Research fueled by: Forschungszentrum Jülich November 11 th, 2009 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 A road to next generation technologies through basic research: Nanoelectronics, spintronics, and materials control in multiband complex systems University of W ü rzburg Laurens Molenkamp, E. Hankiewiecz, et al University of Nottingham Bryan Gallagher, Richard Campion, et al.

2 Nanoelectronics, spintronics, and materials control by spin-orbit coupling I. Role of basic research in technology development Control of material and transport properties through spin-orbit coupling: II. Ferromagnetic semiconductors III. Tunneling anisotropic magnetoresistance IV. Anomalous Hall effect and spin-dependent Hall effects Spin-injection Hall effect device concept Spin based FET: old and new paradigm in charge-spin transport Theory expectations and modeling Experimental results Future challenges and perspectives Nanoelectronics, spintronics, and materials control in multiband complex systems through spin-orbit coupling

3 Nanoelectronics, spintronics, and materials control by spin-orbit coupling Circuit heat generation is one key limiting factor for scaling device speed Industry has been successful in doubling of transistor numbers on a chip approximately every 18 months (Moore’s law). Although expected to continue for several decades several major challenges will need to be faced. The need for basic research in technology development

4 Nanoelectronics, spintronics, and materials control by spin-orbit coupling International Technology Roadmap for Semiconductors Basic Research Inc. 1D systems Single electron systems (FETs) Spin dependent physics Ferromagnetic transport Molecular systems New materials Strongly correlated systems Nanoelectronics The need for basic research in technology development Nanoelectronics Research Initiative SWAN MIND INDEX WIN Advanced Micro Devices, Inc. IBM Corporation Intel Corporationation MICRON Technology, Inc.MICRON Te Texas Instruments Incorporatedexas Instruments Advanced Micro Devices, Inc. IBM Corporation Intel Corporationation MICRON Technology, Inc.MICRON Te Texas Instruments Incorporatedexas Instruments Sinova Texas A&M

5 Nanoelectronics, spintronics, and materials control by spin-orbit coupling I. Role of basic research in technology development Control of material and transport properties through spin-orbit coupling: Ferromagnetic semiconductors Tunneling anisotropic magnetoresistance Anomalous Hall effect and spin-dependent Hall effects Spin-injection Hall effect device concept Spin based FET: old and new paradigm in charge-spin transport Theory expectations and modeling Experimental results Future challenges and perspectives Nanoelectronics, spintronics, and materials control in multiband complex systems through spin-orbit coupling

6 Nanoelectronics, spintronics, and materials control by spin-orbit coupling Spin-orbit coupling interaction (one of the few echoes of relativistic physics in the solid state) This gives an effective interaction with the electron’s magnetic moment Consequences Effective quantization axis of the spin depends on the momentum of the electron. Band structure (group velocities, scattering rates, etc.) mixed strongly in multi-band systems If treated as scattering the electron gets asymmetrically scattered to the left or to the right depending on its “spin” Classical explanation (in reality it is quantum mechanics + relativity ) “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 and effective magnetic field Motion of an electron

7 Nanoelectronics, spintronics, and materials control by spin-orbit coupling Control of materials and transport properties via spin-orbit couplingAs Ga Mn New magnetic materials Nano- transport Spintronic Hall effects Magneto- transport Caloritronics Topological transport effects Effects of spin-orbit coupling in multiband systems

8 Nanoelectronics, spintronics, and materials control by spin-orbit coupling Nano- transport Spintronic Hall effects Magneto- transport Caloritronics Topological transport effects Effects of spin-orbit coupling in multiband systems Ferromagnetic Semiconductors Need true FSs not FM inclusions in SCs Mn Ga As Mn GaAs - standard III-V semiconductor + Group-II Mn - dilute magnetic moments & holes (Ga,Mn)As - ferromagnetic semiconductor Control of materials and transport properties via spin-orbit coupling

9 Nanoelectronics, spintronics, and materials control by spin-orbit coupling As Ga Mn New magnetic materials Nano- transport Spintronic Hall effects Magneto- transport Caloritronics Topological transport effects Effects of spin-orbit coupling in multiband systems Transition to a ferromagnet when Mn concentration > % DOS spin ↓ spin ↑ valence band As-p-like holes Mn Ga As Mn >2% Mn EFEF ferromagnetism onset near MIT when localization length is longer than Mn-Mn spacing. Zener type model Control of materials and transport properties via spin-orbit coupling Jungwirth, Sinova, et al RMP 06

