1. Research fueled by: 8th International Workshop on Nanomagnetism & Superconductivity Coma-ruga July 2 nd, 2012 Expecting the unexpected in the spin Hall effect: from fundamental to practical JAIRO SINOVA Texas A&M University Institute of Physics ASCR Hitachi Cambridge Joerg W ü nderlich, A. Irvine, et al Institute of Physics ASCR Tomas Jungwirth, Vít Novák, et al 1 U. of Wurzberg Laurens Molenkamp, E. Hankiewicz, et al

2. 2 Talk dedicated to “THE” dream team CAMPEONES! OLE !

3. 3 Nanoelectronics, spintronics, and materials control by spin-orbit coupling I. Introduction: Basics of AHE: SOC origins and mechanism SHE phenomenology II. Spin Hall effect: the early days First proposals: from theory to experiment First observations of the extrinsic and intrinsic (optical) Inverse spin Hall effect: SHE as a spin current detector Direct iSHE in metals Spin pumping and iSHE Intrinsic mesoscopic SHE SHE-FET: first steps towards practicality (but perhaps not) Spin Hall injection and spin precession manipulation iSHE device with spin-accumulation modulation FMR measurement of SHE angle: giant SHE as a spin current generator FMR and SHE angle Giant intrinsic SHE and STT: Future MRAM technology? Conclusion Expecting the unexpected in the spin Hall effect: from fundamental to practical

4. 4 Simple electrical measurement of out of plane magnetization (or spin polarization ~ n ↑ -n ↓ ) InMnAs Spin dependent “force” deflects like-spin particles ρ H =R 0 B ┴ +4π R s M ┴ Anomalous Hall Effect: the basics I _ F SO _ _ _ majority minority V M⊥M⊥ AHE is does NOT originate from any internal magnetic field created by M ⊥ ; the field would have to be of the order of 100T!!! Nagaosa, Sinova, Onoda, MacDonald, Ong, Rev. Mod. Phys. 2010

5. 5 e-e-e-e- Nanoelectronics, spintronics, and materials control by spin-orbit coupling (one of the few echoes of relativistic physics in the solid state) This gives an effective interaction with the electron’s magnetic moment Classical explanation (in reality it arises from a second order expansion of Dirac equation around the non-relativistic limit) “Impurity” potential V(r) Produces an electric field ∇V∇V B eff p s In the rest frame of an electron the electric field generates an effective magnetic field Motion of an electron Consequence #1: Spin or the band-structure Bloch states are linked to the momentum. Coupled multi-band system. Internal communication between spin and charge:spin- orbit coupling interaction Consequence #2: Mott scattering

6. 6 Cartoon of the mechanisms contributing to AHE independent of impurity density Electrons have an “anomalous” velocity perpendicular to the electric field related to their Berry’s phase curvature which is nonzero when they have spin-orbit coupling. Electrons deflect to the right or to the left as they are accelerated by an electric field ONLY because of the spin-orbit coupling in the periodic potential (electronics structure) E SO coupled quasiparticles Intrinsic deflection B Electrons deflect first to one side due to the field created by the impurity and deflect back when they leave the impurity since the field is opposite resulting in a side step. They however come out in a different band so this gives rise to an anomalous velocity through scattering rates times side jump. independent of impurity density Side jump scattering V imp (r) (Δso>ħ/τ)  ∝ λ* ∇ V imp (r) (Δso<ħ/τ) B Skew scattering Asymmetric scattering due to the spin-orbit coupling of the electron or the impurity. Known as Mott scattering. ~σ~1/n i V imp (r) (Δso>ħ/τ)  ∝ λ* ∇ V imp (r) (Δso<ħ/τ) A

7. 7 Spin Hall effect like-spin Take now a PARAMAGNET instead of a FERROMAGNET: Spin-orbit coupling “force” deflects like-spin particles I _ F SO _ _ _ V=0 non-magnetic Transverse spin-current generation in paramagnets without external magnetic fields by spin-depedent deflection of electrons Carriers with same charge but opposite spin are deflected by the spin-orbit coupling to opposite sides.

