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Magneto-transport anisotropy phenomena in GaMnAs and beyond

Magneto-transport anisotropy phenomena in GaMnAs and beyond. Tom as Jungwirth. Universit y of Nottingham Bryan Gallagher, Richard Campion, Kevin Edmonds , Andrew Rushforth, Tom Foxon, et al. Institute of Physics ASCR

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Magneto-transport anisotropy phenomena in GaMnAs and beyond

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  1. Magneto-transport anisotropy phenomena in GaMnAs and beyond Tomas Jungwirth University of Nottingham Bryan Gallagher, Richard Campion, Kevin Edmonds, Andrew Rushforth, Tom Foxon, et al. Institute of Physics ASCR Karel Výborný, Alexander Shick. Jan Zemen, Jan Mašek, Vít Novák,KamilOlejník,, et al. University & Hitachi Cambridge Jorg Wunderlich, Andrew Irvine,Elisa de Ranieri, Byonguk Park, etal. Texas A&M Jairo Sinova, et al. University of Texas Allan MaDonald, Maxim Trushin,et al. University of Wuerzburg Charles. Gould, Laurens Molenkamp, et al.

  2. Observations made from studies of AMR phenomena in GaMnAs (outline) 1. More than just bulk AMR in ohmic devices: TAMR, CBAMR 2. In DMSs bulk AMR has the simplest intuitive picture 3. TAMR and CBAMR are transferable to room-T metal FMs & AFMs

  3. Experimental observation of (ohmic) AMR magnetization Lord Kelvin 1857 current AMR sensors: dawn of spintronics Inductive read elements Magnetoresistive read elements 1980’s-1990’s Now often replaced by GMR or TMR but still extensively used in e.g. automotive industry Problems with small magnitude and scaling

  4. ss sd ss sd itinerant 4s: no exch.-split no SO localized 3d: exch. split SO coupled Theory of AMR: current response to magnetization via spin-orbit coupling Model for transition metal FMs: Smit 1951 ? Miscroscopic theory: relativistic LDA & Kubo formula theory experiment FeNi Banhart&Ebert EPL‘95

  5. 7% 2.5% >1.5% Mn ~ 1% x=0.07% Renewed research interest in AMR due to FS like (Ga,Mn)As Ohno. Science ’98 MnGa acceptor: electrical conduction similar to conventional p-doped GaAs metallic ~0.1% Mn insulating <<0.1% Mn Jungwirth et al. PRB ’07

  6. >1% Mn ~  h+  h+ Renewed research interest in AMR due to FS like (Ga,Mn)As (Ga,Mn)As Mn moment: Ferromagnetism reminiscent of conventional metal band FMs (Fe, Co, Ni,..) d/dT~cv Ni Tc (Ga,Mn)As ferromagnetic Tc Novak et al. PRL ’08

  7. Renewed research interest in AMR due to FS like (Ga,Mn)As AMR’s of order ~1-10%: - routine characterization tool - semi-quantitatively described assuming scattering of valence-band holes Baxter et al. PRB ’02, Jungwirth et al. APL’02, ‘03

  8. Magnetic anisotropies in (Ga,Mn)As valence band degenerate HH bands and LH bands in GaAs: anisotropic surface and spin-texture due to crystal and SO coupling in As(Ga) p-orbitals j=3/2 HH HH & LH Fermi surfaces exchange-split HH bands and LH bands in (Ga,Mn)As: anisotropic due to crystal, SO coupling and FM exchange field HH HH M Dietl et al. PRB ’01, Abolfath et al. PRB ‘01

  9. DOS Simple direct link between band structure and transport Magnitude, control, and tuneability of MR Complexity of the device design SET Chemical potential  CBAMR micro-structures MTJ Tunneling DOS  TAMR heterostructures bulk Scattering lifetimes  ohmic AMR Resistor

  10. TAMR: spectroscopy of tunneling DOS anisotropy k - resolved tunneling DOS electrode barrier GaMnAs Vbias Binpl M Giddings et al. PRL ’04 M Selectivity tuned by choice of barrier, counter-electrode, or external fields

  11. Au AlOx GaMnAs TAMR: spectroscopy of tunneling DOS anisotropy Gould et al. PRL ’04 M M Non-selective barrier and counter-electrode  only a few % TAMR

  12. p-(Ga,Mn)As n-GaAs:Si TAMR: spectroscopy of tunneling DOS anisotropy M M Giraud et al. APL ’05, Sankowski et al. PRB’07, Ciorga et al.NJP’07, Jerng JKPS ‘09 Very selective p-n Zener diode MTJs Binpl Giraud et al. Spintech ’09

  13. p-(Ga,Mn)As n-GaAs:Si TAMR: spectroscopy of tunneling DOS anisotropy M M Extra-momentum due to Lorentz force during tunneling Very selective p-n Zener diode MTJs Binpl Giraud et al. Spintech ’09

  14. Q VD Source Drain Gate VG CBAMR: M-dependent electro-chemical potentials in a FM SET Wunderlich et al. PRL ’06 [110] M  [100] [110] [010] magnetic electric & control of CB oscillations

  15. Huge MRs controlled by low-gate-voltage: likely the most sensitive spintronic transistorsto date Wunderlich et al. PRL ’06 Schlapps et al. PRB ‘09

