Magnetoresistance and spin-transfer torque in magnetic tunnel junctions S. Yuasa IEEE Distinguished Lecturer 2012 E-mail: [email protected] Collaborations Toshiba Corp. (H. Yoda) (STT-MRAM) Canon Anelva Corp. (sputtering deposition process) Osaka University (Y. Suzuki) (rf & high-speed experiments) CNRS/Thales(A.Fert, V.Cros, J.Grollier) (spin-torque oscillator) Outline (1) Spintronics (2) Tunnel magnetoresistance (TMR) ・Magnetoresistance ・Tunnel magnetoresistance in magnetic tunnel junction (MTJ) ・Giant TMR in MgO-based MTJ ・CoFeB/ MgO/ CoFeB structure for device applications (3) Spin-transfer torque (STT) ・Physics of spin-transfer torque ・Spin-transfer torque MRAM (STT-RAM or Spin-RAM) ・Microwave applications What is spintronics ? N Electric Charge -e Spin (a small magnet) S Electronics Electron ・power amplification Magnetics ・magnetic recording ・logic operation TMR non-volatile basically volatile & STT Transistor, LSI Hard Disk Drive (HDD) Spintronics Both charge and spin of electrons are utilized for new functions. Difference between conventional magnetics and spintronics ? Magnetics Coupling between charge and spin by induction coil Most of the energy is wasted. N Spintronics Coupling between charge and spin -e by quantum mechanical effects (e.g. tunnel magnetoresistance, spin-transfer torque) S Highly efficient ! Outline (1) Spintronics (2) Tunnel magnetoresistance (TMR) ・Magnetoresistance ・Tunnel magnetoresistance in magnetic tunnel junction (MTJ) ・Giant TMR in MgO-based MTJ ・CoFeB/ MgO/ CoFeB structure for device applications (3) Spin-transfer torque (STT) ・Physics of spin-transfer torque ・Spin-transfer torque MRAM (STT-RAM or Spin-RAM) ・Microwave applications Magneto-Resistance (MR) Change in electric resistance induced by magnetic field. R Magneto-resistance ratio (MR ratio) Magnetic field H Resistance, Resistance, required to induce MR 0 Magnetic field, H MR converts magnetic signals into electric signals. (cf. STT converts electric signals into magnetic signals.) MR ratio at RT & a low H (~1 mT) is important for device applications. Magnetoresistance MR ratio @RT&low H Year AMR effect 1857 Lord Kelvin MR = 1 - 2% 1985 GMR effect A. Fert, P. Grünberg MR = 5 - 15% (Nobel Prize 2007) 1990 TMR effect 1995 (Al-O barrier) T.Miyazaki, J. Moodera MR = 20 - 70% 2000 2005 2010 Outline (1) Spintronics (2) Tunnel magnetoresistance (TMR) ・Magnetoresistance ・Tunnel magnetoresistance in magnetic tunnel junction (MTJ) ・Giant TMR in MgO-based MTJ ・CoFeB/ MgO/ CoFeB structure for device applications (3) Spin-transfer torque (STT) ・Physics of spin-transfer torque ・Spin-transfer torque MRAM (STT-RAM or Spin-RAM) ・Microwave applications Tunnel MagnetoResistance (TMR) effect in magnetic tunnel junction (MT J) Ferromagnetic electrode e e e e e e Insulator (nm –thick) (tunnel barrier) Ferromagnetic electrode Parallel (P) state Antiparallel (AP) state Resistance RP : low Resistance RAP : high MR ratio (RAP – RP)/RP×100% (performance index) 3d ferromagnet Amorphous Al-O MR ratios of 20 – 70% at RT 3d ferromagnet Read head of hard disk drive (HDD) Sense current Magnetic field signal Recording media Read head Head Medium （magnetic sensor） ） 2 AMR GMR TMR 1000 Higher MR ratio GMRヘッドの出現 ■ ■ is required for 100 ■ ■■ Gbit/in ■ （ ■■ > 200 Gbit/inch2. ■■■ 10 ■ ■ ■ ■ TMR head 1 ■ (amorphous ■ ■ tunnel barrier) GMR head 0.1 ■ 0.01 Recording density 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 Year Non-volatile Magnetoresistive Random Access Memory (MRAM) Digit line Bit line Bit line MTJ Parallel ：“0” or Antiparallel：“1” MTJ Write line Non-volatile Digit line magnetic memory CMOS + n+ p n CMOS Read out Writing Review: Science 294, 1488 (2001). Non-volatile Magnetoresistive Random Access Memory (MRAM) Freescale (US)’s 4 Mbit – MRAM based on Al-O MTJs (volume production since 2006) <Advantages> Non-volatile, high speed, write endurance > 1016 <Disadvantage> High-density MRAM is difficult to develop. Three important properties for memory device: speed, density, and write endurance 1 Write endurance > 1016 (virtually unlimited) Work Volatile SRAM memory 10 MRAM DRAM Non-volatile FeRAM Write endurance <1012 NOR 100 PRAM Storage NAND Access speed (nsec) HDD 1000 1 Mbit 1 Gbit 1 Tbit Density (bit/chip) Three important properties for memory device: speed, density, and write endurance 1 Write endurance > 1016 High-speed Work (virtually unlimited) MRAM Volatile SRAM memory 10 MRAM DRAMGbit-scale MRAM Non-volatile FeRAM Write endurance <1012 NOR 100 PRAM Storage NAND Access speed (nsec) HDD 1000 1 Mbit 1 Gbit 1 Tbit Density (bit/chip) How can we develop Gbit-scale MRAM ? Major requirement for Gbit-MRAM ・Large output voltage MR ratio >> 100% Breakthrough ・Reduction of writing power spin-torque switching 1G DRAM 1M MRAM Capacity of MRAM cannot Capacity (Capacity bit ) go beyond 64 Mbit. 1K 2000 2002 2004 2006 2008 Year 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 (Al-O barrier) MR = 20 ~ 70 % GMR head 2000 Memory TMR head 2005 MRAM Much higher MR ratio is required 2010 for next-generation devices. Simple model for TMR effect : Julliere’s model Tunnel Tunnel FM 1 Barrier FM 2 FM 1Barrier FM 2 e e DOS DOS DOS DOS E F EF EF EF D D1 D2 2 D D1 D D D1 1 2 2 P alignment AP alignment 2 ↑ ↓ ≡ , ,1,2 1 ↑ ↓ Spin Polarization, P How can we attain giant MR ratio ? (1) Electrode material with full spin-polarization Energy |P| = 1 EF “Half Metal” (E.g.) some Heusler alloys, Fe3O4, CrO2, LaSrMnO3 perovskite Room-temperature MR ratios for half-metal electrodes have never exceeded those for simple 3d alloys such as Co-Fe. (2) Crystalline tunnel barrier such as MgO(001) Outline (1) Spintronics (2) Tunnel magnetoresistance (TMR) ・Magnetoresistance ・Tunnel magnetoresistance in magnetic tunnel junction (MTJ) ・Giant TMR in MgO-based MTJ ・CoFeB/ MgO/ CoFeB structure for device applications (3) Spin-transfer torque (STT) ・Physics of spin-transfer torque ・Spin-transfer torque MRAM (STT-RAM or Spin-RAM) ・Microwave applications Amorphous Al-O barrier vs. crystal MgO barrier (theory) Amorphous Al-O barrier Crystal MgO(001) barrier (conventional MTJ) 2 2 5 5 Fe(001) 1 Fe(001) 1 Amorphous MgO(001) 1 Al-O Fe(001) 1 Incoherent tunneling of Dominant tunneling of fully various Bloch states. spin-polarized 1 Bloch states MR < 100% at RT MR >> 1000% (theory) Butler et al. PRB 2001; Mathon ibid 2001. What we learn about “tunneling effect” at an undergraduate course Barrier Potential barrier is width, t assumed to be vacuum. Free-electron wave exp(ik·z ) Barrier height EF Electrode Tunnel Electrode barrier Exponential decay of DOS Tunneling transmittance (T) decays exponentially as a function of t. (WKB approx.) 2× T exp( 8m/ t) m: effective mass Realistic tunneling effect Conduction Bloch states band Band EF gap Valence band Electrode Barrier Electrode Evanescent states in the band gap Both Bloch states and evanescent states have (i) specific orbital symmetry & (ii) specific band dispersion. (complex wave vector k = kr + i ) Bloch states and evanescent states couple at interface. Decay of evanescent state largely depends on orbital symmetry. 1 (spd) 5 (pd) Fe MgO Fe 2’ (d) Butler (2001). 1 : s + pz + d 2 z 2 – x 2 –y 2 Coupling between Bloch states and evanescent states Ideal coherent tunneling for k// = 0 direction Fe(001) MgO(001) Fe(001) 1 1 1 k k// 5 5 5 2 2’ 2 Fully spin-polarized 1 band in bcc Fe(001) 1.5 1 majority spin 1.0 minority spin ( eV ( eV ) 0.5 1 F E - E 0.0 EF -0.5 (001) direction H Fully spin-polarized 1 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 1 band. (e.g. bcc Fe1-xCox , some Heusler alloys) Importance of interface (theory) X.-G. Zhang, et al., PRB 68, 092402 (2003). Excess O atom k LDOS of states k// 1 Ideal interface Oxidized interface Fe- 1 states couple with Fe- 1 states do not couple MgO-1 states at k// . with MgO-1 states at k// = 0. MR ratio > 1000% MR ratio < 100% Fully epitaxial Fe/MgO/Fe MTJ grown by MBE Yuasa et al., Nature Mater. 3, 868 (2004). Fe(001) (Pinned layer) MgO(001) Fe(001) (Free layer) 2 nm TEM image Experimental demonstrations of giant TMR Single-crystal MgO(001) barrier Textured MgO(001) barrier 300 180 T = 20 K 160 MR = 247% 140 200 120 T = 293 K 100 MR = 180% 80 100 MR ratio(%) 60 MR ratio(%) 40 20 0 0 -200 -100 0 100 200 -1000 -500 0 H (Oe) H (Oe) Yuasa et al., Nature Mater. 3, 868 (2004). Parkin (IBM), Nature Mater. 3, 862 (2004). 600 500 “Giant TMR effect” 400 Crystal MgO(001) tunnel barrier 300 MgO(001) FeCo(001) MR ratioat RT (%) IBM [3] 200 Textured MTJ AIST [2] MgO(001) Amorphous Al-O AIST [1] 100 tunnel barrier Fe(001) Fully epitaxial MTJ 0 1995 2000 2005 Year [1] Yuasa, Jpn.J.Appl.Phys.43,L558 (2004). [2] Yuasa, Nature Mater.3,868 (2004). [3] Parkin, Nature Mater.3, 862 (2004). Fundamental problem on thin film growth MTJ structure for practical applications Free layer or MTJ 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 600 Crystal MgO(001) barrier 500 : Single crystal MgO(001) : Poly-crystal MgO(001) 400 : CoFeB/MgO/CoFeB 300 Canon-Anelva & AIST [4] MgO(001) FeCo(001) MR ratioat RT (%) IBM [3] 200 Textured MTJ AIST [2] MgO(001) AIST [1] 100 AmorphousAl-O Fe(001) tunnel barrier Fully epitaxial MTJ 0 1995 2000 2005 Year [1] Yuasa, Jpn.J.Appl.Phys.43,L558 (2004).