHE, Yvete TOIVOLA, John C. MARCHACK, Eugene J. O’SULLIVAN, Jeong-Heon DUDA, Michael T. JOHNSON, Sunita PARK, Mark REUTER, Ray P. ROBERTAZZI, Philip L. GANGOPADHYAY, Seagate Technology, USA. TROUILLOUD, and Yu ZHU, IBM-Samsung MRAM Alliance, IBM TJ Watson Research Center, USA. 2. 9:30 AM: Measurement of Nanoscale HDI Heat Transfer Using a PMR Head with Contact Sensor, Yuan MA and David B. BOGY, 2. 1:45PM: Spin Orbit Torque Switching of CoFeB C Magnetic Free Layers with Pt and Ta Heavy UC Berkeley, Berkeley, USA. E Metals for SOT MRAM Development, Goran MIHAJLOVIĆ, Oleksandr MOSENDZ, Young-Suk CHOI, 3. 10:00 AM: Graphene Coating for Heat- Lei WAN, Patrick BRAGANCA, Neil SMITH and Assisted Magnetic Recording, Neeraj Jordan KATINE, HGST, A Western Digital Company, C DWIVEDI1, Anna K. OTT2, Reuben J. YEO1, Chunmeng DOU2, Ugo SASSI2, Domenico DE USA. FAZIO2, A. C. FERRARI2 and C. S. BHATIA1, 1) 3. 2:15PM: Mag-Flip Spin Torque Oscillator Using National University of Singapore, 2) Cambridge Highly Spin Polarized Heusler Alloy Spin University, UK. E Injection Layer for Microwave Assisted Magnetic Recording, S. BOSU, H. SEPEHERI-AMIN, 10: 0 - 10: 5 AM: Break 3 4 Y. SAKURABA, S. KASAI, M. HAYASHI, and K. HON, National Institute for Materials Science, Tsukuba, Japan. 10:45 AM - 11:45 AM Session D: MRAM-I 2:45 - 3:00 PM: Break Chair: Usha Varshney, National Science 4. 3:00PM: Spin Orbit Engineering for Low Power Foundation Computing, K. H. CAO1, 2, S. Z. PENG1, 2, L. SU1, 2, M. X. E WANG1, 2, X. X. ZHAO1, 2, J. Q. ZHOU1, 2, X. Y. LIN1, 2, Z. H. WANG1, 2, W. S. ZHAO1, 2, 1) Fert Beijing Institute, 1. 10:45 AM: Technical Considerations for Commercial MRAM Products, J.M. SLAUGHTER, Beihang University, Beijing, P.R. China, 2) School of Electronic and Information Engineering, Beihang D R. WHIG, H.-J. CHIA, F.B. MANCOFF, S. IKEGAWA, S. AGGARWAL, M. DEHERRERA, S. DESHPANDE, J. University, Beijing, P.R. China. JANESKY, M. LIN, D. HOUSSAMEDDINE, K. NAGEL, J.J. SUN, Everspin Technologies, Arizona, USA. 5. 3:30PM: Temperature Dependence of Critical Device Parameters in 1 Gbit Perpendicular 2. 11:15 AM: Dynamical Magnetoelectric E Magnetic Tunnel Junction Arrays for STT- 1 1 2 2 Switching of Perpendicular Exchange Bias, Yu MRAM, C. PARK , J.J. KAN , C. CHING , J. AHN , L. 2 2 2 2 2 SHIRATSUCHI1, Nguyen Thi Van ANH1, Ryoichi XUE , R. WANG , A. KONTOS , S. LIANG , M. BANGAR , D 2 2 1 2 NAKATANI1, Yoshinori KOTANI2 and Tetsuya H.CHEN , S. HASSAN , S. KIM , M. PAKALA , and S. H. 1 NAKAMURA2, 1) Osaka Univ., Osaka, Japan 2) KANG , 1) Qualcomm Technologies, Inc., San Diego, JASRI/SPring-8, Hyogo, Japan. California 92121, USA, 2) Silicon Systems Group, Applied Materials, Inc., Sunnyvale, California, USA.

. 4:00PM: Antiferromagnetic Spintronics, X. MARTI1,, I. FINA2 1) Institute of Physics ASCR, Czech E6 Republic., 2) Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Barcelona, Spain.

16 TMRC 2016 INVITED PRESENTATIONS Friday AM, August 19th Friday PM, August 19th 8:30 AM - 11:45 AM 1:15 PM - 5:00 PM Session F: BPMR, MAMR and Advanced Session G: BPMR and TDMR Readers Co-chairs: Pr. Jian-Gang Zhu, Carnegie Mellon Co-chairs: Roger Wood, HGST Gerardo Bertero, Western Digital Pr. Randall Victora, University of Minnesota

1. 8:30 AM: New Block Copolymers Enable G1. 1:15 PM: Characterization of Write Signal 5nm BPM Patterning, Grant WILLSON1, Austin Efficiency Utilizing Write Current Dynamic Wave- F LANE1, Michael MAHER1, Gregory BLACHUT1, Shaping, John T. CONTRERAS, Jonas GOODE, Yusuke ASANO1, Yasunobu SOMEYA1, Stephen Alexander TARATORIN, Xinzhi XING, Jianping CHEN, SIRARD2 and Xiaomin YANG3, 1) The Univ. of Texas, Austin, TX, USA, 2) Lam Research, Western Digital Corporation, San Jose, CA, USA. Fremont, CA, USA, 3) Seagate Technology, Fremont, CA, USA. G2. 1:45 PM: Spatially-Coupled Codes for Advanced Magnetic Recording, Lara DOLECEK, Homa 2. 9:00 AM: High Recording Performance of ESFAHANIZADEH, and Ahmed HAREEDY, University Bit-Patterned Media with Two-Layer of California, Los Angeles, Los Angeles, CA, USA. F Inclined Anisotropy ECC Dots, Naoki HONDA1, Kiyoshi YAMAKAWA2, 1) Tohoku Institute of Technology, Sendai, Japan, 2) Akita Industrial G3. 2:15 PM: Exploring TDMR Gain Constraints, Technology Center, Akita, Japan. James ALEXANDER, Tue NGO, Shafa DAHANDEH, Western Digital, USA. 3. 9:30 AM: Micromagnetic Model Analysis of Spin-Transfer Torque Oscillator and Write 2:45 - 3:00 PM: Break F Heads for Microwave-Assisted Magnetic Recording, Yasushi KANAI1, Kazuetsu YOSHIDA2, Simon GREAVES3 and Hiroaki MURAOKA3, 1) G4. 3:00 PM: Joint Timing Recovery and Signal Niigata Institute of Technology, Kashiwazaki, Detection for Two-Dimensional Magnetic Japan, 2) Kogakuin University, Tokyo, Japan, 3) Recording, Chaitanya Kumar MATCHA and Shayan Tohoku University, Sendai, Japan. Garani SRINIVASA, Department of Electronics Systems Engineering, Indian Institute of Science, India. 10:00 - 10:15 AM: Break

4. 10:15 AM: Multiple Layer Microwave-Assisted 5. 3:30PM: Two-Dimensional Modulation Codes Magnetic Recording, Simon GREAVES1, Yasushi Using Polar Coding Techniques, Hidetoshi SAITO, 2 1 F KANAI , and Hiroaki MURAOKA , 1) RIEC, Tohoku G Kogakuin University, Tokyo, Japan. University, Sendai, Japan, 2) IEE, Niigata Institute of Technology, Kashiwazaki, Japan. 6. 4:00 PM: High Density Turbo TDMR Detection 5. 10:45 AM: High Output CPP-GMR Using with Local Area Influence Probabilistic Model, New Spacer Materials with Half-Metallic G Xueliang SUN1, Krishnamoorthy SIVAKUMAR1, F Heusler Electrodes, Y. SAKURABA, J.W. JUNG, Benjamin J. BELZER1, and Roger WOOD2, 1) S. LI, Y. DU, J. CHEN, T.T. SASAKI, Y. MIURA, T. M. NAKATANI, T. FURUBAYASHI, Y.K. TAKAHASHI, Washington State University, Pullman, WA, USA, 2) and K. HONO, National Institute for Materials HGST, San Jose, CA, USA. Science (NIMS), Tsukuba, Japan. 7. 4:30 PM: Two-Dimensional Equalization and 6. 11:15 AM: Design Study of High Resolution Detection or Two Tracks n Array-Reader and High Reliability TMR Reader with f i G 1 F Recessed Pin Layer Structure, Satoshi Based Magnetic Recording, Jun YAO , Euiseok MIURA, Kenzo MAKINO, Takahiko MACHITA, HWANG2, B. V. K. Vijaya KUMAR3, and George Naomichi DEGAWA, Takumi UESUGI and Takeo MATHEW1, 1) Broadcom Ltd., San Jose, CA, USA, 2) KAGAMI, TDK Corporation, Nagano, Japan. Gwangju Institute of Science and Technology, South Korea, 3) Carnegie Mellon University, Pittsburgh, PA, USA.

17 TMRC 2016 POSTER SESSIONS

Poster Session H Wednesday August 17th, 2016, 4:45 – 6: PM Poster session H features all invited papers from oral sessions A, B, C and D, as well as the contributed posters below. 15

H1) Definition of an Areal Density Metric for Magnetic Recording Systems Steven D. GRANZ1, Tim RAUSCH1, Richard BROCKIE2, Roger WOOD2, Gerardo BERTERO2, and Edward C. GAGE1, 1) Seagate Technology, Shakopee, Minnesota, USA, 2) Western Digital Corporation, San Jose, California, USA

H2) 3D Product Codes for Magnetic Tape Recording Roy D. CIDECIYAN, Simeon FURRER, and Mark A. LANTZ, IBM Research – Zurich, 8803 Rüschlikon, Switzerland

H3) Signal Processing and Coding System for TDMR Data from Grain Flipping Probability Model Morteza MEHRNOUSH1, Krishnamoorthy SIVAKUMAR1, Benjamin J. BELZER1, Sari Shafidah SHAFI'EE2, Kheong Sann CHAN2, 1) Washington State University, Pullman, WA, USA, 2) Data Storage Institute, Singapore, 117608

H4) Interface Modification to Improve the Friction and Wear Resistance of Ultrathin Tape Head Overcoats Reuben J. YEO, Neeraj DWIVEDI and C. S. BHATIA, Department of Electrical and Computer Engineering, National University of Singapore, Singapore

Poster Session I Thursday August 18th, 2016, 4:45 – 6: PM Poster session I features all invited papers from oral sessions E, F, and G, as well as the contributed posters listed below. 15

I1) Ion Beam Patterning of High Density STT-Ram Devices, Vincent IP1, Shuogang HUANG2, Santino D. CARNEVALE1, Ivan L. BERRY2, Katrina ROOK1, Thorsten B. LILL2, Ajit P. PARANJPE3 and Frank CERIO1, 1) Veeco Instruments, Plainview, NY, USA, 2) Lam Research Corporation, Fremont, CA, USA, 3) Veeco Instruments, Somerset, NJ, USA

I2) Design of Cooling NFT System using SPP Waveguide for HAMR Y. HAYASHI, K. TAMURA, Y. ASHIZAWA, S. OHNUKI, and K. NAKAGAWA, Nihon University, Tokyo, Japan

I3) Electric Switchin g of Magnetization using Magnetoelectric Cr2O3 Film T. NOZAKI, M. AL-MAHDAWI, S. P. PATI, S. YE, Y. SHIOKAWA, and M. SAHASHI, Department of Electronic Engineering, Tohoku University, Sendai 980-0845, Japan

I4) Effect of MgO Oxidation Level on Spin-Orbit Torque Magnetization Switching of CoFeB/MgO Nanodots Noriyuki SATO, Robert M. WHITE and Shan X. WANG, Stanford University, Stanford, USA

I5) Achieving 10 nm Full Pitch Lines by Directed Self-Assembly of Block CopolymersCA, on Large Area for HDMR Media XiaoMin YANG,1 Shuaigang XIAO,1 Austin LANE,2 Gregory BLACHUT,2 Michael MAHER,2 Yusuke ASANO,2 Yautzong HSU,1 Zhaoning YU,1 Michael FELDBAUM,1 Stephen SIRARD,3 Philip STEINER,1 Koichi WAGO,1 Kim LEE,1 Diane HYMES,4 David KUO,1 and Grant WILLSON2 , 1) Fremont Research Center, Seagate Technology, Fremont, CA 94538, 2) Department of Chemical Engineering, University of Texas at Austin, Austin, TX 78712, 3) Lam Research Corporation, Austin, TX 78753, 4) Lam Research Corporation, Fremont, CA 94538, USA

18 A1

HIGH TRACK PITCH CAPABILITY FOR HAMR RECORDING

Chris REA1, Pradeep SUBEDI1, Hua ZHOU1, Douglas A. SAUNDERS1, Michael CORDLE2, Pu- Ling LU2, Peter J CZOSCHKE1, Stephanie HERNANDEZ2, Minjie MA2, Radek LOPUSNIK1, Yingguo PENG3, Jan-Ulrich THIELE3, Alexander Q. WU3, Ganping JU3, Tim RAUSCH2, Mike SEIGLER1, Edward GAGE2 1Recording Head Operations, Seagate Technology, Bloomington, MN 55435 USA 2Storage Research Group, Seagate Technology, Shakopee, MN 55379 USA 3Seagate Technology, 47010 Kato Road, Fremont, CA 94538 USA

ABSTRACT With the recent 1.0[1] and 1.5[2,5] Tb/inch2 basic technology demonstration, and drive level demonstration [3,4] Heat-Assisted Magnetic Recording (HAMR) [5] has proven to be a viable and promising technology for future magnetic data-storage products. The commercialization of HAMR presents some significant technical challenges that need to be resolved before the widespread adoption of the technology can begin. The recent demonstrations illustrate high track density performance for this new technology compared to conventional Perpendicular Magnetic Recording (PMR). HAMR

Conventional PMR Figure 1:Micromagnetic modeling for the HAMR (upper) and PMR (lower) write process using a 128-bit pseudo random sequence at 2000 kilo-flux changes per inch (KFCI). Only a subset of the pattern is shown. The upper and lower images use different color schemes, attached at the right, to indicate the level of saturation in the perpendicular direction.

Differences in the areal density capability limits for HAMR and PMR are explored using spinstand measurements, drive footprinting and micromagnetic modeling. The written track curvature is measured with a special technique that mitigates cross-track averaging effects due to finite read sensor width. Tracks written with HAMR heads are shown to have more curvature compared to those written with modern PMR writers. Mitigation of written track curvature is demonstrated with two different HAMR writer designs. The curvature effect appears to challenge not only the downtrack bit resolution during readback, but also the cross-track written width with increased linear density (Fig 2). Experimental measurements of constant bit error rate for different linear and track densities indicate a significant opportunity for high track density recording using HAMR (Fig 3). The difference appears to be related to the ability for HAMR to address high track pitches with a minimal increase in risk of adjacent track interference compared to PMR.

Chris Rea E-mail: [email protected] 19 tel: +001-9524025919 A1

Figure 2 : Track width normalized to that at 300 KFCI for a PMR head (green), and two different HAMR designs. Inset: the measured curvature for all 3 designs, showing higher curvature for the HAMR heads, with HAMR B (red) being worse than HAMR A.

Figure 3: BER loss between tracks written with and without squeeze tracks (one write each) at the specified TD using one head from the PMR and HAMR groups in fig 2 In this paper we explore the recording characteristics in more depth that appear to drive the differences between HAMR and PMR recording and pose a few questions for the community to explore.

REFERENCES 1) A. Wu et al., “HAMR1.0 T/in2 demo” IEEE Transactions on Magnetics 49 (2) , art. no. 6417009 , pp. 779-782 2) G. Ju, Y. Peng et al.” Heat Assisted Magnetic Recording Media and Advanced Characterization – Progress and Challenges”, IEEE Transactions on Magnetics,51, 11, (2015), pp 1 - 9, 3) T. Rausch et al., “HAMR drive performance and integration challenges”, IEEE Transactions on Magnetics Volume 49, Issue 2, 2013, Article number6416994, P730-733. 4) C. Rea and J. Hohlfeld et al. “HAMR Performance and Integration Challenges”, IEEE Trans. Magn. 50, (2014), 62-66 5) C. Rea and P. Subedi et al “Areal density Limits for Heat Assisted Magnetic Recording and Perpendicular Magnetic Recording”, IEEE Trans. Magn,PP,(2016),1-3, DOI: 10.1109/TMAG.2016.2527735

20 A2

AREA DENSITY IMPACT OF TRANSITION CURVATURE IN HEAT ASSISTED MAGNETIC RECORDING

Jian-Gang (Jimmy) ZHU and Hai LI Data Storage Systems Center, Carnegie Mellon University, Pittsburgh, U.S.A., [email protected]

I. INTRODUCTION This paper/talk presents a systematic micromagnetic modeling investigation on the area density impact of curved thermal profile in the media for heat assisted magnetic recording (HAMR). Present NFT designs offer a variety of thermal profiles with various degrees of curvature [1][2]. Previous studies have shown that curved transition front causes various performance degradations due to a combination of both read and write effects [3][4]. Here, we mainly focus on the write effect of curved thermal profile on its recording performance degradations as a function of cross-track position and impact on linear density. The curved thermal profile is represented by a perfectly circular one while a rectangular thermal profile is used for straight transitions without any curvature. The thermal gradient in the radius direction for the circular profile is assumed to be the same as the one for the rectangular profile in the down track direction. A previously developed HAMR micromagnetic model is utilized [5]. A spatially uniform head field with a o field angle of 45 is used for recording. The recording media is assumed to be granular FePt-L10 media with room temperature Hk = 90kOe and a σHk = 10%. The track width of the thermal profile, Trec to Trec, is 40nm for all the cases presented here. Figure 1 shows a set of recorded magnetization patterns, simulated with the circular and rectangular thermal profiles, at two different linear densities. An infinitely wide straight transition patterns were recorded prior to the narrow track recording. For all the cases shown in this digest, a thermal gradient of 6 K/nm is assumed.

II. RESULTS AND DISCUSSION In HAMR, transition jitter noise arises due to finite grain size, grain Curie temperature distribution, and anisotropy field distribution. Medium noise also arises at track edges as shown in Figs 2 and 3 for recording with the rectangular and circular thermal profiles, respectively. Figure 4 shows the medium SNR at a linear density of 2500KFCI with a box read function of 2nm track width. The narrow reader function essentially removes the read-effect of the curved transition. For the circular thermal profile, SNR significantly degrades quickly at cross-track positions away from track center. This is due to the following two factors. One, for circular, the thermal gradient is the highest along the radius direction. However, at distance z away from the track center, the thermal gradient, TG, along the down track direction degrades:

TG(z) = TG (0)⋅ 1 − (2z /W )2 for z

21 A2

[1] N. Zhou, et al, “Plasmonic near-field transducer for heat-assisted magnetic recording,” Nanophotonics, vol. 3, no. 3, pp. 141–155, 2014. [2] J. Gosciniak, et al, “Novel droplet near-field transducer for heat-assisted magnetic recording,” Nanophotonics, vol. 4, no. 1, pp. 503–510, 2015. [3] C. Rea, et al, “Writer and Reader Head-to-Media Spacing Sensitivity Assessment in HAMR,” IEEE Trans. Magn., vol. 52, no. 2, 2016. [4] G. Ju, et al, “High Density Heat-Assisted Magnetic Recording Media and Advanced Characterization - Progress and Challenges,” IEEE Trans. Magn., vol. 51, no. 11, pp. 1–9, 2015. [5] J.-G. Zhu & H. Li, “Understanding Signal and Noise in Heat Assisted Magnetic Recording,” IEEE Trans. Magn., 49 (2), 765-772, 2013.

Fig. 1. Simulated recording patterns with the circular Fig. 2. Noise power distribution along a HAMR track and rectangular thermal profiles. recorded with the rectangular thermal profile.

Fig. 3. Noise power distribution along a HAMR track Fig. 4. Narrow-read SNR plotted as a function of recorded with the circular thermal profile. cross-track position for the circular (circles) and rectangular (triangles) thermal profiles at 1500KFCI linear density.

Fig. 5. Illustration of shortened distance between Fig. 6. Narrow read-track signal power plotted as a subsequent transitions with curved thermal recording function of cross-track position for the rectangular profile at cross-track positions away from track center. profile (upper) and the circular profile (lower)

22 A3

IMAGING OF THE THERMAL SPOT IN HEAT ASSISTED MAGNETIC RECORDING

Robert EATON1, Andreas MOSER2 and Daniel WOLF3 1) Western Digital., San Jose, USA, [email protected] 2) Western Digital., San Jose, USA, [email protected] 3) Western Digital., San Jose, USA, [email protected]

I. INTRODUCTION The curved shape of transitions written in HAMR has a large impact on the recording performance. The curvature maps the contour of the trailing edge of thermal spot profile when it passed through the refreeze temperature during recording. We will discuss a new technique designed to measure the contour of the transition with high resolution. The quantitative imaging technique is performed at drive level and used for determining the shape of the refreeze contour of the thermal spot profile

II. METHODOLOGY A ballistic seek cross-track phase profile is acquired for highest accuracy in the measurement of the average transition shape. In addition to the ballistic seek profiles a transverse micro track trim is introduced to significantly remove cross track reader convolution. The combination of these techniques allows for a fast, phase coherent version of the iterative concentric micro track trim methods, which were previously used to examine curvature of written transitions [1]. The cross track phase profiles are acquired in two steps. First, a digital oscilloscope is used to acquire the on-track reference trace. This trace contains periodic data in the data sector as well as the sync marks in the servo fields at either end. The data portion is analyzed to find the exact frequency of the periodic data. Second, a periodic sinusoidal position offset is introduced such that the reader crosses the data track with a radial velocity of approximately 200 nm per wedge to wedge interval. The next acquired trace records the crossing of the data and the servo fields. The servo sync fields are used to correct the timing differences between the reference trace and the data trace. The phase delta between the crossing trace and the reference trace versus the off-track position map out the transition shape. The 2D image of a single period of the test pattern can be constructed from the crossing data trace. The data trace is resampled such that there are an integer number of samples per period. Next the trace is arranged as 2D image data with one period per row. Finally, the raw image is averaged in the radial direction and down-sampled to produce an image with equal scaling in the radial and down-track directions. This typically produces an image with ~1nm square pixels. This method solves the problem of aligning the phases of adjacent scan lines by using a single phase reference for all of the scan lines in the image. The images produced with the phase profiles faithfully reproduce the reader’s 2-D voltage response to the magnetization pattern in the media. However, it is often desirable to know the 2-D media magnetization that generated reader response. To do this the influence of the cross track reader convolution needs to be minimized. This has been accomplished previously with concentric micro track profiles. The method presented in this paper creates a single transverse micro track that runs from one side of the track to the other. The reader then follows down the center of this micro track as the phase profile is acquired. As a result the phase shift in the phase profile is localized to a very narrow (~20nm) set of off-track positions.

III. RESULTS Figure 1 is a series of images showing the effect of applying the transverse micro track over a range of skew angles (-12.2 to 11.3 degrees). In comparison to the untrimmed track (Fig 1 B.) the micro track images (Fig.1A) show a smaller radius of curvature and the ‘nose’ of the curvature moves from one side of the track to

Robert Eaton E-mail: [email protected] tel: +01-408-363-5509 23 A3

the other as a function of skew. The fact that the transition line is a function of skew indicates that the recorded transitions are not circular. The transition line in the micro track images can be fitted with an ellipse that is rotated with the skew angle. Figure 2 show the effect of increasing laser current on the phase profile images. The radius of curvature in the untrimmed images (Figure 2A) appears to get smaller with increasing laser current. This is a consequence of the cross track reader convolution [2]. The images prepared with transverse micro track show the expected trend of increasing radius of curvature with increasing laser current.

REFERENCES 1) Chris Rea, et al. “Areal density Limits for Heat Assisted Magnetic Recording and Perpendicular Magnetic Recording”, IEEE Transactions on Magnetics , DOI 10.1109/TMAG.2016.2527735 2) G. Bertero, M. Alex, D. Wolf, B. Valcu, R. Eaton, “HAMR Recording Challenges at High Linear Densities”, Intermag 2015, Beijing, China, Paper AA-03

Figure 1 Images of recorded patterns shown versus skew and transverse trim A. Transverse trim micro track 45% MRW. B. Untrimmed

Figure 2. Recorded pattern shape vs. laser current. A. Untrimmed B. Transverse trim micro track

24 A4

USING ENSEMBLE WAVEFORM SIGNAL-TO-NOISE ANALYSIS TO COMPARE HEAT ASSISTED MAGNETIC RECORDING CHARACTERISTICS OF MODELED AND MEASURED SIGNALS

Stephanie HERNANDEZ1, Pu-Ling LU1, Pavol KRIVOSIK2, Pin-Wei HUANG3, Walter EPPLER2, Tim RAUSCH1 and Edward GAGE1 1) Storage Research Group, Seagate Technology, Shakopee MN 2) Recording Head Operations, Seagate Technology, Bloomington MN 3) Recording Media Operations, Seagate Technology, Fremont CA

I. INTRODUCTION Heat assisted magnetic recording (HAMR) [1] is the technology most likely to surpass conventional perpendicular magnetic recording (PMR) in terms of areal density capability (ADC). However, written-in media noise is a dominant noise source in HAMR that must be addressed before the multiple terabit per square inch ADC predicted can be reached. Micromagnetic analysis has long proven to be a useful tool in understanding media noise in PMR and, more recently, in HAMR [2]. Recently [3], signal to noise characteristics obtained using the autocorrelation signal to noise method [4] matched well between micromagnetically modeled HAMR waveforms and waveforms obtained from spin-stand measurements. This analysis included the breakdown of transition and remanence signal to noise ratios (SNRs) as a function of laser power and linear density. In this paper, micromagnetically modeled HAMR waveforms are compared to waveforms obtained from both drive and spin-stand measurements operating at similar conditions using the same auto-correlation signal to noise method.

II. RESULTS

Fig. 1: Transition SNR as a function of ambient temperature for both drive measurement and model. Figure 1 shows total spatial SNR as a function of ambient temperature at constant track width for both modeled waveforms and waveforms obtained from drive measurements. The drive measurements were performed at a linear density of 1270 kilobits per square inch (KBPI) and a track density of 375 kilotracks per square inch (KTPI). The near field transducer (NFT) peg width in the experiment is 50 nm and the reader

Stephanie Hernandez [email protected] 952-402-2348 25 A4

width (RW) is 30 nm. The modeled waveforms were obtained using the renormalized LLG method. In the model the linear density is 1588 KBPI and the track density is 375 KTPI. The NFT peg width is 60 nm and the RW is 30 nm. Both the experiment and the model show that transition SNR is relatively flat as a function of ambient temperature at the conditions tested.

Figure 2: CBD as a function of cross-track offset/track width for different peg widths.

Given the good agreement between model and experiment using this signal to noise analysis, we suggest using this method as a way to easily characterize recording behavior at any range of conditions and media/head properties. For example, figure 2 shows channel bit density (CBD) versus cross-track reader offset as a percentage of track width for modeled waveforms. Two different peg widths are included (30 nm and 60 nm), writing at the same track width. CBD is defined as the PW50 (pulse width at the 50% amplitude point of the differentiated reader response to an isolated transition) divided by the bit length. It is estimated from the dibit response method as described in [5]. The ambient temperature in the simulation is 300 K for both peg widths, and the recording conditions are as stated above. The CBD inflation with cross-track reader position is much steeper for the 30 nm peg width compared to the 60 nm peg width at the same written track width. This is indicative of how much the curvature increases as the peg width decreases.

REFERENCES 1) M. Kryder, E. Gage, T. McDaniel, W. Challener, R. Rottmayer, G. Ju, Y. Hsia, F. Erden, “Heat Assisted Magnetic Recording”, Proc. IEEE 96, 1810-1835 (2008) 2) J.-G. Zhu, H. Li, “Understanding Signal and Noise in Heat Assisted Magnetic Recording”, IEEE Trans. Magn. 49, 765-772 (2013) 3) S. Hernandez, P. Krivosik, P.W. Huang, W. Eppler, T. Rausch, E. Gage, “Parametric Comparison of Modeled and Measured Heat Assisted Magnetic Recording Using a Common Signal to Noise Metric”, IEEE Trans. Magn. (Accepted) 4) G. Mian, T. Howell, “Determining a Signal to Noise Ratio for an Arbitrary Data Sequence by a Time Domain Analysis”, IEEE Trans. Magn. 29, 3999-4001 (1993) 5) Synchronization-Free Dibit Response Extraction From PRBS Waveforms," Inci Ozgunes and Walter R. Eppler, IEEE Trans. Magn. 39, 2225 (2003)

26 A5

WRITING PROCESS MODELLING AND IDENTIFICATION FOR HEAT-ASSISTED MAGNETIC RECORDING

Kun MA1,2, Wai Ee WONG2, Jianyi WANG2, Guoxiao GUO3, and Youyi WANG1 1) School of Electrical & Electronic Engineering, Nanyang Technological University, Singapore 639798 2) Western Digital Singapore HDD R&D Center, 89 Science Park Drive #03-05, Singapore 118261 3) Western Digital Technologies, 3355 Michelson Dr. Suite 100, Irvine, CA 92612, USA

I. INTRODUCITON Heat-assisted magnetic recording (HAMR) technology is expected to bring the areal density (AD) to beyond a few Tb/in2. The written-in signal quality for HAMR is dominated by the hotspot thermal profile on the media [1]. Given a head and media design, the thermal profile is mainly determined by the input laser power and the spacing between near field transducer (NFT) and the media, referred as NFT-to-media spacing (NMS) in this paper. NMS keeps changing during the writing process due to laser heat dissipation generated protrusions [2], on top of the usual spacing change due to thermal fly-height control (TFC) and writing current. In this paper, we construct a dynamic HAMR writing process model and take input laser power and TFC as the inputs and the signal magnitude as output as shown in Fig. 1. The parameters of this model are identified by minimizing the difference between the model output and drive measurement data using Gaussian-Newton method.

II. METHODOLOGY The writing process model consists four parts. Namely, the NFT protrusions whose time constants are represented by cp, cs, τp and τs; the energy transfer efficiency vs. NMS which is governed by f(dnms); the media heat transient process which is characterized by ch and τh; and finally the thermal power to signal conversion function g(PE). ΔPF is the power applied to the heater for TFC to adjust the default fly-height during write. It is applied long before the actual write happens hence assumed to have reached its steady state when writing starts. The input laser power are divided into two parts. The first part is called prelase PPL which is used to preheat the NFT[1]. This power is applied some time before the actual write, similar to ΔPF. Prelase power PPL should be kept low enough to avoid erasing the servo pattern. The second part is called active laser power PA. Active laser power is only applied during actual write. In our system identification, we keep the PPL constant and sweep PA and ΔPF while writing a single tone signal to the media. PA is chosen to be a step function. For each combination of inputs PA and ΔPF, the written-in signal magnitude vs. time is measured. The signal magnitude SM is derived by moving average of the absolute value of the raw signal waveform captured by a digital oscilloscope. The moving window size is the same as the period of the written-in signal. The true signal magnitude ST is derived by subtracting noise floor N from SM.

In this paper, we assume that the NFT fast protrusion magnitude dp is proportional to the input laser power, and that the temperature at hotspot center of the media is proportional to the thermal power delivered to the media PM. The relationship between effective media thermal power PE and the written-signal magnitude can be approximated by a quadratic function when the thermal spot size is small. With measured signal magnitude at different time and input conditions, together with the above assumptions and approximations, the model parameters can be identified accordingly.

