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Tunnel Magnetoresistance Effect and Its Applications

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Tunnel Magnetoresistance Effect and Its Applications Tunnel Magnetoresistance Effect and Its Applications S. Yuasa, R. Matsumoto, A. Fukushima, H. Kubota, K. Yakushiji, T. Nakamura, Y. Suzuki and K. Ando Collaborators Osaka University (High-frequency experiment) Canon Anelva Corp. (R & D of manufacturing technology) Toshiba Corp. (R & D of Spin-MRAM) Funding agencies Outline (1) Introduction (2) Epitaxial MTJs with a crystalline MgO(001) barrier (3) CoFeB / MgO / CoFeB MTJs for device applications Spintronics N Charge -e Spin S Electron Electronics Magnetics ・diode ・ Magneto- magnetic recording ・transistor ・permanent magnet resistance Since 1988 LSI Spintronics Hard Disk Drive Both charge and spin of (HDD) the electron is utilized for novel functionalities. What is “magnetoresistance” ? A change in resistance by an application of H. Magneto-Resistance ; MR Resistance (R) Magnetoresistance ratio (MR ratio) Magnetic field H required to induce MR change 0 Magnetic field (H) MR ratio at RT & a low H (~1 mT) is important for practical applications. Magnetoresistance MR ratio (RT & low H) Year AMR effect 1857 MR = 1 ~ 2 % Lord Kelvin 1985 GMR effect A. Fert, P. Grünberg MR = 5 ~ 15 % (Nobel Prize 2007) 1990 TMR effect 1995 MR = 20 ~ 70 % T. Miyazaki, J. Moodera 2000 2005 2010 Tunnel magnetoresistance (TMR) effect FM electrode Tunnel barrier FM electrode Parallel (P) state Antiparallel (AP) state Tunnel Resistance RP : low Tunnel Resistance RAP : high Magnetic tunnel junction (MTJ) MR ratio ≡ (RAP – RP)/RP (performance index) Room-temperature TMR in 1995 T. Miyazaki (Tohoku Univ.) J. S. Moodera (MIT) Ferromag. electrode Amorphous Al-O MR ratios of 20 – 70% at RT Ferromag. electrode Al-O – based MTJ Technologies for HDD read head Write head Recording medium S S N N N N S N S S Head Medium S N Rotation Read head ） 2 AMR GMR TMR 1000 GMRヘッドの出現 Next-generation read ■ ■ 100 ■ head is indespensable ■ ■ ■ 2 Gbit / inch ■■ for > 200 Gbit / inch . （ ■■ 10 ■■ ■ ■ TMR head ■ 1 ■ ■ ■ GMR head 0.1 ■ 0.01 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 Recording density Year Magnetoresistive Random Access Memory (MRAM) MTJ “0” Bit Line Ward Line “1” Non-volatile memory Magnetoresistive Random Access Memory (MRAM) Bit Line MTJ Write Line Word Line n+ p n+ CMOS Freescale’s 4 Mbit-MRAM Cross-section structure based on Al-O MTJs Volume production since 2006. <Advantages> Non-volatile, high speed, infinite write endurance, etc. <Disadvantage> High-density MRAM is difficult to develop. MR ratios > 150% at RT are required for developing Gbit-MRAM. MR effects Device applications MR ratio (RT & low H) Year AMR effect HDD head 1857 MR = 1 ~ 2 % Inductive head 1985 GMR effect MR = 5 ~ 15 % 1990 MR head TMR effect 1995 MR = 20 ~ 70 % GMR head 2000 Memory TMR head 2005 MRAM Much higher MR ratios were required 2010 for next-generation devices. Outline (1) Introduction (2) Epitaxial MTJs with a crystalline MgO(001) barrier (3) CoFeB / MgO / CoFeB MTJs for device applications