MSE 7025 Magnetic Materials (and Spintronics) Lecture 14: Spin Transfer Torque And the future of spintronics research Chi-Feng Pai cfpai@ntu.edu.tw
Course Outline Time Table Week Date Lecture 1 Feb 24 Introduction 2 March 2 Magnetic units and basic E&M 3 March 9 Magnetization: From classical to quantum 4 March 16 No class (APS March Meeting, Baltimore) 5 March 23 Category of magnetism 6 March 30 From atom to atoms: Interactions I (oxides) 7 April 6 From atom to atoms: Interactions II (metals) 8 April 13 Magnetic anisotropy 9 April 20 Mid-term exam 10 April 27 Domain and domain walls
Course Outline Time Table Week Date Lecture 11 May 4 Magnetization process (SW or Kondorsky) 12 May 11 Characterization: VSM, MOKE 13 May 18 Characterization: FMR 14 May 25 Transport measurements in materials I: Hall effect 15 June 1 Transport measurements in materials II: MR 16 June 8 MRAM: TMR and spin transfer torque 17 June 15 Spin transfer torque 18 June 22 Final exam
(modified from) From spin transfer torque, the spin Hall torque, to spin-orbit torque: An Experimentalist s Point of View April 11 th, 2016 NTU-IAM Seminar Talk Chi-Feng Pai
Spintronics: The beginning Giant magnetoresistance (GMR) ~3% Tunneling magnetoresistance (TMR) RAP R R ( AP ) P >100%
Spintronics: The beginning Spin valve Magnetic tunnel junction (MTJ) (top view) TMR~180% S. Yuasa et al., Nature Mater. 3 868 (2004)
Spintronics: The beginning HDD read-head sensors
Spin transfer torque 4s (itinerant)-3d (localized) s-d interaction J sˆ S r sd e - L. Berger, Phys. Rev. B 54, 9353 (1996) Ya. B. Bazaliy, B.A. Jones, and S.C. Zhang, Phys. Rev. B 57, R3213 (1998)
Spin transfer torque Landau-Lifshitz-Gilbert-Slonczewski (LLGS) eqn dm dm PI m Heff m ( m m) dt dt 2e M V 0 S L. Berger, Phys. Rev. B 54, 9353 (1996) J. Slonczewski, J. Magn. Magn. Mater. 159, L1 (1996) J. Z. Sun, Phys. Rev. B 62, 570 (2000)
Spin transfer torque Early experimental evidence E. B. Myers et.al. Science 285, 867 (1999)
Spin transfer torque Early experimental evidence E. B. Myers et.al. Science 285, 867 (1999)
Spin transfer torque Early experimental evidence J. A. Katine et.al. PRL 84, 3419 (2000)
Spin transfer torque Early experimental evidence J. A. Katine et.al. PRL 84, 3419 (2000) Current citation: ~1,700 times
Spin torque switching Current induced ST switching Spin valve MTJ 2e JC0 0M St( HC M eff / 2) ~ 10 10 A/cm P 6 7 2
Spin torque switching Current induced field vs. torque switching Field Spin-torque H JC0r J C0
Spin torque microwave generation Current induced torque dynamics Spin-torque nano-oscillator (STNO) A. M. Deac et.al. Nat. Phys. 4, 803 (2008)
Building blocks Spin torque devices
Building blocks Spin torque devices
STT-MRAM and spin logic But then again, what industry cares about is the possible application in non-volatile memory (NVM) Or maybe all spin-logic (ASL)? Nat. Nanotech. 5, 266 (2010)
Spin Hall effect induced STT Can we use a pure spin current to generate spin-torque instead of using a spin-polarized current? LLGS equation with a spin-polarized current dm dm PI m Heff m ( m m) dt dt e M V 2 0 S LLGS equation for the spin Hall effect induced spin current dm dm SH I m Heff m ( m m) dt dt 2e M V 0 S Replace spin-polarization by the spin Hall angle!
Spin Hall effect induced STT Can we use a pure spin current to generate spin-torque instead of using a spin-polarized current? Critical switching current in a (in-plane-magnetized) F/I/F MTJ 2e JC0 0M St( HC M eff / 2) P If we use the spin Hall effect 2e JC0 0M St( HC M eff / 2) SH Replace spin-polarization by the spin Hall angle!