III. RESULTS

To verify the accuracy of the model, active laser power PA with overshoot is applied instead of the step function used during system identification. As shown in Fig. 2, a good match between simulation and

Guoxiao Guo E-mail: [email protected] tel: +1-949-6729721 27 A5

measurement can be achieved. As one application example of the model, we can estimate and compensate for the NMS change when writing consecutive number of sectors. Fig. 3 compares the signal magnitude of a few sectors of data and RRO correction field. Data occupies 80% of the wedge to wedge time, and causes slow protrusion ds to increase over time. Such increase in ds does not happen for RRO write which occupies 2% of wedge to wedge time. As a result, the RRO signal magnitude is lower than data even the same laser power is applied. With NMS compensation, RRO field magnitude can be improved to the same level as data when slow protrusion ds reaches the steady state as shown in Fig. 4.

REFERENCES [1] M. H. Kryder, E. C. Gage, T. W. McDaniel, W. A. Challener, R. E. Rottmayer, G. Ju, Y.-T. Hsia, and M. F. Erden, "Heat Assisted Magnetic Recording", Proc. IEEE, 96(11), pp. 1810-1835, (2008). [2] P. Haralson, H. Ruan, G. Chia, P. Thayamballi, K. Anderson, W. Cain, S. Dahandeh, J. Alexander, C. Macchioni, G. Bertero, "HAMR Drive Integration Features and Challenges", Digest of TMRC 2015, paper A1.

Fig. 1. HAMR writing model.

Fig. 2. Good match between simulation and drive measurement signal magnitude ST given input laser current shown. NFT protrusion is around 4 nm.

Fig. 3. Simulation of data (>80% duty cycle) and RRO (<2% duty cycle) writing for consecutive number of wedges using laser setting optimized based on data writing conditions. Amplitude for wedges #1, 3, 7, 15 and 30 are overlapped for comparison. Right hand side is the zoom-in view of the left figure to Fig. 4. With NMS compensation, RRO writing can show the RRO and initial part of data clearly. achieve the same signal quality as data writing when slow protrusion reaches steady state.

28 B1

THERMAL GRADIENT MEASUREMENTS FOR HEAT-ASSISTED MAGNETIC RECORDING: METHOD AND APPLICATIONS Douglas A. SAUNDERS, Julius HOHLFELD, Xuan ZHENG, Tim RAUSCH, and Chris REA Seagate Technology, Bloomington MN, USA

I. INTRODUCTION The linear density limit of a magnetic recording system is largely determined by the grain size of the recording media and the effective write gradient of the recording head. In a Heat-Assisted Magnetic Recording (HAMR) system, the effective write gradient is dominated by the trailing edge gradient of the heated spot – the thermal gradient. In this presentation, a new method for measuring thermal gradient is reviewed, and examples of its usefulness in design feedback are shown.

II. THERMAL GRADIENT MEASUREMENTS Thermal gradient (TG) is usually measured by modulating the power of the laser that provides the light used to heat the media via the Near-field Transducer (NFT). The power modulation modulates the size of the thermal spot on the meda. During magnetic pattern writing, this modulates the width of the written track and the downtrack positions of the written transitions (Figure 1). By measuring how much the width changes or how far the transitions shift downtrack per fractional power change, the gradient, ∂T/∂x, can be inferred. [1,2] For the downtrack case:

∂T ΔIop ()−TT aW = ⋅ (1) ∂x Iop Δx

where Δx is the downtrack modulation of the written transition from its nominal position, Iop is the nominal laser operating current above the lasing threshold, ΔIop is the current deviation due to modulation, TW is the writing temperature, and Ta is ambient temperature. For the crosstrack case, half of the change in width is substituted for Δx in Eq. 1, producing the crosstrack gradient at the edge of the track. The downtrack measurement is most relevant to determining linear density while the crosstrack is most relevant to determining track density. To measure the transition shifting due to laser modulation, the signal is usually digitized and the locations of the transitions are determined by zero-crossings [1,2]. An alternative method introduced here is to detect the spectral sidebands caused by the transition shifting, or phase modulation. This can be done with the readily available narrow band or “overwrite” filters of a typical magnetic recording spinstand.

For a sinusoidal laser modulation at frequency ωm which produces a transition displacement modulation Δx of a carrier signal written at ωc with head-media velocity v, it will be shown that the ratio of the first spectral sideband amplitude to the signal amplitude (in voltage) is a ratio of Bessel functions. At low modulation levels, the ratio simplifies to be directly proportional to Δx:

. . . . Sideband_Ratio (SBR) = J0(Δx ωc / v ) / J1(Δx ωc / v ) = ~ 0.5 Δx ωc / v (2) This measurement of Δx can be implemented on any tester by adding an inexpensive oscillator to provide calibrated modulation for the laser current. Consideration of calibration and optimum test frequencies will be covered.

Douglas Saunders E-mail: [email protected] tel: +1-952-402-7162 29 B1

III. APPLICATIONS With only one write and two reads, the simplicity and quickness of the test makes it useful at the production level where thousands of heads can be measured and compared. Such data are shown in Figure 2, illustrating how thermal gradient determines a fundamental recording property such as PW50. Other examples will include dimensional targeting and design comparisons.

REFERENCES 1) J. Hohlfeld, X. Zheng, M. Benakli, “Measuring temperature and field profiles in heat assisted magnetic recording,” J. Appl. Phys. 118, 064501,( 2015). 2) H. J. Richter, C. C. Poon, G. Parker, M. Staffaroni, O. Mosendz, R. Zakai, and B. C. Stipe, “Direct measurement of the thermal gradient in heat assisted magnetic recording,” IEEE Trans. Magn., vol. 49, no 10, pp. 5378-5381, October 2013.

Fig. 1. A change in laser power changes the width of the track and the downtrack location of the transition. The incremental changes will be inversely proportional to the thermal gradient.

Fig. 2 Large thermal gradient is necessary for low pulse width.

30 B2

OPTIMIZING THE OPTICAL AND THERMAL DESIGN OF HEAT ASSISTED MAGNETIC RECORDING MEDIA

Pierre-Olivier JUBERT and Michael K. GROBIS Western Digital Corporation, San Jose, CA, USA, [email protected]

Heat assisted magnetic recording (HAMR) is a promising technology for extending data recording densities to well beyond 1 Tb/in2 [1-2]. HAMR recording involves locally heating magnetic media to temperatures above the Curie temperature using intense electric fields generated by laser excitation of a near field transducer (NFT) located on the write head. The characteristics of the resulting thermal hot spot in the media play a large role in determining the areal density capability of the media. High areal densities require high thermal gradients and narrow thermal write widths to enable good on track performance and high track density. The thermal profile stems from both the optical characteristics of the coupled NFT-media system, as well as the thermal transport properties of the media. As such, both the optical and thermal properties of the medium stack need to be optimized for maximizing performance for a given NFT [3]. The media optimization has several additional constraints. First, the media stack needs to function as a recording media in a hard disk drive, which limits the choices in the media design and adds a convective term to the heat flow. Second, the NFT lifetime will decrease with increased thermal load, which necessitates properly weighing potential tradeoffs between recording performance and laser power. In this work we report on a systematic modeling optimization of the optical and thermal properties of a simplified HAMR media stack that helps reveal basic design principles for a HAMR recording environment. Optical and thermal profiles are calculated for a four-layer stack that, above a glass substrate, includes a heatsink, an underlayer, the recording layer and a capping layer. The thickness t, refractive index n, extinction coefficient k and thermal conductivity κ of each layer are varied systematically. The optimization searches for the media parameters that maximize the temperature gradient in the recording layer, at a fixed track width and with the constraint of keeping the laser power below a threshold Pmax. The latter is imposed by NFT lifetime considerations. Fig. 1 illustrates the variation of gradient and power at fixed write width for a media stack with varying underlayer thermal conductivity, for recording velocity ranging from 0 to 30 m/s. We introduce a weighted gradient and power to properly account for the recording performance and power required to write sectors across the disk. These parameters are used to rank the different media property combinations

!" !" !" !" !" (��⁄��) = ∫ ! (�)! ∗ � ∗ ��!∫ � ∗ �� � ! = � � ∗ �� �� (1) ! !" !" !" !" !"

The optimization results are summarized in Table I. Low n, large k plasmonic materials provide an ideal reflecting mirror to the NFT, forming a resonant cavity where the electromagnetic field is optimally confined. The overcoat and recording layer are optimum for n~1 and k~1, compromise between maximizing the absorption and minimizing the attenuation of the electromagnetic field. Low in-plane thermal conductivity is essential to limit lateral heat flow in the recording layer. Low effective thermal boundary conductance provide a boost in temperature gradient at constant required laser power. Stronger heatsinking increases the temperature gradient but at the expense of a proportional increase of the required laser power. The heatsink thickness and thermal conductivity can therefore be tuned to achieve the target written track width at the maximum laser power Pmax. Practically however, it is challenging to build a HAMR stack that satisfies all the requirements of Table I. The optical properties and thermal properties of materials are not independent. In addition, the recording layer need to have the right microstructure and magnetic orientation, which imposes constraints on the available underlayer and heatsink materials. These constraints necessarily lead to material compromises for the HAMR stack. It is important to understand and quantify the impact of each optical or thermal media parameter. This optimization study provides such analysis and the paper will present quantitative comparisons of different

Pierre-Olivier JUBERT E-mail: [email protected] tel: +1-408-7176071 31 B2

optical and thermal tradeoffs/variations. REFERENCES 1) A. Q. Wu, et al. “HAMR Areal Density Demonstration of 1+ Tbpsi on Spinstand”, IEEE Trans Magn. vol. 49, no. 2, pp. 779-782, Feb. 2013. 2) G. Ju, et al., “High Density Heat-Assisted Magnetic Recording Media and Advanced Characterization—Progress and Challenges”, IEEE Trans Magn. vol. 51, no. 11, p. 3201709, November. 2015. 3) W. A. Challener et al., “Heat-assisted magnetic recording by a near-field transducer with efficient optical energy transfer,” Nature Photon., vol. 3, pp. 220–224, Mar. 2009; B. C. Stipe et al., “Magnetic recording at 1.5 Pb m−2 using an integrated plasmonic antenna,” Nature Photon., vol. 4, pp. 484–488, Jun. 2010; T. Matsumoto, et al., “Integrated head design using a nanobeak antenna for thermally assisted magnetic recording”, Photonics Express, vol. 20, no. 17, pp. 18946-18954, 2012.

Table I: Optical and thermal optimization results for a simple four-layer HAMR stack. In brackets are the variation range of each parameter. Optimum optical properties Optimum thermal properties

Layer t (nm) n k κ (W/mK), h (W/m2K)

overcoat 5 [fixed] 1 in [0.1:5] 1 in [0:8] 1.3 [fixed]

mag. layer 10 [fixed] 1 in [0.1:5] 1 in [0:8] κip →0 [0.001:5] / κop > 4 [1:21]

-1 interface mag-UL hMag-UL heff = (1/ hMag-UL+ 1/ hUL-HS+tUL/κUL)

underlayer tUL→0 0.1 in [0.1:5] 8 in [0:8] κUL heff = 20 in [20:200]

interface UL-HS hUL-HS

heatsink >60 0.1 in [0.1:5] 8 in [0:8] κHS > 200 W/mK [1:400] (to be adjusted with tHS to achieve target track width at Pmax)

Fig. 1 – Evolution of the temperature gradient and of the required laser power to write at track of 60 nm when the effective thermal conductance varies from 380 MW/m2K to 38 MW/m2K (right to left). Open symbols correspond to different disk velocities. Stars show the velocity weighted power and gradients.

32 B3

NFT MATERIAL Q: ASSUMPTIONS AND REALITY

M. STAFFARONI and T. MALETZKY TDK Headway Technologies Inc., Milpitas, USA [email protected], [email protected]

I. CONVENTIONAL NFT MATERIAL Q ASSUMPTIONS Gold (Au) has been in vogue for some time as the choice material for implementing HAMR near-field transducers (NFTs) [1,2] owing to its chemical inertness and superb optical quality in the spectral region of interest for HAMR, which is typically around 1.5eV [2-5]. Refractive index measurements of Au thin films employed in NFT fabrication have been found to always yield values consistent with literature [7-10], with material quality factor (Q = ɛ’/ɛ”) in the range of 12-30, the exact value depending on the choice of film deposition technique and conditions. It is thus commonplace in NFT design and modeling to assume textbook Au optical properties [2,5-6].

II. RECOVERING EXPERIMENTAL TRENDS WITH TEXTBOOK GOLD The validity of assuming textbook Au optical properties for NFT modeling can be tested by checking model predictions against spin-stand data. In Fig. 1 we show such a comparison for 54 different NFT configurations with the same head overcoat tested on the same media under identical conditions with MWW(6T) = squeeze track pitch ±3nm. The left panel of Fig.1 shows a plot of measured squeeze BER vs. model cross-track thermal gradient. The right panel of Fig. 1 is a plot of observed NFT power requirement vs. model expectation for the parts from the left panel (note the spread is due entirely to difference in NFT dimensions or geometry). Each data point corresponds to the median experimental value for a group of 10-50 nominally identical NFTs, with vertical bars representing each group’s measured 2σ. The model geometries were built based on average NFT dimensions measured from SEMs of 10 virgin parts for each NFT configuration.

Fig. 1: Comparison of spin-stand data to model predictions assuming textbook Au optical properties.

We find models assuming textbook Au optical properties can recover the expected link between high thermal gradient and good recording performance; however they fail to reproduce experimentally observed NFT power requirements (with the exception of the solid markers which will be discussed is Section III).

Matteo Staffaroni [email protected] +1-408-934-5507 33 B3

III. RECOVERING EXPERIMENTAL TRENDS WITH REVISED GOLD The model-experiment gap in the NFT power requirement can be closed (as illustrated in Fig. 2) by revising the optical properties of the NFT Au in the model such that the material Q is reduced by a factor of ~4x (achieved by increasing the real part of the index of refraction alone). Physically, this downward revision may be attributed to the ubiquitous use of metallic wetting layers between NFT Au and the surrounding dielectrics [11], or to process steps involved in giving the NFT its shape. As expected, a three-fold reduction in NFT material Q adversely affects the maximum achievable thermal gradient, but the trend of higher gradient yielding better performance persist.

Fig. 2: Comparison of spin-stand data to model predictions assuming revised Au optical properties.

The reduction in optical quality of the NFT Au has significant implications for NFT design strategy as high thermal gradients remain attainable but the path to attaining them involves unconventional NFT design choices. It is also worth noting that the significant reduction in the material quality factor of current Au-based NFTs opens the door for use of other materials previously scorned for being optically inferior to Au. The solid markers in Fig.1 & 2 correspond to NFTs that are, for the most part, not made of Au. Note that they remain close to the unity line in the NFT power requirement plots of both Fig. 1 and Fig. 2 since they only minimally rely on Au to achieve their optical resonance, and are thus not affected by the model assumptions for the Au optical properties.

REFERENCES 1) W.A. Challener, et al., Nature Photonics, 3(4), 220-224, (2009). 2) B.C. Stipe, et al., Nature Photonics, 4(7), 484-488, (2010). 3) E.F. Rejda, and K.W. Wierman, U.S. Patent. No. 9,025,281, (2015). 4) W.A. Challener, U.S. Patent No. 7,272,079, (2007). 5) T. Zhao, et al., U.S. Patent No. 8,427,925, (2013). 6) W.A. Challener, U.S. Patent No. 7,106,935, (2006). 7) M.L. Thèye, Physical Review B, 2(8), 3060 (1970). 8) P.B. Johnson, and R.W. Christy, Physical Review B, 6(12), 4370 (1972). 9) D.E. Aspnes, E. Kinsbron, and D.D. Bacon, Physical Review B, 21(8), 3290, (1980). 10) E.D. Palik, Handbook of Optical Constants of Solids, Academic Press (1998). 11) H. Balamane, et al., U.S. Patent No. 8,988,827, (2015).

34 B4

DESIGNS AND REQUIREMENTS FOR HEAT ASSISTED RECORDING FOR 10 TBIT/INCH²

D. SUESS1 and C. VOGLER2 1) Christian Doppler Laboratory for Advanced Magnetic Sensing and Materials, Institute for Solid State Physics, TU Wien, Wiedner Hauptstrasse 8-10, 1040 Vienna, Austria, [email protected]

2) Institute of Solid State Physics, TU Wien, Wiedner Hauptstrasse 8-10, 1040 Vienna, Austria, [email protected]

I. Introduction In heat assisted recording central questions are (i) how thermally written in errors [1] and transition jitter can be minimized (ii) if proper media designs [2] can lower the required temperature of the NFC (iii) and how the recording speed influence the error rate. In order to tackle these questions we performed simulations based on a coarse-grained LLB approach [3], which shows excellent agreement with atomistic simulations [4]. Different recording schemes such as pulsed laser spot recording (PLSR) and continuous laser spot recording (CLSR) are discussed. For each recording scheme shingled as well as conventional (centered track) recording are analyzed. The switching behaviors of monolayers (HM) as well as exchange spring structures with graded Curie temperature [2] (HM2/SM) under different external conditions are examined by means of these models. With the simulation results we extract (i) for bit patterned magnetic recording (BPM) the bit error rates and (ii) for granular recording the transition jitter in cross and down track direction as well as written in errors. II. Results

In the presented calculations distributions of the Curie temperature (Tc), grain size and grain positions are considered according to the ASTC guidelines. The average grain size is d = 5 nm and the thickness of the media t = 10nm. We investigated one island of two different media design. One hard magnetic island (HM), where the media parameter are as following (HM1): K1 = 6.6 MJ/m³, Js = 1.43 T, damping λ = 0.1, Tc = 536 K. The direction of the easy axis is z. In order to decrease the thermally written in errors a modulated Tc media was investigated (HM2/SM), where the upper part (first 5 nm) of the grain is softer and has a high Curie temperature than the lower part (second 5 nm). The upper part has the parameter: K1 = 0 MJ/m³, Js = 2.16 T, damping λ = 0.1, Tc = 1140 K. The lower part of the media has the following parameters: K1 = 6.6 MJ/m³, Js = 1.43 T, damping λ = 0.1, Tc = 744 K. The recording process of continuous laser spot recording is modeled by moving a Gaussian heat pulse across the media with a constant maximum temperature Tpeak. The full width halve maximum of the temperature pulse is, FWHM = 20nm. In addition to the heat pulse a time varying field is applied with a frequency of 1 GHz, pointing alternately in the +z and –z direction with a maximum field strength of µ0Hmax = 0.8T. Switching from +z to –z occurs within a time of 0.2 ns.

The effect of the head velocity vhead on the switching probability is shown in Fig. 1 for these two media designs. The phase diagrams can be seen as footprints of the recording head. At high peak temperatures, above TC, the switching behavior of the HM grain becomes worse for increasing vhead. This can be explained by the duration of the heat pulse, which becomes shorter for higher velocities. If the pulse duration becomes comparable to the time the bits need for magnetization reversal, the switching probability decreases. This decrease starts at very high temperatures. The reason is that the effective time window for reversal starts with the cooldown below the Curie point. For high peak temperatures, the thermal gradient increases and the effective time window slightly below Tc narrows. The same effect was observed for PLSR (see Fig. 4a) or for granular media in [5].

Dieter Suess E-mail: [email protected] tel: +43-1-58801-13746 35 B4

2 For a head speed of vhead = 7.5 m/s we obtain for the (HM) BPM an areal storage density of 13.23 Tb/in for shingled and 6.62Tb/in2 for conventional recording for a bit error rate of BER < 10-3, respectively [6]. For higher head speeds (vhead = 20 m/s) due to the significant thermally written in errors for the media (HM) graded Curie temperature media (HM2/SM) have to be used in order to achieve BER < 10-3. How an increase of the head velocity and a wider FWHM influences the recording performance is discussed in detail in the talk. For granular recording the expected jitter in down-cross and cross-track direction is as low as 0.28 nm and 0.6nm for a head velocity of 7.5 m/s. The talk is concluded with an outlook for the potential of heat assisted recording utilizing first order phase transition, which have the potential to allow for small AC and DC errors at high data rate..

1) H. J. Richter, A. Lyberatos, U. Nowak, R. F. L. Evans, und R. W. Chantrell,'The thermodynamic limits of magnetic recording', J. Appl. Phys., 111, 33909, (2012). 2) D. Suess und T. Schrefl,'Breaking the thermally induced write error in heat assisted recording by using low and high Tc material', Appl. Phys. Lett., 102, 162405 (2013). 3) R. F. L. Evans, W. J. Fan, P. Chureemart, T. A. Ostler, M. O. A. Ellis, und R. W. Chantrell,'Atomistic spin model simulations of magnetic nanomaterial', Journal of Physics: Condensed Matter, 26, 103202 (2014). 4) C. Vogler, C. Abert, F. Bruckner, und D. Suess,'Landau-Lifshitz-Bloch equation for exchange-coupled grain', Phys. Rev. B, 90, 214431 (2014). 5) J. G. Zhu und H. Li,'Medium Optimization for Lowering Head Field and Heating Requirements in Heat-Assisted Magnetic Recordin', IEEE Mag. Lett., 6, 1 (2015). 6) C. Vogler, C. Abert, F. Bruckner, D. Suess, und D. Praetorius,'Heat-assisted magnetic recording of bit-patterned media beyond 10 Tb/in', Appl. Phys. Lett., 108, 102406 (2016).

Fig. 1 Switching probability phase diagram (footprint of written bit pattern) of the a) HM and b) HM2/SM grains under an applied field of 0.8T. Various head velocities vhead are compared.

36 B5 HEATED DOT MAGNETIC RECORDING MEDIA – THE ULTIMATE RECORDING DENSITY

David S. KUO1, Kim Y. LEE1, Xiaomin YANG1, Shuaigang XIAO1, Yautzong HSU1, Zhaoning YU1, Michael FELDBAUM1, Tim KLEMMER1, Yukiko KUBOTA1, Jan-Ulrich THIELE1, Philip STEINER1, Koichi WAGO1,

Stefano DALLORTO2,3, and Deirdre OLYNICK2

(1) Seagate Media Research, Fremont, California, USA. [email protected] (2) Molecular Foundry, Lawrence Berkeley National Lab, Berkeley, California, USA. (3) Oxford Instruments, Concord, Massachusetts, USA.

I. INTRODUCTION Areal-density growth is critical for the hard disk drive industry to compete with solid-state memory in client computing and to meet increasing cloud demand in big data. As perpendicular magnetic recording (PMR) approaches its limitation in areal density, heat-assisted magnetic recording (HAMR) is the next technology focus to elevate the areal density to beyond 1.5-2 Tbpsi. After HAMR, heated dot magnetic recording (HDMR) is the only conceivable path to reach beyond 10Tbpsi. HDMR media consists of lithographically defined high anisotropy (Ku) magnetic islands. Its recording requires synchronous writing to individual dots with laser to raise the magnetic island’s temperature beyond the Curie temperature while applying a magnetic field to switch the dots. To fabricate HDMR media, one starts with a well-oriented continuous FePtX film and utilizes low-cost nano-imprint lithography and advanced pattern transfer techniques to form magnetic islands of single-digit nm size. HDMR presents huge challenges to media fabrication due to extremely small (< 10 nm) and tight -tolerance feature sizes and pitches. It also requires custom-made production equipment to achieve throughput and cost targets. Furthermore, the incorporation of servo features adds to process complexity. Though challenging, recently the Seagate team has successfully demonstrated 5Tdpsi FePt based HDMR media with 15K Oe coercivity (Figure 1).

II. RESULTS AND DISCUSSION

Fabrication of HDMR involves two major process blocks. One is quartz template fabrication and the other is the media patterning process. Quartz template fabrication utilizes a rotating e-beam system to prepare a low density seed pattern for both servo and data. Directed self-assembly (DSA) of block copolymers is then used to achieve higher density and the final pattern. This mix-and-match DSA and conventional lithography scheme has been successfully developed to incorporate both servo and data pattern in imprint templates as illustrated in Figure 2. Both 1Tdpsi and 2Tdpsi densities have been demonstrated. Subsequent media fabrication involves pattern transfer by nano-imprint lithography (NIL) and ion-beam etching (IBE). 1.5Tdpsi HDMR and 2.0 Tdpsi BPM media with fully functional integrated servo features have been successfully fabricated on 2.5” disks.

David S Kuo [email protected] 37 +1-510-353-4923 B5 Track-following capability and basic read-write functionality for a 1.5 Tdpsi HDMR media (Figure 3) are shown using a spinstand.

Although Seagate has achieved significant progress in HDMR technology, many challenges remain. The hexagonal patterned media have severe limitations in recording margin, skew mitigation, and bit aspect ratio. Patterned rectangular bits with lamellae-forming block co-polymers (BCP) and double patterning process can address these concerns. Our team has recently demonstrated 10 nm full-pitch lamellae-forming BCP lines as well as 8 nm full-pitched lines by combining BCP and pattern doubling process. These techniques are the building blocks to achieve 10Tdpsi and beyond HDMR media.

REFERENCE

1) X.-M. Yang, S. Xiao, Y. Hsu, H. Wang, J. Hwu, P. Steiner, K. Wago, K. Lee, D. Kuo, "Fabrication of servo integrated template for 1.5 Teradot/inch2 bit patterned media with block copolymer directed assembly ", J. Micro/Nanolith. MEMS MOEMS 13(3), 031307 (Jul–Sep 2014)

2) Kim Y. Lee, XiaoMin Yang, Shuaigang Xiao, Yautzong Hsu, Zhaoning Yu, Michael Feldbaum, Philip Steiner, Koichi Wago, Ning Li, and David Kuo, “Fabrication and Characterization of Bit Patterned Media at 1.5 Tdots/in2 and Beyond”, presented at Intermag in Beijing (May 2015)

Figure 2. Hexagonal HDMR Servo Integration Using Figure 1. 5 Tdpsi FePt islands (12.2 nm pitch) by an Overlay-Controlled Hybrid DSA Method self-assembly of PS-b-PDMS

Figure 3. Adjacent track overwrite effect

38 B6

FePt DAMPING: PHYSICAL ORIGINS AND RECORDING IMPACT ON HAMR AND STT-RAM

Randall H. VICTORA1,2 and Tao QU2 1) Electrical and Computer Engineering, University of Minnesota, Minneapolis, MN , USA, [email protected] 2) School of Physics and Astronomy, University of Minnesota, Minneapolis, MN, USA,[email protected]

I. INTRODUCTION In view of recent technology and drive level demonstrations, HAMR continues to be a viable and promising technology for future magnetic data-storage products. L10 FePt has become the leading candidate for HAMR media, owing to its stability against thermal fluctuations even when scaling storage grains to the nano-scale. L10 FePt(001) granular films with grain size of 5-6 nm have been fabricated using an MgO underlayer[1]. In magnetic recording, the damping constant α affects the writing speed because it reflects energy lost in the switching dynamics. When applying an external magnetic field to write a storage grain, large α is preferred to increase the writing speed and reduce the noise. For grain pitch 6.55nm, transition jitter is decreased more than 30% when room temperature damping is tuned from 0.005 to 0.02 [2], as shown in Fig 1. Conversely, STT-RAM requires the minimization of α to increase energy efficiency. The experimentally reported intrinsic damping diverges among investigators [3,4], probably due to varying fabrication techniques. Here, we apply Kambersky’s torque correlation technique [5], within the tight binding method, to study the damping of L10 FePt in both ordered and chemically disordered states. II. RESULTS

The damping of L10 ordered FePt is predicted to be 0.02 at room temperature. The computed damping and magnetization in Table I are consistent with experiment [5]. As damping represents the loss of magnetic energy from the spin to the lattice through the spin orbit interaction (SOI), the damping is affected by the intrinsic SOI from both Fe and Pt elements. Fig. 2 shows that the damping depends strongly on the SOI of the non-magnetic element for values beyond 0.2eV. Artificially shifting the Fermi level, as might be accomplished by doping with impurity atoms, shows that α follows the density of states (DOS) at the Fermi level, as shown in Fig. 3. We introduce lattice defects (inevitable in experiment) through exchanging the positions of 3d and non-3d transition elements in 36 atom supercells. The damping increases with reduced degree of chemical order, as shown in Fig. 4, owing to the enhanced spin-flip channel allowed by the broken symmetry. This prediction is confirmed by measurements in FePt [3]. It is demonstrated that this corresponds to an enhanced DOS at the Fermi level, owing to the rounding of the DOS with loss of long-range order. L10 alloys with Pt have larger damping values than those with Pd, due to the larger SOI in Pt (~0.54eV) rather than Pd (~0.15eV). L10 FePd has the lowest damping value of 0.009, making it more suitable for STT-MRAM. L10 COPt has the largest damping of 0.067, that should reduce magnetic recording noise but has the disadvantage of higher Curie temperature. In our most recent work, we extend to the multilayer structures (Fe/MgO). An extra interfacial damping term contributes to the total damping due to the broken symmetry at the interface. For Fe/MgO, the interfacial damping is 0.0265, and could be a significant fraction of the total damping in a thin film. REFERENCES

1) L. Zhang et. al, "L10-ordered FePtAg–C granular thin film for thermally assisted magnetic recording media", J. Appl. Phys. 109, 07B703 (2011). 2) Pin-Wei Huang and R. H. Victora, "Heat assisted magnetic recording: Grain size dependency, enhanced damping, and a simulation/experiment comparison", J. Appl. Phys. 115, 17B710 (2014). 3) S. Mizukami et. al, "Fast magnetization precession observed in L10-FePt epitaxial thin film", Appl. Phys. Lett. 98, 052501 (2011). Randall H. Victora E-mail: [email protected] tel: (612)625-1825 39 B6

4) P. He et. al, "Quadratic Scaling of Intrinsic Gilbert Damping with Spin-Orbital Coupling in L10 FePdPt Films: Experiments and Ab Initio Calculations", Phys. Rev. Lett. 110, 077203 (2013). 5) T. Qu and R. H. Victora, “Effect of substitutional defects on Kambersky damping in L10 magnetic materials”, Appl. Phys. Lett. 106, 072404 (2015)

SFe SPt LFe LPt Mt Mexp αR.T FePt 2.663 0.454 0.18 0.06 3.358 3.4 0.02

Table I. The columns represent the spin and orbital moment for Fe and Pt specifically, calculated and experimental magnetic moment, and damping values at R.T. of L10 ordered FePt. The unit of all the moments is Bohr magneton µB.

0.05

0.04

0.03

total α total 0.02

0.01

0.00 0.0 0.2 0.4 0.6 0.8 SOI ξ (eV) Fig. 1. Transition jitter versus grain pitch with two different sets of damping constants.[2] Fig. 2. The total damping in L10 FePt vs SOI strength ξPt when retaining the original value of Fe SOI.[5] 10 total Ef0 α 0.1 FePd

0.01 α DOS 1 1E-3

1E-4 -2 -1 0 1 2 Fermi_en_shift (eV)

Fig. 3. The total damping and DOS versus Fermi level in L10 FePt.[5] Fig. 4. Total damping at room temperature vs chemical degree of order in disordered (a) FePt, (b) FePd, (c) CoPt, and (d) CoPd with varying number of substitutional defects.[5]

40 C1

WRITE-INDUCED HEAD CONTAMINATION IN HAMR

James D. KIELY1, Paul M. JONES2, Y. YANG2, John L. BRAND1, Manuel ANAYA-DUFRESNE1, Patrick C. FLETCHER1, Florin ZAVALICHE2, Yvete TOIVOLA1, John C. DUDA1, Michael T. JOHNSON1, Sunita GANGOPADHYAY1 1) Seagate Technology LLC, 7801 Computer Ave S., Bloomington, MN 55435 2) Seagate Technology LLC, 47010 Kato Road, Fremont, CA 94538

I. INTRODUCTION Among the challenges of Heat-Assisted Magnetic Recording (HAMR) is maintaining the stability and cleanliness of the head-disk interface. From the advent of high-temperature recording technologies, lubricant evaporation, degradation, and transfer to the recording head has been a primary concern. In previous embodiments of optical recording, accumulation on the head was identified as one of the major challenges, and contamination of the near-field recording head was an additional challenge [1]. In HAMR, prior publications have described mechanisms for lubricant evaporation [2, 3], flow and thinning [4], and transfer to the head [5]. In addition to lubricant transfer, other thermo-chemical reactions can take place that would not occur at conventional magnetic recording temperatures. These reactions may result in a higher level of contamination on the HAMR recording head. Understanding the factors that contribute to the accumulation of head contamination, the impact of accumulation on recording performance, and how it might affect device reliability is critically important to the development of HAMR.