  16. DOS Simple direct link between band structure and transport Chemical potential  CBAMR SET Tunneling DOS  TAMR MTJ Scattering lifetimes  AMR Resistor

  17. - - Simplicity of the microscopic picture of AMR in (Ga,Mn)As SET CBAMR,TAMR: SO & FM polarized bands M MTJ MnGa MnGa ohmic AMR: main impurities – FM polarized random MnGa can consider bands with SO coupling only Resistor

  18. Simplicity of the microscopic physical picture in (Ga,Mn)As current SET CBAMR: only el.-chem potentials  no M vs current term M cryst. axis TAMR: current direction is cryst. distinct  inseparable M vs current term current M MTJ cryst. axis current M AMR: M vs current (non-crystalline) term can be separated and dominates in (Ga,Mn)As cryst. axis Resistor

  19. Key mechanism for AMR in (Ga,Mn)As: FM impurities & SO carriers in non-cryst.-like spherical bands KL Hamiltonian in spherical approximation MGa Heavy holes current Electro-magnetic impurity potential of MnGa acceptor Rushforth PRL’07, Trushin et al. PRB ‘09, Vyborny et al. PRB ‘09

  20. - - Pure magnetic MnGa impiruties: positive AMR, Backward-scattering matrix elements current

  21. - - Electro-magnetic MnGa impiruties: negative AMR, current Backward-scattering matrix elements

  22. p [1021 cm-3] AMR 202-1 AMR= - 244-2 4+1 - - Electro-magnetic MnGa impiruties: negative AMR,  ~ screened Coulomb potential  current all scatt. backward scatt.

  23. p [1021 cm-3] AMR 202-1 AMR= - 244-2 4+1 - - Electro-magnetic MnGa impiruties: negative AMR,  ~ screened Coulomb potential  current all scatt. backward scatt.

  24. current current Negative and positive and crystalline AMR in R&D 2D system Dresselhaus Rashba

  25. AMR in 2D R&D and 3D KL system from exact solution to integral Boltzmann eq. contains only cos and sin harmonics analytical solution to the integral Boltzmann eq.

  26. ss sd ss sd itinerant 4s: no exch.-split no SO localized 3d: exch. split SO coupled AMR in transition/noble metals Model for transition metal FMs: Smit 1951 ? Miscroscopic theory: relativistic LDA & Kubo formula theory experiment FeNi Banhart&Ebert EPL‘95

  27. TAMR and CBAMR predictions for metals ab intio theory Wunderlich et al., PRL ’06,Shick, et al, PRB '06 Anisotropy in chemical potential Anisotropy in DOS

  28. Experimental observation of large and bias dependent TAMR Shick et al PRB ’06, Parkin et al PRL ‘07, Park et al PRL '08 ab intio theory TAMR in SO-coupled FMs experiment Park et al PRL '08

  29. Experimental observation of CBAMR in metals Bernand-Mantel et al Nat. Phys.‘09

  30. spontaneous moment magnetic susceptibility spin-orbit coupling Optimizing TAMR/CBAMR in transition-metal structures Consider TM combinations containing Mn e.g. FM Mn/W  upto ~100% TAMR Shick, et al PRB ‘08 But most transition/noble metals with Mn are AFMs!

  31. AFM spintronics Zero stray field in compensated AFMs Ultrafast dynamics of spin excitations

  32. Mn2Au spin-dn spin-up Predicted strong AFM with no frustration

  33. MnIr spin-dn spin-up Conventional AFM

  34. Magnetic moments (mB) Element specific MAE (meV) *MAE accuracy ~0.01 meV LocalMn-atom moment contributes only little to the MAE Most of the MAE comes from zero moment Au, Ir atoms

  35. Each of localized 3d(Mn)- sublattices  induces the magnetic moment on 5d-site Strong 5d-SOC produces the MAE Summing over 3d(Mn)- sublattices  - non-zero! = 0 complies with t-reversal symmetry of AFM Strong 5d-SOC x 3d(Mn)-exchange filed x local susceptibility produce the MAE

  36. TAMR and CBAMR ADOS([q,f]-[q’,f’])= [DOS[q, f]–DOS[q’,f’]]/ DOS[q’,f’] andATDOS= [TDOS[q, f]–TDOS[q’,f’]]/TDOS[q’,f’] Hard [001]-to-easy [110] ADOS([001]-[110]) ~ 50 % ATDOS([001]-[110]) ~ 20 % =Ef[001]-Ef[110]=-2.5 mV Sizable TAMR and CBAMR in AFMs

  37. Effect of in-plane strain – moment reorientations and TAMR [010] Easy [110]  Easy [010] at <1% strain [100] 1% strain Strain-induced TAMR ADOS([110]-[010]) ~ 20 % ATDOS([110]-[010]) ~ 20 %

  38. GMR/TMR and spin-torque relay on coherence & quality of interfaces  in principle possible but likely very difficult to build AFM spintronics on these effects Instead bulid AFM spintronics on a set of magnetic anisotropy phenomena Piezo- (or other) electric control of AF moment orientation & TAMR (CBAMR)

  39. Observations made from studies of AMR phenomena in GaMnAs (summary) 1. More than just bulk AMR in ohmic devices: TAMR, CBAMR 2. In DMSs bulk AMR has the simplest intuitive picture 3. TAMR and CBAMR are transferable to room-T metal FMs & AFMs

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