Recent developments in spintronic

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1 Recent developments in spintronic Tomas Jungwirth nstitute of Physics ASCR, Prague University of Nottingham in collaboration with Hitachi Cambridge, University of Texas, Texas A&M University

2 - Spintronics ics in footsteps of classical electronics from resistors and diodes to transistors - Spintronics - ferromagnetism & spin-orbit coupling

3 Resistor classical spintronic external manipulation of charge & spin internal communication between charge & spin e - replaced sensors in magnetic storage

4 Non-relativistic many-body e - Pauli exclusion principle & Coulomb repulsion Ferromagnetism total wf antisymmetric = orbital wf antisymmetric * spin wf symmetric (aligned) FERO MAG NET Robust (can be as strong as bonding in solids) Strong coupling to magnetic field (weak fields = anisotropy fields needed only to reorient macroscopic moment)

5 Relativistic "single-particle" e - Spin-orbit coupling (Dirac eq. in external field V(r) & 2nd-order in v /c around non-relativistic limit) V B eff s p r H r r = s SO B eff 1 = 2 2m c Beff 2 r ( V ) p B ex B eff B ex + B eff FM without SO-coupling GaAs valence band As p-orbitals large SO GaMnAs valence band tunable FM & large SO

6 AMR Ferromagnetism: sensitivity to magnetic field SO-coupling: anisotropies in Ohmic transport characteristics GaMnAs M <100> M <010> Band structure depends on M

7 Diode classical spin-valve TMR Based on ferromagnetism only no (few) spin-up DOS available at E F large spin-up DOS available at E F

8 Tunneling AMR: anisotropic tunneling DOS due to SO-coupling (Ga,Mn)As MRAM Au Au [100] [100] [010] [100] [010] [010] - no exchange-bias needed - spin-valve with ritcher phenomenology than TMR Gould, Ruster, Jungwirth, et al., PRL '04, '05

9 Wavevector dependent tunnelling probabilityt (k y, k z ) in GaMnAs Red high T; blue low T. y thin film j t Magnetisation in plane Magnetization perp. to plane z x y constriction z j t x Magnetization in-plane Giddings, Khalid, Jungwirth Wunderlich et al., PRL '05

10 TAMR in metals ab-initio calculations Shick, Maca, Masek, Jungwirth, PRB '06 NiFe TAMR TMR Bolotin,Kemmeth, Ralph, cond-mat/ TMR ~TAMR >>AMR Viret et al., cond-mat/ Fe, Co break junctions TAMR >TMR

11 Spintronic transistor - magnetoresistance controlled by gate voltage D S G Narrow channel & side gate - to enhance transistor action in this highly-doped semiconductor Ferromagnetic semiconductor - natural choice of material Coulomb Blockade AMR: SET & anisotropic chemical potential due to SO-coupling

12 Source Q V D Single Electron Transistor Drain V g = 0 Gate V G U 2 e 2C Q ' ' = dq VD( Q )&VD = Q / CΣ U = Σ 0 > k B T Coulomb blockade V g 0 ( Q + Q ) U = = 2C 2 0 &Q0 C V G G Σ Q 2C 2 Σ Q ind 0 = (n+1/2)( n+1/2)e Q ind Q ind Q0ind = ne Q=ne - discrete Q 0 =C g V g - continuous Q 0 =-ne blocked ΔE C n-1 n n+1 n+2 e 2 /2C Σ Q 0 =-(n+1/2)e open

13 Coulomb blockade anisotropic magnetoresistance Spin-orbit coupling r M chemical potential depends on chemical potential U If lead and dot different (different carrier concentrations in our (Ga,Mn)As SET) = Q ' dq VD 0 ( Q ' r QΔμ( M ) r r ) + & Δμ( M ) = μl( M ) μd( e r M ) U = ( Q + Q 2C Σ 0 ) 2 &Q 0 = C G [V G + V M ( r M )] &V M = r Δμ( M ) e C C Σ G electric & magnetic control of Coulomb blockade oscillations

14 Wunderlich, Jungwirth, Kaestner, et al., preprint CBAMR if change of Δμ(M) Δμ ~ e 2 /2C Σ In (Ga,Mn)As( ~ mev (~ 10 Kelvin) In room-t ferromagnet Δμ(M) ~100K Δμ change ofo Programmable logic with CBAMR p- or n-type FET-like transistor in a single nano-sized CBAMR device (a) R C [ MΩ ] V G0 0 1 R C [ MΩ ] M 0 M V G [ V ] magnetic non-volatile mode electric mode V G [ V ] V G1

15 SPIN HALL EFFECT no ferromagnetism, spin-orbit coupling only all-electric spintronics F SO F SO non-magnetic I V=0 Spin-current generation in non-magnetic systems without applying external magnetic fields lateral LED CP photon emission I lateral LED Spin accumulation without charge accumulation excludes simple electrical detection

16 Intrinsic SHE: : present in perfect crystal and when spin-orbit coupling >> impurity scattering rate Murakami, Nagaosa, Zhang, Science '03 Sinova, Culcer, Niu, Sinitsyn, Jungwirth, MacDonald, Phys. Rev. Lett. '04 r Heuristic picture H r = s r SO B eff 1 2 2m c Beff = 2 ( V ) r p =0

17 Microscopic theory and some interpretation experimentally detected spin * velocity non-conserving (ambiguous) theoretical quantity S z edge L so ~ j z bulk t so t so =h/δ so : (intrinsic) spin-precession time L so =v F t so : spin-precession length Nomura, Wunderlich, Sinova, Kaestner, MacDonald, Jungwirth, Phys. Rev. B '05

18 Spin and Anomalous Hall effects Spin-orbit coupling force deflects like-spin particles F SO minority F SO majority I ρ = R B + H 0 4πR s M V Simple electrical measurement of magnetization InMnAs

19 Intrinsic AHE approach explains many experiments (Ga,Mn)As systems [Jungwirth et al. PRL 02, APL 03, Chun et al. condmat/ ] Fe [Yao, Kleinman, MacDonald, Sinova, Jungwirth et al PRL 04] Co [Kotzler and Gil PRB 05] Layered 2D ferromagnets such as SrRuO3 and pyrochlore ferromagnets [Onoda and Nagaosa, J. Phys. Soc. Jap. 01,Taguchi et al., Science 01, Fang et al Science 03, Shindou and Nagaosa, PRL 01] Ferromagnetic spinel CuCrSeBr [Lee et al. Science 04]

20 Spintronics with spin-orbit coupling New effects and rich phenomenology (TAMR,CBAMR) As strong effects as effects based on FM only p-type (Ga,Mn)As( ideal systems - better understanding of old effects (AMR, AHE), searching for new generic effects Spintronic transistors (CBAMR-SET) and spin generation and manipulation with electric fields only (SHE) p 1.5 μm channel n n y z x SHE microchip, 100μA high-field lab. equipment 100 A

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