II. RESULTS Lubricant transfer between the head and media has been discussed previously [5]. In steady-state flight, a thin layer of media lubricant is transferred to the recording head. This head layer is in flux with the media lubricant layer, and as this mass flux is occurring, adsorbed contaminants from the media, head, and environment are included in the desorption/adsorption process. These contaminants do not desorb at the same temperature as lubricant, and air shear at the surface of the slider may move this unwanted film away from the locations where the head temperature is high enough to cause desorption. This dynamic process leads to a residual film of contaminant material that affects the functioning of the optical coupling between the active elements in the head and the media. An example of head contamination is shown in Fig. 1, which is an atomic force microscope scan of the surface heights of a HAMR transducer after writing on HAMR media. The onset of head contamination is very rapid, with contamination being observed within milliseconds of the initiation of writing, as shown in Fig 2. With continued writing, the total contamination volume grows. The contamination height is limited by the head-media clearance, and the contamination height does not change significantly after hundreds of hours of writing. The impact of contamination on recording is dependent upon the optical properties of the contamination. It is reasonable to expect that a portion of the contamination is media lubricant, which has a refractive index between 1.1 and 1.5. Additional lubricant in the head-disk gap increases energy transfer efficiency [6], which is expected to benefit recording performance, especially in those cases where efficiency is poor. One result of this increased energy transfer efficiency is a reduction in the laser power necessary for recording. We report how contamination is believed to impact recording characteristics using models to predict plasmonic coupling across interfaces with different levels of contamination. A continuing challenge for HAMR is transducer reliability, and therefore any reduction in required laser power, and thus head temperature, caused by contamination can have a significant impact on device lifetime.

James D. Kiely E-mail: [email protected] tel: +01- 952-402-8028 41 C1

Any benefit is likely to be balanced, however, with a change in the thermal properties of the head-disk interface. Without contamination in the head-disk interface, the heat transfer from the media to the head is relatively small [7] and the media hot spot increases the head temperature by approximately 10K. If the separation between the head and disk is filled with lubricant instead of pressurized air, the heat transfer coefficient will be higher because the thermal conductivity of lubricant is much higher than the thermal conductivity of air. In this paper, we review modeling and measurements of how head contamination may impact heat transfer from the media to the head, increasing head temperatures and reducing reliability.

Figure 1. Atomic Force Microscope height mapping of write-induced head contamination on the slider Figure 2. Contamination height as a function of laser surface of a HAMR writer. writing time.

REFERENCES 1) V. Novotny and R. Hajjar, “Optical storage system with head cleaning mechanism based on a position-controllable optical interfacing surface in an optical head”, U.S. Patent 6307832 B1, (2001) 2) M.S. Lim and A.J Gellman , " Kinetics of laser induced desorption and decomposition of Fomblin Zdol on carbon overcoats", Tribology Internat., 38, 554-561 (2005). 3) P.M. Jones, et al., “Laser-Induced Thermo-Desorption of Perfluoropolyether Lubricant from the Surface of a Heat-Assisted Magnetic Recording Disk: Lubricant Evaporation and Diffusion”, Tribology Lett., 59, 33, (2015). 4) J.B. Dahl and D.B. Bogy, “Lubricant Flow and Evaporation Model for Heat-Assisted Magnetic Recording Including Functional End-Group Effects and Thin Film Viscosity”, Tribol. Lett., 52, (2013) 5) Y. Yang , et al.," Head–Disk Lubricant Transfer and Deposition During Heat-Assisted Magnetic Recording Write Operations", IEEE Trans. Magn., 51(11), (2015). 6) K. Sendur and P.M. Jones, “Effect of Fly Height and Refractive Index on the Transmission Efficiency of Near-Field Optical Recording”, Appl. Phys. Lett., 88, (2006). 7) L. Juang, et al, “HAMR Thermal Modeling including Media Hot Spot”, IEEE Trans. Magn., 49 (6), (2013).

42 C2

MEASUREMENT OF NANOSCALE HDI HEAT TRANSFER USING A PMR HEAD WITH CONTACT SENSOR

Yuan MA1 and David B. BOGY2 1) UC Berkeley, Berkeley, USA, [email protected] 2) UC Berkeley, Berkeley, USA, [email protected]

I. INTRODUCTION In the hard disk drive (HDD) industry, new technologies including heat assisted magnetic recording (HAMR) and two-dimensional magnetic recording (TDMR) are being developed to meet the increasing demand for storage capacity. The implementation of these technologies is closely related to the understanding of heat transfer at the nanoscale. In nanoscale heat transfer, one of the key factors is the measurement of the heat transfer coefficient across the head-disk interface (HDI) gap. It has been theoretically predicted that the heat transfer coefficient in a nanoscale HDI gap is much larger than traditionally expected. This, in turn, could greatly affect the reliability design of HAMR heads and media. To measure the heat transfer coefficient of the HDI as the gap closes to zero we designed a series of experiments that allows us to control the clearance between the PMR head temperature sensor and a rotating or static disk from around 10 nm to contact. II. EXPERIMENTAL DETAILS The experimental setup is shown in Figure 1, which uses a production PMR read-write head system. The initial clearance between the temperature sensor and the static disk is controlled with the help of the crown feature of the slider. The crown has a smooth convex surface with a height of around 7 nm. After loading the head onto the disk, the initial clearance can be adjusted by moving the disk forward or backward using a micro stage. The thermal fly-height control (TFC) power is then supplied step by step to drive the protrusion into contact. The resistive embedded contact sensor (ECS) in the slider is used to measure the head temperature. Production lubed and delubed disks are used in the experiments. III. RESULTS AND DISCUSSIONS On rotating and static lubed disks, the change of the ECS resistance against TFC power is plotted in Fig. 2 and Fig. 3. In both of these figures, the ECS resistance first increases with TFC power, then decreases and finally increases again. According to [1], TFC heating and frictional heating cause the first and second rising segments of the curve in Fig. 2, and the falling segment is solely due to air cooling. However, this explanation fails for Fig. 3, because on a non-rotating disk, there is no air flow and no extra pressure and pressure gradients in the head-disk interface, resulting in a mean free path much larger than the gap size, which is only a couple of nm. Under this static condition a conventional simple air conduction model for the gap cannot be applied because within the 2 nm gap there are not enough air molecules to support the air conduction model. An alternate explanation is put forth here. According to wave based heat conduction theory [2-3], the heat transfer coefficient across a closing gap increases drastically due to phonon tunneling when the gap size is within only a couple of nanometers. As the HDI clearance reduces to around 1 or 2 nm, the heat flux between the head and disk will increase strongly, causing a drop in the ECS temperature. After contact with the disk is achieved, the ECS temperature will again increase due to further TFC heating. The result on a delubed disk is plotted in Figure 4. As the TFC power is increased here we see a sharp temperature drop followed by a small bump. This small bump could be due to a contamination particle in the contact region. Several more results will be presented in the paper.

Yuan Ma E-mail: [email protected] tel: +1-510-599-7276 43 C2

Figure 1 Experimental setup Figure 2 Contact sensor resistance change with increasing TFC on a rotating disk and its air-cooling-based explanation

Figure 3 Contact sensor resistance change with Figure 4 Contact sensor resistance change with increasing TFC on a static lubed disk and its increasing TFC on a static delubed disk phonon-conduction-based explanation

REFERENCES 1) Xu, J., Shimizu, Y., Furukawa, M., "Contact/Clearance Sensor for HDI Subnanometer Regime," Magnetics, IEEE Transactions On, 50(3) 114-118, (2014). 2) Budaev, B. V., and Bogy, D. B., "Computation of Radiative Heat Transport Across a Nanoscale Vacuum Gap," Applied Physics Letters, 104(6) pp. 061109, (2014). 3) Budaev, B. V., and Bogy, D. B., "Heat Transport by Phonon Tunneling Across Layered Structures used in Heat Assisted Magnetic Recording," Journal of Applied Physics, 117(10) pp. 104512, (2015).

44 C3

GRAPHENE COATING FOR HEAT-ASSISTED MAGNETIC RECORDING

Neeraj DWIVEDI1, Anna K. OTT2, Reuben J. YEO1, Chunmeng DOU2, Ugo SASSI2, Domenico DE FAZIO2, A. C. FERRARI2 and C. S. BHATIA1,* 1) Department of Electrical and Computer Engineering, National University of Singapore, Singapore 2) Cambridge Graphene Centre, University of Cambridge, Cambridge, UK

I. INTRODUCTION Ultrathin (~2.5–3.0nm) amorphous carbon overcoats (COCs) are used in commercial hard disk drive (HDD) media, for the purpose of protecting the underlying magnetic media from corrosion and wear [1,2]. To achieve an areal density beyond 1 Tb/in2, the COC should be < 2nm [3]. At these thicknesses, conventional COCs have issues in tribology [3] and corrosion at the head-disk interface [4,5]. Furthermore, they degrade under heat-assisted magnetic recording (HAMR)-like conditions [6], making them unsuitable for HAMR application. Graphene is a promising material for protective coatings owing to its excellent tribological, mechanical and anti-corrosion properties [7,8]. These, combined with its atomic thinness, make it an attractive material for future media overcoats. Here, we present and discuss the friction, wear and corrosion characteristics of single layer graphene (SLG)-coated hard disks and compare its performance with thicker commercial COCs. We also discuss the stability of graphene on Si under laser irradiation conditions comparable to those expected in HAMR.

II. RESULTS AND DISCUSSION

SLG is grown on Cu [9] and then transferred on bare commercial CoCrPt:oxide media and Si/SiO2 via a standard wet transfer process [10]. Raman spectroscopy shows that the transfer process does not damage the SLG [11], Figure 1a). The friction and wear properties are assessed by ball-on-disk tribological tests, and compared with a ~2.7nm commercial COC on media (CM), as well as uncoated media with no lubricant (BM), Figure 2. SLG shows the lowest friction and wear, despite its atomic thinness. Electrochemical corrosion measurements are used to evaluate and compare the corrosion resistance. The measured corrosion current densities (jcorr), which vary inversely with corrosion resistance [12], show that SLG improves the corrosion resistance of uncoated media by ~2.5 times. CM shows the highest corrosion resistance because of the larger COC thickness. However, the use of two layers of graphene (2LG) significantly improves the corrosion resistance, closer to that of CM. The friction and wear properties of 2LG are found to be slightly better than SLG. Finally, the stability of SLG on Si is evaluated by exposing the sample to a 785 nm laser with increasing power densities from 1.3 to 21.8 mW/µm2 for 200s, chosen to be comparable with HAMR conditions [13]. Raman spectra and optical images are recorded before and after irradiation, showing no degradation, Fig. 1b).

III. CONCLUSIONS Graphene shows potential as an ultrathin media overcoat material for HAMR. Investigations are ongoing on few-layer graphene overcoats to examine their effectiveness in further improving the wear and corrosion performance, as well as studying the laser irradiation stability of graphene on FePt media.

REFERENCES 1) A. C. Ferrari, “Diamond-like carbon for magnetic storage disks”, Surf. Coat. Technol. 180 –181, 190 (2004). 2) C. Casiraghi, J. Robertson, and A. C. Ferrari, “Diamond-like carbon for data and beer storage”, Mat. Today 10, 44 (2007). 3) B. Marchon et al. “The Head-Disk Interface Roadmap to an Areal Density of Tbit/in²”, Adv. Tribol. 2013, 62329 (2013). 4) N. Dwivedi et al., “Ultrathin carbon with interspersed graphene/fullerene-like nanostructures: a durable protective overcoat for high density magnetic storage”, Sci. Rep. 5, 11607 (2015). *Corresponding Author: C. S. Bhatia E-mail: [email protected] tel: +65-65167216 Topic: HAMR 45 C3

5) C. M. Mate et al., “New methodologies for measuring film thickness, coverage and topography”, IEEE Trans. Magn. 36, 110 (2000). 6) P. M. Jones, J. Ahner, C.L. Platt, H. Tang, and J. Hohlfeld, “Understanding disk carbon loss kinetics for heat assisted magnetic recording”, IEEE Trans. Magn. 50, 3300704 (2014). 7) D. Berman, S. A. Deshmukh, S. K. R. S. Subramaniam, A. Erdemir, and A. Sumant, “Extraordinary macroscale wear resistance of one atom thick graphene layer”, Adv. Funct. Mater. 24, 6640 (2014). 8) D. Prasai, J. C. Tuberquia, R. R. Harl, G. K. Jennings, and K. I. Bolotin, “Graphene: corrosion-inhibiting coating”, ACS Nano 6, 1102 (2012). 9) S. Bae et al., “Wafer scale synthesis and transfer of graphene films”, Nat. Nanotechnol. 5, 574 (2010). 10) F. Bonaccorso et al., “Production and processing of graphene and 2d crystals”, Mater. Today 15, 564 (2012). 11) A. C. Ferrari and D. M. Basko, “Raman spectroscopy as a versatile tool for studying the properties of graphene”, Nat. Nanotechnol. 8, 235 (2013). 12) D. T. Sawyer, A. Sobkowiak, and J. L. Roberts, “Electrochemistry for chemists”, John Wiley, NY, (1995). 13) S. Kundu et al., “Probing the role of carbon microstructure on the thermal stability and performance of ultrathin (< 2nm) overcoats on L10 FePt media for heat-assisted magnetic recording”, ACS Appl. Mater. Inter. 7, 158 (2015).

12000 a) SLG on media 2D 2000 b) 1.3mW/µm² λ=785nm =514.5 nm G 10000 λ 3.1mW/µm² D SLG/media 2D' 6.3mW/µm² 1500 15.7mW/µm² background subtracted 8000 21.8mW/µm 21.8mW/µ2m²

2 6000 SLG/media 1000 15.7mW/µm

2 4000 N 6.3mW/µm O 2 2 media 500 2 2000 3.1mW/µm Intensity (arb. units) Intensity (arb. Intensity (arb. units) Intensity (arb. SLG/Cu 1.3mW/µm2 0 0 500 1000 1500 2000 2500 3000 3500 1000 1200 1400 1600 1800 2000 Raman shift (cm-1) Raman shift (cm-1)

Fig. 1: a) Raman spectra of SLG grown on Cu foil (PL background subtracted), BM substrate, graphene transferred on BM substrate, and graphene on BM substrate after background subtraction. All spectra are recorded at 514.5nm. b) Raman spectra of SLG on a Si/SiO2 substrate irradiated with different laser power densities at 785nm excitation, normalized to the G peak intensity. No damage can be seen.

(b) Ball Wear Track 25 1.2 (a) (c) BM 1.0 BM 20 ) 2 0.8 15

0.6 CM CM (nA/cm 10 0.4 corr j

Coefficient of Friction 0.2 SLG 5 SLG 0.0 0 2000 4000 6000 8000 10000 BM CM SLG 2LG Number of Cycles Samples

Fig. 2: (a) Frictional curves, (b) optical images of balls and wear tracks after ball-on-disk tribological test and (c) corrosion current density (jcorr) for different samples. Scale bar in optical images represents 100 µm.

46 D1

TECHNICAL CONSIDERATIONS FOR COMMERCIAL MRAM PRODUCTS

J.M. SLAUGHTER, R. WHIG, H.-J. CHIA, F.B. MANCOFF, S. IKEGAWA, S. AGGARWAL, M. DEHERRERA, S. DESHPANDE, J. JANESKY, M. LIN, D. HOUSSAMEDDINE, K. NAGEL, J.J. SUN Everspin Technologies, Inc., Chandler, Arizona, USA

I. BACKGROUND AND MRAM PRODUCT GENERATIONS Magnetoresistive Random Access Memory (MRAM) is a high-speed nonvolatile memory that can provide unique solutions which improve overall system performance in a variety of areas including data storage, industrial controls, networking, and others. While the development of magnetic tunnel junction (MTJ) materials with very high magnetoresistance was fundamental in enabling a fast, reliable read operation in MRAM circuits, it is the method of writing the magnetic state of the storage device that distinguishes one technology generation from another. Each new generation offers the potential for much higher memory density and lower power operation. Here we review the key properties that enable MRAM products, and present recent results on ST-MRAM with pMTJ devices for higher memory densities and lower write energy for reduced operating power. Field-switched MRAM, generally considered the first generation, is mass produced by Everspin in densities up to 16Mb. This “Toggle” MRAM employs a field switching innovation called “Savtchenko switching” to enable reliable programming of bits in the array.[1] Other key A major feature of Toggle MRAM is the essentially unlimited write endurance, since there is no wear-out mechanism related to switching the magnetization state of the free layer with magnetic fields. During the read operation, the bias applied to the MTJ is at a level far below the breakdown voltage of the device, resulting in a high reliability product. The robust switching mode and long-term stability of optimized MTJ devices enable Toggle MRAM to address a range of markets from general-use commercial to industrial and automotive markets requiring higher-temperature operation up to 125C. The second MRAM generation employs magnetic switching by spin-torque-transfer, enabling smaller MTJ devices for higher density. A 64Mb DDR3 MRAM product based on spin-torque switching is currently in early production at Everspin.[2] This 90nm-node product is passed on MTJ devices with in-plane magnetization (iMTJ), and this technology has recently been extended to a 40nm-node 256Mb chip. Spin-torque switching of MTJ devices with magnetization perpendicular to the film plane (pMTJ) will further extend the density of ST-MRAM, and reduce the write energy, to enable Gb-class memories in the near future.

II. SPIN-TORQUE MRAM RECENT RESULTS: IN-PLANE TO PERPENDICULAR Challenges in developing manufacturable ST-MRAM include achieving certain key MTJ device parameters as well as a strong emphasis on control of bit-to-bit distributions in the memory array. Examples include: scaling the critical current for switching (Ic) while maintaining energy barrier to thermal reversal (Eb), controlling the switching for high Eb and tight distributions, and separation of the critical voltage (Vc) distribution from the breakdown voltage (Vbd) distribution. Switching distributions measured for large arrays of bits include contributions from the single-bit thermal distribution and bit-to-bit variation from shape or material defects. Both thermal variation and extrinsic effects are typically dependent on the pulse duration tp, usually becoming worse for higher speed switching. A schematic of the important bit-to-bit distributions in ST-MRAM arrays is shown in Figure 1. The critical voltage (Vc) is the mean bias required to switch 50% of the bits and Vbd is the mean breakdown

Jon Slaughter E-mail: [email protected] tel: +1-480 347-1111 47 D1

voltage. The write bias applied by the write circuit (Vwrite) must be well separated from both the Vc distribution and the Vbd distribution to ensure reliable writing of the highest Vc MTJs while also avoiding the breakdown of any MTJs. The read bias applied by the read circuit must be low enough to avoid unintended switching of MTJs in the low tail of the Vc distribution. As shown in the figure, the read and write operating regions for ST-MRAM are determined by the separation of these distributions. Having large separation between the mean values is important but having narrow distributions is just as important. Optimization of the free layer materials and patterning process are critical to obtaining the desired well-behaved distributions. The free layer materials and interfaces are engineered to obtain the desired mean properties as well as the desired distributions. For example, interfacial perpendicular magnetic anisotropy (PMA) was used to lower the mean switching voltage of the in-plane free layer. The perpendicular free layer is surprisingly similar to the in-plan free layer but with the perpendicular free layer designed to have stronger interfacial PMA and lower moment to make it strongly perpendicular. In addition to the high PMA free layer, the process and materials were optimized to achieve tight bit-to-bit switching distributions, and minimize the level of magnetic defects that can cause non-normal Vc distributions.[3] Data from 64Mb devices with our optimized stack show excellent agreement with a single Normal distribution for both iMTJ and pMTJ arrays with optimized free layer design and process. Figure 2 is a comparison of experimental Vc distribution widths for in-plane and perpendicular integrated arrays over a range of write pulse widths (tp) for devices with similar areas. We find that the relative pMTJ distribution width (σ/Vc) is narrower than for iMTJ arrays, and the increase in σ/Vc with decreasing tp is much less pronounced for the pMTJ devices. Additional performance results and product application examples will be presented.

REFERENCES 1) B. N. Engel, et al., IEEE Trans. Magn., vol. 41, no. 1, pp. 132–136, Jan. 2005. 2) N. D. Rizzo, et al., IEEE Trans. Magn., vol. 49, no. 7, pp. 4441–4446, Jul. 2013.

Figure 1. Schematic of the important bit-to-bit Figure 2. ST switching distributions for arrays of distributions in ST-MRAM arrays. The write voltage optimized devices, iMTJ and pMTJ, of similar device (Vwrite) must be well separated from the mean Vc and area. Tighter distributions with better high-speed Vbd. performance is obtained from pMTJ arrays.

48 D2

DYNAMICAL MAGNETOELECTRIC SWITCHING OF PERPENDICULAR EXCHANGE BIAS

Yu SHIRATSUCHI1, Nguyen Thi Van ANH1, Ryoichi NAKATANI1, Yoshinori KOTANI2 and Tetsuya NAKAMURA2 1) Osaka Univ., Osaka, Japan, [email protected] 2) JASRI/SPring-8, Hyogo, Japan

I. INTRODUCTION Electric field control of magnetization possesses the potential application to the high density magnetic storage and/or the high capacity magnetic random access memory. Several methods are now under the investigation in this scheme: voltage-induced magnetic anisotropy change [1], magneto-electric (ME) effect [2-4] etc. First concept, i.e. the voltage-induced magnetic anisotropy change can be induced at the ferromagnetic (FM)/non-magnetic layer interface and thus, the bi-stability of the magnetization direction is maintained. In contrast, the second concept is generally investigated at the FM/antiferromagnetic (AFM) layer interface such as CoFe/BiFeO3 [2] and Co/Cr2O3 [4] layered system where the bi-stability of magnetization direction is broken by the exchange anisotropy which has a key role in this concept. Namely, when the exchange bias field is higher than the coercivity, the FM magnetization direction at a remanent state is defined at a certain direction. In other words, by switching the exchange bias polarity by the electric field, the FM magnetization direction immediately switches. Previously, we reported that at the Co/Cr2O3 interface, the high perpendicular exchange anisotropy above 0.4 erg/cm2 can be induced [5] and demonstrated that the induced exchange anisotropy is dynamically switchable by a pulsed voltage [4]. In this presentation, we will overview the electric field induced switching of exchange bias, in particular, using Cr2O3 as the AFM layer and also present the resent progress of the dynamical switching of the exchange bias in this system mainly based on our own achevements. We also address the switching process in our system approaching using X-ray magnetic circular dichroism using the focused X-ray.

II. EXPERIMENTAL

Typical stacking structure of the sample was Pt(1.2-5.0 nm)/Co(0.4-0.6 nm)/X(0.5 nm)/Cr2O3(150-250 nm)/Pt(20 nm)/α-Al2O3-substrate. The films were fabricated by using the DC magnetron sputtering system with a base pressure below 6×10-6 Pa. The fabricated films were patterned into a Hall-cross structure by means of the photo-lithography and the Ar ion milling. The Hall-cross devices equipped with the bottom electrode to apply the gate voltage to the Cr2O3 layer. The ME switching of the perpendicular exchange bias was characterized using the anomalous Hall effect (AHE). Magnetic domain observations were carried out based on the soft X-ray magnetic circular dichroism measurements using the focused soft X-ray [6]. Utilizing element-specific ability of synchrotron, we carried out the observation for FM Co and AFM Cr independently by tuning the photon energy of the incident soft X-ray at the Co L3 and Cr L3 edges, respectively. The observations were carried out at BL25SU of SPring-8, Japan. Structural characterizations were carried out using the fabricated film before the patterning: reflection high energy electron diffraction, X-ray reflectivity and X-ray diffraction; these characterizations yield the crystallographic orientation normal to the film as Pt(111), Co(111), Cr2O3(0001).

III. RESULTS AND DISCUSSIONS

Yu Shiratsuchi E-mail: [email protected] tel: +81-6-68797489 49 D2

Figure 1(a) shows the AHE loops exhibiting the reversible ME switching. The electric field required to switch the exchange bias shows the clear hysteresis and the threshold electric field decreases with increasing the magnetic field; the required energy gain is proportional to the product of electric (E) and magnetic (H) fields, i.e. EH product. By adopting the pulse voltage to switch the exchange bias polarity, we can access the switching process. In Fig. 1(b), the changes in the exchange bias field and the remanence ratio with the pulse duration are presented. For the investigated EH condition, the exchange bias field fully switched at about 1 micro seconds, which is very slow compared to the spin precession time; the switching process might be dominated by the magnetic domain wall propagation. The FM and AFM magnetic domain observations and their spatial coupling can verify this speculation; if the exchange bias is determined by a domain-by-domain basis and the exchange bias polarity is coupled with the interfacial AFM spin orientation, these findings would support the switching process. In fact, direct observations of the spatial distribution of XMCD for Co and Cr spins show that these patterns are spatially coupled. Besides, when the magnetic hysteresis was observed for this condition, both positive and negative exchange biases were observed. Furthermore, the magnetic domain patterns were fully recovered after removing the magnetic field.

250 (a) EH = -20000 kOe⋅kV/cm 1.0 1.0 (b) 200 0.8 150 0.6 100 0.5 0.4 50 0.2 0.0 0 0.0 -50 -0.2

-100 ratio Remanence -0.5 -0.4 -150 -0.6

Normallized AHE Voltage -200 -0.8 -1.0 Exchange / Oe bias field EH = +25600 kOe⋅kV/cm -250 -1.0 -1000 -750 -500 -250 0 250 500 750 1000 10-8 10-7 10-6 10-5 Magnetic field / Oe Pulse duration / ns Fig. 1 (a) AHE loops exhibiting positive and negative exchange biases after applying the E and H fields above EH > 2.0×104 kOe kV/cm, EH < -1.4×104 kOe kV/cm, respectively. (b) Changes in exchange bias field and remanence ratio with the pulse duration. IV. SUMMARY We achieved the reversible switching of the perpendicular exchange bias by the ME effect of the Cr2O3(0001) film. The dynamical switching using the pulse voltage has also been investigated. Depending on the input energy conditions, the slow switching in the micro second range was observed implying that the switching process was dominated by the AFM domain wall propagations. The FM and AFM domain observations support this discussion. ACKNOWLEDGEMENTS This work is partly supported by JSPS KAKENHI (Grant Nos. 25706007, 26630296, 16H03832, 16H02389), ImPACT program, the Photonics Advanced Research Center (PARC) at Osaka University. The XAS/XMCD measurements were performed at the SPring-8 synchrotron radiation facility with the approval of JASRI (Proposal Nos. 2014A0079, 2014B0079, 2015A0079, 2015B0079, 2016A0079).

REFERENCES 1) T. Maruyama et al., Nature Nanotechnology 4, 158-161 (2009). 2) Y-H Chu et al., Nature Materials, 7, 478-482 (2008). 3) X. He et al., Nature Materials 9, 579-585 (2010). 4) Y. Shiratsuchi et al., Phys. Rev. Lett. 109, 077202 (2012). 5) K. Toyoki, Y. Shiratsuchi et al., Appl. Phys. Lett. 106, 162404 (2015). 6) Y. Shiratsuchi et al., AIMS Mater. Sci. 2, 484-496 (2015).

50 E1

SPIN TORQUE MRAM

Daniel C. WORLEDGE, Guohan HU, Junghyuk LEE, Janusz J. NOWAK, Jonathan Z. SUN, Anthony ANNUNZIATA, Stephen BROWN, Younghyun KIM, Chandrasekharan KOTHANDARAMAN, Gen LAUER, Nathan MARCHACK, Eugene J. O’SULLIVAN, Jeong-Heon PARK, Mark REUTER, Ray P. ROBERTAZZI, Philip L. TROUILLOUD, and Yu ZHU

IBM-Samsung MRAM Alliance, IBM TJ Watson Research Center, Yorktown Heights, NY

I. INTRODUCTION Spin Torque MRAM is an emerging memory which possesses a unique combination of density, speed, and non-volatility.[1] Potential applications include use as an embedded-Flash replacement below the 28 nm node, a low-power non-volatile working memory in mobile applications, and a fast, dense cache memory. Spin Torque MRAM was invented by John Slonczewski at IBM, who developed the theory for spin-transfer torque in the late-80’s and early-90’s, predicting that a spin-polarized current of sufficient magnitude could switch a small magnetic bit back and forth.[2] In related work, Luc Berger predicted that a spin-polarized current in a metallic multilayer could move a domain wall and stimulate emission of spin waves.[3] Low temperature experimental verification of these predictions came in 1998 when Tsoi et al. observed current-induced excitations in metallic multilayers [4], and in 1999 when Jonathan Sun of IBM used spin torque to switch a particle in a manganite tunnel junction [5] and Myers et al. switched a magnetic domain in a point contact in a metallic multilayer.[6] In 2000, Jonathan Sun developed the single-domain model of spin torque switching, including an analytic formula for the switching current as a function of materials parameters[7]. In 2004, Huai et al. [8] and then Fuchs et al. [9] demonstrated the first spin torque switching of magnetic tunnel junctions, using AlOx tunnel barriers. The discovery of high magnetoresistance in MgO tunnel junctions at IBM by Stuart Parkin [10] and later Shinji Yuasa at AIST [11], accelerated progress on Spin Torque MRAM. Sony demonstrated the first CMOS-based Spin Torque MRAM in 2005, showing individual MgO-based magnetic tunnel junctions writing in 2 ns in a 4 kbit 180-nm node chip. [12] All work up to this point used in-plane magnetized materials. It was well known since Slonczewski’s early work [2] that spin torque switching would be more efficient for perpendicularly magnetized materials, but it was challenging to implement tunnel junctions using perpendicular magnetic anisotropy. Early progress was made by Toshiba, who demonstrated spin torque switching of the first perpendicularly magnetized tunnel junctions, using L10 alloys.[13] These materials required growth at high temperature, leading to rough films; typically only single device data were published. The discovery that CoFeB/MgO could be perpendicular was made at IBM in 2009 [14], and shared with our partner TDK. This materials system was also independently discovered by the Ohno group at Tohoku University [15]. The discovery of perpendicular magnetic anisotropy in CoFeB/MgO enabled perpendicular magnetic tunnel junctions with high TMR using simple materials, and led to a dramatic growth in activity on Spin Torque MRAM. Using these materials, IBM demonstrated in 2010 low write-voltage with steep write-error-rate curves and tight bit-to-bit distributions estimated to be sufficient to yield a 64 Mb chip, the first statistical demonstration that Spin Torque MRAM could work reliably.[16] IBM also demonstrated scaling down to 20 nm [17], ultra-low write-error-rates below 1e-11 [18], the existence of sub-volume activation [19], the development of stable reference layers [20], and record high spin torque switching efficiency [21]. IBM designed an 8 Mb Spin Torque MRAM circuit in 90 nm technology, [22] and shared the design with our partner TDK. Using CMOS layers fabricated at IBM and magnetic tunnel junction layers fabricated at TDK, TDK and IBM demonstrated writability, endurance, and data retention of a full 8 Mb array [23]. Using perpendicular CoFeB/MgO, TDK also demonstrated stability after annealing at 400 C. [24] Recently, Samsung has demonstrated perpendicular magnetic tunnel junctions with magnetoresistance above 350%. [25] Daniel WORLEDGE E-mail: [email protected] 51 tel: 408-927-1345 E1

II. DOUBLE MAGNETIC TUNNEL JUNCTIONS One of the key remaining challenges in developing Spin Torque MRAM is to reduce the switching current, which enables denser MRAM chips due to the smaller required cell transistor. One method to reduce the switching current is to use a double magnetic tunnel junction, which uses two reference layers and two tunnel barriers, one on each side of the free layer. This allows spin torque to be applied to both the top and bottom interfaces of the free layer. Since the write current travels into one side of the free layer and out of the other side, the reference layers must be set anti-parallel to each other, so that the two spin torques add constructively (see Fig. 1). Using double magnetic tunnel junctions we have demonstrated a factor of two improvement in spin torque switching efficiency. [26]

1) A. D. Kent and D. C. Worledge, Nature Nano. 10, 187 (2015). 2) J. C. Slonczewski, Phys. Rev. B 39, 6995 (1989). J. C. Slonczewski, J. Magn. Magn. Mater. 159, L1 (1996). US patent 5695864, J. C. Slonczewski, 1997. 3) L. Berger, Phys. Rev. B 54, 9353 (1996). 4) M. Tsoi, A. G. M. Jansen, J. Bass, W.-C. Chiang, M. Seck, V. Tsoi, P. Wyder, Phys. Rev. Lett. 80 (1998) 4281. 5) J. Z. Sun, J. Mag. Mag. Mat. 202, 157 (1999). 6) E. B. Myers, D. C. Ralph, J. A. Katine, R. N. Louie, R. A. Buhrman, Science 285, 867 (1999). J. A. Katine, F. J. Albert, R. A. Buhrman, E. B. Myers, and D. C. Ralph, Phys. Rev. Lett. 84, 3149 (2000). 7) J. Z. Sun, Phys. Rev. B 62, 570 (2000). 8) Y. Huai, F. Albert, P. Nguyen, M. Pakala, and T. Valet, Appl. Phys. Lett. 84, 3118 (2004). 9) G. D. Fuchs, et al., Appl. Phys. Lett. 85, 1205 (2004). 10) S. S. P. Parkin, et al., Nature Mater. 3, 862 (2004). 11) S. Yuasa, T. Nagahama, A. Fukushima, Y. Suzuki, and K. Ando, Nature Mater. 3, 868 (2004). 12) M. Hosomi et al., Proc. IEDM Tech. Dig. 459 (2005). 13) T. Kishi, et al., Proc. IEDM Tech. Dig. 1 (2008). H. Yoda, et al., Curr. Appl. Phys. 10, 87 (2010). 14) D. C. Worledge, US patent application US 20110303995 A1, filed June 15, 2010. D. C. Worledge, et al., Appl. Phys. Lett. 98, 022501 (2011). D. C. Worledge, et al., J. Appl. Phys. 115, 172601 (2014). 15) S. Ikeda, et al., Nature Mater. 9, 721 (2010). 16) D. C. Worledge, et al., IEDM Tech. Dig. 296 (2010). 17) M. Gajek, et al., Appl. Phys. Lett. 100, 132408 (2012). 18) J. J. Nowak, et al., IEEE Magnetics Letters 2, 3000204 (2011). 19) J. Z. Sun, et al., Phys. Rev. B 84, 064413 (2011). J. Z. Sun, et al., J. Appl. Phys. 111, 07C711 (2012). 20) G. Hu, et al., IEEE Mag. Lett. 4, 3000104 (2013) 21) J. Z. Sun, et al., Phys. Rev. B 88, 104426 (2013). 22) J. DeBrosse, et al., Custom Integrated Circuits Conference, 2015, 1. 23) Y. J. Lee, et al., VLSI Technology, 2013, 1. 24) L. Thomas, et al., J. Appl. Phys. 115, 172615 (2014). 25) M. Krounbi, et al., ECS Trans. 69, 119 (2015). 26) G. Hu, et al., IEDM Tech. Dig. 668 (2015).