10 Nanoelectronics, spintronics, and materials control by spin-orbit coupling As Ga Mn New magnetic materials Nano- transport Spintronic Hall effects Magneto- transport Caloritronics Topological transport effects Effects of spin-orbit coupling in multiband systems Transition to a ferromagnet when Mn concentration increases DOS spin ↓ spin ↑ valence band As-p-like holes Mn Ga As Mn >2% Mn EFEF ferromagnetism onset near MIT when localization length is longer than Mn-Mn spacing. Zener type model Mn Ga As Mn Ferromagnetic Ga 1-x Mn x As x>1.5% Ferromagnetism mediated by delocalized band states: polarized carriers with large spin-orbit coupling pxpx pypy ∇V∇V H so p s Many useful properties FM dependence on doping Low saturation magnetization What are the consequences of the strong spin-orbit coupling of the carriers “gluing” the localized Mn moments ? Control of materials and transport properties via spin-orbit coupling

11 Effects of spin-orbit coupling in multiband systems Nanoelectronics, spintronics, and materials control by spin-orbit coupling As Ga Mn New magnetic materials Nano- transport Spintronic Hall effects Magneto- transport Caloritronics Topological transport effects Effects of spin-orbit coupling in multiband systems Mn Ga As Mn Ferromagnetic Ga 1-x Mn x As x>1.5% Ferromagnetism mediated by delocalized band states: polarized carriers with large spin-orbit coupling pxpx pypy ∇V∇V H so p s What are the consequences of the strong spin-orbit coupling of the carriers “gluing” the localized Mn moments ? Many useful properties FM dependence on doping Low saturation magnetization Control of magnetic anisotropy Strain & SO ↓ Strain induces changes in the band structure and, in turn, change the ferromagnetic easy axis. Piezoelectric devices: fast magnetization switching Wunderlich, Sinova, et al PRB 06 Tensile strainCompressive strain M → M → Control of materials and transport properties via spin-orbit coupling

12 Effects of spin-orbit coupling in multiband systems Nanoelectronics, spintronics, and materials control by spin-orbit coupling As Ga Mn New magnetic materials Nano- transport Spintronic Hall effects Magneto- transport Caloritronics Topological transport effects Magneto-transport in GaMnAs G(T)MR ~ 100% MR effect Control of materials and transport properties via spin-orbit coupling Fert, Grunberg et al. 1988

13 Nanoelectronics, spintronics, and materials control by spin-orbit coupling As Ga Mn New magnetic materials Nano- transport Spintronic Hall effects Magneto- transport Caloritronics Topological transport effects Effects of spin-orbit coupling in multiband systems Magneto-transport in GaMnAs G(T)MR ~ 100% MR effect Control of materials and transport properties via spin-orbit coupling Exchange split bands: σ~ TDOS(↑↓) < TDOS(↑↑) Fert, Grunberg et al. 1988

14 Nanoelectronics, spintronics, and materials control by spin-orbit coupling As Ga Mn New magnetic materials Nano- transport Spintronic Hall effects Magneto- transport Caloritronics Topological transport effects Effects of spin-orbit coupling in multiband systems Magneto-transport in GaMnAs TMR ~ 100% MR effect Control of materials and transport properties via spin-orbit coupling Exchange split bands: σ~ TDOS(↑↓) < TDOS(↑↑) TAMR Tunneling Anisotropic Magnetoresistance discovered in (Ga,Mn)As Gold et al. PRL’04 Au σ ~ TDOS (M) → TAMR can be enormous depending on doping Now discovered in FM metals !! Ruster, JS, et al PRL05

15 Nanoelectronics, spintronics, and materials control by spin-orbit coupling As Ga Mn New magnetic materials Nano- transport Spintronic Hall effects Magneto- transport Caloritronics Topological transport effects Effects of spin-orbit coupling in multiband systems Control of materials and transport properties via spin-orbit coupling Magneto-transport in GaMnAs TAMR Tunneling Anisotropic Magnetoresistance discovered in (Ga,Mn)As Gold et al. PRL’04 Au σ ~ TDOS (M) → TAMR can be enormous depending on doping Ruster, JS, et al PRL05 Now discovered in FM metals !!