8. 8 Spin Hall Effect (Dyaknov and Perel 1971) Interband Coherent Response  (E F  ) 0 Occupation # Response `Skew Scattering‘ [Hirsch, S.F. Zhang] 2000 Intrinsic `Berry Phase’ [Murakami et al, Sinova et al] 2003 Influence of Disorder [Inoue et al, Misckenko et al, Chalaev et al…]

9. 9 First experimental observations at the end of 2004 Wunderlich, Kästner, Sinova, Jungwirth, cond-mat/0410295 PRL January 05 Experimental observation of the spin-Hall effect in a two dimensional spin-orbit coupled semiconductor system Co-planar spin LED in GaAs 2D hole gas: ~1% polarization Kato, Myars, Gossard, Awschalom, Science Nov 04 Observation of the spin Hall effect bulk in semiconductors Local Kerr effect in n-type GaAs and InGaAs: ~0.03% polarization (weaker SO-coupling, stronger disorder)

10. 10 I F SO _ majority minority V I _ F SO _ _ _ V=0 non-magnetic I spin F SO _ V non-magnetic MzMz M z =0 I=0 M z =0 magnetic optical detection AHE SHE SHE -1 ✓ ✓ Completing the spin dependent Hall family: SHE -1

11. 11 Valenzuela, S. O. & Tinkham, M, Nature‘06 x y z BzBz Electrical non-local spin valve detection by FM and by iSHE extrinsic SHE -1 in metals iSHE NL spin detection

12. 12 SHE -1 magnetoresistance measurement Kimura et al PRL 98, 156601 (2007) (SHE angle HIGHLY underestimated) Magnetoresistance signals from SHE and inverse SHE θ SH ~ 0.0037 Originally proposed by Shufeng Zhang 2000

13. 13 4 nm thick Pt 80 nm thick Cu 100 nm junction width Reason for the underestimate of θ SH by Kimura et al. The Cu shunts most of the charge current, so the charge current density in the Pt was much smaller than assumed. Courtesy of D.C. Ralph

14. 14 Spin pumping and SHE -1 Saitoh et al APL 06 Theory based on ref: Silsbee, Janossy, and Monod, PRB 19, 4382 (1979)

15. 15 sample layout Brune et al, Nature Physics insulating p-conducting n-conducting Mesoscopic intrinsic SHE -1 in HgTe theory

16. 16 I. Introduction: Basics of AHE: SOC origins and mechanism SHE phenomenology Spin Hall effect: the early days First proposals: from theory to experiment First observations of the extrinsic and intrinsic (optical) Inverse spin Hall effect: SHE as a spin current detector Direct iSHE in metals Spin pumping and iSHE Intrinsic mesoscopic SHE SHE-FET: first steps towards practicality (but perhaps not) Spin Hall injection and spin precession manipulation iSHE device with spin-accumulation modulation FMR measurement of SHE angle: giant and SHE as a spin current generator FMR and SHE angle Giant intrinsic SHE: Future MRAM technology? Conclusion Expecting the unexpected in the spin Hall effect: from fundamental to practical

17. 17 Problem: Rashba SO coupling in the Datta-Das SFET is used for manipulation of spin (precession) BUT it dephases the spin too quickly (DP mechanism). From DD-FET to new paradigm using SO coupling 1) Can we use SO coupling to manipulate spin AND increase spin-coherence? Can we detect the spin in a non-destructive way electrically? 3) Can this effect be exploited to create a spin-FET logic device? Use the persistent spin-Helix state or quasi-1D-spin channels and control of SO coupling strength (Bernevig et al 06, Weber et al 07, Wünderlich et al 09, Zarbo et al 10) Use AHE to measure injected current polarization electrically (Wünderlich, et al Nature Physics. 09, PRL 04) Spin-Hall AND-gate device (Wünderlich, Jungwirth, et al Science 2010) DD-FET

18. 18 V H2 I VbVb V H1 x V H2 VbVb V H1 x Spin Hall effect transistor: Wunderlich, Jungwirth, et al, Science 2010 SiHE inverse SHE iSHE transistor

19. 19 Spin-FET with two gates → logic AND function Wunderlich et al., Science.‘10 SHE transistor AND gate

20. 20 Electrical spin modulator B x =0 Olejník, K. et al. arxiv.org/abs/1202.0881, to appear in PRL (2012).

21. 21 I. Introduction: Basics of AHE: SOC origins and mechanism SHE phenomenology Spin Hall effect: the early days First proposals: from theory to experiment First observations of the extrinsic and intrinsic (optical) Inverse spin Hall effect: SHE as a spin current detector Direct iSHE in metals Spin pumping and iSHE Intrinsic mesoscopic SHE SHE-FET: first steps towards practicality (but perhaps not) Spin Hall injection and spin precession manipulation iSHE device with spin-accumulation modulation FMR measurement of SHE angle: giant and SHE as a spin current generator FMR and SHE angle Giant intrinsic SHE: Future MRAM technology? Conclusion Expecting the unexpected in the spin Hall effect: from fundamental to practical

