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Magnetic Anisotropy and Exchange in (001) Textured Fept-Based Nanostructures

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Magnetic Anisotropy and Exchange in (001) Textured Fept-Based Nanostructures University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln Theses, Dissertations, and Student Research: Department of Physics and Astronomy Physics and Astronomy, Department of 12-2013 Magnetic Anisotropy and Exchange in (001) Textured FePt-based Nanostructures Tom George University of Nebraska-Lincoln, [email protected] Follow this and additional works at: https://digitalcommons.unl.edu/physicsdiss Part of the Condensed Matter Physics Commons George, Tom, "Magnetic Anisotropy and Exchange in (001) Textured FePt-based Nanostructures" (2013). Theses, Dissertations, and Student Research: Department of Physics and Astronomy. 31. https://digitalcommons.unl.edu/physicsdiss/31 This Article is brought to you for free and open access by the Physics and Astronomy, Department of at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Theses, Dissertations, and Student Research: Department of Physics and Astronomy by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. MAGNETIC ANISOTROPY AND EXCHANGE IN (001) TEXTURED FePt-BASED NANOSTRUCTURES by Tom Ainsley George A DISSERTATION Presented to the Faculty of The Graduate College at the University of Nebraska In Partial Fulfillment of Requirements For the Degree of Doctor of Philosophy Major: Physics and Astronomy Under the Supervision of Professor David J. Sellmyer Lincoln, Nebraska December, 2013 MAGNETIC ANISOTROPY AND EXCHANGE IN (001) TEXTURED FePt-BASED NANOSTRUCTURES Tom Ainsley George, Ph.D. University of Nebraska, 2013 Adviser: David J. Sellmyer Hard-magnetic L10 phase FePt has been demonstrated as a promising candidate for future nanomagnetic applications, especially magnetic recording at areal densities approaching 10 Tb/in2. Realization of FePt’s potential in recording media requires control of grain size and intergranular exchange interactions in films with high degrees of L10 order and (001) crystalline texture, including high perpendicular magnetic anisotropy. Furthermore, a write-assist mechanism must be employed to overcome the high coercivity of L10 FePt nanograins. The research described in this dissertation examines potential solutions to the aforementioned problems. Specifically, a nonepitaxial method of fabricating highly (001) textured thin films is investigated by careful tuning of the as-deposited structure. Such highly textured films could be useful as a template for bit-patterned media. Secondly, control of grain size and intergranular magnetic interactions is demonstrated using non-magnetic additions of Al2O3, C, and Au. Finally, large reductions in the coercivity of high anisotropy, epitaxially grown L10 FePt islands are achieved in an exchange-coupled composite system by adding an exchange coupled layer of FePt:SiO2 with moderate anisotropy. The results show promise for the implementation of L10 FePt in future magnetic recording media and other nanomagnetic applications. iii Copyright 2013, Tom A. George. iv DEDICATION To my late father, Professor Thomas Adrian George, whose quiet inspiration, modesty, and commitment to excellence have provided me with the model of the man I forever strive to be. To my wonderful wife, Annie, without whose encouragement I would not have succeeded. I sincerely thank her for her patience and understanding through some very difficult times. v ACKNOWLEDGMENTS I am indebted to many people who have helped me along this venture. In the following I will include as many as I can, but this will be by no means exhaustive. First and foremost, I want to sincerely thank my adviser, Professor David Sellmyer, for his unwavering support and guidance. His patient tutelage has allowed me to become the scientist I am today. I have learned so much from not only his scientific expertise but his affable yet professional character. It has been an honor to be a part of his research group and I will forever value his advice. I would also like to sincerely thank my supervisory committee of Professors Roger Kirby, Ralph Skomski, and Jeff Shield, for their challenging questions, useful guidance, and patience in helping me complete my Ph.D. work. In particular, I would like to thank Professor Skomski for providing complementary theoretical models with which to interpret my experimental data and for including me in his related projects. As a member of Professor Sellmyer’s group I have had the opportunity to work with many excellent scientists. I want to thank Dr. Jian Zhou, Dr. Minglang Yan, Dr. Yingfan Xu, Dr. Korey Sorge, Dr. Maria Daniil, Dr. Parashu Kharel, and Dr. Bala Balasubramanian for their mentoring at various stages of my development. I would like to thank Professor Sy-Hwang