Fig. 1 Structure of a double magnetic tunnel junction. The spin torques from the top and bottom tunnel barriers add constructively.

52 E2

SPIN ORBIT TORQUE SWITCHING OF CoFeB MAGNETIC FREE LAYERS WITH Pt AND Ta HEAVY METALS FOR SOT MRAM DEVELOPMENT

Goran MIHAJLOVIĆ, Oleksandr MOSENDZ, Young-Suk CHOI, Lei WAN, Patrick BRAGANCA, Neil SMITH and Jordan KATINE San Jose Research Center, Western Digital Company, San Jose, CA 95135, USA

I. INTRODUCTION Spin-orbit torque (SOT) has been recently proposed as an alternative to spin-transfer torque (STT) for writing bits in magnetic random access memory (MRAM) cells [1]. A proposed cell consists of a SOT layer (e.g. a heavy metal like Pt or Ta) in contact with the free (storage) layer of a magnetic tunnel junction (MTJ) and is written and read via two separate current paths controlled by two transistors [2]. In order to write, current is passed through SOT layer and the switching torque on the adjacent free layer is exerted either via the spin Hall or Rashba effect [3]. Since the write current does not flow through the tunnel barrier of the MTJ, the SOT MRAM cell is expected to provide further improvements for MRAM technology in terms of reliability, endurance, write energy and write speed [4].

II. RESULTS In this talk we will present our results on the development of SOT MRAM devices based on Pt (or Ta)/CoFeB/MgO trilayers where CoFeB is patterned into an elliptical free layer and is magnetized in plane. We will describe a method for detecting the magnetic state of an in-plane magnetized elliptical free layer using the differential planar Hall effect which significantly simplifies fabrication of test devices for studying SOT writing. Using this technique, we observe SOT switching of 50 x 150 nm2 CoFeB free layers with DC currents of opposite polarity for Pt and Ta SOT layers respectively, as well as with current pulses down to 10 ns pulse widths. By studying the dependence of the switching current on magnetic layer material, thickness and coercivity, as well as an applied external magnetic field, we demonstrate that switching is consistent with the theoretical picture of a spin Hall effect driven anti-damping like switching mechanism in both cases.

We also measure switching probability distributions for both field and current induced switching and 7 2 8 2 determine J ≈ 4 x 10 A/cm for Ta/CoFeB and J ≈ 1 x 10 A/cm for Pt/CoFeB devices with thermal c0 c0 stability factors Δ in the range of 60 - 90. Both results encourage further development of SOT MRAM for 2 in-plane magnetic layer switching. In this respect, Pt appears to be more promising due to its lower ρ(Jc0) product, due to about 10 times lower ρ. We expect that further Pt/CoFeB interface optimization can lead to significant reduction in J . We also show that J is thickness independent for Pt down to t = 4 nm, which c0 c0 allows a smaller write transistor and consequently smaller bit cell footprint.

REFERENCES 1) L. Liu et al., Science 336. 555 (2012). 2) Y. Kim et al., IEEE Trans. Electron Devices 62, 561 (2015) 3) A. Manchon and S. Zhang, Phys. Rev. B 78, 212405 (2008) 4) F. Oboril et al., IEEE Trans. CAD 34, 367 (2015)

Goran MIHAJLOVIC E-mail: [email protected] San Jose Research Center 53 HGST, a Wester n Digital company tel: 408 717-6000 3403 Yerba Buena Rd. San Jose, CA 95135 E2

Fig 1. (a) An SEM image of SOT writing test device adapted to describe differential planar Hall effect measurement; (b) current switching probability measured for 2 nm thick 50 x 150 nm2 CoFeB free layer on 10 nm thick Ta SOT layer; (c) field and d) current switching for the Pt (6 nm)/CoFeB (2 nm)/MgO SOT test device with 50 x 150 nm2 free layer.

54 E3 MAG-FLIP SPIN TORQUE OSCILLATOR USING HIGHLY SPIN POLARIZED HEUSLER ALLOY SPIN INJECTION LAYER FOR MICROWAVE ASSISTED MAGNETIC RECORDING

S. BOSU, H. SEPEHERI-AMIN, Y. SAKURABA, S. KASAI, M. HAYASHI, and K. HONO

National Institute for Materials Science, Tsukuba, Japan [email protected]

I. BACKGROUND Energy-assisted recording techniques such as heat assisted magnetic recording (HAMR) and microwave assisted magnetic recording (MAMR) have been proposed to increase the effective head field gradient to overcome the writability problem in the hard disk drives (HDD) for recording density beyond 2 Tbit/in2. In MAMR, ac magnetic field (µ0Hac) generated from a spin torque oscillator (STO) is applied to the recording media for lowering the switching field of magnetic grains in perpendicular media. A major challenge of MAMR is the development of a mag-flip spin torque oscillator (STO) [1] with a cross section area of ~ 40 × 40 nm2 or less consisting of an in-plane magnetized field generating layer (FGL) and a perpendicularly magnetized spin-injection layer (SIL) that is able to generate a large µ0Hac > 0.1 T from FGL with a frequency f over 20 GHz at small critical bias current density JC < 1.0 × 108 A/cm2 [2]. Recently, we demonstrated a mag-flip STO using highly spin polarized Heusler alloy Co2FeGa0.5Ge0.5 (CFGG) as a spin injection layer (SIL) [3] for the reduction of JC. We reported the usage of FePt/CFGG SIL reduces JC by ~50% compared to that using a FePt/Fe2Co SIL. Our micromagnetic simulations employing a new micromagnetic simulation code [4] that solves the coupled dynamics and spin accumulation simultaneously also implied that JC can be reduced by the spin accumulation at SIL interface [3]. In the previous study, CFGG layer with µ0Ms ~ 1 T was used as FGL, however, it was observed that [3] with only a small increment of J the FGL oscillation turned from a stable to chaotic mode before going to large angle (φ) uniform oscillation perpendicular to plane (OPP) because of the possible magnetization switching of CFGG FGL for enhanced spin transfer torque (STT) from higher J. Therefore, to generate a stable OPP mode as well as to achieve a high µ0Hac ∞ µ0Msl×sinφ (l is the thickness of FGL), we employed Fe2Co with µ0Ms ~ 2.3 T as FGL in combination with a highly spin polarized CFGG (l=3 nm) SIL perpendicularly magnetized with FePt (10 nm).

II. RESULTS AND DISCUSSION

To estimate the generated µ0Hac from STOs with different D on perpendicular media, we first performed micromagnetic simulations based on the LLG equation. The schematic diagram of an oscillating STO separated by 10 nm orthogonally from a perpendicular recording media is shown in Fig. 1(a). The estimated µ0Hac distribution on the perpendicular media from an oscillating Fe2Co FGL with µ0Ms ~ 2.3 T are summarized in Fig. 1(b) for D ~ 20 to 60 nm. µ0Hac varies from 0.12 T for D ~ 20 nm to 0.25 T for D ~ 60 nm when the angle of the OPP mode is considered to be its maximum, i.e., φ = 90°.

Therefore, as of requirement for MAMR, (a) (b) Fe2Co FGL with even D ~30 to 40 nm can Mag-flip STO generate for µ0Hac ~ 0.15 to 0.2 T. Fig. 2(a) shows the schematic diagram of D = 20-60 nm the experimental CPP nano-pillar STO devices. The sign of dc current Idc is defined negative when electrons flow from the top electrode to the bottom one. We prepared cylindrical shape

STO devices with D ~ 29 and 42 nm to 10 nm investigate STO properties. The SEM images Media for D ~ 42, and 29 nm STO devices are presented in Fig. 2(b). ΔR- µ0Hext curves for Idc = -0.5 and -0.2 mA for D ~ 42 and 29 nm in Fig. 2(a) and (b) measured by the two Figure 1: (a) Schematic illustration of STO and probe method with a perpendicular (θH = 0) perpendicular recording media setup for ac field calculation µ0Hac, (b) calculated stray field distributions Subrojati BOSU from STO’s with different sizes, D = 20 to 60 nm. E-mail: [email protected]

Tel: +81-29-851-3354 (PHS:3918) 55

55 E3

(b) µ0Hext ~1.1 T show the FGL is (a) almost/completely saturated out of the plane Ta/Au Ag/Ru cap e- (Idc < 0) θH = 0° Fe Co (7 nm) (cartoon 1 in red curves). At I = –3.5 for D = 2 dc Ag(5 nm) CFGG(3 nm) 42 nm and Idc = –0.5 for D = 29 nm the sudden FePt (10 nm) μ0Hext rise of intermediate resistance state at high Cr/Au- buffer layer D = 29 nm µ0Hext (cartoon 3) corresponds to the increase in MgO(100)-substrate D = 42 nm the relative angle between FGL and SIL by ST excitations. The increase in ΔR at high µ0Hext (d) 4 (c) D = 42 nm 4 D = 29 nm θH = 0° I = -0.2 mA with increasing Idc corresponds to the 120 Idc= -0.5 mA 150 dc = -3.5 mA = -0.5 mA = -2.5 mA = -6 mA enlargement of oscillation trajectories. At Idc = Hext 8 2 80 3 100 –6mA (|J| ~ 4.3 × 10 A/cm ) for D ~ 42 nm and 3 8 2

(mOhm)

(mOhm) at I ~ –2.5 mA(|J| ~ 3.7 × 10 A/cm ) for D ~ R

dc R 40 2 ∆ 50 2

∆ 29 nm, the change in ΔR (cartoon 4) is 1 1 comparable to that of the zero field state 0 0 -1.0 -0.5 0.0 0.5 1.0 -1.0 -0.5 0.0 0.5 1.0 (cartoon 2) which suggests the appearance of µ H (T) µ0Hext(T) 0 ext very large angle OPP mode with magnetization Figure 2: Schematic diagram of the STO device structure of FGL lying almost in the film plane (cartoon (a), SEM images of reference devices with D ~ 42 and 29 4). However, for uniform OPP mode, rf nm (b). ΔR-µ0Hext curves at different Idc with µ0Hext applied spectrum can not be detected when θH = 0 since perpendicular to film plane, i.e., θH ~ 0° for D ~ 42 nm (c), the relative angle between the FGL and SIL is and D ~ 29 nm STO devices (d). Inset cartoons represent constant during oscillation, i.e., the variation of relative magnetization orientations of FGL and SIL as well ΔR with time ΔR (t) is expected to be zero. as oscillation states of FGL. Therefore, to obtain a finite ΔR(t), i.e., to detect the f spectrum, it is necessary to tilt the (a) D = 42 nm (c) 120 I = -0.5 mA dc Idc = -6 mA f ~ 21 GHz θH of µ0Hext from the film normal. = -6 mA θH = 4 to 5° 1.094 T Figures 3(a) and (b) show the ΔR- µ0Hext 40 80 1.072 T curves with µ0Hext applied at a slight tilting θH μ0Hext 1.022 T ~ 4 to 5° for D~ 42 nm and 29 nm, 20 R(mOhm) 40 1.0 T

∆ respectively. Large ΔR rise at high µ0Hext is PSD(nV/rtHz) comparable to that at µ0Hext ~ 0, which 0.93 T 0 0 corresponds to large angle oscillations that -1.0 -0.5 0.0 0.5 1.0 10 20 30 40 8 2 D = 29 nm f(GHz) appears at Idc = –6 mA (|J| ~ 4.3 × 10 A/cm ) µ H (T) 0 ext (d) and – 3.5 mA (|J| ~ 5.3 × 108 A/cm2) for D ~ (b) 150 θ = 4 to 5° Idc = -3.5 mA f ~ 25.5 GHz Idc = -0.2 mA H 42 nm and 29 nm, respectively. Corresponding =-3.5 mA 1.1 T 40 power spectra with maximum f ~ 21 and 25.5 μ0Hext 100 1. 065 T GHz around µ0Hext ~ 1.1 T for D ~ 42 and 29 1.042 T nm, respectively, in Figs. 4(c) and (d) are in 20 R(mOhm) 50

PSD(nV/rtHz) the OPP mode. The f peak position decreases ∆ 1.012 T with decreasing µ0Hext. However, J is still 0.98 T 0 0 higher than the requirements for practical -1.0 -0.5 0.0 0.5 1.0 10 20 30 40 MAMR STOs, which could be reduced by µ0Hext(T) f(GHz) improving the chemical ordering as well as Figure 3: ΔR-µ0Hext curves with µ0Hext applied at tilting spin polarization of Heusler alloy SIL. angle, θH ~ 4-5° from film normal for (a) D ~ 42 nm and (b) D In summary, we have successfully ~ 29 nm STO devices. Inset cartoons represent oscillation prepared STOs with small diameters of D ~ 29 states of FGL. rf power spectrum (c) for D = 42 nm at Idc = and 42 nm as required size for a mag-flip STO. -6mA, and (d) for D = 29 nm at Idc = -3.5 mA. f ~ 21 GHz and We have achieved high frequency, f ~ 21-25 f ~ 25.5 GHz has been observed for D = 42 nm and D = 29 GHz large angle stable OPP mode spin torque nm, respectively at µ0Hext ~ 1.1 T. precession in a Fe2Co (7 nm) FGL at |J| in the order of 108 A/cm2 using highly spin polarized FePt(10 nm)/Co2FeGa0.5Ge0.5(3 nm) SIL. Our micromagnetic simulation results also imply that large ac magnetic field µ0Hac ~ 0.2 T can be generated from the STOs with a pillar size of D ~ 30 to 40 nm using Fe2Co (7 nm) FGL with high µ0Ms ~ 2.3 T.

This work was supported by ASRC MAMR project. References: [1] Zhu J. et al., IEEE Trans. Magn. 44, 125 (2008), [2] Takeo A. et al., Intermag Conference 2014 (AD-02),[3] Bosu S. et al., Appl. Phys. Lett. 108,072403 (2016) [4] Abert C. et al., Sci. Rep. 5, 14855 (2015) E4

SPIN ORBIT ENGINEERING FOR LOW POWER COMPUTING

K. H. CAO1, 2, S. Z. PENG1, 2, L. SU1, 2, M. X. WANG1, 2, X. X. ZHAO1, 2, J. Q. ZHOU1, 2, X. Y. LIN1, 2, Z. H. WANG1, 2, W. S. ZHAO1, 2 * 1) Fert Beijing Institute, Beihang University, 100191, Beijing, P.R. China 2) School of Electronic and Information Engineering, Beihang University, 100191, Beijing, P.R. China, [email protected]

I. ABSTRACT Low power is desirable for battery-powered electronics, such as mobile phone and internet of things (IOTs) devices etc. Spintronics is considered as an emerging technology that can offer this property based on its data non-volatility/fast operation/easy integration with CMOS. And spin transfer torque random access memory (STT-MRAM) has attracted much attention from academics and industries field. However, this technology has met some challenges in term of switching power, data stability and density etc. Spin orbit engineering may contribute of its improvement and this paper will give four examples: 1) heavy metal with strong spin orbit coupling for strong perpendicular magnetic anisotropy (PMA); 2) assistance of Spin-Hall Effect for fast Spin Transfer Torque; 3) Skyrmion racetrack memory with voltage control pinning; 4) all spin logic with weak spin orbit coupling channel. These could allow a full spin computing system with ultra-low power in the future. II. SPIN ORBIT ENGINEERING FOR STRONG PERPENDICULAR MAGNETIC ANISOTROPY Compared to in-plane magnetic anisotropy, PMA-based STT-MRAM has the potential to fulfill the properties of high-density, high thermal stability and low critical current. The core device in STT-MRAM is a CoFe(B)-MgO based magnetic tunnel junction (MTJ), which possesses a strong interfacial PMA due to hybridization of both d and p orbitals via spin-orbit coupling (SOC) [1, 2]. Further experiments revealed that a capping or seed layer adjacent to CoFe(B) has an essential influence on the PMA value, but the mechanism of such effect remains unclear. We investigated the origin of PMA in MgO/CoFe/metallic capping layer structures by using a first- principles computation scheme [3]. We found that the sum of PMAs at those two interfaces approximately equals to the PMA in the MgO/CoFe/X (X = Ta, Ru, Hf) structure and a large PMA can be obtained at the CoFe/X interface. This work confirmed that PMA in those three layer structures can be divided into two parts and analyzed separately, and the PMA could be tuned by choosing a proper capping materials or using double-barrier structure. Also, it can benefit the design of PMA-based MTJs for advanced applications. III. PMA-MTJ SWITCHING BY SPIN-HALL-EFFECT ASSISTED SPIN TRANSFER TORQUE The two-terminal MTJ devices based STT-MRAM suffers from energy and speed bottlenecks due to dependent write/read paths and barrier breakdown issue. Recently, a new effect named spin-orbit torque (SOT) is leading to the development of the concepts of three-terminal MTJ devices at lower energy and higher speed than conventional two-terminal MTJ, some experiments demonstrated current-induced in-plane and perpendicular magnetization switching can be realized SOT in the heavy metal/ferromagnetic bilayer system. However, for the PMA-MTJ, an additional magnetic field is required to achieve deterministic switching. We investigated the magnetization switching induced by Spin-Hall-assisted STT in a three-terminal device that consists of a PMA-MTJ and a β-W strip through solving numerically a modified Landau-Lifshitz-Gilbert equation [4]. We found that Spin-Hall assisted STT write schemes can significantly improve the switching speed of a STT-MTJ (reduced to < 1ns). Finally, we developed an electrical model of three terminal MTJ/β-W device with Verilog-A language and we performed transient simulation of switching a 4T/1MTJ/β-W memory cell with Spectre simulator. Simulation results demonstrated that spin-Hall-assisted STT-MTJ has advantages over conventional STT-MTJ in write speed and energy.

*Weisheng Zhao E-mail: [email protected] tel: +86-010-82314875 57 E4

IV. SKYRMION RACETRACK MEMORY WITH VOLTAGE CONTROL Magnetic skyrmion is appealing as an information carrier for future nanoelectronics, owing to its stability, small size and extremely low driving current density. One of the most promising applications of skyrmion is to build racetrack memory (RM). Compared to domain wall-based RM (DW-RM), skyrmion-based RM (Sky-RM) possesses quite a few benefits in term of energy, density and speed etc. Until now, the fundamental behaviors, including nucleation/annihilation, motion and detection of skyrmion have been intensively investigated. However, pinning/depinning of skyrmion still remains an open question and has to be addressed before applying skyrmion for RM. In order to promote the development of Sky-RM from fundamental physics to realistic electronics, we investigated the pinning/depinning characteristics of Skyrmion in a nanotrack with the voltage-controlled magnetic anisotropy (VCMA) effect (interfacial SOC). Results showed a skyrmion can be pinned/depinned by opening/closing VCMA-gates with an always-on driven current or decreasing/increasing driven current density. Then, we proposed a compact model and designed framework of Sky-RM for electrical evaluation. This work made a significant step for the development of Sky-RM [5]. V. ALL-SPIN LOGIC BASED ON 2D MATERIALS Spintronics devices have been proposed to perform ultra-low power logic with nearly zero standby power consumption, but most of them suffer from the large dynamic power in the requirement to frequently transform data between electrical and magnetic states. All-spin logic device (ASLD) is one of the most promising candidates to overcome the power challenge of traditional CMOS, since it can store as well as compute with spin information and logic-in-memory structure. The main part of ASLD is the structure of the Lateral Nonlocal Spin Valve (LNLSV), which is composed of PMA input and output Nano-magnet connected by a nonmagnetic channel (weak spin orbit coupling, such as copper or graphene). We developed a physical-based compact model that integrates STT switching of output Nano-magnet, Spin transport properties of LNLSV, and breakdown characteristic of channel. Results showed that the asymmetric is the most effective for ASLD in terms of current limitation [6]. We also presented a graphene-based all-spin logic gate (G-ASLG) and developed a spin-current compact model for spice simulation [7]. The optimization of ASLD and G-ASLG showed the possibility to implement ultra-low power and fast spintronics application. REFERENCES 1) S. Ikeda et al. "A perpendicular-anisotropy CoFeB–MgO magnetic tunnel junction", Nature Materials, 9(9) 721-724, (2010). 2) H. X. Yang et al, "First-principles investigation of the very large perpendicular magnetic anisotropy at Fe|MgO and Co|MgO interfaces", Physical Review B, 84(5) 054401, (2011). 3) S. Z. Peng et al, "Origin of interfacial perpendicular magnetic anisotropy in MgO/CoFe/metallic capping layer structures", Scientific reports, 5:18173, (2015). 4) Z. H. Wang et al. "Perpendicular-anisotropy magnetic tunnel junction switched by spin-Hall-assisted spin-transfer torque", Journal of Physics D: Applied Physics, 48(6) 065001, (2015) 5) W. Kang et al. "Voltage Controlled Magnetic Skyrmion Motion for Racetrack Memory", Scientific reports, 6:23164, (2016). 6) L. Su et al, "Current-limiting challenges for all-spin logic devices", Scientific reports, 5:14905, (2015). 7) L. Su et al, " Proposal for a graphene-based all-spin logic gate ", Applied Physics Letters, 106(7) 072407, (2015).

58 E5

TEMPERATURE DEPENDENCE OF CRITICAL DEVICE PARAMETERS IN 1 GBIT PERPENDICULAR MAGNETIC TUNNEL JUNCTION ARRAYS FOR STT-MRAM

C. PARK1, J.J. KAN1, C. CHING2, J. AHN2, L. XUE2, R. WANG2, A. KONTOS2, S. LIANG2, M. BANGAR2, H.ChEN2, S. HASSAN2, S. KIM1, M. PAKALA2, and S. H. KANG1 1Qualcomm Technologies, Inc., San Diego, California 92121, USA. 2Silicon Systems Group, Applied Materials, Inc., Sunnyvale, California 94085, USA.

I. Introduction Perpendicular STT-MRAM is well-positioned to serve as a nonvolatile working memory for embedded applications due to the combination of high performance, high endurance, and logic compatibility [1]. To achieve robust STT-MRAM designs, it is essential to maintain adequate read and write margins over a wide range of operating temperatures. Since both the tunneling magnetoresistance ratio (TMR) and the thermal stability factor (Δ) decrease with the increase in temperature, the read margin could be eroded at elevated temperatures [2]. It has also been reported that the probability of read disturbance errors increases with the reduction of Δ [2]. In contrast, the write margin improves at elevated temperatures due to the reduction in switching current (Ic) [3]. These behaviors are illustrated in Fig. 1. Accordingly, one of the critical challenges for the realization of STT-MRAM products is the preservation of critical device parameters such as Ic, Δ, Hc (coercivity), TMR and VBD (breakdown voltage) within the specified design margin over a wide range of operating temperatures (typically, from -30 °C to 125 °C). In this work, we investigate the temperature-dependent attributes of pMTJs in a 1 Gbit array from 25 °C to 125 °C. We systematically correlate the properties of pMTJ films to the critical attributes of the devices in order to demonstrate the robustness of such devices at elevated temperatures

II. Device fabrication and characterization Co/Pt based pMTJ stacks with dual MgO interfaces were deposited by an Applied Materials’ Endura® sputtering system on 300 mm wafers. pMTJ patterning was conducted using 193 nm dry lithography and advanced etching tools to fabricate 1 Gbit arrays consisting of 40-50 nm diameter pMTJs with 130-200 nm pitch. The details of the array structures were described in prior work [4,5]. III. Results

The representative R-H loops are shown in Fig. 2, revealing that Hc and TMR decrease with increasing temperature, while Hoff is generally unaffected. Note that reasonably high Hc (> 750 Oe) is still achieved at 125 °C. Since the temperature sensitivity is closely related to the magnetic material properties, pMTJs with different free layers (FL) exhibiting different magnetic moments were investigated. The stack with high FL moment typically maintains TMR > 120 % at 125 °C, which can meet our circuit design margin for fast, error-free read operations, as shown in Fig. 3. Thermal stability Δ was extracted by fitting a uniaxial thermal reversal model to the RDR (read disturbance rate) [2]. The extracted values of Δ were 74 and 51 at 25 °C and 125 °C respectively. These are in close agreement with the calculated values of Δ (EB/kBT) as a function of temperature, assuming only thermal fluctuations of a single domain, as shown in Fig. 4. The slight discrepancy at 125 °C is attributed to the temperature dependence of materials parameters such as moment and magnetic anisotropy. These results demonstrated that even at 125 °C, critical pMTJ device requirements were concurrently satisfied, assuring that perpendicular STT-MRAM is a robust and competitive embedded nonvolatile memory.

59 E5

REFERENCES [1] S. H. Kang, Symp. VLSI Tech. 2014, p.36. [2] K. Lee et al, IEEE MAGNETICS LETTERS, Vol 3, 3000604 (2012). [3] K. Lee and S.H. Kang, IEEE Trans. Magn, vol. 46, 1537 (2010). [4] C. Park et al, IEDM15-664 (2015). [5] L. Xue et al., IEEE Trans. Magn, vol. 51, 4401503 (2015).

Fig. 1. Illustration of relevant design margins for Fig. 2. Resistance vs. magnetic field as a read, write and break down voltages. function of temperature (from 25 °C to 125°C).

Fig. 3. Macro level circuit simulation of bit Fig. 4 Comparison of calculated Δ and error rate vs sensing time at varying TMR median measured Δs. values.

60 E6

ANTIFERROMAGNETIC SPINTRONICS

X. MARTI1,2, I. FINA3 1) Institute of Physics ASCR, v.v.i., Cukrovarnicka 10, 162 53 Praha 6, Czech Republic. [email protected] 2) IGSresearch, C/ La Coma, Nave 8, 43140 La Pobla de Mafumet (Tarragona), Spain. 3) Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Campus UAB, Bellaterra 08193, Barcelona, Spain, [email protected]

Spintronics has been largely based on ferromagnetic materials. In the past five years, many of the fundamental ferromagnetic spintronics (FS) procedures have been reproduced using antiferromagnets giving rise to an independent field of research called antiferromagnetic spintronics (AFS) [1]. AFS is based on exploiting the different resistance states that are concomitant the orientation of the antiferromagnetically coupled magnetic moments respect to the current or the crystallographic unit cell. The first demonstration of such bistable memory effect was performed mimicking the ferromagnetic magnetic tunnel junctions (MTJ) in 2011. In this experiment, one insulating MgO barrier was separating and antiferromagnetic (IrMn) and a non magnetic (Pt) electrodes and, at cryogenic temperatures, evidenced the existence of two resistance states separated by more than 100% [2]. Later in 2014, a two state memory bistability was observed at room temperature using for reading the antiferromagnetic anisotropic magnetoresistance as a reading method [3]. This seminal experiment employed for writing the AFS bit a combination of heat and external magnetic fields thus rendering the application unfeasible for realistic applications despite the high academic interest. This year, one revolutionary experiment has been performed in which electrical currents have been successfully employed to manipulate, at room temperature and with no additional magnetic fields, several stable magnetic configurations that can echo in several distinct, stable and switchable resistance states [4]. Each AFS cell reproduces the same resistance increment upon identical small electrical pulses thus a single memory cell could act as a counter [4]. Between the lowest resistance state and the highest resistance state one can, at the moment of writing, encode up to 16 memory steps. One fundamental question is: what is the next step in AFS. AFS has traveled along the footsteps of ferromagnets in the past 5 years. On the strong points side, AFS provides robustness against external magnetic field perturbations (up to 12 T could not perturbate the antiferromagnetic memory bits [1,3,4]), the absence of stray fields that may interact with neighbouring bits and a theoretically 3 order of magnitude higher switching speeds than in the case of ferromagnetic analogous blocks. On the weak aspects, AFS is a very young technological proposal that is so far limited to a rather small list of candidate materials which encompass certain complexities in the growth and sample preparation. This weakness is being mitigated by the increasing number of research groups that devote efforts into this new field as it can be seen in the now stable presence of a focussed symposia on AFS in the key international conferences. It is worth noting here that AFM spintronics has already achieved a significant goal in technology transfer: the ability to electrically switch the memory states at room temperature and read the resulting value with simple Ohmic resistance measurements allowed the presentation of hand-size USB-based memory demonstration devices with very low capacity (~1-10 bits) to be shown real-time to the audience of DPG conference held in Regensburg, Germany, last 2016. It is remarkable how after the seminal theoretical proposal published in 2010 [5], the AFS field has converted into a compact USB-powered device for the broad audience in just 6 years. However, the fail or success of a new technology proposal depends on how the existing technologies leave some niche spaces for it to develop. In this talk, we will present two strategies to render the AFS technologies viable despite the tight competition of magnetic memories and other non-magnetic based techonologies.