The spin Hall effect (revisit) J S SH e The spin Hall angle ˆ J e - ˆ Spin-orbit interaction J / J SH s e J s J e e - e - e - M. I. Dyakonov and V. I. Perel, JETP 13 467 (1971) J. E. Hirsch, Phys. Rev. Lett. 83 1834 (1999)
The SHE in transition metals The spin Hall conductivity calculated for 4d 5d elements Tanaka, T. et al, Phys. Rev. B 77, 165117 (2008) ab initio calculation: θ SH (Ta)<0 and θ SH (Pt)>0 for highly resistive case, θ SH (Ta) can be large
Spin Torque-Ferromagnetic Resonance 2 f H0 H0 4 M eff Vmix IRF RRF dm dm m H m J ( m m) dt dt e M t eff e SH 2 0 S Spin current in plane torque τ ST symmetric peak Oersted field perpendicular torque τ H antisymmetric peak
Spin Torque-Ferromagnetic Resonance mˆ ˆ mˆ ˆm H + Vmix IRF RRF S J A HRF Je // S Spin current in plane torque τ ST symmetric peak Oersted field perpendicular torque τ H antisymmetric peak
V mix ( V) ST-FMR results of Ta and Pt S J S + + A J e 10 S J S A J e 4 2 0-2 f = 9 GHz CoFeB (4nm)/Ta (8nm) 0 40 80 120 160 B ext (mt) CoFeB/β-Ta V mix ( V) 0-10 f = 9 GHz CoFeB (3 nm)/ Pt (6 nm) -20 0 40 80 120 160 B ext (mt) CoFeB/Pt
V mix ( V) ST-FMR results of Ta and Pt + + S J S A J e 10 S J S A J e 4 2 0-2 f = 9 GHz CoFeB (4nm)/Ta (8nm) 0 40 80 120 160 B ext (mt) β-ta SH 0.15 V mix ( V) 0-10 f = 9 GHz CoFeB (3 nm)/ Pt (6 nm) -20 0 40 80 120 160 B ext (mt) Pt SH 0.07
Three-terminal devices
dv/di (k ) dv/di (k ) Three-terminal devices External field induced switching 100 100 DC current induced SHE-ST switching B ext = -3.5 mt 90 I DC =0 ma 80 70 60 dv/di (k ) 100 90 80 70 60-20 0 20 B ext (mt) -15-10 -5 0 5 10 15 20 B ext (mt) 80 60-1.5-1.0-0.5 0.0 0.5 1.0 1.5 I DC (ma)
dv/di (k ) Switching Current (ma) Three-terminal devices 1.5 1.0 0.5 0.0 I C AP to P I C P to AP 100 DC current induced SHE-ST switching B ext = -3.5 mt -0.5-1.0-1.5 1E-3 0.01 0.1 1 Ramp Rate (ma/s) 80 2e JC0 0M St( HC M eff / 2) SH 0.12 0.04 -Ta SH 60-1.5-1.0-0.5 0.0 0.5 1.0 1.5 I DC (ma)
5d transition metals Significant spin-orbit interactions 4d 5d 41 42 43 44 45 46 47 Nb Mo Tc Ru Rh Pd Ag 73 74 75 76 77 78 79 Ta W Re Os Ir Pt Au β-ta SH 0.15 (ST-FMR, ST-switchings) Liu et al., Science 336, 555 (2012) 0.068 0.005 Pt SH (ST-FMR, ST-switchings) Liu et al., Phys. Rev. Lett. 106 036601 (2011) Liu et al., Phys. Rev. Lett. 109 096602 (2012)
α-w Tungsten (W) BCC Conductive (20-40 μω cm) β-w A15 cubic Resistive (150-350 μω cm)
Intensity (counts) Sputtered W films Thickness-dependent resistivity and phase 300 250 4000 3500 W(6nm) W(8nm) XRD ( cm) 200 150 100 50 0 4 8 12 16 20 W thickness (nm) 3000 2500 2000 1500 1000 -W (200) -W (110) -W (210)/ -W (110) -W (211) Pai et al, Appl. Phys. Lett. 101, 122404 (2012) 30 35 40 45 50 2 (degree)
Sputtered W films Thickness-dependent resistivity and phase STEM imaging EELS composition survey W Fe Mg Hf Ta Chi-Feng Pai et al (unpublished data)
V mix ( V) ST-FMR from W(4-20nm)/Py(5nm) 40 0-40 -80-120 -160 Py(5nm)/W(4nm) Py(5nm)/W(10nm) 50 100 150 Chi-Feng Pai et al (unpublished data) B ext (mt) f 9 GHz
ST-FMR from W(4-20nm)/Py(5nm) SH 0.30 0.25 0.20 0.15 0.10 0.05 Chi-Feng Pai et al (unpublished data) SH-W ~ 0.3 4 8 12 16 20 W Thickness (nm)
Three-terminal devices 2e JC0 0M St( HC M eff / 2) SH Pai et al, Appl. Phys. Lett. 101, 122404 (2012) 0.33 0.06 -W SH
Three-terminal devices 3-terminal devices with different W thicknesses Thickness (nm) Resistivity (μω cm) Phase θ SH 5.2 260 β 0.33 6.2 80 α+β 0.18 15 21 α <0.07 In agreement with ST-FMR results The spin Hall angle is phase/resistivity dependent
The ever-growing spin Hall angle Reported spin Hall angles or spin Hall efficiencies
The ever-growing spin Hall angle Reported spin Hall angles or spin Hall efficiencies
The ever-growing spin Hall angle Reported spin Hall angles or spin Hall efficiencies
The ever-growing spin Hall angle What s next? Topological insulators? Bi 2 Se 3 effective spin Hall angle ~ 80% Liu et al, Phys. Rev. B 91, 235437 (2015)
The ever-growing spin Hall angle What s next? Topological insulators? Bi 2 Se 3 effective spin Hall angle ~ 200-350% Mellnik et al, Nature 511, 449 (2014)
The ever-growing spin Hall angle What s next? Topological insulators? (Bi 0.5 Sb 0.5 ) 2 Te 3 effective spin Hall angle ~ 14000%-42500%!!! Mellnik et al, Nature 511, 449 (2014)
The ever-growing spin Hall angle What s next? 2D materials? WTe 2 effective spin Hall angle?? MacNeill et al, arxiv (2016)