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Spintronic transistors: magnetic anisotropy and direct charge depletion concepts

Spintronic transistors: magnetic anisotropy and direct charge depletion concepts. Tom as Jungwirth. Universit y of Nottingham Bryan Gallagher, Tom Foxon, Richard Campion, Kevin Edmonds, Andrew Rushforth, et al. Institute of Physics ASCR

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Spintronic transistors: magnetic anisotropy and direct charge depletion concepts

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  1. Spintronic transistors: magnetic anisotropy and direct charge depletion concepts Tomas Jungwirth University of Nottingham Bryan Gallagher, Tom Foxon, Richard Campion, Kevin Edmonds, Andrew Rushforth, et al. Institute of Physics ASCR Alexander Shick, Karel Výborný, Jan Zemen, Jan Masek, Vít Novák,Kamil Olejník, et al. Hitachi Cambridge, Univ. Cambridge Jorg Wunderlich, Andrew Irvine, David Williams, Elisa de Ranieri, Byonguk Park, Sam Owen, etal. Texas A&M Jairo Sinova, et al. University of Texas Allan MaDonald, et al.

  2. Electric field controlled spintronics HDD, MRAM controlled by Magnetic field Spintronic Transistor Low-V 3-terminal devices STT MRAM spin-polarized charge current 1) indirect via magnetic anisotropy 2) direct charge depletion effects

  3. Au AMR TMR parallel state FM exchange int.: Spin-orbit int.: antiparallel state TAMR FM exchange int.: Discovered in GaMnAs Gould et al. PRL’04

  4. Bias-dependent magnitude and sign of TAMR Shick et al PRB ’06, Parkin et al PRL ‘07, Park et al PRL '08 ab intio theory TAMR is generic to SO-coupled FMs experiment Park et al PRL '08

  5. spontaneous moment magnetic susceptibility spin-orbit coupling Optimizing TAMR in transition-metal structures Consider uncommon TM combinations e.g. Mn/W  voltage-dependent upto ~100% TAMR Shick, et al PRB ‘08

  6. Q VD Source Drain M [010] [110] Gate [100] VG [110] [010] Devices utilizing M-dependent electro-chemical potentials: FM SET SO-coupling  (M) ~ mV in GaMnAs ~ 10mV in FePt magnetic electric & control of CB oscillations Wunderlich et al, PRL '06

  7. CB oscillations shifted by changing M(CBAMR) (Ga,Mn)As nano-constriction SET Electric-gate controlled magnitude and sign of magnetoresistance  spintronic transistor & Magnetization controlled transistor characteristic (p or n-type)  programmable logic

  8. Ga Mn As Mn Ferromagnetic semiconductor GaAs:Mn Exchange-split, SO-coupled, & itinerant holes EF spin  ~1% Mn << 1% Mn >2% Mn DOS Energy spin  onset of ferromagnetism near MIT As-p-like holes localized on Mn acceptors valence band As-p-like holes As-p-like holes • - random dilutemoment FM  • difficult to achieve high Tc • intrinsically very disordered • system • - heavily-doped SC • difficult to grow and gate Mn-d-like local moments

  9. FM & transport in the disordered GaMnAs DMS Ordered magnetic semiconductors Disordered DMSs Eu - chalcogenides Broad peak near Tc and disappeares with annealing (higher uniformity) Sharp critical contribution to resistivity at Tc ~ magnetic susceptibility

  10. Scattering off correlated spin-fluctuations Fisher&Langer, PRL‘68 singular singular Ni, Fe Eu0.95Cd0.05S Tc

  11. In GaMnAs F~d-  sharp singularity at Tc in d/dT Annealing sequence Optimized GaMnAs materials with x~4-12% and Tc~80-185K: very well behaved FMs Novak et al., PRL ‚08

  12. Low-voltage gating of the highly doped (Ga,Mn)As 10’s-100’s Volts in conventional MOS FETs Ohno et al. Nature ’00, APL ‘06 p-n junction FET p-n junction depletion simulations 2x 1019 cm-3 ~25-50% depletion feasible at low voltages Owen, et al. arXiv:0807.0906

  13. Complete spintronic FET characteristics Tc Tc

  14. Magnetization switching by short low-Vg pulses Due to voltage-controlled Kc and Ku anisotropies -1V +3 V  depletion/accumulation & high-frequency studies of DMS materials and spintronics semiquantitative microscpic theory understanding

  15. Tc Conclusion 1) Studies in GaMnAs suggest new generic approaches to electric field controlled spintronics via magnetic anisotropies - TAMR - CBAMR 2) Optimized GaMnAs is excellent itinerant FM; low-voltage charge depletion effects on electric&magnetic properties demonstrated in all-semiconductor p-n junction transistor - d/dT singularity at Tc - GaMnAs junction FET

  16. (Ga,Mn)As growth high-T growth optimal-T growth • Low-T MBE to avoid precipitation & high enough T to maintain 2D growth • need to optimize T & stoichiometry for each Mn-doping Detrimental interstitial AF-coupled Mn-donors  need to anneal out (Tc can increase by more than 100K) Annealing also needs to be optimized for each Mn-doping

  17. No indication for reaching technological or physical Tc limit in (Ga,Mn)As yet Tc up to 187 K at 12% Mn doping Novak et al. PRL ‘08 2005 Growth & post-growth optimized GaMnAs films 1998

  18. Other (III,Mn)V’s DMSs Kudrnovsky et al. PRB 07 Delocalized holes long-range coupl. Weak hybrid. Mean-field but low TcMF InSb d5 Impurity-band holes short-range coupl. Strong hybrid. Large TcMF but low stiffness GaP GaAs seems close to the optimal III-V host

  19. coupling strength / Fermi energy band-electron density / local-moment density Magnetism in systems with coupled dilute moments and delocalized band electrons Jungwirth et al, RMP '06

  20. Other DMS candidates III = I + II  Ga = Li + Zn GaAs and LiZnAs are twin SC (Ga,Mn)As and Li(Zn,Mn)As should be twin ferromagnetic SC • But Mn isovalent in Li(Zn,Mn)As • no Mn concentration limit and self-compensation • possibly both p-type and n-type ferromagnetic SC (Li / Zn stoichiometry) Masek et al. PRL 07

  21. Sharp d/dT singularity in GaMnAs at Tc – consistent with F~d- Novak, et al. PRL‘08

  22. Ga Mn As Mn p s V Beff Strong spin-orbit coupling  favorable for spintronics As-p-like holes Strong SO due to the As p-shell(L=1) character of the top of the valence band

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