22. 22 T. Kimura et al. PRL 98, 156601 (2007) Magnetoresistance signals from SHE and inverse SHE θ SH ~ 0.0037 K. Ando et al. PRL 101, 036601 (2008) Effect of inverse SHE on magnetic damping θ SH ~ 0.08 SHE angle measurements in Pt Vary by a Factor of 20 O. Mosendz et al. PRL 104, 046601 (2010), PRB 82, 214403 (2010) Magnetically-excited Py precession produces voltage by inverse SHE θ SH ~ 0.013 (assumes λ SF = 10 nm in Pt) Courtesy of D.C. Ralph

23. 23 DC-Detected Spin-Transfer-Driven Ferromagnetic Resonance (ST-FMR) Resonant resistance oscillations generate a DC voltage component by mixing S V mix DC circuitry Main source of signal at low bias: Related work: Tulapurkar et al., Nature 438, 339 (2005) Courtesy of D.C. Ralph

24. 24 Slonczewski torque: symmetric Lorentzian A. A. Tulapurkar, et al., Nature 438, 339 (2005). J. N. Kupferschmidt et al., PRB 74, 134416 (2006). A. A. Kovalev et al., PRB 75, 104403 (2007). V mix If both Slonczewski and out-of-plane spin- torque components are present then the FMR response is a simple sum of two contributions. Out-of-Plane torque: antisymmetric Lorentzian Out-of-Plane Torque Slonczewski Torque M free M fixed Accurate measurement of SHE angle Courtesy of D.C. Ralph FMR Peak Shape Analysis

25. 25 Spin torque FMR measurement of the SHE Spin currentin plane torque τ ST symmetric peak Oersted fieldperpendicular torque τ H antisymmetric peak The two driving forces induce oscillations with 90° phase difference DC readout of the FMR signal using the anisotropic magnetoresistance of Py Courtesy of D.C. Ralph

26. 26 Pt(1.5-15 nm)/Py(2-15 nm), room temperature + = Luqiao Liu et al., PRL 106, 036601 (2011) Spin torque FMR measurement of the SHE Results: θ SH = 0.068 ± 0.005 for Pt This is big! control tests Courtesy of D.C. Ralph

27. 27 Why is J S /J C ~ 0.06 is Big? The spin Hall angle is a relationship between current densities. To calculate the efficiency of total spin current generation, must take into account a difference in areas A = Lw t a = tw can be >> 1 even with θ SH ~ 0.06 With the spin Hall effect, the traversal of one electron through the sample can transfer more than angular momentum to a magnet! Courtesy of D.C. Ralph

28. 28 spin Hall effect in Ta ab initio calculation: θ SH (Ta) has opposite sign compared to θ SH (Pt) for highly resistive case, θ SH (Ta) can be very large Tanaka, T. et al, Phys. Rev. B 77, 165117 (2008) INTRINSIC spin Hall conductivity calculated for 4d, 5d elements

29. 29 ST-FMR induced by the SHE in Ta CoFeB/TaCoFeB/Pt antisymmetric peak, same sign (Oersted field) symmetric peaks, opposite sign (spin torque) J S /J C = 0.15 ± 0.04! Narrower linewidth – less added damping from Ta compared to Pt Courtesy of D.C. Ralph Liu et al. Science 336, 555 (2012)

30. 30 SHE as a source for spin current JcJc JsJs FM NM Spin Hall Device Conventional Magnetic Tunnel Junction J S and J C travel perpendicular paths J S and J C travel the same path What isin various metals? Courtesy of D.C. Ralph Text

31. 31 Using the Spin Hall Torque to Switch In-Plane-Polarized Magnetic Layers -- A 3-Terminal Device Ta strip 1 μm wide MTJ 100×300 nm 2 DC current in Ta strip to write Resistance measurement across the MTJ to read Liu et al. Science 336, 555 (2012)  Switch the magnetic moment using the SHE via an anti-damping mechanism  Use a magnetic tunnel junction to read out the magnetic orientation

32. 32 DC current induced switching Ramp-rate measurement of critical currents: I c0 = 2.0 mA E 0 ~ 46 k B T J S /J C ≈ 0.12 ± 0.03 agrees with ST-FMR and perpendicular switching measurements No barrier degradation, better read-out signal compared to conventional devices. Switching currents well below 100 μA should be possible. Liu et al. Science 336, 555 (2012) May 4th 2012 Courtesy of D.C. Ralph

33. 33 Spin Hall Effect: from fundamentals to applications SOC Effects SHE AHE AMR CIT Current induced torque CIT Current induced torque SHE -1 Intrinsic Extrinsic Optical Electrical Spin current detector Spin current generator FMR Spin Pumping Spin Caoloritronics Spin Caoloritronics STT SHE-MRAM?? Jungwirth, Wunderlich, Olejnik, Nat. Mat. 11,382 (2012)