Theoretical prediction of giant TMR effect in Fe/MgO/Fe Fe(001) MgO(001) Fe(001) Fully epitaxial MTJ < First-principle calculations > ・Butler et al., Phys. Rev. B 63, 056614 (2001). ・Mathon & Umerski, Phys. Rev. B 63, 220403 (2001). MR ratio > 1000% Spin polarization P Tunnel Tunnel FM 1 barrier FM 2 FM 1 barrier FM 2 e e Energy Energy Energy Energy E E F EF F EF D D D D1↑ D1↓ 2↑ 2↓ D1↑ D1↓ D2↑ 2↓ Parallel (P) state Antiparallel (AP) state Tunnel resistnce: RP Tunnel resistnce: RAP ( D (E ) − D (E ) ) = α↑ F α↓ F α = 1, 2. MR ≡ (RAP – RP) / RP = 2P1P2 / (1 – P1P2), Pα , (Dα↑(EF ) + Dα↓(EF ) ) Spin polarization P Tunneling process in MTJs Amorphous Al-O barrier Crystalline MgO(001) barrier No symmetry 4-fold symmetry Δ 2’ Δ5 Δ2’ Δ5 Fe(001) Fe(001) Δ1 Δ1 Al-O MgO(001) Δ1 Fe(001) Δ1 Various Bloch states Only the Bloch states with Δ1 tunnel incoherently. symmetry tunnel dominantly. MR ratio < 100% at RT Fully spin-polarized Δ1 band in bcc Fe(001) 1.5 majority spin Δ1↓ 1.0 minority spin Δ ( eV ) 0.5 1↑ F E - E 0.0 EF -0.5 Γ (001) direction H Fully spin-polarized Δ1 band ⇒ Giant MR ratio is theoretically expected. Not only bcc Fe but also many other bcc alloys based on Fe or Co have fully spin-polarized Δ1 band. (e.g. bcc Fe1-xCox , Heusler alloys) Fully epitaxial Fe/MgO/Fe MTJ grown by MBE Fe(001) (Pinned layer) MgO(001) Fe(001) (Free layer) 2 nm TEM image S. Yuasa et al., Nature Materials 3, 868 (2004). Magnetoresistance of epitaxial Fe/MgO/Fe MTJ 300 tMgO = 2.3 nm T = 20 K 200 MR = 247% T = 293 K MR = 180% MR ratio ( % ) 100 0 -200 -100 0 100 200 H ( Oe ) MTJs with a single-crystal MgO(001) barrier S. Yuasa et al., Nature Materials 3, 868 (2004). Magnetoresistance of textured MgO-based MTJ MTJs with a (001)-oriented poly-crystal (textured) MgO barrier S. S. P. Parkin et al., Nature Materials 3, 862 (2004). Up to 600% at RT 260 240 “Giant TMR effect” 220 IBM [3] 200 AIST [2] 180 Crystal MgO(001) 160 tunnel barrier 140 120 100 Amorphous Al-O AIST [1] 80 MR ratio (%) at RT tunnel barrier 60 Nancy 40 20 CNRS-CSIC 0 1995 2000 2005 Year [1] Yuasa, Jpn. J. Appl. Phys. 43, L558 (2004). [2] Parkin, Nature Mater. 3, 862 (2004). [3] Yuasa, Nature Mater. 3, 868 (2004). Outline (1) Introduction (2) Epitaxial MTJs with a crystalline MgO(001) barrier (3) CoFeB / MgO / CoFeB MTJs for device applications MgO(001) 260 Amorphous CoFeB 240 Anelva - AIST [4] 220 IBM [3] MgO(001) 200 FeCo(001) Crystal MgO(001) AIST [2] 180 Textured MTJ tunnel barrier 160 140 MgO(001) 120 Fe(001) 100 Amorphous Al-O AIST [1] Fully epitaxial 80 MTJ MR ratio (%) at RT tunnel barrier 60 Nancy 40 20 CNRS-CSIC 0 1995 2000 2005 Year [1] Yuasa, Jpn. J. Appl. Phys. 43, L558 (2004). [2] Parkin, Nature Mater. 3, 862 (2004). [3] Yuasa, Nature Mater. 3, 868 (2004). [4] Djayaprawira, SY, APL 86, 092502 (2005). MTJ structure for practical applications For MRAM & HDD read head Free layer or Tunnel barrier Pinned layer Ru This structure is FM (Co-Fe) based on fcc (111). AF layer (Pt-Mn or Ir-Mn) for exchange biasing MgO(001) cannot be grown on fcc (111). 