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. Hitachi Cambridge, Univ. Cambridge Jorg Wunderlich, Andrew Irvine, David Williams, Elisa de Ranieri, Byonguk Park, Sam Owen, et al. Institute of Physics ASCR Alexander Shick, Karel Výborný, Jan Zemen, Jan Masek, Vít Novák, Kamil Olejník, et al. University of Texas Allan MaDonald, et al. Texas A&M Jairo Sinova, et al.

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

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

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

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

& electric & magnetic control of CB oscillations SourceDrain Gate VGVG VDVD Q Devices utilizing M-dependent electro-chemical potentials: FM SET SO-coupling   (M) [ 010 ] M [ 110 ] [ 100 ] [ 110 ] [ 010 ] ~ mV in GaMnAs ~ 10mV in FePt 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

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

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

Eu 0.95 Cd 0.05 S Fisher&Langer, PRL‘68 singular TcTc Ni, Fe singular Scattering off correlated spin-fluctuations

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

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

Complete spintronic FET characteristics TcTc TcTc

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

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

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

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

Weak hybrid. Delocalized holes long-range coupl. Strong hybrid. Impurity-band holes short-range coupl. InSb GaP d5d5 GaAs seems close to the optimal III-V host Other (III,Mn)V’s DMSs Mean-field but low T c MF Large T c MF but low stiffness Kudrnovsky et al. PRB 07

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

III = I + II  Ga = Li + Zn Other DMS candidates Masek et al. PRL 07 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) GaAs and LiZnAs are twin SC (Ga,Mn)As and Li(Zn,Mn)As should be twin ferromagnetic SC

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

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