16 Nanoelectronics, spintronics, and materials control by spin-orbit coupling Effects of spin-orbit coupling in multiband systems As Ga Mn New magnetic materials Nano- transport Spintronic Hall effects Magneto- transport Caloritronics Topological transport effects Control of materials and transport properties via spin-orbit coupling

17 Nanoelectronics, spintronics, and materials control by spin-orbit coupling Effects of spin-orbit coupling in multiband systems As Ga Mn New magnetic materials Nano- transport Spintronic Hall effects Magneto- transport Caloritronics Topological transport effects Control of materials and transport properties via spin-orbit coupling Anomalous Hall effects I F SO majority minority V Nagaosa, Sinova, Onoda, MacDonald, Ong, RMP 10

18 Nanoelectronics, spintronics, and materials control by spin-orbit coupling Simple electrical measurement of out of plane magnetization 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

19 Nanoelectronics, spintronics, and materials control by spin-orbit coupling 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

20 Nanoelectronics, spintronics, and materials control by spin-orbit coupling Contributions understood in simple metallic 2D models Semi-classical approach: Gauge invariant formulation Sinitsyn, Sinvoa, et al PRB 05, PRL 06, PRB 07 Kubo microscopic approach: in agreement with semiclassical Borunda, Sinova, et al PRL 07, Nunner, JS, et al PRB 08 Non-Equilibrium Green’s Function (NEGF) microscopic approach Kovalev, Sinova et al PRB 08, Onoda PRL 06, PRB 08

21 Nanoelectronics, spintronics, and materials control by spin-orbit coupling Review of AHE (to appear in RMP 2010), Nagaosa, Sinova, Onoda, MacDonald, Ong Phenomenological scaling regimes of AHE Scattering independent regime Q: is the scattering independent regime dominated by the intrinsic AHE? 1.A high conductivity regime for σ xx >10 6 (Ωcm) -1 in which AHE is skew dominated 2.A good metal regime for σ xx ~ (Ωcm) -1 in which σ xy AH ~ const 3.A bad metal/hopping regime for σ xx 1 Skew dominated regime

22 Nanoelectronics, spintronics, and materials control by spin-orbit coupling n, q n’≠n, q Intrinsic AHE approach in comparing to experiment: phenomenological “proof” DMS systems (Jungwirth et al PRL 2002, Jungwirth, Sinova, et al APL 03) layered 2D ferromagnets e.g. SrRuO 3 ferromagnets (Taguchi et al, Science 01, Fang et al, Science 03) CuCrSeBr compounds ( Lee et al, Science 04) Fe (Yao et al PRL 04) Experiment: σ AH ∼ 1000 (Ω cm) -1 Theory: σ AH ∼ 750 (Ω cm) -1 AHE in Fe AHE in GaMnAs

23 Nanoelectronics, spintronics, and materials control by spin-orbit coupling Valenzuela et al Nature 06 Inverse SHE Anomalous Hall effect: more than meets the eye Wunderlich, Kaestner, Sinova, Jungwirth PRL 04 Kato et al Science 03 Intrinsic Extrinsic V Mesoscopic Spin Hall Effect Intrinsic Brune,Roth, Hankiewicz, Sinova, Molenkamp, et al 09 Wunderlich, Irvine, Sinova, Jungwirth, et al, Nature Physics 09 Spin-injection Hall Effect Anomalous Hall Effect I _ FSOFSO FSOFSO _ _ majority minority V Spin Hall Effect I _ FSOFSO FSOFSO _ _ V

24 Nanoelectronics, spintronics, and materials control by spin-orbit coupling I. Role of basic research in technology development Control of material and transport properties through spin-orbit coupling: Ferromagnetic semiconductors Tunneling anisotropic magnetoresistance Anomalous Hall effect and spin-dependent Hall effects Spin-injection Hall effect device concept Spin based FET: old and new paradigm in charge-spin transport Theory expectations and modeling Experimental results Future challenges and perspectives Nanoelectronics, spintronics, and materials control in multiband complex systems through spin-orbit coupling Wunderlich, Irvine, Sinova, Jungwirth, et al, Nature Physics 09 Spin-injection Hall Effect

