Transcript Slide 1
Spintronic transistors: magnetic anisotropy and direct charge depletion concepts Tomas Jungwirth 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, et al. University of Nottingham Bryan Gallagher, Tom Foxon, Richard Campion, Kevin Edmonds, Andrew Rushforth, et al. Texas A&M Jairo Sinova, et al. University of Texas Allan MaDonald, et al. Electric field controlled spintronics HDD, MRAM STT MRAM Spintronic Transistor controlled by Magnetic field spin-polarized charge current Low-V 3-terminal devices 1) indirect via magnetic anisotropy 2) direct charge depletion effects AMR TMR parallel state FM exchange int.: Spin-orbit int.: M ~ vg (M vs. I ) antiparallel state TAMR TDOS(M ) Au FM exchange int.: TDOS() TDOS() Discovered in GaMnAs Gould et al. PRL’04 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 Park et al PRL '08 experiment Optimizing TAMR in transition-metal structures spontaneous moment Consider uncommon TM combinations e.g. Mn/W voltage-dependent upto ~100% TAMR Shick, et al PRB ‘08 Devices utilizing M-dependent electro-chemical potentials: FM SET [110] [010] M Source Drain Gate VG [100] [110] Q VD [010] SO-coupling (M) ~ mV in GaMnAs ~ 10mV in FePt (Q Q0 ) 2 U & Q0 CG [VG VM ( M )] 2C ( M ) C & VM e CG electric & magnetic control of CB oscillations Wunderlich et al, PRL '06 (Ga,Mn)As nano-constriction SET CB oscillations shifted by changing M (CBAMR) Electric-gate controlled magnitude and sign of magnetoresistance spintronic transistor & Magnetization controlled transistor characteristic (p or n-type) programmable logic Ferromagnetic semiconductor GaAs:Mn DOS spin Exchange-split, SOcoupled, & itinerant holes EF << 1% Mn ~1% Mn >2% Mn Energy spin onset of ferromagnetism near MIT As-p-like holes localized on Mn acceptors valence band As-p-like holes - random dilute moment FM difficult to achieve high Tc Ga As-p-like holes Mn - intrinsically very disordered system - heavily-doped SC difficult to grow and gate Mn-d-like local moments Mn As FM & transport in the disordered GaMnAs DMS Ordered magnetic semiconductors Disordered DMSs Eu - chalcogenides Sharp critical contribution to resistivity at Tc ~ magnetic susceptibility Broad peak near Tc and disappeares with annealing (higher uniformity) Scattering off correlated spin-fluctuations Fisher&Langer, PRL‘68 2 (T ) ~ ( Ri , T ) ~ J pd [ Si S0 Si S0 ] singular (F d ) ~ (F ~ d ) ~ U singular d / dT ~ dU / dT cv Ni, Fe Eu0.95Cd0.05S Tc 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 Low-voltage gating of the highly doped (Ga,Mn)As 10’s-100’s Volts in conventional MOS FETs p-n junction FET Ohno et al. Nature ’00, APL ‘06 p-n junction depletion simulations 2x 1019 cm-3 ~25-50% depletion feasible at low voltages Owen, et al. arXiv:0807.0906 Complete spintronic FET characteristics Tc Tc Magnetization switching by short low-Vg pulses depletion/accumulation & high-frequency studies of DMS materials and spintronics Due to voltage-controlled Kc and Ku anisotropies -1V +3 V semiquantitative microscpic theory understanding 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 Tc (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 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 ‘08 180 160 2005 Growth & post-growth optimized GaMnAs films 140 120 1998 TC(K) 100 80 60 40 20 0 0 1 2 3 4 5 6 Mntotal(%) 7 8 9 10 Other (III,Mn)V’s DMSs Kudrnovsky et al. PRB 07 Weak hybrid. Mean-field but low TcMF InSb Strong hybrid. Large TcMF but low stiffness GaP GaAs seems close to the optimal III-V host Delocalized holes long-range coupl. d5 Impurity-band holes short-range coupl. coupling strength / Fermi energy Magnetism in systems with coupled dilute moments and delocalized band electrons band-electron density / local-moment density Jungwirth et al, RMP '06 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 Masek et al. PRL 07 no Mn concentration limit and self-compensation possibly both p-type and n-type ferromagnetic SC (Li / Zn stoichiometry) Sharp d/dT singularity in GaMnAs at Tc – consistent with F~d- Novak, et al. PRL‘08 Strong spin-orbit coupling favorable for spintronics Ga Mn As Mn As-p-like holes H SO eS p 1 dV (r ) r Beff S L mc mc er dr V s Beff Strong SO due to the As p-shell (L=1) character of the top of the valence band p