X. Martí E-mail: [email protected] tel: +42 657 74 12 81 61 E6

The current strategy for AFS is to stimulate two branches at the same time: on one hand, the classic non-volatile memory is taken as a long-term vision while, on the other hand, at a much more near future, it is expected to test this year some unique functionalities of AFS in the emerging field of Internet of Things, where the hardware demands are relatively small since the majority of the value of the product comes from the fact that it is connected to the internet. It is important to give an illustative example here: in a network of vehicle counters the need for in-situ (on each lane for instance) memory is very low (most of the products transmit data almost in real-time). On these grounds, the memory segment of <1 kByte non-volatile memory is the first target for AFS. The ability of AFS to encode over 16 incremental resistance changes in a non-volatile manner combined with the possibility of stimulating the AFS bit with piezoelectric ~1 V pulses allows addressing a fundamental problem in smart cities: how to transit from a active scheme (electrical consumption is happenning continuously) into a completely passive scheme (electrical consumption only happens when the data is collected). By using an array of ~8 AFS memory cells with an inherent capacity of ~16 memory states the “AFS vehicle counter coupled to a piezoelectric stimulus” could count wheels in a virtually complete passive way. We believe that the energy saving, manifested via the maintenance costs of the network of batteries, will echo in a significant economical saving which will give profit margin for the AFS technology choice. At the same time, a mesh of over 100’s of AFS devices deployed in cities with political and economical significance can boost the AFS technology in a way that could not be achieved by an eventual direct competition with existing competitors in large capacity memory chips. We remark that our short-term strategy is to address emerging markets in which the novelty is accepted relatively easier than in the case of well established markets such us the large-capacity non-volatile memory technologies. On a similar page, there are aspects, such as information security and new counterfeiting methods, that have a significnat echo in data storage. Here, AFS present a quite interesting property: they are magnetically invisible because they produce no external stray fields. On these grounds, one could envisage devices in which the magnetic data is stored at room-temperature in the form of antiferromagnetic memory bits while it is momentarily awakened to be edited as a ferromagnetic bit. FeRh-based proof-of-concept has been already fabricated [6]. We use of the magnetic phase boundary to subsequently write and cloak information. The phase transition can be driven Figure 1 Sketch of the magnetic cloackable memory thermally or electrically thus allowing either electrically and heat controlled. contact-less or contact manipulation. Again, we address the security market as the pay per bit is much higher than in the traditional large capacity memories and, more importantly, the market is demanding very low capacity devices (~256 bits is competitive) which are feasible in the first stages of a new memory proposal REFERENCES 1) T. Jungwirth et al., “Antiferromagnetic spintronics, Nature Nanotechnology 11, 231–241 (2016) 2) B.G. Park et al. “A spin-valve-like magnetoresistance of an antiferromagnet-based tunnel junction”, Nature Materials 10, 347 (2011) 3) X. Marti, et al., “Room-temperature antiferromagnetic memory resistor.” Nature Materials 13, 367 (2014). 4) P. Wadley, et al., “Electrical switching of an antiferromagnet”, Science 10.1126/science.aab1031 5) A.B. Shick, et al., “Spin–orbit coupling induced anisotropy effects in bimetallic antiferromagnets: A route towards antiferromagnetic spintronics”, Phys. Rev. B 81, 212409 (2010) 6) J. Clarkson, et al., “An invisible non-volatile solid-state memory”, arXiv:1604.03383

62 F1

NEW BLOCK COPOLYMERS ENABLE 5nm BPM PATTERNING

1 1 1 1 1 Grant WILLSON , Austin LANE , Michael MAHER , Gregory BLACHUT , Yusuke ASANO , Yasunobu SOMEYA1, Stephen SIRARD2 and Xiaomin YANG3 1) The Univ. of Texas, Austin, TX, USA, [email protected] 2) Lam Research, Fremont, CA, USA, [email protected] 3) Seagate Technology, Fremont, CA, USA, [email protected]

I EXTENDED ABSTRACT A series of silicon containing block copolymers has been designed, synthesized and evaluated for use in production of templates for replication of bit patterned media by imprint lithography. The goal of this work was to enable efficient production of patterns with minimum features less than 10nm in width. Several new monomers and block copolymers were synthesized and characterized for this study. Incorporation of silicon into one of the blocks enables very high etch contrast between the blocks under both oxidizing and reducing conditions1. This large difference in the etch rate of the blocks provides high resolution, high aspect ratio images1. The lamella in these polymers do not naturally orient perpendicular to the substrate, so a new method had to be developed to enable their orientation. This process depends on new “polarity switching” top coat polymers and carefully tuned bottom coatings2. A simple method was devised that allows fast optimization of both the top and bottom coatings and these coatings were optimized for each of the new polymers3. The minimum features that can be produced by these polymers is dictated by the degree of polymerization and the interaction parameter (χ), which describes the energy penalty associated with mixing the blocks. The greater the tendency for phase separation (the higher the χ) the higher the resolution of these systems. The χ value of the polymers was measured using the x-ray scattering methodology as previously described4. Many new polymers were synthesized and auditioned and the study has now produced materials that enable directed self-assembly of lamellae with 5nm lines and spaces as shown in Figure 1 below. Etch processes have been developed that enable selective removal of the hydrocarbon block in these structures to provide high aspect ratio patterns1. Progress has been made in developing an understanding the directed assembly of the high resolution silicon containing polymers through detailed studies at The University of Texas, at Seagate in the USA and at IMEC in Belgium. A methodology that enables quantification of the effect of process variables on the perfection of assembly has been developed and applied to minimizing the defectivity in a representative system. Focused ion beam milling experiments have provided thin cross sections of the assembled patterns and these were studied by transmission electron microscopy and electron energy loss spectroscopy to provide excellent elemental maps that detail the interaction between the block copolymer segments and the guiding structures. These studies have led to a new understanding of the importance of the interaction between the sidewall of the guiding patterns and the block copolymer lamellae5. Material and process design enables establishment of selectivity in the reaction between the surface treatment with the substrate and with the sidewall of the guide pattern, which in turn, offers the opportunity to greatly reduce the resolution demands on the lithography used to produce the guide patterns. At this time, the efforts have yielded a stable process for producing well resolved and aligned 50 Angstrom (5nm) lines and spaces. The polymers required to carry out this process can be acquired from Nissan Chemical industries, Ltd. Toyama, Japan. Work continues toward developing new materials and processes to support demonstration of a 40 Angstrom patterning process.

GRANT WILLSON E-mail: [email protected] tel: 1-512-471-4342 63 F1

II REFERENCES 1. Sirard, Stephen, Laurent Aza ; William Durand, Michael Maher, Kazunori Mori, Gregory Blachut, Dustin Janes, Yusuke Asano, Yasunobu Someya, Diane Hymes, David Graves, Chris Ellison and Grant Willson “Plasma etch of block copolymers for lithography,” Proc. SPIE 9782, Advanced Etch Technology for Nanopatterning V, 97820K (March 23, 2016); doi:10.1117/12.2220305 2. Bates, Christopher M.; Seshimo, Takehiro; Maher, Michael J.; Durand, William J.; Cushen, Julia D.; Dean, Leon M.; Blachut, Gregory; Ellison, Christopher J.; Willson, C. Grant “Polarity-Switching Top Coats Enable Orientation of Sub-10-nm Block Copolymer Domains,” Science 338(6108) 775 (2012) 3. Maher, Michael J. ; Bates, Christopher M.; Blachut, Gregory; Sirard, Stephen; Self, Jeffrey L.; Carlson, Matthew C.; Dean, Leon M.; Cushen, Julia D.; et al. “Interfacial Design for Block Copolymer Thin Films,” Chemistry of Materials 26(3) 1471-1479 (2014) 4. Durand, William J.; Blachut, Gregory; Maher, Michael J.; Sirard, Stephen; Tein, Summer; Carlson, Matthew C.; Asano, Yusuke; Zhou, Sunshine X.; Lane, Austin P.; Bates, Ch “Design of high-χ block copolymers for lithography,” Journal of Polymer Science, Part A: Polymer Chemistry 53(2) 344-352 (2015) 5. Cushen, Julia D.; Lei Wan, Gregory Blachut, Michael J. Maher, Thomas R. Albrecht, Christopher J. Ellison, C. Grant Willson, and Ricardo Ruiz “Double-Patterned Sidewall Directed Self-Assembly and Pattern Transfer of Sub-10 nm PTMSS-b-PMOST.,” ACS applied materials & interfaces 7(24) 13476-13483 (2015)

IV. ILLUSTRATIONS

Fig. 1 Fifty Angstrom (5nm) lines and spaces in block copolymer assembled over guide lines in imprint resist. The contrast has been enhanced by a brief exposure to oxygen reactive ion etching. This work was done in collaboration with Seagate Technologies and Lam Research .

64 F2

HIGH RECORDING PERFORMANCE OF BIT-PATTERNED MEDIA WITH TWO-LAYER INCLINED ANISOTROPY ECC DOTS

Naoki HONDA1, Kiyoshi YAMAKAWA2 1) Tohoku Institute of Technology, Sendai, Japan, [email protected] 2) Akita Industrial Technology Center, Akita, Japan, [email protected] I. INTRODUCTION The exchange coupled composite (ECC) dot structure [1] is effective to reduce the switching field while maintaining the thermal stability. We have extended the study from the original 2-layer to 4-layer structure, and proposed a 3-layer structure which exhibited not only a small switching field but also a small applied field angle dependence of the switching field [2]. A shingle write recording simulation on a bit-patterned media (BPM) with the 3-layer ECC dot array indicated possibility of a high areal density recording beyond 4 Tdot/in2 [3]. Small applied field angle dependence, however, is obtained only for 3 or 4-layer structures. The layer structure of such ECC dots is not simple and requires precise compensation of the magnetic properties. We have found that a small switching field as well as a small applied field angel dependence of the field is obtainable even for 2-layer ECC dot when the anisotropy axis is inclined. II. SWITCHING PROPERTIES First, we have studied the switching properties of 2-layer ECC dots with inclined anisotropy axis using a simple spin mode. A small normalized switching field of less than 0.5 was indicated for 2-layer ECC dot when the anisotropy axis is inclined to 10˚ with an increased anisotropy field for the soft layer, as shown in Fig. 1. The spin model analysis also indicated a small applied field angle dependence of the field. The switching properties of the 2-layer ECC dots were confirmed by simulation. The ECC dot was 3 3 modeled as stacked cubes with a size of 5×5×5 nm , Ms of 950 emu/cm , an anisotropy inclination angle of 10˚. The anisotropy fields for the hard and soft layers were 44 and 2.6 kOe, respectively, which assures the thermal stability of the dot with the additional anisotropy of the stacking. Dispersion of 5% in the anisotropy fields, and a standard deviation of 1.5˚ in the anisotropy axis were assumed. The simulation was performed for 32×32 dots with a dot spacing of 30 nm. A minimum normalized switching field, h*, of 0.51 was obtained as shown in Fig. 2, which is consistent with that of the spin model analysis. Small applied field angle dependences were also obtained for both applied field directions (Fig. 3), while that of the normal ECC dot indicated much larger dependence with the angle. III. RECORDING SIMULATION Recording simulation was performed for BPM with the dot arrays of the 2-layer ECC dots using a shielded planar head field [4]. The BPM was modeled as a 8×64 dot array in a square lattice with a dot pitch of 12.5 nm placed on a soft magnetic underlayer (SUL), which corresponds to an areal dot density of 4 Tdot/in2. The SUL was introduced as a mirror image plane. The write field of a shielded planar head with a multi-tapered main pole with a pole surface size of 20 nm×50 nm and MP-SUL and MP-medium surface separation of 14 and 4 nm, respectively, was used, which is the same condition as used in ref. [3]. Three-dimensional head fields were applied Fig. 1. Normalized second layer anisotropy to each layer center of the dot. field dependence of the minimum normalized switching fields for 2-layer inclined anisotropy ECC dots compared with a 2-layer and 3-layer ECC dots with perpendicular anisotropy. Naoki HONDA E-mail: [email protected] tel: +81-22-305-3220 65 F2

Fig. 4 shows the BPM model and the recorded pattern with a 2032 kFRPI. As the pole width of the write head was 20 nm, three tracks were partially recorded simultaneously. But the dots of the track at one pole edge were successfully recorded with a recorded liner density of 2032 kFRPI. A down track write shift margin of 2.5 nm was obtained. The error free (BER < 0.016) region in the cross track direction was around 2.5 nm, while the region was reduced to as small as 1 nm for the dot array with the normal ECC dots. The difference in the cross track shift margin between the two ECC dots is expected to come from the difference in the applied field angle dependence in the switching field. The write shift margins were greatly increased when fictitious inter-dot exchange coupling was introduced to the soft layer, Fig. 2. Interlayer exchange coupling field which is shown in Fig. 5. Write shift margins differ from each dependence of the switching field for the other corresponding to the applied field angle dependence of inclined and normal 2-layer ECC dots obtained the switching field shown in Fig. 3. Inclination to the trailing by simulation. direction (φ =100˚) might be the best. Since the DT write shift margin is greater than 6 nm, the linear density of the dot might be increased so that the areal density is increased to 5 Tdot/in2. The dots would also be applied to granular media for high density recording. REFERENCES 1) R. H. Victora, X. Shen, “Exchange coupled composite media for perpendicular magnetic recording,” IEEE Trans. Magn., 41(2), 537-542, 2005. 2) N. Honda, “Analysis of Magnetic Switching of 2 to 4 Layered Exchange Coupled Composite Structures,” J. Magn. Soc. Jpn., 37, 126-131, 2013. 3) N. Honda, K. Yamakawa, “High-Areal Density Recording Simulation of Three-Layered ECC Bit-Patterned Media with a Fig. 3.Applied field angle dependence of the Shielded Planar Head,” IEEE Trans. Magn., 50(11), 3002504, switching field for the inclined and normal 2014. 2-layer ECC dots obtained by simulation. 4) K. Yamakawa, Y. Ohsawa, S. Greaves, H. Muraoka, "Pole design optimization of shielded planar writer for 2 Tbit/in2 recording," J. Appl. Phys., 105, 07B728-3, 2009.

Fig. 5.Error free region map for the BPM with an Fig. 4. BPM model and the recorded pattern with a 2032 inter-dot exchange coupling field of 3 kOe. kFRPI.

66 F3

MICROMAGNETIC MODEL ANALYSIS OF SPIN-TRANSFER TORQUE OSCILLATOR AND WRITE HEADS FOR MICROWAVE-ASSISTED MAGNETIC RECORDING

Yasushi KANAI1, Kazuetsu YOSHIDA2, Simon GREAVES3 and Hiroaki MURAOKA3 1) Niigata Institute of Technology, Kashiwazaki, Japan, [email protected] 2) Kogakuin University, Tokyo, Japan, [email protected] 3) Tohoku University, Sendai, Japan, {simon, muraoka}@riec.tohoku.ac.jp

I. NTRODUCTION Microwave-assisted magnetic recording (MAMR) [1] is one candidate scheme for next-generation perpendicular magnetic recording and is considered to bridge the gap between current perpendicular magnetic recording (PMR, shingled recording etc.) and heat assisted magnetic recording (HAMR). A micromagnetic model analysis was used to investigate spin-transfer torque oscillators (STO) and magnetic write heads for microwave-assisted magnetic recording. Our goal is to propose STO and head configurations that allow the STO element to operate at a low injected current density for higher reliability.

II. MODELING Previously we considered an isolated STO element (without the write head) and a STO integrated into a write head. By neglecting the magnetostatic interactions in the integrated models the injected current density was reduced to around half compared to a model including interactions, meaning that the injected current density could be lowered if interactions were reduced [2]. It was also found that the optimum injected current densities (Jopt) were dependent on the STO geometry as well as the STO material characteristics, where Jopt was defined as the current needed to make the FGL magnetization rotate in the film plane. Various STO structures (cylinder, cylinder with cut and cuboid with cut) were considered and inserted into the

write head as shown in Figs. 1 and 2. Unless stated otherwise the FGL material properties were: 4πMs = 16 kG, Hk = -6 -6 31.4 Oe, A = 2.0×10 erg/cm and α = 0.02. The reference layer (RL) had: 4πMs = 8 kG, Hk = 20 kOe, A = 1.0×10 -6 erg/cm and α = 0.02. The write head had: 4πMs = 24 kG, Hk = 31.4 Oe, A = 3.0×10 erg/cm and α = 0.02. The antiferromagnetic coupling (AFC) constant in the soft magnetic underlayer (SUL) was -0.2 erg/cm2. The applied coil current was 0.06 AT0p DC. The electrons flowed from the FGL into the RL and the FGL oscillation arose from the reflected spin-torque at the RL interface. The whole space including the write head, STO and SUL was treated micromagnetically. The STO was divided into cubes with 2.5 nm sides and the remaining material was divided into tetrahedra. III. RESULTS

In Fig. 3, Jopt were derived for various write head structures. With regard to the STO geometry, Jopt was lowest for the cylindrical STO and highest for the cuboid with cut. The tilted main pole – trailing shield gap model had

lower Jopt compared with the perpendicular gap head due to less STO – head interaction. When FGL 4πMs = 24 kG, a large injected current was necessary, in-plane FGL oscillation was quite hard to obtain and the relationship between the FGL angle and the injected current was not clear, especially for FGL with cut. It was difficult to find head material parameters (anisotropy field, exchange constant, saturation magnetization and Gilbert damping factor) that could reduce Jopt whilst maintaining large STO and head fields. We will perform media recording simulations to determine how the STO - optimized head works in the full paper.

We acknowledge a part of this work was financially supported by ASRC, Japan.

Yasushi KANAI E-mail: [email protected] tel: +81-257-22-8111 67 F3

X Z STO Y

MP TS ABS

30 nm (a) Cuboid (b) Cylinder (a) Perpendicular gap (Perp cuboid) model 30 nm X Z RL: 5 nm STO Y

IL: 2 nm 30 deg FGL: 10 nm MP TS ABS 30 deg 24.2 nm 30 nm (c) Cuboid with cut (b) Recessed cuboid/cylinder models X 34 nm Z RL: 5 nm Y IL: 2 nm STO

MP 30 deg FGL: 10 nm TS ABS 30 deg 28.2 nm 30 nm

(d) Cylinder with cut (c) Tilted cuboid/cylinder with cut (Tilted cut) models Fig. 1 Various STO structures used for calculations. Fig. 2 Various integrated STO models.

Cuboid 30×60 Perp Cuboid Tilted cut Recessed cuboid Cylinder cut Recessed cylinder 110 110

90 90

70 70

50 50 FGL angle [deg] angle FGL FGL angle [deg] angle FGL 30 30

10 10 1.00 1.25 1.50 1.75 2.00 1.5 2.0 2.5 3.0 3.5 4.0 Injected current denisity, J [×108 A/cm2] Injected current density, J [×108 A/cm2]

Fig. 3 FGL angle vs. injected current density for various integrated STO models. FGL 4πMs = 16 kG (left) and 24 kG (right). Cuboid 30×60 denotes main pole width (MPW) = 30 nm and main pole throat height (TH) = 60 nm.

REFERENCES 1) J.-G. Zhu, X. Zhu, and Y. Tang, “Microwave assisted magnetic recording,” IEEE Trans. Magn., 44(1), 125-131, (2008). 2) T. Katayama, Y. Kanai, K. Yoshida, S. J. Greaves, and H. Muraoka, “Model analysis of tilted spin-torque oscillator with magnetic write head for shingled microwave-assisted magnetic recording,” IEEE Trans.Magn., 50 (11), 3001904, (2014). 3) Y. Kanai, T. Katayama, K. Yoshida, S. Greaves, and H. Muraoka, “Micromagnetic simulation of spin-torque oscillator (STO) for microwave-assisted magnetic recording: Interaction between write head and STO and optimum injected current,” IEEE Trans. Magn., (2016) (in press).

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MULTIPLE LAYER MICROWAVE ASSISTED MAGNETIC RECORDING

Simon GREAVES1, Yasushi KANAI2 and Hiroaki MURAOKA1 1) RIEC, Tohoku University, Sendai, Japan, [email protected] 2) IEE, Niigata Institute of Technology, Kashiwazaki, Japan, [email protected]

I. INTRODUCTION

The resonance frequency of a magnetic material is proportional to the anisotropy field, Hk. The switching field of a magnetic grain can be reduced by applying a high frequency (HF) magnetic field close to the resonance frequency. Such microwave-assisted magnetic recording (MAMR) can also be used to selectively record on different layers within a recording medium [1], [2]. In this work we investigate two and three layer recording media.

II. RESULTS

Fig. 1 shows an example of a 3 layer recording medium. The head field distribution used in this work was calculated assuming the spacing between the main pole ABS and the SUL was 25 nm. The vertical head fields under the trailing edge of the main pole are also shown in fig. 1 as a function of distance from the ABS. Each of the three recording layers sees a different head field and, if the Hk of each layer varies in proportion to the head field acting on the layer, the resonance frequency of each layer will be different and selective recording on each layer may be possible. If the average grain size in each layer is assumed to be the same the layer thickness should increase with distance from the head to maintain the same thermal stability.

To optimise the properties of each layer the maximum switchable Hk was calculated as a function of HF field frequency and head position. Fig. 2 shows an example for a 2 layer medium in which RL1 is 8 nm thick, RL2 is 4 nm thick and the spacing between the layers is 8 nm. Each recording layer has an exchange coupled composite (ECC) structure with a hard layer and a soft layer. The maximum hard layer Hk that could be switched is plotted for two HF field frequencies: 17 GHz and 60 GHz. For RL2 the maximum switchable Hk was about 245 kOe at 60 GHz and 130 kOe at 17 GHz. This range is indicated by the shaded region in fig. 2. If a Hk value in the middle of this range is chosen recording should be possible at 60 GHz, but not possible at 17 GHz. The same applies to RL1, but with a different Hk range and the HF frequencies reversed.

Fig. 3 shows the results of writing tracks on a 2 layer medium in which the grains were initially randomly magnetised up or down. Tracks written on RL2 with a 60 GHz HF field had high SNR and the magnetisation of RL2 was unchanged when writing on RL1 at 17 GHz. For RL1 high SNR was only possible for low Ku, but this also led to the magnetisation of RL1 changing when the HF field frequency was 60 GHz. Magnetostatic interactions between the layers and between grains within each layer were responsible. It would still be possible to store different information in each layer by first writing on RL2 and then on RL1. Alternatively, a bit patterned medium with similar properties would have much lower magnetostatic interactions and the recording performance of RL1 should improve.

The 3 layer design shown in fig. 1 was also optimised. Fig. 4 shows the switching probabilities of grains in each layer as a function of HF field frequency for a 3 layer, single grain stack. Selective switching of each layer was possible, although the switching probability of RL1, the layer furthest from the head, was not quite 100%. Tracks written on this 3 layer medium showed high SNR (~15 dB) for all three layers. As for the 2 layer medium, selective writing of each layer was complicated by magnetostatic interactions. Writing in the sequence RL3, RL2, RL1 gave the best results as layers nearer the head with higher resonance frequencies were not affected by lower frequency HF fields. III. ISSUES

The key to realising multiple layer recording is control of magnetostatic interactions between layers, i.e. as large a

Simon Greaves RIEC, Tohoku University, Katahira 2-1-1, Aoba ku, Sendai, 980-8577 69 E-mail: [email protected] Tel: +81-22-217-5458 F4

separation between the layers as possible. However, the strength of the HF field rapidly decreases with distance from the STO. Increasing the size and thickness of the STO field generation layer (FGL) helps, but the maximum size is limited by the need for coherent oscillation of the FGL magnetisation. Another issue is how to read out the stored information. These topics will be addressed in the presentation.

Fig. 1 Example of a 3 layer medium and head field Fig. 2 Maximum switchable Hk for each layer in a 2 as a function of distance from the ABS. layer medium. Hk refers to the hard layer.

Fig. 3 SNR of tracks written on a 2 layer medium as Fig. 4 Switching probability of grains in a 3 layer a function of the hard layer Ku of RL1. medium as a function of HF field frequency.

REFERENCES

[1] G. Winkler, D. Suess, J. Lee, J. Fidler, M. A. Bashir, J. Dean, A. Goncharov, G. Hrkac, S. Bance and T. Schrefl, "Microwave-assisted three-dimensional multilayer magnetic recording", Appl. Phys. Lett. 94, 232501, (2009).

[2] S. J. Greaves, Y. Kanai and H. Muraoka, "Recording on dual-layer and dual-thickness bit patterned media using microwave assisted magnetic recording", IEEE Transactions on Magnetics 51 (2016), accepted.

70 F5

HIGH OUTPUT CPP-GMR USING NEW SPACER MATERIALS WITH HALF-METALLIC HEUSLER ELECTRODES

Y. SAKURABA, J.W. JUNG, S. LI, Y. DU, J. CHEN, T.T. SASAKI, Y. MIURA, T. M. NAKATANI, T. FURUBAYASHI, Y.K. TAKAHASHI, and K. HONO National Institute for Materials Science (NIMS), Tsukuba, Japan [email protected]

I. INTRODUCTION Tunnel magnetoresistance (TMR) read sensors with MgO tunneling barrier have played an important role to enhance a recording density of HDD for this decade because of its high MR ratio. However, it is expected that TMR read head cannot reach small device resistance-area product (RA) less than about 0.3 Ω·µm2 that is required to obtain a sufficient S/N ratio for the recording density beyond 2 Tbit/in2. Therefore, to develop new read head that satisfies required MR ratio and RA will be a serious issue if recording density would be close to 2 Tbit/in2. Current-perpendicular-to-plane giant magnetoresistive (CPP-GMR) device using half-metallic Heusler electrodes is a potential candidate for next generation read head because of its small RA and expected large MR ratio arising from high spin-polarization of Heusler electrodes. Previous extensive studies have successfully realized large MR ratio over 50% and ΔRA over 10 mΩ·µm2 in the epitaxial CPP-GMR devices with Heusler electrodes and Ag spacer[1-3]. However, there are many remaining issues in Heusler-based CPP-GMR for a practical read head. One important issue is too small RA, i.e., RAs obtained in Ag spacer-based devices are about 20 to 30 mΩ·µm2, which is smaller than optimum RA (~ 100 mΩµm2) for read head application[4]. Therefore, searching new spacer materials or multilayer spacer structure leading to both higher RA and MR ratio is strongly desired. In addition, the most important task is to develop a poly-crystalline device showing comparable MR property with epitaxial devices. In previous study we have reported large ΔRA of 21 mΩ·µm2 with a relatively large RA of 30-50 mΩ·µm2 in the epitaxial device with AgZn spacer[5]. Therefore, in recent study, we applied AgZn spacer to poly-crystalline devices with Heusler electrode. On the other hand, as a new type of multilayer spacer, we employed B2-NiAl/NM/B2-NiAl trilayer spacer. Because first-principle calculation showed B2-NiAl has better electronic band matching with Heusler electrode, the interface of B2-NiAl layer with Heusler electrode is expected to enhance interfacial spin-dependent electron scattering.

II. EXPRIMENTAL METHOD For the poly-crystalline devices with AgZn spacer, we used thermally oxidized Si-substrate with a thick Ta/Cu/Ta electrode. The surface of Ta/Cu/Ta was polished by CMP to obtain flat surface. Co2FeGe0.5Ga0.5(CFGG)/AgZn/CFGG trilayer is deposited by sputtering at ambient temperature and then annealed at 400°C. For the epitaxial devices with B2-NiAl/NM/B2-NiAl spacer, we used MgO single crystalline substrate. (001)-oriented epitaxial CFGG/NiAl/Ag/NiAl/CFGG layers were grown on Cr/Ag buffer layers at ambient temperature and then annealed at 550°C. We prepared the devices with different thickness (tNiAl) of NiAl layers, 0, 0.21, 0.42 to 0.63 nm.

III. EXPRIMENTAL RESULTS Fig.1 shows a TEM image for poly-crystalline CFGG/AgZn/CFGG devices annealed at 400°C. We found relatively flat interfaces with a (001) oriented texture growth. Averaged RA of 40-50 mΩ·µm2 and the highest ΔRA of 9.5 mΩ·µm2 were observed, which are nearly twice of the poly-crystalline CFGG/Ag/CFGG.[6].

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Fig.2 shows NiAl thickness dependence of RA, ΔRA and MR ratio in CFGG/NiAl/Ag/NiAl/CFGG epitaxial 2 CPP-GMR devices. RA monotonically increases with NiAl thickness and reaches 55 mΩ·µm for tNiAl = 0.63 nm, although the mechanism of enlargement of RA is not simply understood by a two-current diffusion model. We found the highest MR ratio of 82% at RT and 285% at 10 K for tNiAl = 0.21 nm, which is the largest MR ratio in CPP-GMR ever reported. Since EDS element mapping confirmed that inserted thin NiAl layers remains at the CFGG interfaces without a remarkable inter-diffusion, it seems that NiAl layers has an effect to enhance interfacial spin-dependent scattering.[7] These MR ratio and RA values satisfy the required property for 2Tbit/in2 as plotted in Fig.3[4], therefore, NiAl/Ag/NiAl are promising spacer for next generation read head based on CPP-GMR devices.

CFGG AgZn CFGG CoFe Cr

Poly- CFGG/AgZn/CFGG MR ratio = 19% 2 62 ΔRA = 9.5mΩµm

60 MR ratio=19% ΔRA=9.5 mΩµm2 )

2 58 m µ Ω 56 (m

RA 54

52

50 -2 -1 0 1 2 Figure 2 NiAl thickness tNiAl dependence of the variation of (a) RPA, (b) ΔRA, H (kOe) and (c) observed MR values. The data points are averaged values of the devices on one substrate, and error bars show the observed highest and lowest values. Figure 1 TEM image and MR curve for the (d) RT and (e) 10 K MR curves of the CPP-GMR device with tNiAl = 0.21 nm poly-crystalline CFGG/AgZn/CFGG device on showing the highest MR ratio. The open circle indicates the second sample for Si-substrate. tNiAl = 0.21 nm.[7]

REFERENCES NiAl/Ag/NiAl [1] Iwase et al., APEX 2, 063003 (2009). [2] Sakuraba, et al, Appl. Phys. Lett. 101, 252408 (2012). [3] Li et al., Appl. Phys. Lett. 103, 042405 (2013) [4] Takagishi et al., IEEE Trans. Magn. 46, 2086 (2010). poly-AgZn [5] Du et al., Appl. Phys. Lett. 107, 112405 (2015). [6] Du et al., Appl. Phys. Lett. 103, 202401 (2013). [7] Jung et al., Appl. Phys. Lett.108, 102408 (2016).

Figure 3, Required MR ratio and RA for the read head of 2Tbit/in2 and the reported values in CPP-GMR and TMR devices.

F6

DESIGN STUDY OF HIGH RESOLUTION AND HIGH RELIABILITY TMR READER WITH RECESSED PIN LAYER STRUCTURE

Satoshi MIURA, Kenzo MAKINO, Takahiko MACHITA, Naomichi DEGAWA, Takumi UESUGI and Takeo KAGAMI

Magnetic Heads & Sensors Business Company, TDK Corporation 543 Otai, Saku-shi, Nagano 385-8555, Japan

I. INTRODUCTION Recently, the market of hard disk drive (HDD) is shifting from mobile usage to data center usage. This change strongly requests an improved reader head with higher performance and higher reliability endurance. We had proposed the design of what is called “Junction shield (JS)” as side reading reduction in the TMRC 2013 [1]. The JS reader showed an advantage for higher track density recording due to narrower micro-track profile by side shield effect. Next our challenge in the performance point of view is reduction of the shield-to-shield (S-to-S) spacing to obtain high signal resolution for higher linear density and high data transfer rate recording. Anti-ferromagnetic (AFM) layer has thicker thickness than other layers in TMR film to keep magnetic moment direction in pinned layer against high temperature process and environment, which is one of factors to limit the S-to-S spacing. One of the candidates for narrower S-to-S spacing is to apply an ordered Mn3Ir which has large blocking temperature to AFM layer [2]. It can allow to utilize thin AFM layer without pinning performance drawback. In this time, we propose a new concept design by recessing AFM layer from ABS as further S-to-S spacing reduction comparing with the TMR with Mn3Ir to achieve the S-to-S spacing of less than 20nm potentially. This design is compatible with the JS, thus we can get both benefits of side reading reduction and high signal resolution by combining it with the JS design. Furthermore the recessing AFM design brings pin related reliability improvement of reader.