25 Nanoelectronics, spintronics, and materials control by spin-orbit coupling Can we achieve direct spin polarization injection, detection, and manipulation by electrical means in an all paramagnetic semiconductor system? Long standing paradigm: Datta-Das FET Unfortunately it has not worked : no reliable detection of spin-polarization in a diagonal transport configuration No long spin-coherence in a Rashba spin- orbit coupled system (Dyakonov-Perel mechanism) Towards a realistic spin-based non-magnetic FET device

26 Nanoelectronics, spintronics, and materials control by spin-orbit coupling 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). New paradigm using SO coupling: SO not so bad for dephasing 1) Can we use SO coupling to manipulate spin AND increase spin-coherence? Can we detect the spin in a non-destructive way electrically? Use the persistent spin-Helix state and control of SO coupling strength (Bernevig et al 06, Weber et al 07, Wünderlich et al 09) Use AHE to measure injected current polarization at the nano-scale electrically (Wünderlich, et al 09, 04)

27 Nanoelectronics, spintronics, and materials control by spin-orbit coupling Spin-dynamics in 2D electron gas with Rashba and Dresselhauss SO coupling a 2DEG is well described by the effective Hamiltonian: Something interesting occurs when spin along the [110] direction is conserved long lived precessing spin wave for spin perpendicular to [110] The nesting property of the Fermi surface: Bernevig et al PRL 06, Weber et al. PRL 07 Schliemann et al PRL 04

28 Nanoelectronics, spintronics, and materials control by spin-orbit coupling Effects of Rashba and Dresselhaus SO coupling  = -  [110] _ k y [010] k x [100]  > 0,  = 0 [110] _ k y [010] k x [100]  = 0,  < 0 [110] _ k y [010] k x [100]

29 Nanoelectronics, spintronics, and materials control by spin-orbit coupling Spin-dynamics in 2D systems with Rashba and Dresselhauss SO coupling For the same distance traveled along [1-10], the spin precesses by exactly the same angle. [110] _ _

30 Nanoelectronics, spintronics, and materials control by spin-orbit coupling Spatial variation scale consistent with the one observed in SIHE Spin-helix state when α ≠ β Wunderlich, Irvine, Sinova, Jungwirth, et al, Nature Physics 09

31 Nanoelectronics, spintronics, and materials control by spin-orbit coupling Type (i) contribution much smaller in the weak SO coupled regime where the SO- coupled bands are not resolved, dominant contribution from type (ii) Crepieux et al PRB 01 Nozier et al J. Phys. 79 Two types of contributions: i)S.O. from band structure interacting with the field (external and internal) Bloch electrons interacting with S.O. part of the disorder Lower bound estimate of skew scatt. contribution AHE contribution to Spin-injection Hall effect Wunderlich, Irvine, Sinova, Jungwirth, et al, Nature Physics 09

32 Nanoelectronics, spintronics, and materials control by spin-orbit coupling Local spin-polarization → calculation of AHE signal Weak SO coupling regime → extrinsic skew-scattering term is dominant Lower bound estimate Spin-injection Hall effect: theoretical expectations

33 Nanoelectronics, spintronics, and materials control by spin-orbit coupling 2DHG 2DEG e h e e ee e h h h h h VsVs VdVd VHVH Spin-injection Hall effect device schematics For our 2DEG system: Hence α ≈ -β

34 Nanoelectronics, spintronics, and materials control by spin-orbit coupling Spin-injection Hall device measurements trans. signal σoσoσoσo σ+σ+σ+σ+ σ-σ-σ-σ- σoσoσoσo VLVL

35 Nanoelectronics, spintronics, and materials control by spin-orbit coupling Spin-injection Hall device measurements trans. signal σoσoσoσo σ+σ+σ+σ+ σ-σ-σ-σ- σoσoσoσo VLVL SIHE ↔ Anomalous Hall Local Hall voltage changes sign and magnitude along a channel of 6 μm

