MSE 7025 Magnetic Materials (and Spintronics)

Transcription

1 MSE 7025 Magnetic Materials (and Spintronics) Lecture 14: Spin Transfer Torque And the future of spintronics research Chi-Feng Pai

2 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

3 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

4 (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

5 Spintronics: The beginning Giant magnetoresistance (GMR) ~3% Tunneling magnetoresistance (TMR) RAP R R ( AP ) P >100%

6 Spintronics: The beginning Spin valve Magnetic tunnel junction (MTJ) (top view) TMR~180% S. Yuasa et al., Nature Mater (2004)

7 Spintronics: The beginning HDD read-head sensors

8 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)

9 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)

10 Spin transfer torque Early experimental evidence E. B. Myers et.al. Science 285, 867 (1999)

11 Spin transfer torque Early experimental evidence E. B. Myers et.al. Science 285, 867 (1999)

12 Spin transfer torque Early experimental evidence J. A. Katine et.al. PRL 84, 3419 (2000)

13 Spin transfer torque Early experimental evidence J. A. Katine et.al. PRL 84, 3419 (2000) Current citation: ~1,700 times

14 Spin torque switching Current induced ST switching Spin valve MTJ 2e JC0 0M St( HC M eff / 2) ~ A/cm P 6 7 2

15 Spin torque switching Current induced field vs. torque switching Field Spin-torque H JC0r J C0

16 Spin torque microwave generation Current induced torque dynamics Spin-torque nano-oscillator (STNO) A. M. Deac et.al. Nat. Phys. 4, 803 (2008)

17 Building blocks Spin torque devices

18 Building blocks Spin torque devices

19 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)

20 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!

21 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!

22 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 (1971) J. E. Hirsch, Phys. Rev. Lett (1999)

23 The SHE in transition metals The spin Hall conductivity calculated for 4d 5d elements Tanaka, T. et al, Phys. Rev. B 77, (2008) ab initio calculation: θ SH (Ta)<0 and θ SH (Pt)>0 for highly resistive case, θ SH (Ta) can be large

24 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

25 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

26 V mix ( V) ST-FMR results of Ta and Pt S J S + + A J e 10 S J S A J e f = 9 GHz CoFeB (4nm)/Ta (8nm) B ext (mt) CoFeB/β-Ta V mix ( V) 0-10 f = 9 GHz CoFeB (3 nm)/ Pt (6 nm) B ext (mt) CoFeB/Pt

27 V mix ( V) ST-FMR results of Ta and Pt + + S J S A J e 10 S J S A J e f = 9 GHz CoFeB (4nm)/Ta (8nm) B ext (mt) β-ta SH 0.15 V mix ( V) 0-10 f = 9 GHz CoFeB (3 nm)/ Pt (6 nm) B ext (mt) Pt SH 0.07

28 Three-terminal devices

29 dv/di (k ) dv/di (k ) Three-terminal devices External field induced switching DC current induced SHE-ST switching B ext = -3.5 mt 90 I DC =0 ma dv/di (k ) B ext (mt) B ext (mt) I DC (ma)

30 dv/di (k ) Switching Current (ma) Three-terminal devices I C AP to P I C P to AP 100 DC current induced SHE-ST switching B ext = -3.5 mt E Ramp Rate (ma/s) 80 2e JC0 0M St( HC M eff / 2) SH Ta SH I DC (ma)

31 5d transition metals Significant spin-orbit interactions 4d 5d Nb Mo Tc Ru Rh Pd Ag Ta W Re Os Ir Pt Au β-ta SH 0.15 (ST-FMR, ST-switchings) Liu et al., Science 336, 555 (2012) Pt SH (ST-FMR, ST-switchings) Liu et al., Phys. Rev. Lett (2011) Liu et al., Phys. Rev. Lett (2012)

32 α-w Tungsten (W) BCC Conductive (20-40 μω cm) β-w A15 cubic Resistive ( μω cm)

33 Intensity (counts) Sputtered W films Thickness-dependent resistivity and phase W(6nm) W(8nm) XRD ( cm) W thickness (nm) W (200) -W (110) -W (210)/ -W (110) -W (211) Pai et al, Appl. Phys. Lett. 101, (2012) (degree)

34 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)

35 V mix ( V) ST-FMR from W(4-20nm)/Py(5nm) Py(5nm)/W(4nm) Py(5nm)/W(10nm) Chi-Feng Pai et al (unpublished data) B ext (mt) f 9 GHz

36 ST-FMR from W(4-20nm)/Py(5nm) SH Chi-Feng Pai et al (unpublished data) SH-W ~ W Thickness (nm)

37 Three-terminal devices 2e JC0 0M St( HC M eff / 2) SH Pai et al, Appl. Phys. Lett. 101, (2012) W SH

38 Three-terminal devices 3-terminal devices with different W thicknesses Thickness (nm) Resistivity (μω cm) Phase θ SH β α+β α <0.07 In agreement with ST-FMR results The spin Hall angle is phase/resistivity dependent

39 The ever-growing spin Hall angle Reported spin Hall angles or spin Hall efficiencies

40 The ever-growing spin Hall angle Reported spin Hall angles or spin Hall efficiencies

41 The ever-growing spin Hall angle Reported spin Hall angles or spin Hall efficiencies

42 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, (2015)

43 The ever-growing spin Hall angle What s next? Topological insulators? Bi 2 Se 3 effective spin Hall angle ~ % Mellnik et al, Nature 511, 449 (2014)

44 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)

45 The ever-growing spin Hall angle What s next? 2D materials? WTe 2 effective spin Hall angle?? MacNeill et al, arxiv (2016)