II. DESIGN CONCEPT A cross-sectional schematic diagram of the new concept reader design we made is shown in Fig.1 (a) comparing with the conventional reader (Fig.1 (b)). The pin layers including AFM layer are recessed from ABS to reduce the S-to-S spacing at ABS. We call it “Recessed Pin Layer (RPL)”. An additional shield layer instead of the AFM layer can be set under a pinned layer via nonmagnetic buffer layer near ABS. If the additional shield layer has an enough shield function, the narrower S-to-S spacing can be realized. The magnetic moment direction in the pinned layer near ABS should be fixed by the recessed AFM layer. Namely it is essential to control the AFM offset from free layer. Reliability aspects wise, the RPL reader can be expected to improve not only corrosion robustness but also ESD and so on, because the AFM layer escapes from main bias current path.

III. EXPERIMENTAL RESULTS AND DISCUSSION Resolution and Micro Track Sharpness (MTS) evaluated by a spin-stand dynamic performance measurement are shown in Figs.2 and 3. Here, the resolution is a ratio of signal amplitude (TAAM) at high frequency (2T) to one (TAAL) at low frequency (6T). MTS is a ratio of width of micro track at 10% (MT10)

Satoshi Miura E-mail: [email protected] tel: +81-267-68-5111 73 F6

to 50% (MT50) of maximum amplitude. The RPL readers show higher resolution and smaller MTS than the conventional readers with JS under similar TAAL and MT50 respectively. These results indicate that the additional shield works as a magnetic shield and can help to reduce the S-to-S spacing. Fig.4 shows the ESD profiles of the RPL and the conventional reader. In the conventional one, fluctuation of resistance was observed just before breakdown voltage of more than 0.5 V. On the other hand, no clear fluctuation of resistance was observed in the RPL. It implies that magnetization in pinned layer is stable even just before breakdown in which high bias current flows through TMR film. It means the recessed AFM structure has robustness against bias current induced instability to pinning strength. In summary, the RPL structure we proposed here showed not only high performance which has an advantage for higher BPI and TPI recording but only high reliability. We believe the RPL design is suitable to satisfy any technological requirements for the data center usage HDD.

REFERENCES 1) Takumi Uesugi, et al., “STUDY OF SIDE SHIELDED READER FOR ULTRA-HIGH TPI MAGNETIC RECORDING”, TMRC 2013 digest, B2 (2013). 2) Ken-ichi Imakita, et al., “Giant exchange anisotropy observed in Mn-Ir/Co-Fe bilayers containing ordered Mn3Ir phase”, Appl. Phys. Lett., 85, 3812 (2004).

(a) (b)

Fig. 1 Cross-sectional schematic diagram of (a) the new concept reader with Recessed Pin Layer (RPL) and (b) the conventional reader

Fig. 2 Resolution vs TAAL Fig. 3 MTS vs MT50 Fig. 4 ESD profiles of (a) the RPL and (b) the conventional reader

74 G1

CHARACTERIZATION OF WRITE SIGNAL EFFICIENCY UTILIZING WRITE CURRENT DYNAMIC WAVE-SHAPING

John CONTRERAS, Jonas GOODE, Alexander TARATORIN, Xinzhi XING, Jianping CHEN Western Digital Corporation, Recording Subsystems, San Jose, CA 95135

I. WRITE FREQUENCY EFFECTS ON MEDIA TRACK WIDTH As disk storage areal densities continue to increase, a constant increase of the tracks per inch (TPI) is required. A measurement directly related to the TPI is the magnetic core-width (MCW) that is an imprint of the write head’s magnetization into the disk media. The MCW is determined by media properties, write head, write current level settings [1,2], and the write data frequency. For a fixed write current settings, the low-frequency and dc write patterns, the track widths are the widest, while the high-frequency patterns produce narrow track widths. Figure1 below shows different track widths for the low and high frequency data patterns for different baseline dc current, Iw, level settings. The vertical axis is the overshoot amplitude (OSA) of the write current that is above the Iw settings. The horizontal axis is the data-bit spacing or can be consider as a multiple of the read channel’s clock frequency (Bit Spacing-T*clock period). These contours are typical profiles of PMR head-media system that clearly shows that higher low-frequency Iw settings creates vertical contours. Where lower Iw settings have more sweeping contours. The sweeping contours provide the motivation to a write current OSA that has dynamic wave-shaping (DWS) as function of the bit-spacing. The red-dashed line shows potential of keeping a uniform Figure 1: MCW for different baseline current settings, track width as a function of the Iw: a) Iw=20, b) Iw=25, c) Iw=30, d) Iw=35 bit-spacing.

Figure 2 shows the bit-error rate (BER) contours as a function of the bit-spacing. Superimposed on the BER contours is the MCW values that provides evidence that having a DWS write current signal can create a constant acceptable BER, for example -2.5, while maintaining a constant track width.

These findings provided the motivation to architect and build a preamp with a write-driver that has DWS capability. There are two means to create the logic to control the overshoot. One means is to have the preamp with internal timers and such that the bit-spacing can be measured and determine the proper overshot. But this circuit architecture can only look back (LB) at transitions. Another architecture is to have the read channel that is on the SoC, provide DWS digital (DWSd) signal levels through a read multiplexed line that can

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enable when the write current overshoot should be enabled or disabled. This logic scheme can look ahead (LA) at upcoming write transition signals and determine if the OSA should be high for short bit spacing or low for long-bit spacing. In addition, this same architecture that is used for LA can be configured to operate LB, where only past bit-spacing determine if DWSd logic level is high or low.

The hardware implementation of DWS for a disk drive configuration has been constructed for this investigation, and the implementation creates component dependencies between the write precomp and the DWS peak currents. Managing these components’ dependencies have challenges in selecting the write current’s optimum settings, which is a vital part in utilizing DWS.

This presentation will give a review and a discussion of the above material. Specifically, the presentation will expound on the MCW measurements and its relationship to the BER, architecture that implement DWS, and DWS application and results as applied to SMR and PMR recording technologies. We will also review an electrical to head’s magnetic system response.

REFERENCES 68nm [1] Daniel Z. Bai, Peng Luo, Adam Torabi, Dave Terrill, James Wang, Kroum Stoev, Francis Liu, Matthew Moneck, Yuhui Tang, and Jian-Gang Zhu, “Return Field-Induced Partial Erasure in Perpendicular Recording Using Trailing-Edge 64.5nm Shielded Writers”, IEEE Trans. Magn., vol. 43, pp. 600-604, 2007. [2] A. Taratorin and K. Klaassen, “Media saturation and overwrite in perpendicular recording,” IEEE Trans. Magn., vol. 42, no. 2, pp. 157–162, Feb. 2006. Figure 2: BER curve of a PRBS signal with the MCW lines superimposed.

76 G2

SPATIALLY-COUPLED CODES FOR ADVANCED MAGNETIC RECORDING

Lara DOLECEK1, Homa ESFAHANIZADEH2, and Ahmed HAREEDY3 1) University of California, Los Angeles (UCLA), Los Angeles, CA, 90095, [email protected] 2) University of California, Los Angeles (UCLA), Los Angeles, CA, 90095, [email protected] 3) University of California, Los Angeles (UCLA), Los Angeles, CA, 90095, [email protected]

I. INTRODUCTION Spatially coupled (SC) codes are a class of sparse graph-based codes known to have capacity-approaching performance in the limit of very long block lengths. SC codes are constructed based on underlying LDPC (low density parity check) codes, by first partitioning this LDPC code and then piecing the components together. Significant recent research efforts have been devoted to the asymptotic, ensemble-averaged study of SC codes, as these coupled variants of existing LDPC codes offer excellent properties in a variety of settings. While the asymptotic analysis is important, due to simplifying assumptions and averaging effects, results from the asymptotic domain are not readily translatable to the practical, finite-length setting. Despite this chasm, finite-length analysis of SC codes is still largely unexplored. In this work, we tackle the problem of optimized design of SC codes in the context of magnetic recording (MR) applications. In particular, we identify combinatorial structures in the graphical representation of the code that are detrimental in the MR setting. An intriguing observation is that for the same SC/LDPC code, the problematic objects for the MR channels are topologically different than from the AWGN setting, thus necessitating a careful code design development for the MR applications. We first demonstrate that the choice of the so-called cutting vector, which guides the code partitioning in the SC code design, directly affects the cardinality of these problematic objects. In particular, we show that the count is the highest -- and consequently that the performance is the worst – in the case of the degenerate cutting vector, which precisely corresponds to uncoupled LDPC block codes. We therefore show that coupling always improves the performance and that the degree of improvement is dependant on the choice of the cutting vector. We then extend our analysis to a broader scope of code designs, including non-binary SC codes, multiple cutting vectors, and a novel class of multi-dimensional SC codes that are especially well suited for two-dimensional magnetic recording (TDMR). We demonstrate high performance of the proposed designs and universality of the proposed code design methodology.

II. OVERVIEW OF SC CODES AND GRAPHICAL STRUCTURES SC codes are constructed by coupling together into a chain a number of identical LDPC (block) codes [1]. The parity-check viewpoint is shown in Figure 1. In this example, a cutting vector partitions the parity check matrix of the underlying code into components H0 and H1. L replicas of the partitioned matrix are then pieced together, as shown in Figure 1 for L=7 components. The advantage of the resultant design is that it introduces “structured irregularity” while maintaining decoding complexity comparable to the decoding complexity of the underlying LDPC constituents.

We make two important observations. First, the choice of the cutting vector determines the cardinality of the structures that are detrimental in the finite-length case. Second, the type of the detrimental structures depends on the channel, and in particular, objects that dominate performance of graph-based codes on AWGN channels and objects that dominate performance of graph-based codes on PR channels are mathematically distinct. These two observations serve as a foundation of our SC code optimization framework for MR applications. In particular, we

LARA DOLECEK E-mail: [email protected] tel: +001-310-825-2108 77 G2

combinatorially characterize dominant decoding errors as certain absorbing sets: an (a,b) absorbing set is a configuration that has a variable nodes and b unsatisfied check nodes, and is such that all variable nodes have strictly more satisfied than unsatisfied check nodes, [2].

III. ANALYSIS AND DESIGN OF SC CODES FOR MR CHANNELS For SC codes built out of LDPC codes of column degree 3, through extensive simulations, we show that the majority of decoding errors for MR contain a (4,4) absorbing set – this configuration is in fact a subgraph of several different seemingly unrelated topologies that can all result in a decoding error. As a result, a code design methodology that focuses on reduction or elimination of (4,4) absorbing sets is the most effective.

We also note that (4,4) absorbing sets are not objects on interest in the design and analysis of column weight 3 LDPC codes when used in the canonical AWGN setting; in this setting, instead, the predominant decoding errors are due to (3,3) absorbing sets. This seemingly subtle difference has important consequences: code optimization techniques must be channel aware. We then develop a code design methodology that selects the cutting vector of an SC code as the one that precisely minimizes (4,4) absorbing sets. In the non-binary setting, combined with optimized non-binary edge weight assignments, our codes enjoy 4 orders of magnitude performance improvement over block-based counterparts and 2 orders of magnitude improvement over uniformed selection of the cutting vector and the edge weight assignment, as shown in Figure 2 for array-based (AB) LDPC code length 1058 bits and L=5, over GF(4). Top curve is the block code.

Building on this successful result, we then discuss several novel extensions. First, we investigate the use of multiple cutting vectors. We also develop multi-dimensional code designs with carefully placed check nodes that exploit cross track diversity in TDMR. Initial results are encouraging.

REFERENCES 1) D. J. Costello et al., "Spatially-Coupled Sparse Codes: Theory and Practice,” IEEE Comm. Magazine, 52(7), 168-176, (2014). 2) L. Dolecek et al., "Analysis of Absorbing Sets and Fully Absorbing Sets for Array-Based LDPC Codes", IEEE Trans. Info. Theory, 56(1) 181-201, (2010).

−2 10 NB−AB NB−AB−SC, Code 1 −3 10 NB−AB−SC, Code 2 NB−AB−SC, Code 3

−4 10

−5 10

−6 10 FER normalized over L

−7 10

−8 10 16 16.5 17 17.5 18 SNR (dB)

Fig. 1 Parity check matrix of a block LDPC code (left) Fig. 2 Representative performance comparison. and the resultant SC code (right). Code 1: random cutting vector and edge weight Code 2: optimal cutting vector and random edge weights, Code 3: optimal cutting vector and edge weights.

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EXPLORING TDMR GAIN CONSTRAINTS James ALEXANDER1, Tue NGO2, Shafa DAHANDEH1 1)Western Digital, Irvine, CA 92612 2)Western Digital, San Jose, CA 95138

I. INTRODUCTION Since the inception two-dimensional magnetic recording strategy (TDMR) [1], there has been an increased focus on signal-processing and systems-driven approach to improving recording density of magnetic recording technologies. Such an approach is of interest since its gains are complementary to other technologies striving for recording-density-gains, e.g., heat-assisted magnetic recording (HAMR) [2], and bit-patterned media (BPM) [3]. It is worth mentioning that gains projected by TDMR are due to multi-track signal processing (e.g., multi-track detection and coding) using a multi-sensor array, and involve solving system- related issues hitherto not encountered (e.g., multi-track timing recovery, write synchronization, etc.). Multi-sensor-array-based read channels that process single data tracks are a pre-cursor to TDMR technology. In the presentation we utilize OTC bathtub curves to illustrate how reader geometries, placement and performance affect TDMR gains. The gain estimation methodology uses waveforms from spin-stand in conjunction with an FPGA-based read channel to estimate areal density (AD) gain provided by the proposed multi-sensor-array based read channel. This methodology was used to evaluate the AD gains on a shingled magnetic recording (SMR) system. The gain estimation procedure was also performed on a non-shingled perpendicular magnetic recording (PMR) system, where adjacent-track interference (ATI) imposes constraints on areal density improvements.

II. CRITERIA USED FOR QUANTIFYING AREAL DENSITY CAPABILITY (ADC) In this digest, we use two different scenarios to quantify AD as described below and in [4]: 1. Squeeze-to-Death (SQ2D) based ADC: This corresponds to squeezing the tracks until the best error rate – Bit Error Rate (BER) or Sector Failure Rate (SFR) – within a track is no longer better than a predetermined threshold target. In other words, this scenario makes sure that the tip of the error rate bathtub is not worse than the target error rate. 2. Off-Track-Capability (OTC) based ADC: Unlike the previous one, this scenario focuses on the worst error rate, and squeezes the tracks until the worst error rate – Bit Error Rate (BER) or Sector Failure Rate (SFR) – within a track is no longer better than a predetermined threshold target, and makes sure that the width of the bathtub curve defined by those worst error rate points satisfies the required OTC. 3. Cross Track Separation (CTS) is the center to center distance across the track between the two readers at zero skew angle. 4. Down Track Separation (DTS) is the center to center distance down the track between the two readers at zero skew angle.

OTC criterion is used during On-The-Fly (OTF) error correction making sure that even the worst case error rate is sufficient to have converged sectors with operations utilizing OTF. SQ2D criterion, on the other hand, relies on steps in Error Recovery (ER) mode, and makes sure that those sectors, which did not converge during OTF, can still converge with ER steps. However, every time the drive goes through ER mode, it takes time for the sector to converge, hence reduces drive performance. Thus, the rate of a drive going through ER steps is market segment dependent, hence which criterion to use highly depends on a given market segment. Here, we will use both criteria instead of choosing a specific one for a given market segment.

III. NUMERICAL RESULTS In this work, we used a spin stand to quantify the ADC advantages of the first version of TDMR architecture. The spin stand testing includes full build heads with dual readers and a writer capable of full stroke testing, we show that AD gain depends on the criteria applied (SQ2D) or (OTC), the reference kFCI, CTS and DTS, combined reader width to track pitch ratio and reader balance. More details will be presented during the conference.

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REFERENCES [1] R. Wood , M. Williams , A. Kavcic and J. Miles "The feasibility of magnetic recording at 10 terabits per square inch on conventional media", IEEE Trans. Magn., vol. 45, no. 2, pp.917 -923 2009 [2] R. E. Rottmayer "Heat-assisted magnetic recording", IEEE Trans. Magn., vol. 42, no. 10, pp.2417 -2421 2006 [3] B. Terris , T. Thomson and G. Hu "Patterned media for future magnetic data storage", Microsyst. Technol., vol. 13, no. 2, pp.189 -196 2006

[4] S. Dahandeh, M. F. Erden, R. Wood, “Areal-Density gains and Technology Roadmap for Two-Dimensional Magnetic Recording” TMRC 2015 paper

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JOINT TIMING RECOVERY AND SIGNAL DETECTION FOR TWO- DIMENSIONAL MAGNETIC RECORDING Chaitanya Kumar MATCHA and Shayan Garani SRINIVASA Department of Electronics Systems Engineering, Indian Institute of Science, Bengaluru: 560012 Correspondence Email: [email protected]

I. INTRODUCTION Two-dimensional magnetic recording (TDMR) aims at achieving user bit densities > 1 Tb/in2 by reducing the size of a bit and using sophisticated signal processing algorithms. With the reduction in bit size, it is very important to sample the readback signal with accurate timing. In addition to the frequency offsets and jitter often seen in 1D magnetic recording, the timing errors in TDMR contain the frequency offset in the downtrack direction due to a timing error in the cross-track direction and vice-versa. Interpolative timing recovery (ITR) scheme [1] is a convenient scheme to recover the timing errors seen in TDMR. In this scheme, the readback signal is sampled at slightly higher than Nyquist rate and the samples are interpolated to obtain the samples at ideal locations. In this paper, a) we model the timing errors as a random walk where the timing errors are discretized, b) we propose a 2D joint timing and signal detector by extending the 2D soft-output Viterbi algorithm (SOVA) in [2] to include the timing errors in its state information, c) we propose an architecture where two such joint timing and signal detectors exchange information in a turbo fashion to achieve a gain in raw bit error rate (BER) performance. We study the efficacy of the algorithm via simulations over a Voronoi based TDMR channel model in [2] with channel bit density of 1 Tb/in2 for a grain size of 10 nm and comprising of generalized partial response (GPR) equalization and data dependent noise prediction (DDNP) to handle the media noise.

II. RESULTS Figure 1(a) shows the architecture of joint timing and signal detector. In this architecture, the over-sampled readback signal is interpolated to get the signal at the estimated timing errors. The likelihood probability of a local span of received samples is computed for a discrete set of timing errors over these local samples as well as for all possible data bit patterns. The likelihood probability conditioned on the timing errors is computed by interpolating the samples to the conditioned timing errors. Figure 1(b) shows the definition of a state of the 2D SOVA that includes timing offsets in addition to the bit values where the timing offsets are defined as a random walk from the current timing estimate. The best combination of timing errors and bit patterns will be used to update the current timing estimates and make the decisions on the bits. Figure 2 shows a turbo scheme where two such joint timing and signal detectors operate in a turbo loop exchanging the soft decisions on bits as well as timing estimates. One detector operates in raster scan order (top-left to bottom-right) within a 2D page and other detector operates in the reverse-raster scan order (bottom-right to top-left) to achieve SNR gains in the raw BER performance. We simulate the TDMR channel using a Voronoi based media model with CTC = 10 nm, bit size = 25x25 nm achieving channel bit density of 1 Tb/in2. The discrete time offsets for the maximum likelihood (ML) metric computation is taken from {+2.5 nm, 0 nm, -2.5 nm} in both downtrack and crosstrack directions. Using simulations, we show that we are able to recover timing errors with 5% oversampling of the readback signal. Figure 3 shows that performance of the proposed algorithm in a single iteration is ~0.5 dB better than the ITR timing errors estimated using Mueller and Muller method followed by signal detection using 2D SOVA. Figure 4 shows that the turbo scheme results in improvement in BER performance with multiple iterations achieving an overall performance gain of > 1 dB in SNR when measured over a simple AWGN 2D ISI channel with ideal timing, quantifying the efficacy of our detector.

REFERENCES 1) B. P. Reddy, S. G. Srinivasa and S. Dahandeh, “Timing Recovery Algorithms and Architectures for Two- Dimensional Magnetic Recording Systems,” in IEEE Trans. Magn., Apr. 2015. 2) C. K. Matcha, and S. G. Srinivasa, “Generalized Partial Response Equalization and Data-Dependent Noise Predictive Signal Detection Over Media Models for TDMR,” in IEEE Trans. Magn., Oct. 2015. 3) S. Datta and S. G. Srinivasa, “Design Architecture of a Two-Dimensional Separable Iterative Soft Output Viterbi Detector,” in IEEE Trans. Magn., 2016.

SHAYAN GARANI SRINIVASA* Department of Electronic Systems Engineering Indian Institute of Science Fax: +91-80-2293-2290 Tel: +91-80-2360-0810 (232) Email: [email protected] 81 Bengaluru, 560012, INDIA G4

(a) Architecture of 2D joint timing and signal detection. Timing errors are estimated by minimizing a ML metric.

(b) The state of the 2D SOVA is updated to include timing offsets that are modeled as random walk around the current timing estimate. Fig. 1 Joint timing and signal detection scheme estimates timing offsets by minimizing ML metric of a local span of received samples. The ML metric is defined for a state of 2D SOVA. The state includes timing offset information along with the bit values. The timing offsets are modeled as a random walk around the current timing estimate of the sample.

Fig. 2 Turbo scheme in which two joint timing and signal detectors exchange information to iteratively achieve gain in BER performance.

Fig. 3 The joint timing and detection scheme Fig. 4 The 2D SOVA in turbo loop gave performs ~0.5 dB better than M&M + 2D SOVA improvement of 1 dB in SNR as compared to the over single iteration for 2D SOVA. Voronoi performance of open-loop 2D SOVA. A simple based TDMR channel model with CTC = 10 nm, 2D ISI-AWGN channel is used. bit size = 25x25 nm at 1 Tb/in2 is used.

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TWO-DIMENSIONAL MODULATION CODES USING POLAR CODING TECHNIQUES

Hidetoshi SAITO Department of Applied Physics, School of Advanced Engineering, Kogakuin University 1-24-2 Nishi-shinjuku, Shinjuku-ku, Tokyo, 163-8677, Japan

I. INTRODUCTION In the last decade, it has been considered that various future magnetic recording technologies, such as heat-assisted magnetic recording, microwave-assisted magnetic recording, bit-patterned magnetic recording etc., to overcome the problem of physical and engineering limits for the exiting perpendicular magnetic recording technology. These recording technologies are expected to extend the storage density of magnetic recording systems using two-dimensional magnetic recording (TDMR) schemes towards 10 terabits per square inch (Tb/in2). If we use a TDMR scheme, it is natural to deal with or treat two-dimensional (2D) recording sequences. But, if a 2D maximum likelihood (ML) sequence detector is used to decode 2D sequences, it is known that there is a difficult computational complexity problem related to NP hard for decoding complexity [1]. Therefore, in general, designing methods of 2D modulation codes are restricted by coding rate, encoding or decoding complexity from the view point of signal processing. In this research, it shows a new design method for 2D modulation codes. This design method of the proposed 2D modulation codes are based on the polar coding technique [2]. Especially, using the fast transform methods in signal processing, it is known that complexity of a polar encoder or decoder is O(� log �) if a given polar code has a code length of � = 2!. This characteristics of complexity is preferable for a TDMR scheme. II. TWO-DIMENSIONAL MODULATION CODES BASED ON A POLAR CODING SCHEME Polar codes are linear codes and introduced by Arıkan in [2] for channel coding. The polar transform is to apply ⨂! ! the transform matrix �! ≡ �! , the n-th Kronecker power of �! = �!�! to the block of 2 bits where ! ! �! = (1 1) , �! = (0 1) and the notation T represents a matrix transposition. The polar encoder chooses a set of ! ℓ rows of the matrix �� to a form of ℓ×2 matrix which is used as the generator matrix in the encoding procedure. In this encoding process, the information bits which correspond to this set is called “frozen bits”. The way of choosing this set is dependent on the channel � and uses a phenomenon called “channel polarization”. ! ! ! ! Channel polarization is an operation which produces 2 channels {�!! : 1 ≤ i ≤ 2 } from 2 independent copies of a symmetric binary discrete memoryless channel such that the new parallel channels are polarized in the sense that their mutual information is either close to 0 (completely noisy channels) or close to 1 (perfectly noiseless channels). In this research, the defined 2D modulation codes are generated as follows. Fig.1 Each frozen bit is set to “0” which is a dummy bit. In polar coding, �!! = �!!×�!! where �!!, �!! are a codeword and a binary information ! ! ! sequence with length of 2 bits, respectively. Here, it satisfies that �!!×�!! = �!! where �!! is the 2 ×2 identity matrix. Then, these frozen bits are considered to be parity symbols in a codeword. If a dummy symbol is inserted periodically into a codeword, these dummy symbols give a (0, �!, �!: �′) constraint, where a (0, �!)-RLL constraint is defined for the down-track direction of each track and a �! constraint is defined for the cross-track direction of every �′ tracks [3]. In decoding, a codeword without dummy symbols is decoded by a successive cancellation (SC) decoder [2] which decodes the bits �!! in order. If �!! = �!! ×�!! , it is able to obtain estimated information bits. III. TDMR SCHEME USING A PROPOSED 2D MODULATION CODE Fig. 1 shows the block diagram of the bit patterned magnetic recording (BPMR) system. This BPMR system is

Hidetoshi Saito E-mail: [email protected] 83 tel: +81-3-3340-2831 fax: +81-3-3348-3486 G5

based on the proposed TDMR scheme using bit patterned media (BPM) with 2D generalized partial response (GPR) equalization and one-dimensional (1D) a posteriori probability (APP) detector. In Fig. 1, 2� + 1-track recording is assumed for the BPMR system. For the readback BPMR channel, the readback signal of BPM is represented by the

2D Gaussian pulse response given by [4] and the normalized peak of the pulse amplitude is Ap. The noise sequence ! is additive white Gaussian noise (AWGN) with zero mean and variance �! . IV. ERROR RTAE PERFORMANCES OF TWO-DIMENSIONAL MODULATION CODES Fig. 2 shows the bit error rate (BER) performances of the coded GPR systems. In Fig.2, the dashed and solid lines show the BER performances of the proposed 2D modulation coding with the (0,62,20:3), (0,82,20:4) constraints using four-track recording and three-track recording, respectively. These proposed 2D modulation codes have the

code rate �!= 12156/12486, 16208/16584 and the effective transmission rate �!= 2.92, 3.91, respectively. The dotted line shows the BER performances of the compared 1D coding with the single 1D APP detector for the desired single-track recording. The recoding code satisfy the (0,8) constraint for down- track direction and a codeword sequence is precoded by the precoder which has polynomial 1/(1+D) (mod 2) where the symbol D represents a unit

shift in the down-track direction. These compared recording code have �!= 64/65 and �!= 0.9846. The recording condition corresponds to the areal density of 4.0 Tb/in2 given by [4]. In this simulation, the SNR is defined as SNR

= 20log!" Ap/�� [dB]. As can be seen Fig. 2, the performances of the BPMR systems with proposed 2D modulation coding using three tracks and four tracks outperforms that of 1D RLL coding using a single track by about 7.4 dB, 12.8 dB of SNR gains at a BER of 10-5, respectively. V. CONCLUSIONS In this research, new 2D modulation codes are proposed for a TDMR scheme. In concrete terms, the BPMR system based on the TDMR scheme with proposed modulation codes are assumed and the BER performances are evaluated. Research results imply that the performances of these 2D modulation codes using polar coding techniques are superior to the performance of the conventional high rate 1D RLL modulation code. REFERENCES 1) E. Ordentlich and R. M. Roth, IEEE Tran. Inf. Theory, vol.57, no.12, pp.7661-7670, Dec. 2011. 2) E. Arikan, IEEE Trans. Inf. Theory, vol.55, no.7, pp.3051-3073, July 2009. 3) R. E. Swanson and J. K. Wolf, IEEE Trans. Magn., vol.28, no.6, pp.3407-3416, Nov. 1992. 4) T. Wu, M. A. Armand and J. R. Cruz, IEEE Trans. Magn., vol.50, no.1, pp.1-11, Jan. 2014.

Fig. 1 Block diagram of TDMR scheme. Fig. 2 Evaluated BER performances.

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HIGH DENSITY TURBO TDMR DETECTION WITH LOCAL AREA INFLUENCE PROBABILISTIC MODEL

Xueliang SUN1, Krishnamoorthy SIVAKUMAR1, Benjamin J. BELZER1, and Roger WOOD2 1) Washington State University, Pullman, WA, USA, xsun,siva,[email protected] 2) HGST, San Jose, CA, USA, [email protected]

I. INTRODUCTION This talk describes probabilistic message-passing TDMR turbo-detection algorithms that locally estimate magnetic grain interactions with coded data bits, and thus iteratively assist channel decoding. Such local area influence probabilistic (LAIP) techniques are especially effective at densities ranging from about 4 magnetic grains per coded bit (GPB) down to about 1 GPB, where interaction between grains and bits is significant and occasionally a bit will not be written on any grain, and hence will effectively be “overwritten” by bits on surrounding grains. Sometimes non-overwritten bits can also be influenced so severely that their values read from the magnetic media have wrong signs. By modeling the interaction among grains, LAIP enables detection of both overwritten bits and wrong-sign non-overwritten bits so that their log-likelihood ratios (LLRs) are assigned small magnitudes for overwritten bits and correct signs for non-overwritten bits. As it is easier for a channel decoder to fill in erasures than to correct errors, and the number of errors is reduced, LAIP-based detectors can potentially achieve targeted bit error rate (BER) performance at higher channel coding rates, resulting in higher user information areal densities. An earlier dynamic grain state estimation (DGSE) paper [1] utilizes the generalized belief propagation (GBP) algorithm [2] for the discretized-nuclei Voronoi grain model (DNVGM), which is based on the random Voronoi grain model (RVGM) of [3]. Results in [1] showed that information densities greater than 0.4966 user bits/grain (U/G) could be achieved on the DNVGM. For the fixed-boundary Voronoi grain model (FBVGM), which is the model of [3] with known boundary bits and grain shapes, [1] suggested that 0.4515 U/G could be achieved. Motivated by these results, we propose a novel detector based on the LAIP model. This new detector employs both the RVGM of [3] and the FBVGM. Simulation results show that the LAIP-based detector can accurately detect both overwritten bits and severely influenced non-overwritten bits, and hence higher user densities can be achieved compared to [1]. In addition, the LAIP-based detector’s computer run time is 10000 times faster than the GBP-based detector in [1]. II. AFFECTED AREA We assume the centroid write model, where a given Voronoi grain cell is magnetized (to a value of ±1) by the channel bit containing that grain cell’s centroid. We assume a soft-bit read model that computes the value yi read at the center of the ith bit cell as the area integral of magnetizations of all grains contained within the bit cell. We introduce the affected area α to represent the influence from adjacent bits, as shown in Fig. 1. The total influence from surrounding bits on the read value of a coded bit U is calculated as , where stands for the affected area with magnetization total +++= ααααα DCBA αj from bit j, and the influences from diagonally adjacent bits are neglected due to their relatively smaller values. We investigate two different boundary assumptions Fig. 1. Portions of central for the random Voronoi grain model [3]: FBVGM and RVGM. FBVGM assumes coded bit U affected by that every boundary grain nucleus is fixed at every boundary bit cell center, and that vertically and horizontally boundary bits have a polarity of 0; hence, boundary bits have no influence on their adjacent bits B, C, and D. adjacent bits. In the more realistic RVGM, the boundary grain nuclei are randomly distributed and the boundary bits take random polarities ±1. Thus, the RVGM boundary bits can influence adjacent

bits. Both models assume random nuclei locations inside a 3 × N block of coded bits, and both have a ratio σA/µA of grain area standard deviation to mean of about 0.25, consistent with [3].