36 Nanoelectronics, spintronics, and materials control by spin-orbit coupling Further experimental tests of the observed SIHE

37 Nanoelectronics, spintronics, and materials control by spin-orbit coupling Non public slides deleted. Please contact Sinova if interested

38 Nanoelectronics, spintronics, and materials control by spin-orbit coupling Summary of spin-injection Hall effect  Basic studies of spin-charge dynamics and Hall effect in non-magnetic systems with SO coupling  Spin-photovoltaic cell: solid state polarimeter on a semiconductor chip requiring no magnetic elements, external magnetic field, or bias  SIHE can be tuned electrically by external gate and combined with electrical spin-injection from a ferromagnet (e.g. Fe/Ga(Mn)As structures)

39 Nanoelectronics, spintronics, and materials control by spin-orbit coupling I. Role of basic research in technology development Control of material and transport properties through spin-orbit coupling: Ferromagnetic semiconductors Tunneling anisotropic magnetoresistance Anomalous Hall effect and spin-dependent Hall effects Spin-injection Hall effect device concept Spin based FET: old and new paradigm in charge-spin transport Theory expectations and modeling Experimental results Future challenges and perspectives Nanoelectronics, spintronics, and materials control in multiband complex systems through spin-orbit coupling

40 Nanoelectronics, spintronics, and materials control by spin-orbit coupling Effects of spin-orbit coupling in multiband systems As Ga Mn New magnetic materials Nano- transport Spintronic Hall effects Magneto- transport Caloritronics Topological transport effects Theory techniques and development in our studies First principles DFT (LDA,LDA+U) Tight-binding Phenomenological k.p models Exact diagonalization study of disorder Non-equilibrium Green’s function formalism in real space Landuaer-Buttiker formalism phenomenological k.p DOS NEGF tunneling formalism Semiclassical transport NEGF/Keldysh formalism Kubo microscopic method Numerical diagonalization studies

41 Nanoelectronics, spintronics, and materials control by spin-orbit coupling Non public slides deleted. Please contact Sinova if interested

42 Nanoelectronics, spintronics, and materials control by spin-orbit coupling Allan MacDonald U of Texas Tomas Jungwirth Texas A&M U. Inst. of Phys. ASCR U. of Nottingham Joerg Wunderlich Cambridge-Hitachi Laurens Molenkamp Würzburg Xiong-Jun Liu Texas A&M U. Mario Borunda Texas A&M Univ. Harvard Univ. Nikolai Sinitsyn Texas A&M U. U. of Texas LANL Alexey Kovalev Texas A&M U. UCLA Liviu Zarbo Texas A&M Univ. Xin Liu Texas A&M U. Ewelina Hankiewicz (Texas A&M Univ.) Würzburg University Sinova’s group Principal Collaborators Gerrit Bauer TU Delft Bryan Gallagher U. of Nottingham and many others

43 Nanoelectronics, spintronics, and materials control by spin-orbit coupling

44 Nanoelectronics, spintronics, and materials control by spin-orbit coupling Mesoscopic SHE

Nanoelectronics, spintronics, and materials control by spin-orbit coupling Advantages: No worries about spin-current definition. Defined in leads where SO=0 Well established formalism valid in linear and nonlinear regime Easy to see what is going on locally Fermi surface transport Charge based measurements of SHE: SHE -1 Non-equilibrium Green’s function formalism (Keldysh-LB) 59 Hankiewiecz,JS, Molenkamp et al PRB 05

Nanoelectronics, spintronics, and materials control by spin-orbit coupling 60 H-bar structures for detection of Spin-Hall-Effect (electrical detection through inverse SHE) E.M. Hankiewicz et al., PRB 70, R (2004)

Nanoelectronics, spintronics, and materials control by spin-orbit coupling sample layout Molenkamp et al (unpublished) insulating p-conducting n-conducting Strong SHE -1 in HgTe theory

48 Nanoelectronics, spintronics, and materials control by spin-orbit coupling Strong SHE -1 in HgTe Molenkamp et al (unpublished)

49 Nanoelectronics, spintronics, and materials control by spin-orbit coupling Strong SHE -1 in HgTe Molenkamp et al (unpublished) Narrower sample (non- inverted)