Spintronic transistors magnetic anisotropy and direct charge depletion

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 Ma. Donald, et al.

Electric field controlled spintronics HDD, MRAM controlled by Magnetic field Spintronic Transistor STT MRAM 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. : antiparallel state TAMR Au FM exchange int. : Discovered in Ga. Mn. As 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 rbit cou g plin ility eptib susc n-o spi netic mag 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) ~ m. V in Ga. Mn. As ~ 10 m. V in Fe. Pt 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 Ga. As: 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 As-p-like holes Mn - intrinsically very disordered system - heavily-doped SC difficult to grow and gate Ga Mn-d-like local moments Mn As

FM & transport in the disordered Ga. Mn. As 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 singular Fisher&Langer, PRL‘ 68 singular Ni, Fe Eu 0. 95 Cd 0. 05 S Tc

In Ga. Mn. As F~d - sharp singularity at Tc in d /d. T Annealing sequence Optimized Ga. Mn. As materials with x~4 -12% and Tc~80 -185 K: 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 2 x 1019 cm-3 ~25 -50% depletion feasible at low voltages Owen, et al. ar. Xiv: 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 -1 V +3 V semiquantitative microscpic theory understanding

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

(Ga, Mn)As growth high-T growth optimal-T growth Low-T MBE to avoid precipitation & high enough T to maintain 2 D 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 100 K) 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 2005 Growth & post-growth optimized Ga. Mn. As films 1998

Other (III, Mn)V’s DMSs Kudrnovsky et al. PRB 07 Weak hybrid. Mean-field but low Tc. MF In. Sb Strong hybrid. Large Tc. MF but low stiffness Ga. P Ga. As seems close to the optimal III-V host Delocalized holes long-range coupl. d 5 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 Ga. As and Li. Zn. As 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 /d. T singularity in Ga. Mn. As at Tc – consistent with F~d - Novak, et al. PRL‘ 08

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