Liou, Professor Sitaram Jaswal, and Dr. Yi Liu for numerous useful discussions regarding experimental and theoretical aspects of my work and physics in general. In addition, I have profited greatly from interactions with visiting scholars Dr. Emmanuel Kockrick, Dr. Johan Engelen, Professor Mircea Chipara, and Dr. Damien LeRoy, to all of whom I extend my sincerest gratitude. I was fortunate enough to experience a mutually beneficial mentorship of several undergraduates and I would be remiss not to thank Steven Ray, Craig Meeks, Adam Attig, Anatol Hoemke, and Justin Baize for their hard work in the lab and the friendships that resulted. vi I would like to thank Professor Alexei Gruverman and Dr. Haidong Lu for their collaboration on the electric modulation of magnetization in BTO/LSMO interfaces. A Ph.D. program is a difficult journey and I am thankful for the companionship of my classmates and close colleagues. I would like to thank fellow group members Dr. Zhen Li, Dr. Xiaohui Wei, Dr. Yao Zhao, Dr. Rui Zhang, Dr. Yongsheng Yu, Xiaolu Yin, and Baskhar Das for sharing their journeys with me and becoming part of mine. Thanks also goes to the many other friends made within the department through shared classes or just enjoying their company; thank you Professor Christian Binek, Dr. Shawn Hilbert, Shawn Langan, Verona Skomski, Dr. Andrew Baruth, Dr. J.D. Burton, Dr. Srinivas Polisetty, Steven Rosenboom, Dr. Toney Kelly, Chad Peterson, Dr. Dale Johsnton, Kayle DeVaughn, Dr. Marcus Natta, Dr. Xi He, Dr. Yi Wang, Dr. Abhijit Mardana, and Dr. Tathagata Mukherjee. My PhD experience was thoroughly enriched by the friendships forged along the way. I would like to thank present and former members of the wonderful support staff in the Physics Department and the Nebraska Center for Materials and Nanomagnetism at UNL: Shelli Krupicka, Theresa Sis, Kay Haley, Jenny Becic, Therese Janovec, Cindia Carlson-Tsuda, Patty Fleek, Joyce McNeil, Marge Wolfe, Jen Barnason, Cyndy Petersen, Les Marquart, Mike Jensen, Keith Placek, Pat Pribil, Bob Rhynalds, Brian Farleigh, and Dr. John Kelty. Scientific research is not possible without funding. I would like to acknowledge support from the following agencies: NSF-MRSEC (NSF-DMR-0820521), DOE-BES (DE-FG02-04ER46152), INSIC, and NCMN. Last but not least I would like to thank my family: my mom, Denise, and sister, Katie Rasmussen, for providing consistent and unconditional support; my wife, Annie, and two wonderful children, Aran and Bella, for providing necessary and welcomed distractions from the rigors of work; and my in-laws, Pat and Mary Rowan. vii CONTENTS List of Figures ................................................................................................................................... x List of Tables .................................................................................................................................. xiv CHAPTER 1 INTRODUCTION ............................................................................................................. 1 References ................................................................................................................................. 11 CHAPTER 2 BACKGROUND AND THEORY ....................................................................................... 12 2.1 Magnetism in Nanostructured Thin Films ........................................................................... 12 2.1.1 General Concepts .......................................................................................................... 13 2.1.2 Field Dependent Behavior ............................................................................................ 16 2.1.3 Stoner-Wohlfarth and General Multi-domain Magnets ............................................... 20 2.1.4 Granular Magnets ......................................................................................................... 25 2.1.5 Exchange-Coupled Magnets ......................................................................................... 29 2.1.6 Thermal Effects ............................................................................................................. 34 2.2 Fabricating Thin-Film Nanostructures ................................................................................. 36 2.2.1 Thin-Film Deposition by Magnetron Sputtering ........................................................... 37 2.2.2 Thin-Film Growth .......................................................................................................... 39 2.2.3 Post-Deposition Processing .......................................................................................... 42 2.3 L10-Phase FePt...................................................................................................................... 45 2.3.1 Crystal Structure
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