Benjamin J. Belzer E-mail: [email protected] 85 tel: 1-509-335-4970 Work funded in part by NSF CCF-1218885, and by ASTC. G6

III. LOCAL AREA INFLUENCE ESTIMATION BASED TDMR TURBO-DETECTOR

The output LLR for the coded bit U, given soft-bit yU output from the read model, is estimated as

LLRU = log[ αtotal < U αtotal > yPyP U )()( ]. Because αtotal < yU implies that the read value is greater than total influence from surrounding bits, bit U is most likely magnetized with positive polarity, i.e., U = 1, whereas αtotal > yU implies that U = −1. The a-posteriori probability (APP) of the total affected area αtotal conditioned on the vector of read values y = yyyyy DCBAU ],,,,[ for bit U and its adjacent bits A, B, C, and D is estimated as

P α total y = α UAA ∗ α UBB ∗ α UCC ∗ α yyPyyPyyPyyP UDD ),|(),|(),|(),|()|( , where * indicates convolution, and independence is assumed among affected areas from different bits. The PMFs α yyP Uii ),|( are obtained by Monte-Carlo simulations using the random Voronoi model in [3], with discretized α and y variables. In the LAIP based TDMR turbo-detector shown in Fig. 2, which we call the “Y-effect” detector, channel bits u from an irregular repeat accumulate (IRA) encoder are written/read from the FBVGM or RVGM, resulting in sample vector y, which flows into the Y-effect detector. The detector computes LLRs, which are weighted and then flow into .Fig. 2. Block diagram of LAIP TDMR turbo-detector the IRA decoder, which returns output LLRs to the detector to iteratively assist the detector’s estimation and therefore Fig. 2. Block diagram of Y-effect TDMR turbo-detector. improve the accuracy of detection and decoding. IV. SIMULATION RESULTS Table 1. BER performance of GBP and Y-effect turbo-detector at 0.4515 U/G Table 1 shows Detector BER (1st BER (2nd BER (3rd BER (4th BER (5th BER (6th Raw BER simulation results Design iteration) iteration) iteration) iteration) iteration) iteration) comparing the GBP 9.22e-02 4.30e-03 3.00e-03 3.00e-03 performance of the Y-effect turbo- detector Y-effect 9.10e-02 8.23e-04 6.40e-04 6.91e-04 6.46e-04 5.78e-04 5.34e-04 to the GBP-based detector of [1] at 1.1 GPB under the FBVGM, for the same IRA code of rate 0.4966 and code word size 294 bits. The Y-effect detector gives a BER decrease of almost an order of magnitude, and its running time is around 1/10000 that of the GBP-based detector. Table 2 Table 2. BER/FER performance of Y-effect turbo-detector with longer code word length; U/G = 0.4862 shows 1st iteration 2nd iteration 3rd iteration 4th iteration 5th iteration Code rate Raw BER simulation BER, FER BER, FER BER, FER BER, FER BER, FER results with 5.5e-5, 6.8e-6, 1.5e-5, 7.3e-6, 9.4e-6, a stronger 0.5348 9.18e-02 IRA code of 1.2e-2 3.3e-3 4.0e-3 2.7e-3 2.7e-3 length 3009 bits; frame error rate (FER) is also included. A 7.7% density gain compared to the shorter code word length in Table 1 at the same grain density is significant, and suggests that increasing the code word length to about 30 kilobits should give even better performance.

REFERENCES 1) X.Sun, B.Belzer, and K.Sivakumar, “Dynamic grain state estimation for high-density TDMR: Progress and future directions,” IEEE Trans. Magn., 52(2), article 9400107, (2016). 2) J. S. Yedidia, W. T. Freeman, and Y.Weiss, “Constructing free-energy approximations and generalized belief propagation algorithms,” IEEE Trans. Inform. Theory, 51(7), 2282–2312, (2005). 3) R. M. Todd, E. Jiang, R. Galbraith, J. R. Cruz, and R. W. Wood, “Two-dimensional Voronoi-based model and detection for shingled magnetic recording,” IEEE Trans. Magn., 48(11), 4594–4597, (2012).

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TWO-DIMENSIONAL EQUALIZATION AND DETECTION FOR TWO TRACKS IN ARRAY-READER BASED MAGNETIC RECORDING

Jun YAO1, Euiseok HWANG2, B. V. K. Vijaya KUMAR3, and George MATHEW1

1) Broadcom Ltd., San Jose, CA, USA, [email protected] [email protected] 2) Gwangju Institute of Science and Technology, South Korea, [email protected] 3) Carnegie Mellon University, Pittsburgh, PA, USA, [email protected]

I. ABSTRACT Array-reader based two-dimensional magnetic recording (TDMR) has been shown to provide diversity gain against noise and to enable the handling of inter-track interference (ITI) [1][2]. In this work, a three- reader two-track (3R2T) detection system is studied where two 2-D equalizers are used to jointly process array readback signal streams and the equalized outputs are jointly processed by a 2-D detector to detect data from two tracks. Because the noise streams in the outputs from the 2-D equalizers are correlated, we propose de- correlation based equalization and detection algorithms to achieve improved detection performance. The pattern-dependence nature of the noise in each stream is also analyzed and a pattern-dependent noise- predictive (PDNP) detector is presented. Bit-error-rate (BER) performance is evaluated to compare the proposed approaches with an existing scheme. It is shown that tangible performance gains can be achieved by the proposed schemes.

II. INTRODUCTION In the 3R2T detection system, an array-reader with 3 read elements are used to read data from 2 tracks. The readback signal streams produced by the 3 readers contain not only the data written on the 2 tracks but also impairments such as media noise, ITI and electronic noise. The signal streams are passed through an analog front end (AFE) and sampled at baud-rate before feeding to the 2-D equalizers, as shown in Fig. 1.

Fig. 1. Schematic of 3R2T detection system.

The motivation and benefits of using 2-D equalizer with 2-D partial-response (PR) target followed by a symbol-based detector have been shown [2]. However, the approach in [2] ignores the correlation between the two equalizers’ outputs and treats the signal streams from 2-D equalizers as independent when doing data

Jun Yao E-mail: [email protected] tel: +86-21-20607237 87 G7

detection by the symbol-based joint detector. Since there exists significant correlation between the two equalizers’ outputs, in this paper, we present modified equalization and detection schemes to achieve improved performance. We handle the correlation in three different ways: (i) jointly designing the two 2-D equalizers such that the noises at the two equalizers’ output are maximally de-correlated, (ii) modifying the branch metric of the 2-D detection algorithm to account for the correlation in the 2 streams and (iii) designing PDNP detector such that the residual noises in the two streams are maximally de-correlated.

III. RESULTS AND DISCUSSION Fig. 2 shows the BER performance of various equalization and detection schemes. The red curve with circles (de-correlation 2D equalizer and PDNP detector without correlation consideration), blue curve with squares (traditional 2D equalizer and PDNP detector with correlation accounted for in the branch metric), and light-blue curve with stars (traditional 2D equalizer and mean-adjusted de-correlation PDNP detector) correspond to the proposed three different ways of handling the correlation. We also include the best results from [2] (the black curve with asterisks) for comparison. It can be observed that all three proposed schemes achieve performance gains over the existing scheme. The best performing scheme is the one using mean- adjusted de-correlation PDNP detector which also bears the highest complexity among these schemes.

Fig. 2. Performance comparison of various equalization and detection schemes.

REFERENCES

1) G. Mathew, E. Hwang, J. Park, G. Garfunkel, and D. Hu, “Capacity advantage of array-reader-based magnetic recording (ARMR) for next generation hard disk drives,” IEEE Trans. Magn., vol. 50, no. 3, Mar. 2014. 2) J. Yao, E. Hwang, B. V. K. V. Kumar, and G. Mathew, “Two-track joint detection for two-dimensional magnetic recording (TDMR),” in Proc. IEEE Int. Conf. Commun., 2015 (ICC’2015).

88 H1

DEFINITION OF AN AREAL DENSITY METRIC FOR MAGNETIC RECORDING SYSTEMS

Steven D. GRANZ1, Tim RAUSCH1, Richard BROCKIE2, Roger WOOD2, Gerardo BERTERO2, and Edward C. GAGE1 1) Seagate Technology, Shakopee, Minnesota, USA 2) Western Digital Corporation, San Jose, California, USA

I. INTRODUCTION The areal density capability (ADC) of a magnetic recording disk drive is highly dependent on the market segment and how the disk drive will be used. Disk drives used in data centers typically require high input/output performance and are often subject to high levels of vibration from adjacent drives and cooling fans. In contrast, disk drives used in consumer applications like DVRs typically favor high capacity over performance and have little stress from external vibration. The same generation of magnetic recording technology will therefore end up with very different product areal densities. In this paper we define a new areal density metric which represents what areal density is possible under very ideal recording conditions. How much of this ideal areal density that can be harvested in a real product will depend on the application. This approach will allow us to measure the progress of a technology without having to worry about the particulars of different market segments which is often considered confidential information. This avoids a problem seen in earlier methods, which, for example, included a requirement for 15% off-track read capability intended to allow for a certain amount of track-following error due to vibration [1].

II. AREAL DENSITY METRIC There are two common architectures for the layout of tracks in a hard disk drive. In a conventional magnetic recording system (CMR) any track can be written at any time and neighboring tracks do not intentionally overlap. In a shingled magnetic recording system (SMR) the tracks are written sequentially in bands and the tracks intentionally overlap like the shingles on a roof. In SMR, an adjacent track is written only once, but in CMR it may be written many times. This difference is reflected in the following definition. For CMR the test data track is written first then the two adjacent tracks are written 32 times on each side. For SMR, one adjacent track is written before the test track and the other is written after the test track. Unlike CMR, the tracks are written only once. For both recording schemes, the test track and the adjacent tracks should be written with different encoded random data patterns and should contain at least 50, 4kB data sectors. The tracks are written at exactly the defined track-pitch with no additional squeeze. The recommended velocity for making the measurements is 15 m/s. On read-back, the written data sectors should be re-read until 10,000 consecutive sectors have been read. The reader should be positioned on the test track to give the best possible error rate. This demo condition requires that none of the 10,000 sectors are in error. No additional data-recovery procedures are allowed. The areal-density metric is the highest areal density that can be achieved in this manner, given the freedom to choose any recording conditions including linear-density (BPI), track-density (TPI), and reader position. III. EXPERIMENTAL DETAILS We investigated the areal density capability for CMR Heat Assisted Magnetic Recording (HAMR) and SMR HAMR on a spin-stand using the areal density metric described above. Ten HAMR heads were used. The heads and media are similar to those used in previous studies [2-4]. Spinstand measurements were at 6300 rpm, radius 23 mm, skew 0o, and linear velocity 15.25 m/s. Channel areal density (Tflux-changes/in2) was

Steven D. Granz E-mail: [email protected] tel: 952-402-2739 89 H1

measured and a code rate of 0.88 was assumed to obtain user areal density (Tbit/in2).

IV. RESULTS The areal density capability of the ten HAMR heads at 1 nm active clearance yielded an average HAMR-CMR ADC of 1.21 Tflux-changes /in2 (1.07 Tbit/in2) and HAMR-SMR ADC of 1.53 Tflux-changes /in2 (1.35 Tbit/in2) as seen in Figure 1a. We observed a 26% increase in areal density with HAMR-SMR over HAMR-CMR. For HAMR-CMR and HAMR-SMR, the bit aspect ratio was measured at various active clearances as seen in Figure 1b. For HAMR-SMR the track density was significantly higher than HAMR-CMR due to the single adjacent track write and the single-sided adjacent track interference (ATI) when shingling the tracks. We also see that as head media spacing (HMS) increased, ADC decreased ~60 Gbit/in2 per 1 nm for HAMR-CMR and 70 Gbit/in2 per 1 nm for HAMR-SMR.

V. CONCLUSION This proposed new areal density metric enables the industry to standardize and compare the areal density capability of magnetic recording disk drives across various recording technologies independent of market segment.

REFERENCES 1) Z. Jin et al., “Areal-Density Capability of a Magnetic Recording System using a “747” Test based only on Data-Block Failure-Rate", IEEE Trans. Magn., Vol. MAG-44, No. 11, pp. 3718-3721, Nov. 2008 2) G. Ju et al, “High Density Heat-Assisted Magnetic Recording Media and Advanced Characterization – Progress and Challenges”, IEEE Trans Magn., 51 NO. 11 (2015) 3) C. Rea, et al., "HAMR Performance and Integration Challenges", IEEE Trans. Magn., 50 NO. 3 (2014) 4) T. Rausch, et al., “HAMR Drive Performance and Integration Challenges”, IEEE Trans. Magn.,49 NO. 2, 730-733, (2013)

Fig. 1: ADC Capability: a) HAMR-CMR vs HAMR-SMR at 1 nm Active Clearance b) Variable Bit Aspect Ratio and Head Media Spacing

90 H2

3D PRODUCT CODES FOR MAGNETIC TAPE RECORDING

Roy D. CIDECIYAN, Simeon FURRER, and Mark A. LANTZ IBM Research – Zurich, 8803 Rüschlikon, Switzerland {cid, sfu, mla}@zurich.ibm.com

I. INTRODUCTION Digital magnetic tape storage, first commercialized in the early 1950s with an initial reel capacity of about 1.5 MB, today offers 10 TB of cartridge capacity, and still has huge potential to continue scaling the capacity [1,2]. The combination of this scaling potential, the low cost of ownership, long-term stability, and high reliability makes tape storage particularly well suited for big data. The very high reliability of modern tape drives has been enabled through the use of two-dimensional (2D) product codes that provide excellent error-rate performance and burst-error correction capability. It will be important to continue to improve the error-rate performance as cartridge capacities are scaled, so that the probability of encountering an error in a cartridge remains constant [3]. In this work, we propose three-dimensional (3D) product codes for error-correction coding (ECC) in tape storage as a generalization of the 2D product codes currently used. We evaluate the error-rate performance of the 3D product codes with hard-decision iterative decoding by means of hardware simulations, and compare with the performance limits of the tape storage channel based on computations of the channel capacity and the random coding bound. II. RESULTS The ECC scheme used in state-of-the-art linear tape recording is a 2D product coding scheme, in which each row is an (N1, K1) Reed–Solomon (RS) codeword referred to as a C1 codeword and each column is an (N2, K2) RS codeword referred to as a C2 codeword. An ECC block of P = 256 2D product codewords are deeply interleaved and written on M simultaneously written tracks as one 2D block of encoded data referred to as data set E. For a detailed discussion of the ECC block-to-physical block mapping, the user is referred to [4]. In LTO-7 [5] tapes, the size of a data set is about 6 MB. To further improve the error-rate performance by means of longer code(s), we propose to use one (or a few) 3D product code(s) in a data set E instead of the P interleaved 2D product codewords as in state-of-the-art tape drives. Specifically, a third RS code C3 with parameters (N3, K3) is introduced with N3 = P, so that the data set E consist of one product codeword of length N1N2N3 where E = {(i, j, k) | 0 ≤ i < N1, 0 ≤ j < N2, 0 ≤ k < N3}. Note that every 2D cross section of the 3D product codeword in an (i, j), (i, k), or (j, k) plane is a 2D product codeword. Hardware simulations of hard-decision iterative decoding of product codes implemented in a field-programmable gate array (FPGA) have been used to evaluate the output byte-error rate performance of a

3D product code with RS(246, 240) C1 code, RS(96, 84) C2 code and RS(256, 250) C3 code over GF(256). This 3D product code has the same ECC overhead (code rate 0.832) and dead-track and burst-error-correction capability as the LTO-7 2D product code. For each datapoint corresponding to a specific raw byte-error rate ε of interest, 1×1015 bytes have been simulated in a time period of about ten hours. Figure 1 depicts the output byte-error rate after r decoding steps as a function of the raw byte-error rate ε. One full iteration over a data set

consists of three decoding steps C1 -> C2 -> C3, i.e., r = 3, executed in this order. Two and three full iterations -12 imply r = 6 and r = 9 steps of decoding, respectively. An output byte-error rate of 10 corresponds, after one and two full iterations, to a raw byte-error rate ε of about 1.7×10-2 and 4.7×10-2, respectively. The communication channel between the ECC encoder and decoder can be modeled as a discrete

symmetric memoryless channel with state-transition probabilities P(j | k) between input symbols k and output symbols j, as shown in Fig. 2 for symbols over GF(256). The channel capacity C versus ε can readily be

S. FURRER IBM Research – Zurich Säumerstrasse 4, 8803 Rüschlikon, Switzerland 91 +41 44 724 8613 | [email protected] H2

computed and is depicted in Fig. 2. The result indicates that to reproduce the bytes at the decoder output with a probability of error as small as desired, the maximum possible raw byte-error rate ε for an ECC scheme with rate 0.832 corresponds to 0.106. Note that the byte errors at the decoder input are assumed to be independent

and identically distributed. It has been experimentally demonstrated in [1] that an interleaving depth of I = 8 is sufficient for modeling the distribution of byte errors at the C1 decoder input by the binomial distribution. Furthermore, the random coding bound of information theory [6] was used to compute the raw byte-error rate

for codes of finite length. For the proposed 3D code with length N = 6,045,696 bytes, the random coding bound

evaluates to a raw byte-error rate of 10.46 %, which is very close to capacity.

REFERENCES [1] S. Furrer et al., “85.9 Gb/in2 recording areal density on barium ferrite tape,” IEEE Trans. Magn., vol. 51, no. 4, pp. 1–7, Apr. 2015. [2] M. A. Lantz et al., “123 Gb/in2 recording areal density on barium ferrite tape,” IEEE Trans. Magn., vol. 51, no. 11, pp. 1–4, Nov. 2015. [3] International Magnetic Tape Storage Roadmap, Information Storage Industry Consortium, Dec. 2015. [4] R. D. Cideciyan et al., “Product codes for data storage on magnetic tape”, submitted to IEEE JSAC. [5] LTO Ultrium Technology [Online]. Available: http://www.lto.org. Linear Tape-Open and LTO are registered trademarks of IBM, HP, and Quantum in the U.S. and other countries. [6] R. G. Gallager, Information Theory and Reliable Communication. New York: John Wiley & Sons, 1968.

100 1 1 ✏ 0.9 0 0 ✏/255 2 10 ✏/255 0.8 1 1 ✏ 1 4 10 undec 0.7 ✏/255 C1 r=1 ✏/255 C2 r=2 0.6 6 ✏/255 ✏/255 10 C3 r=3 255 1 ✏ 255 C1 r=4 0.5 C2 r=5 4 8 10 10 8 ⇥ byte-error rate C3 r=6 0.4 C1 r=7 6 capacity (byte/channel use) 10 C2 r=8 0.3 10 C3 r=9 4 0.2

12 2 10 0.1 0 0.99 0.995 1 14 10 0 1 2 10 10 0 0.2 0.4 0.6 0.8 1 raw byte-error rate ✏ raw byte-error rate ✏ Fig. 1. Output byte-error rate performance of the Fig. 2. Discrete memoryless channel model and proposed 3D product coding scheme with RS codes capacity in byte/channel use as a function of the

(N1= 246, K1= 240), (N2 = 96, K2 = 84), (N3= 256, K3= 250). raw byte-error rate ε.

92 H3

SIGNAL PROCESSING AND CODING SYSTEM FOR TDMR DATA FROM GRAIN FLIPPING PROBABILITY MODEL

Morteza MEHRNOUSH1, Krishnamoorthy SIVAKUMAR1, Benjamin J. BELZER1, Sari Shafidah SHAFI'EE2, Kheong Sann CHAN2 1Washington State University, Pullman, WA, USA, (mmehrnou,siva,belzer)@eecs.wsu.edu 2 Data Storage Institute, Singapore, 117608 (sari_s, chan_kheong_sann)@dsi.a-star.edu.sg)

I. INTRODUCTION This poster presents a TDMR signal processing and coding system for processing realistic data generated by a grain flipping probability (GFP) model. The dataset was generated at the Data Storage Institute and will be referred to as the DSI data. A two-dimensional intersymbol interference (2D-ISI) detector is used in a turbo iterative approach with an IRA decoder in the proposed system. We use a coset coding approach in our IRA decoder for decoding the received data, as the source bits are generated randomly. A TDMR log-likelihood ratio (LLR) function is used to pass LLRs from the 2D-ISI detector to the IRA decoder. The read head sensitivity function (2D-ISI mask) is estimated using a least squares (LS) approach based on known data bits for a given set of reader outputs. The GFP model provides fast and accurate 2D readback waveforms that include effects captured from micromagnetic simulations and the statistical effects derived from the granularity of the recording medium [1]. Comparison of signals from micromagnetic simulations, the GFP model, and real hard disk drives (HDDs) show a close match between the GFP model signals and real HDD signals [2]. The 2D-ISI detector system works based on the 2D BCJR algorithm; in [3] it is used with an IRA decoder in a one-shot detection/decoding system for TDMR Voronoi model [4] at high grain density. In [3], an �� image with two known boundary rows and columns on either side of the image is considered. In processing the DSI data, we have a single known boundary row on either side; we need to modify the algorithm for this situation. Moreover, we need to modify the LLR transfer between the row and column detectors. Simulation results show that the proposed signal processing approach can achieve up to 1.7 Tb/in2 density at 18nm track pitch (TP).

II. DSI Data Generation Model The GFP model is used to generate a realistic TDMR channel data set. The GFP requires the micromagnetic simulations to train the model, but it needs much less time than directly simulating the TDMR channel [2]. By using the GFP model, readback signals can be generated relatively quickly to evaluate the performance of proposed signal processing systems. DSI data is generated for 11 different values of TP; 100 shingled blocks are simulated for each TP. Each data block has 5 tracks of coded bits with three readers corresponding to the three central rows. Two samples for each coded bit are generated for 25 different reader positions (RPs) to simulate reader offset. We use RPs 4, 12, and 20, corresponding to the center of the three tracks. The grain density in DSI data is 11.4 Tg/in2. We consider three tracks of the readback signals from the TDMR channel model, which indicate the 2D-ISI effect and noise, as � �, � = !,! ℎ �, � � � − �, � − � + � �, � , where � is a 3×� matrix of readback data (considering only the first sample), � is the AWGN, � is the TDMR input data and h is the 2D-ISI mask. By representing the 2D-ISI operation as a matrix multiplication with an N=41206 appropriate block circulant matrix, we Boundary bits obtain a LS estimate of the mask ℎ based on a (Known) … known data matrix � �, � and the … RP 4, CW 1 corresponding reading � �, � . The L=5 … RP 12, CW 2 estimated 2D-ISI mask for the 18nm TP based on … the first sample for each bit is: RP 20, CW 3 … 0.525 1.98 2.28 Fig. 1. DSI data is an �×� image. Two columns and one row ℎ = 3.19 8.27 4.63 . comprise the known boundary. The considered RPs are 4, 12 and 20. 0.801 1.44 0.494 Each row is considered a separate codeword of length almost 41 kb.

In order to use both readback samples in our detector for improved performance, we also estimate a 3×5 mask as follows: (a) we repeat the known data bits � in the row direction to form a 3×2� matrix of data bits; (b) we use the 3×2� matrix of readback samples (considering both samples). A LS estimate of the 3×5 mask is obtained as before. During convolution,

Krishnamoorthy Sivakumar E-mail: [email protected] 93 tel: 1-509-335-4969 Work funded in part by NSF CCF-1218885, and by ASTC. H3

even shifts of the mask correspond to the first readback sample, whereas odd shifts of the mask corresponds to the second readback sample. Then, we obtain a LS estimate of the 3×5 mask based on this readback data arrangement.

III. Signal Processing and Coding System Fig. 2 shows the block diagram of the proposed turbo iterative detection-decoding system. The block labeled 2D-ISI detector is a 2D BCJR-based equalizer [3] that works in a row-by-row fashion to equalize the 2D-ISI effect of the read head sensitivity function. The IRA coset decoder corrects errors caused by the AWGN and/or the erasure errors due to grain overwrite effects. To account for TDMR bit overwrites, an LLR mapping function is inserted between the detector and decoder. The LLR distribution of the detector output is shown in Fig. 3. We fit the distribution with a Gaussian Mixture Model (GMM) and calculate the TDMR LLR function using the approach presented in [3]. The goal of the turbo iterative approach is to pass soft information between the detector and decoder to achieve a BER of about 10−5 at the highest possible code rate (i.e., highest information density). The LLRs are multiplied by a suitable r h LLR LLR factor (indicated by weight �! in Fig. 2). IRA TDMR LLR RC We propose two turbo iterative systems. The first system bˆ Function uses the 3×3 estimated mask presented in section II, in the IRA Coset 2D-ISI 2D-ISI detector and the first sample of the readback data. The Decoder Detector Fig. 2. Proposed turbo +iterative detectionx -decoding system. second system uses the 3×5 estimated mask in the 2D-ISI w detector and both samples of the readback data. We do several 1 iterations between the detector and decoder. The iteration schedule and weight �! in this system are optimized experimentally.

IV. SIMULATION RESULTS This poster presents simulation results for TP = 18nm. We use all of the 100 data blocks which gives 300 codewords. The one-shot detection P =0.37 P =0.63 system (one 2D-ISI detection followed by IRA decoder), using 1 2 the 3×3 μ1=1.82 μ2=3.47 2 2 mask and first sample in 2D-ISI detector, with the code rate of σ1 =2.42 σ2 =10.88 0.5 achieves 1.63 Tbits/in2 in the media of 11.4 Tg/in2 at the BER of 1.2×10!!. The first turbo iterative system with 3×3 mask and first sample in 2D-ISI detector achieves 1.71 Tbits/in2 at the BER of 3.1×10!! which gives a 4.8% density improvement. The second turbo iterative system with the 3×5 mask and both samples in 2D-ISI detector achieves the same density but smaller BER of 1.9×10!!. In the future, we plan to decrease the system complexity and process DSI data sets for other TP values. Fig. 3. Conditional LLR distribution at the output of 2D-ISI detector with GMM fit. REFERENCES 1) K. S. Chan et al., “Channel Models and Detectors for Two-dimensional Magnetic Recording,” IEEE Trans. on Magn., vol. 46, no. 3, pp. 804–811, Mar. 2010. 2) K. S. Chan et al., “Comparison of Signals from Micromagnetic Simulations, GFP Model, and an HDD Readback,” IEEE Trans. on Magn., vol. 51, no. 11, pp. 1-4, Nov. 2015. 3) M. Mehrnoush, B. Belzer, K. Sivakumar, and R. Wood, “Signal Processing for Two Dimensional Magnetic Recording Using Voronoi Model Averaged Statistics,” in Proc. Conf. on Inform. Sci. and Syst., March 2015, pp. 1–6. 4) R. M. Todd, E. Jiang, R. Galbraith, J. R. Cruz, and R. W. Wood, “Two-dimensional Voronoi-based Model and Detection for Shingled Magnetic Recording,” IEEE Trans. on Magn., vol. 48, no. 11, pp. 4594–4597, Nov. 2012.

94 H4

INTERFACE MODIFICATION TO IMPROVE THE FRICTION AND WEAR RESISTANCE OF ULTRATHIN TAPE HEAD OVERCOATS

Reuben J. YEO, Neeraj DWIVEDI and C. S. BHATIA Department of Electrical and Computer Engineering, National University of Singapore, Singapore

I. INTRODUCTION Tape is seen an attractive option for large scale data storage, particularly for archival or backup purposes, because of its low cost per byte, low power consumption and long-term reliability. Nevertheless, due to the progressive wear of the read/write poles by the tape, pole-tip recession (PTR) remains one of the most important problems affecting the performance and reliability of the tape head. To delay or eliminate the onset of PTR, various types of overcoats have been applied onto the tape head to prevent direct contact between the tape and the read/write poles. However, most of these overcoats suffered from delamination at an early stage and the maximum overcoat wear life is reported to be around 1–3 million meters at thickness ~20 nm [1,2]. Here we employ a technique of interface modification which serves to improve the adhesion of the overcoat with the tape head substrate leading to significant improvement of wear resistance while achieving lower friction.

II. RESULTS AND DISCUSSION

17–20 nm monolithic and bilayer overcoats were fabricated on flat Al2O3/TiC composite (AlTiC) substrates and tape heads by sputtering and filtered cathodic vacuum arc (FCVA) process as described in Table 1 [3]. The ball-on-disk tribological properties of the various overcoats on flat AlTiC substrates (Figure 1) revealed low friction in all the carbon-based overcoats, although the FCVA-processed overcoats demonstrated more stable friction lasting throughout the test. Furthermore, FCVA-processed overcoats 17CF and 3SiN14CF as well as bilayer sputtered overcoat 3SiN17CP showed minimal wear debris and a negligible wear track, demonstrating their resilience to wear. Subsequently, 20 nm FCVA-processed overcoats (20CF and 3SiN17CF) were deposited on tape heads and compared with a tape head coated with a 7 nm sputtered bilayer overcoat (3SiN4CP). All three coated heads were subjected to long-term testing with commercial tape. Optical micrographs capturing the head surface conditions of the heads up to 12.5 million meters of tape wear are presented in Figure 2. It is clear that the 3SiN4CP overcoat was completely removed, while the FCVA-processed overcoats were still present on the heads even after 12.5 million meters. The wear life of 3SiN4CP was found to be ~3 million meters [4]. In comparison, the wear durability of 3SiN17CF was slightly better than 20CF based on the smaller distance that the overcoat had receded from the skiving edge. With further testing, the 3SiN17CF overcoat was shown to possess a wear life of 40–50 million meters [4]. Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) revealed that the FCVA-processed overcoats possessed much higher sp3 carbon (sp3C) content (~70–80%) than the sputtered overcoats. Thus it is observed that high sp3C content contributed significantly to the improved wear durability of the FCVA-processed overcoats in the long-term tape tests. Through XPS depth profiling, the bilayer sputtered and FCVA-processed overcoats revealed the presence of strong interfacial bonds such as Si–C, C=N and (Al,Ti)N at the overcoat-substrate interfaces. It was found from time-of-flight secondary ion mass spectrometry (TOF-SIMS) elemental depth profiles and stopping and range of ions in matter (SRIM) simulations that higher extent of atomic intermixing occurred for the FCVA-processed overcoats than the sputtered overcoats leading to higher interfacial bonding, due to the more energetic FCVA process whereby the carbon atoms had sufficient energy to penetrate through the SiNx layer and interact with the AlTiC substrate. Thus, the best wear resistance in 3SiN17CF can be attributed to the combination of both high sp3 bonding and increased interfacial bonding.

III. CONCLUSIONS The wear resistance and friction performance of ultrathin 17–20 nm carbon overcoats for tape heads were enhanced through overcoat-substrate interface modification by introducing a SiNx adhesion interlayer and through high energy pre-treatment by FCVA. The SiNx interlayer contributed to the formation of additional strong interfacial bonding. On the other hand, the FCVA process not only contributed to high sp3C content in the carbon overlayer, but high energy pre-treatment also created more extensive atomic intermixing, resulting in more extensive interfacial Corresponding Author: C. S. Bhatia E-mail: [email protected] 95 tel: +65 -65167216 Topic: Tape H4

bonding and hence improved adhesion. The synergy of high sp3C and enhanced interfacial adhesion through interface modification was the major contributor to the exceptional wear life of the SiNx/ta-C bilayer overcoat.