4-fold symmetry 3-fold symmetry MTJ structure in as-grown state Collaboration with Canon-Anelva Amorphous CoFeB Textured MgO(001) Amorphous CoFeB TEM image Djayaprawira, SY, Appl. Phys. Lett. 86, 092502 (2005). ◆Ideal for device applications This structure can be grown on any kind of underlayers by sputtering deposition at RT + post - annealing. CoFeB / MgO / CoFeB - MTJ with practical structure Crystalline symmetry Free layer 4-fold Tunnel barrier Pinned layer Amorphous SyF structure AF layer for 3-fold exchange- biasing Standard bottom structure for MRAM and HDD head Crystallization of CoFeB by post - annealing S. Yuasa et al., Appl. Phys. Lett. 87, 242503 (2005). Annealing Amorphous CoFeB above 250 ºC Amorphousbcc CoFeB(001) CoFeB Crystal- Textured MgO(001) Textured MgO(001) lization Amorphous CoFeB Amorphousbcc CoFeB(001) CoFeB As-grown MTJ Crystallization of CoFeB MgO(001) layer acts as a template to crystallize amorphous CoFeB. “Solid Phase Epitaxy” Because the Δ1 band in bcc CoFeB(001) is fully spin-polarized, CoFeB/MgO/CoFeB MTJs show the giant TMR effect. Sputtering deposition Canon-ANELVA C-7100 system φ 8 inch Thermally oxidized Standard sputtering machine Si wafer (8 or 12 inch) in HDD industry 100 wafers a day ! MR effects Industrial applications MR ratio (RT & low H) Year HDD head AMR effect 1857 MR = 1 ~ 2 % Inductive head 1985 GMR effect MR = 5 ~ 15 % 1990 MR head TMR effect 1995 MR = 20 ~ 70 % GMR head 2000 Memory Giant TMR effect TMR head 2005 MR = 200 ~ 600 % MRAM Novel MgO-TMR head devices 2010 Spin-torque MRAM Microwave, etc. Technologies for HDD read head Write head Recording medium S S N N N N S N S S Head Medium S N Rotation Read head ） 2 AMR GMR TMR 1000 GMRヘッドの出現 Next-generation read ■ ■ 100 ■ head is indespensable ■ ■ ■ 2 Gbit / inch ■■ for > 200 Gbit / inch . （ ■■ 10 ■■ ■ ■ TMR head ■ 1 ■ ■ ■ GMR head 0.1 ■ 0.01 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 Recording density Year MgO-TMR head for ultrahigh-density HDD Wafer of MgO-TMR head MgO-TMR head Cut Inte- gration Inte- gration Magnetic shield (top lead) Permanent Permanent magnet magnet MgO–MTJ ◆Commercialized in 2007. 20 nm Magnetic shield ◆Density > 250 Gbit / inch2 achieved. (bottom lead) ◆Applicable up to 1Tbit/inch2. TEM image MR effects Industrial applications MR ratio (RT & low H) Year HDD head AMR effect 1857 MR = 1 ~ 2 % Inductive head 1985 GMR effect MR = 5 ~ 15 % 1990 MR head TMR effect 1995 MR = 20 ~ 70 % GMR head 2000 Memory Giant TMR effect TMR head 2005 MR = 200 ~ 600 % MRAM Novel MgO-TMR head devices 2010 Spin-torque MRAM Microwave, etc. Spin-torque MRAM （SpinRAM） M. Hosomi et al.(Sony), Technical Digest of IEDM 2005, 19.1. MTJ CoFeB MgO CoFeB Ru 1 nm CoFe CMOS 6 2 Write current density, JC0 ～ 2x10 A/cm 5 2 JC0 of 5x10 A/cm is required for Gbit-scale SpinRAM. SpinRAM having perpendicular magnetization T. Kishi (Toshiba), SY et al., IEDM (2008) 12.6. Upper metal or 50 nm Upper electrode MgO(001) Storage layer MTJ MgO Referenc Bottom elctrode e layer 50nm A TEM image of 50 nm-sized MTJ Perpendicularly-magnetized electrodes 6 2 JC0 <10 A/cm achieved ! Perpendicularly magnetized MTJ A CMOS integrated MTJ array is a promising technology for Gbit-scale Spin-RAM.
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