REFERENCES 1) W. W. Scott, B. Bhushan, and A. V. Lakshmikumaran, “Ultrathin diamond-like carbon coatings used for reduction of pole tip recession in magnetic tape heads”, J. Appl. Phys. 87, 6182 (2000). 2) B. Shi, J. L. Sullivan, and S. O. Saied, “A study of thin coating wear in high data density tape heads”, J. ASTM Int. 5, 101192 (2008). 3) R. J. Yeo, N. Dwivedi, S. Tripathy, and C. S. Bhatia, “Excellent wear life of silicon nitride/tetrahedral amorphous carbon bilayer overcoat on functional tape heads”, Appl. Phys. Lett. 106, 091604 (2015). 4) N. Dwivedi et al., “Interface engineering and controlling the friction and wear of ultrathin carbon films: high sp3 versus high sp2 carbons”, Adv. Funct. Mater. 26, 1526 (2016).

Table 1: List of samples containing their respective layer thicknesses and carbon deposition process. Process for Samples SiN (nm) Carbon (nm) x Carbon 20CP - 20 sputtering 3SiN17CP 3 17 sputtering 17CF - 17 FCVA 3SiN14CF 3 14 FCVA *numbers represent the thickness of each layer in nm. CP represents pulsed DC sputtered carbon and CF represents FCVA grown carbon.

Fig. 1: Friction graphs, ball and wear track optical micrographs of the four carbon-based overcoats after ball-on-disk tribological testing. The black scale bars represent 100 µm.

Fig. 2: Optical micrographs showing the skiving edge of the head before the long-term wear test and after 12.5 million meters of tape wear. Light and dark contrasts indicate presence and absence of coating, respectively. The yellow line marks the distance that the overcoat has receded from the edge. The black scale bars represent 50 µm.

96 I1

ION BEAM PATTERNING OF HIGH DENSITY STT-RAM DEVICES

1 2 1 2 1 Vincent IP , Shuogang HUANG , Santino D. CARNEVALE , Ivan L. BERRY , Katrina ROOK , Thorsten B. LILL2, Ajit P. PARANJPE3 and Frank CERIO1 1) Veeco Instruments, Plainview, NY, USA, [email protected] 2) Lam Research Corporation, Fremont, CA, USA, [email protected] 3) Veeco Instruments, Somerset, NJ, USA, [email protected]

I. BACKGROUND STTRAM device patterning has been demonstrated via either: reactive ion etch followed by ion beam etch (IBE); or by a full inert-gas IBE strategy.[1],[2] The etching of high density STTRAM structures requires detailed process optimization, due to multiple requirements such as: high aspect ratio; sufficient residual top electrode; minimization of damage to the magnetic and other active layers; and avoidance of electrical shorting across the MTJ barrier. We present experimental ion beam etch rate data for typical STTRAM stack and hard mask materials versus: incidence angle; ion species (Neon, Argon, Xenon); and ion energy. We utilize these data combined with 2-D etch simulations to present guidelines for addressing each of the STTRAM patterning challenges under a full IBE process scheme. For large CD structures, with wide pitch, a single-step IBE recipe may be sufficient, but for small CD or tight pitch features, a multi-step IBE process appears to be necessary. We simulate optimized combinations of multiple etch steps, and demonstrated effective patterning of STTRAM pillars with mask height ~ 160 nm, and pitch varying from 80 – 800 nm pitch. We demonstrate etched features with ~ 85o sidewall angle and no metal re-deposition across the tunnel barrier.

II. ELECTRICAL SHORTING The primary consideration during the first etch step(s) is to effectively open up the narrow-CD pattern, while minimizing conducting re-deposition across the tunnel barrier. In Figure 1, we display a selection of etch rates for key STT-RAM materials, under Argon ion beam etch at ion energy of 500V. Figure 2 details the angular-dependent rates of one example material, W, indicating a significant impact of ion energy. Our etch simulations incorporate also the spatial distribution of redeposited material, and show re-deposition to be minimized by: etch angle further from normal incidence; using lower mass ion; and/or higher ion energy.

III. HARD MASK SELECTIVITY For some typical hard mask materials, Figure 3 plots the etch rate selectivity to CoFeB versus ion energy. Maximization of mask / stack selectivity is a secondary consideration during STT-RAM patterning, in order to minimize mask thickness and maintain etch aspect ratio as low as possible. Higher ion energy provides improved selectivity. We will further review the dependencies on gas species and etch angle.

IV. MAGNETIC AND ELECTRICAL DAMAGE The primary consideration during the final etch step(s) is to remove any sidewall damaged layer resulting from the earlier step(s), while minimizing further damage.[3] Sidewall damage can manifest itself as: a magnet deterioration (loss of perpendicular magnetic anisotropy); an electrical deterioration (in resistance and/or ΔR/R); or an electrical shorting (via metal inter-mixing into the barrier). We present 3-D etch calculations and SRIM simulations to provide guidelines for the damage cleanup steps, in terms of optimal VINCENT IP Email: [email protected] Tel: +001-516-252-1481 ext. 1126 97 I1

etch angle, ion species and ion energy.[4] We show that sidewall damage cleanup is maximized by etch angle further from normal, while further damage generation is minimized primarily by lower ion energy. In particular, we present ion energies required to maintain damaged layer thicknesses from <1nm upwards.

REFERENCES 1) M. Gajek, J.J. Nowak, J.Z. Sun, P.L. Trouilloud, E.J. O’Sullivan, D.W. Abraham, M.C. Gaidis, G. Hu, S. Brown, Y. Zhu, R.P. Robertazzi, W.J. Gallagher and D.C. Worledge, “Spin torque switching of 20 nm magnetic tunnel junctions with perpendicular anisotropy”, Appl. Phys. Lett.100, 132408 (2012). 2) S. Takahashi, T. Kai, N. Shimomura, T. Ueda, M. Amano, M. Yoshikawa, E. Kitagawa, Y. Asao, S. Ikegawa, T. Kishi, H. Yoda, K. Nagahara, T. Mukai, H. Hada, “Ion-beam-etched profile control of MTJ cells for improving the switching characteristics of high-density MRAM”, IEEE Trans. Mag. 42 (10), 2745-2747, (2006). 3) Y. Ohsawa, H. Yoda, T. Ohkubo and K. Hono, “Precise damage observation in ion-beam etched MTJ”, Intermag 2016, to be published in IEEE Trans. Mag. (2016). 4) J. Ziegler, “Particle interactions with matter”, 1998. Online. Available at http://www.srim.org/.

Fig. 1 Argon IBE etch rates for key STT-RAM materials

Fig. 2: Angular dependence of IBE rates for W Fig. 3: IBE rate selectivity of mask: CoFeB

98 I2

DESIGN OF COOLING NFT SYSTEM USING SPP WAVEGUIDE FOR HAMR

Y. HAYASHI, K. TAMURA, Y. ASHIZAWA, S. OHNUKI, and K. NAKAGAWA Nihon Univ., Tokyo, Japan, [email protected]

I. DESIGN OF NFT FOR HAMR Heat assisted magnetic recording (HAMR) with patterned media is one of the technologies for hard disk drives beyond 10 Tbyte capacity. The temperature rise of a near-field transducer (NFT), however, is not a negligible issue. In general, NFTs are located in a dielectric optical waveguide. The thermal conductivity of dielectric material is so low that it is not easy to cool the NFT when the heating energy is concentrated on the NFT for HAMR writing. We studied the structure that a NFT is embedded in a plasmonic waveguide (PWG) which is made of metal [1]. It is revealed that the structure effectively cools the NFT by computational simulation, because the metallic waveguide takes a role of heat sink.

II. SIMULATION MODEL AND RESULTS The structure of HAMR head for thermal calculation is shown in Fig. 1. A dielectric waveguide is located between leading and trailing magnetic poles. Surface plasmon polaritons (SPPs) are exited at the surface of gold PWG which is placed along with the dielectric waveguide. The optical energy, which is propagated in the dielectric waveguide, can transfer to the PWG, and will be delivered to the bottom of the PWG. At the bottom of the PWG, we placed a gold plate as shown in Fig. 2. The plate is tilted as 5 degrees, and the gap between the plate and the recording media is the narrowest at the trailing pole. The recording media has a structure like patterned media. The calculation of electromagnetic wave is performed by the Finite-Difference Time-Domain (FDTD) method. The temperature rise is calculated including the whole structure shown in Fig. 1. When the recording layer shows 277 degree C with h = 35 nm, the highest temperature of the NFT is 360 degree C. The temperature rise of the NFT is very critical depending on h. Less than 200 degree C can be achieved by a structure with smaller h. We found that NFT embedded in PWG is an effective structure to cool the NFT. REFERENCES 1) K. Tamura, Y. Ashizawa, S. Ohnuki, and K. Nakagawa, J. Magn. Soc. Jpn., 38, 131-134, 2014. Acknowledgment This work is partially supported by a Grant of MEXT-Supported Program for the Strategic Research Foundation at Private Universities, 2013-2017.

1,800 nm 190 nm 3,000 nm Ta O 2 5 Fe Al2O3 Au

2,500 nm FH z x 4 nm y FePt 8 nm z MgO 5 nm Cr 10 nm x y Cu 30 nm SiO2 2,000 nm Fig. 1 Structure of head for thermal calculation. Fig. 2 Temperature distribution at the NFT (h = 35 nm).

Katsuji NAKAGAWA E-mail: [email protected] tel: +81-90-40296058 99 This page intentionally left blank.

100 I3

ELECTRIC SWITCHING OF MAGNETIZATION USING MAGNETOELECTRIC CR2O3 FILM

T. NOZAKI, M. AL-MAHDAWI, S. P. PATI, S. YE, Y. SHIOKAWA, and M. SAHASHI Department of Electronic Engineering, Tohoku University, Sendai 980-0845, Japan

Toward application of an electric writing media, we’ve paid attention to the magnetoelectric (ME) antiferromagnet Cr2O3 sputtered film. The magnetoelectric (ME) effect of Cr2O3, first reported on in the 1960s, can be used to control magnetization through an electric field: By applying parallel (anti-parallel) electric (E) and magnetic field (H), the antiferromagnetic domain of Cr2O3 align as ↑↓↑↓ (↓↑↓↑). If we apply fixed positive H, by changing the direction of E from positive to negative, we can switch the direction of surface spin of Cr2O3 from up (↑) to down (↓). In addition, the surface spin information can be transferred to neighbor ferromagnet via exchange coupling. Thus by combining both the ME effect and exchange coupling, we can achieve electric control of magnetization as shown in Fig. 1. Such an electric control of magnetization of Cr2O3/ferromagnet exchange coupling systems was demonstrated by magnetoelectric field cool (MEFC) process using Cr2O3 bulk single crystals in 2005[1], and using Cr2O3 sputtered films in 2014[2]. Isothermal magnetoelectric switching, i.e. switching at constant temperature, also demonstrated recently[3, 4]. Our group has been worked on the material development, aiming to increase operation temperature, enhance thermal stability, reduce switching energy (product of applied E and H: EH products). Recently we observed unusual positive-exchange bias phenomena for Pt spacer inserted Cr2O3/ Co system[5] and found it can be used to decrease EH products. In this study the EH products reductions were demonstrated using Al2O3/Pt buffer/Cr2O3/Pt spacer/Co ferromagnet/Pt cap systems. By adjusting Pt layer thickness and applied H, the EH product was well controlled (Fig.2).

Fig. 1 Schematics of magnetization switching using ME effect and exchange coupling. Here we assume Fig. 2 Magnetization switching by MEFC for Pt spacer antiferromagnetic coupling between Cr2O3 surface spin 1.3nm sample. Due to positive exchange bias, EH and ferromagnet. product change by changing applied H. Acknowledgement: This work was partly funded by ImPACT Program of Council for Science, Technology and Innovation (Cabinet Office, Japan Government).

References: [1] P. Borisov et al., Phys. Rev. Lett., 94 (2005) 117203. [2] T. Nozaki et al., Appl. Phys. Lett., 105 (2014) 212406. [3] X. He et al., Nat. Mater., 9 (2011) 579.

TOMOHIRO NOZAKI E-mail: [email protected] tel: +81-22-752-2172 101 I3

[4] T. Ashida et al., Appl. Phys. Lett., 106 (2015) 132407. [5] T. Nozaki et al., Appl. Phys. Lett., 105 (2014) 212406.

102 I4

EFFECT OF MgO OXIDATION LEVEL ON SPIN-ORBIT TORQUE MAGNETIZATION SWITCHING OF CoFeB/MgO NANODOTS

Noriyuki SATO1, Robert M. WHITE2 and Shan X. WANG3 1) Stanford Univ., Stanford, USA, [email protected] 2) Stanford Univ., Stanford, USA, [email protected] 3) Stanford Univ., Stanford, USA, [email protected]

Spin-orbit torque magnetic random access memory (SOT-MRAM) has gained great attention as one of the promising next generation devices. In addition to the key features of spin-transfer-torque MRAM (STT-MRAM) such as non-volatility and fast operation, SOT-MRAM allows us to separate its read and write path. This separation, which is achieved by magnetization switching through Rashba and/or spin-Hall-induced torques, prevents unintentional writing during read. Furthermore, a high endurance is expected because only a small reading current flows in the tunneling barrier that could reduce dielectric breakdown. In order to realize such a superior device, it is necessary to understand the mechanism of the SOT in heavy metal (HM) / ferromagnetic metal (FM) / oxide (OX) multilayers. The SOT is classified into field-like and damping-like torques, whose origins have been intensively studied. In particular, recent experimental and theoretical studies provide quantitative understanding of the HM and FM layer dependences on the SOT due to Rashba or spin-Hall effects [1], [2]. On the other hand, the effect of the OX layer properties on the SOT is still unclear due to the limited number of studies. Although it has been experimentally shown that the OX material choice has significant impact on both field-like and damping-like torques [3], further studies are necessary to understand the mechanism which causes this large effect. Therefore, in this study, we systematically investigated the effect of the oxidation level of the OX layer on SOT-induced effective fields and the critical current density by varying the oxidation time in HM/FM/OX. The sample stack used in this work is

Ta(5)/CoFeB(1.1)/Mg(0.7)/Natural oxidation(tox sec.)/Mg(0.3)/Ta(2). The numbers in parentheses are in nm unless specified. The MgO layer was prepared by a DC-sputter deposition of metallic Mg followed by a natural oxidation at 50 mTorr O2. Here we demonstrate this industrially preferred MgO preparation method allows us to readily manipulate the oxidation level of the MgO and produces significant impact on the SOT as well as the perpendicular magnetic anisotropy (PMA).

Figure 1 shows the tox dependence on the coercivity Hc extracted from out-of-plane magnetization curves. Hc is maximized when the oxidation time is 30 sec and this behavior should reflect the strength of the

PMA. The samples with tox = 5 and 400 sec have weak PMA indicated by the small Hc. This result is consistent with previous studies that show under and over oxidation lead to weak PMA due to the lack of Fe-O bonding and the oxidation of CoFeB layer, respectively [4], [5]. Figure 2 shows the SOT-induced effective fields per unit current density J measured by the low-current induced lock-in technique [6]. The transverse and longitudinal effective fields correspond to field-like torque and damping like-torque, respectively. We could perform the measurement only for the samples with strong enough PMA. The effective field transverse to the current direction ∆HT/J, which is predominantly attributed to the Rashba effect, has a negative peak at tox = 30 sec for both +Mz and -Mz magnetization states. When the oxidation of Mg is insufficient, the local potential

103 I4

gradient �� at the Mg(MgO)/CoFeB interface is expected to be small due to the small work function difference between the two layers. Thus, the spin-orbit term of the Hamiltonian for a 2D system (��×�) ∙ � should be small, where � is the momentum operator and � is the Pauli matrices. Once the oxidation becomes

large, the CoFeB layer is also subjected to oxidation and thus ∆HT/J decreases. By contrast, the longitudinal effective field is not affected by the oxidation level (not shown). This is reasonable since the natural oxidation process does not affect the HM layer and therefore the spin-Hall-induced torque originating in HM layer should be practically the same for all the samples. In order to demonstrate that SOT critical current can be actually controlled by the oxidation level, nano-magnets (CoFeB/MgO) were prepared on Hall bars (Ta) by e-beam lithography. The diameter of the nano-magnets is 80 nm and they are expected to be single-domain

states. Figure 3 shows measured and calculated spin-orbit torque switching efficiency Jc/(∆/kBT), where Jc is

the critical current density and ∆/kBT is the thermal stability ratio. The measurement was conducted at room temperature and a longitudinal field of 300 Oe was applied. The calculation followed the model recently

developed in [7]. Jc/(∆/kBT) was minimized at tOX = 30 s where the oxidation level is optimum and the tendency agreed with the calculation. In summary, we have demonstrated that the variation of oxidation level has an impact on the field-like torque and the critical current. Our results indicate the possibility of further enhancement of SOT and reduction of the critical current through manipulation of oxidation level in HM/FM/OX multilayers.

REFERENCES 1) J. Kim, et. al., Nat. Mater., vol. 12, no. 3, pp. 240–5, 2013. 2) P. M. Haney, et. al., Phys. Rev. B - Condens. Matter Mater. Phys., vol. 87, no. 17, pp. 1–13, 2013. 3) D. C. Worledge, et. al., J. Appl. Phys., vol. 115, no. 17, p. 172601, May 2014. 4) W. C. Tsai, et. al., Appl. Phys. Lett., vol. 100, no. 17, p. 172414, 2012. 5) H. X. Yang, et. al., Phys. Rev. B., vol. 84, no. 5, p. 054401, Aug. 2011. 6) U. H. Pi, et. at., Appl. Phys. Lett., vol. 97, no. 16, 2010. 7) T. Taniguchi, et al., Phys. Rev. B, vol. 92, no. 2, p. 024428, 2015.

Fig.1 Coercivity Hc. Fig.2 Transverse effective field Fig.3 Spin-orbit torque switching per mA, ∆HT/I0. efficiency, Jc/(∆/kBT).

104 I5

ACHIEVING 10 NM FULL PITCH LINES BY DIRECTED SELF-ASSEMBLY OF BLOCK COPOLYMERS ON LARGE AREA FOR HDMR MEDIA

XiaoMin YANG,1 Shuaigang XIAO,1 Austin LANE,2 Gregory BLACHUT,2 Michael MAHER,2 Yusuke ASANO,2 Yautzong HSU,1 Zhaoning YU,1 Michael FELDBAUM,1 Stephen SIRARD,3 Philip STEINER,1 Koichi WAGO,1 Kim LEE,1 Diane HYMES,4 David KUO,1 and Grant WILLSON2 1) Fremont Research Center, Seagate Technology, Fremont, CA 94538 2) Department of Chemical Engineering, University of Texas at Austin, Austin, TX 78712 3) Lam Research Corporation, Austin, TX 78753 4) Lam Research Corporation, Fremont, CA 94538

I. INTRODUCTION Heated-dot magnetic recording (HDMR), which combines heat-assisted magnetic recording (HAMR) and bit patterned media (BPM), is expected to extend the areal density to 10 Td/in2 and beyond. However, the main challenge to HDMR lies in the fabrication of the sub-10 nm features needed. Patterning of such high-resolution features has been demonstrated by directed self-assembly (DSA) of block copolymers (BCP) as a viable lithography approach [1-4]. While fabrication of round bit islands arranged in hexagonal arrays is the most direct and efficient way to obtain the highest areal densities from cylinder or sphere forming block copolymers, magnetic recording and simulation data indicate that track misregistration can be reduced and recording performance can be improved by using rectangular bit cells[5]. However, when compared to a hexagonal lattice and in order to achieve the same areal density, a rectangular lattice calls for even smaller lithographic features and a two-step fabrication process involving two sets of orthogonally intersecting stripes made from lamellae forming block copolymers. In this paper, we will present the building blocks for 6 to 10 Td/in2 HDMR fabrication through DSA only and through combination of DSA and self-aligned double patterning (SADP) lithography.

II. DSA PATTERN FOR HDMR MEDIA By utilizing a highly incompatible block copolymer of polystyrene-block-polydimethylsiloxane 2 [6] (PS-b-PDMS), we have previously demonstrated 5 Td/in (L0 = 12 nm) FePt dot arrays . In this paper, a 16-nm-pitch line pattern consisting of aligned lying-down PS-b-PDMS cylinders using a 48-nm-pitch imprinted resist prepattern, is employed for pattern transfer into a masking layer to form a mandrel of the line pattern. SADP utilizing atomic layer deposition (ALD) is then applied resulting in a final line pattern in SiO2 with a half pitch of 4 nm (Figure 1). The resist prepattern is found to be critical for the control of DSA pattern parameters such as line space uniformity/line width roughness/line edge roughness. Also extensive experiment on various etch mask materials and etch process conditions is performed to enhance pattern quality. Very recently, aligned 5 nm BCP lamellae were achieved in thin films by DSA of poly(5-vinylbenzo dioxole-block-pentamethyldisilylstyrene (PVBD-b-PDSS) (L0 = 10 nm) (Figure 2). Guiding features for BCP alignment were patterned by imprint lithography and enabled 4x density multiplication of the pre-pattern lines. Both grapho- and chemo-epitaxial assemblies were obtained by varying the BCP film thickness and the duration of the trim etch process for the imprint lines. Pattern transfer was demonstrated by using reactive ion etching to selectively remove the PVBD block from the BCP film (Figure 2b) and transfer into an underlying chromium thin film using a chlorine-based RIE process. This is the first report of 5 nm lamellae BCP features aligned by DSA and represents a significant advancement towards fabricating

XIAOMIN YANG E-mail: [email protected] Tel: 1-510-624-3670 105 I5

high-density (>5 Td/in2) imprint template for future HDMR media applications. Opportunities and challenges using high-resolution DSA material for HDMR application will be discussed. Acknowledgment: The authors thank Stefano Dallorto and Deirdre L. Olynick at Molecular Foundry, Lawrence Berkeley National Laboratory for their ALD experimental help.

REFERENCES [1] XiaoMin Yang, et al., ACS Nano, 3 (7), 1844 (2008). [2] Shuaigang Xiao, XiaoMin Yang et al., ACS Nano, 8 (11), 11854 (2014). [3] XiaoMin Yang, et al., Journal of Nanomaterials, 1 (2013). [4] XiaoMin Yang, et al., Nanotechnology, 25 (39), 295301 (2014). [5] Thomas Albrecht, Hitesh Arora, et al., IEEE Transactions on Magnetics 51 (5), 0800342 (2015). [6] Philip Steiner, et al., TMRC Digest (2015).

(a) (b)

(c) (d)

50 nm

Fig. 1 shows SEM images of DSA of PS-b-PDMS and double patterning: imprinted guide pattern at 48 nm pitch (a), DSA pattern at 16 nm pitch (b), mandrel mask at 16 nm pitch (c), and SiO lines at 8 nm pitch (d). 2

(a) (b)

(c)

Fig. 2 shows SEM images of 5 nm lamellae in thin film of PVBD-PDSS. (a) fingerprint 106 pattern, (b) tilted image after removing the PVBD block, and (c) DSA result on a large area TMRC 2016│Author index

Author last name First name Paper code Page number Al-Mahdawi M. I3 101 Aggarwal S. D1 47 Ahn J. E5 59 Alexander Jim G3 79 Anaya-Dufresne Manuel C1 41 Anh Nguyen Thi Van D2 49 Annunziata Anthony E1 51 Asano Yusuke F1 63 Asano Yusuke I5 105 Ashizawa Y. I2 99 Bangar M. E5 59 Belzer Benjamin J. G6 85 Belzer Benjamin J. H3 93 Berry Ivan L. I1 97 Bertero Gerardo H1 89 Bhatia C. S. C3 45 Bhatia C. S. H4 95 Blachut Gregory F1 63 Blachut Gregory I5 105 Bogy David B. C2 43 Bosu S. E3 55 Braganca Patrick E2 53 Brand John L. C1 41 Brockie Richard H1 89 Brown Stephen E1 51 Cao K. H. E4 57 Carnevale Santino D. I1 97 Cerio Frank I1 97 Chan Kheong Sann H3 93

107 TMRC 2016│Author index

Author last name First name Paper code Page number Chen H. E5 59 Chen J. F5 71 Chen Jianping G1 75 Chia H.-J. D1 47 Ching C. E5 59 Choi Young-Suk E2 53 Cideciyan Roy D. H2 91 Contreras John G1 75 Cordle Michael A1 19 Czoschke Peter J. A1 19 Dahandeh Shafa G3 79 Dallorto Stefano B5 37 De Fazio Domenico C3 45 Degawa Naomichi F6 73 Deherrera M. D1 47 Deshpande S. D1 47 Dolecek Lara G2 77 Dou Chunmeng C3 45 Du Y. F5 71 Duda John C. C1 41 Dwivedi Neeraj C3 45 Dwivedi Neeraj H4 95 Eaton Robert A3 23 Eppler Walter A4 25 Esfahanizadeh Homa G2 77 Feldbaum Michael B5 37 Feldbaum Michael I5 105 Ferrari A. C. C3 45 Fina I. E6 61 Fletcher Patrick C. C1 41

108 TMRC 2016│Author index

Author last name First name Paper code Page number Furrer Simeon H2 91 Furubayashi T. F5 71 Gage Edward A1 19 Gage Edward A4 25 Gage Edward H1 89 Gangopadhyay Sunita C1 41 Goode Jonas G1 75 Granz Steven D. H1 89 Greaves Simon F3 67 Greaves Simon F4 69 Grobis Michael K. B2 31 Guo Gouxiao A5 27 Hareedy Ahmed G2 77 Hassan S. E5 59 Hayashi M. E3 55 Hayashi Y. I2 99 Hernandez Stephanie A1 19 Hernandez Stephanie A4 25 Hohlfeld Julius B1 29 Honda Naoki F2 65 Hono K. E3 55 Hono K. F5 71 Houssameddine D. D1 47 Hsu Yautzong B5 37 Hsu Yautzong I5 105 Hu Guohan E1 51 Huang 25 Pin-Wei A4 Huang Shaogang I1 97 Hwang Euiseok G7 87 Hymes Diane I5 105

109 TMRC 2016│Author index

Author last name First name Paper code Page number Ikegawa S. D1 47 Ip Vincent I1 97 Janesky J. D1 47 Johnson Michael T. C1 41 Jones Paul M. C1 41 Ju Ganping A1 19 Jubert Pierre-Olivier B2 31 Jung J. W. F5 71 Kagami Takeo F6 73 Kan J. J. E5 59 Kanai Yasushi F3 67 Kanai Yasushi F4 69 Kang S. H. E5 59 Kasai S. E3 55 Katine Jordan E2 53 Kiely James D. C1 41 Kim Younghyun E1 51 Kim S. E5 59 Klemmer Tim B5 37 Kontos A. E5 59 Kotani Yoshinori D2 49 Kothandaraman Chandrasekharan E1 51 Krivosik Pavol A4 25 Kubota Yukiko B5 37 Kumar B. V. K. Vijaya G7 87 Kuo David S. B5 37 Kuo David I5 105 Lane Austin F1 63 Lane Austin I5 105 Lantz Mark A. H2 91

110 TMRC 2016│Author index

Author last name First name Paper code Page number Lauer Gen E1 51 Lee Kim Y. B5 37 Lee Junghyuk E1 51 Lee Kim I5 105 Li Hai A2 21 Li S. F5 71 Liang S. E5 59 Lill Thorsten B. I1 97 Lin M. D1 47 Lin X. Y. E4 57 Lopusnik Radek A1 19 Lu Pu-Ling A1 19 Lu Pu-Ling A4 25 Ma Minjie A1 19 Ma Kun A5 27 Ma Yuan C2 43 Machita Takahiko F6 73 Maher Michael F1 63 Maher Michael I5 105 Makino Kenzo F6 73 Maletzky T. B3 33 Mancoff F. B. D1 47 Marchak Nathan E1 51 Marti X. E6 61 Matcha Chaitanya Kumar G4 81 Mathew George G7 87 Mehrnoush Morteza H3 93 Mihajlović Goran E2 53 Miura Y. F5 71

111 TMRC 2016│Author index

Author last name First name Paper code Page number Miura Satoshi F6 73 Mosendz Oleksandr E2 53 Moser Andreas A3 23 Muraoka Hiroaki F3 67 Muraoka Hiroaki F4 69 Nagel K. D1 47 Nakagawa K. I2 99 Nakamura Tetsuya D2 49 Nakatani Ryoichi D2 49 Nakatani M. F5 71 Ngo Tue G3 79 Nowak Janusz J. E1 51 Nozaki T. I3 101 O’Sullivan Eugene J. E1 51 Ohnuki S. I2 99 Olynick Deirdre B5 37 Ott Anna K. C3 45 Pakala M. E5 59 Paranjpe Ajit P. I1 97 Park Jeong-Heon E1 51 Park C. E5 59 Pati S. P. I3 101 Peng Yingguo A1 19 Peng S. Z. E4 57 Qu Tao B6 39 Rausch Tim A1 19 Rausch Tim A4 25 Rausch Tim B1 29 Rausch Tim H1 89

112 TMRC 2016│Author index

Author last name First name Paper code Page number Rea Chris A1 19 Rea Chris B1 29 Reuter Mark E1 51 Robertazzi Ray P. E1 51 Rook Katrina I1 97 Sahashi M. I3 101 Saito Hidetoshi G5 83 Sakuraba Y. E3 55 Sakuraba Y. F5 71 Sasaki T. T. F5 71 Sassi Ugo C3 45 Sato Noriyuki I4 103 Saunders Douglas A. A1 19 Saunders Douglas A. B1 29 Seigler Mike A1 19 Sepeheri-Amin H. E3 55 Shafi'ee Sari Shafidah H3 93 Shiokawa Y. I3 101 Shiratsuchi Yu D2 49 Sirard Stephen F1 63 Sirard Stephen I5 105 Sivakumar Krishnamoorthy G6 85 Sivakumar Krishnamoorthy H3 93 Slaughter J. M. D1 47 Smith Neil E2 53 Somea Yasunobu F1 63 Srinivasa Shayan Garani G4 81 Staffaroni M. B3 33 Steiner Philip B5 37 Steiner Philip I5 105

113 TMRC 2016│Author index

Author last name First name Paper code Page number Su L. E4 57 Subedi Pradeep A1 19 Suess D. B4 35 Sun J. J. D1 47 Sun Jonathan Z. E1 51 Sun Xueliang G6 85 Takahashi Y. K. F5 71 Tamura K. I2 99 Taratorin Alexander G1 75 Thiele Jan-Ulrich A1 19 Thiele Jan-Ulrich B5 37 Toivola Yvete C1 41 Trouilloud Philip L. E1 51 Uesugi, Takumi F6 73 Victora Randall H. B6 39 Vogler C. B4 35 Wago Koichi B5 37 Wago Koichi I5 105 Wan Lei E2 53 A5 Wang Jianyi 27 Wang Youyi A5 27 Wang M. X. E4 57 Wang Z. H. E4 57 Wang R. E5 59 Wang Shan X. I4 103 47 Whig R. D1 White Robert M. I4 103

114 TMRC 2016│Author index

Author last name First name Paper code Page number Willson Grant F1 63 Willson Grant I5 105 Wolf Daniel A3 23 Wong Wai Ee A5 27 Wood Roger G6 85 Wood Roger H1 89 Worledge Daniel C. E1 51 Wu Alexander Q. A1 19 Xiao Shuaigang B5 37 Xiao Shuaigang I5 105 Xing Xinzhi G1 75 Xue L. E5 59 Yamakawa Kiyoshi F2 65 Yang XiaoMin B5 37 Yang Y. C1 41 Yang Xiaomin F1 63 Yang XiaoMin I5 105 Yao Jun G7 87 Ye S. I3 101 Yeo Reuben J. C3 45 Yeo Reuben J. H4 95 Yoshida Kazuetsu F3 67 Yu Zhaoning B5 37 Yu Zhaoning I5 105 Zavaliche Florin C1 41 Zhao X. X. E4 57 Zhao W. S. E4 57 Zheng Xuan B1 29

115 TMRC 2016│Author index

Author last name First name Paper code Page number Zhou Hua A1 19 Zhou J. Q. E4 57 Zhu Jian-Gang (Jimmy) A2 21 Zhu Yu E1 51

116