Ferromagnetic semiconductor materials and spintronic transistors Tomas Jungwirth

Ferromagnetic semiconductor materials and spintronic transistors 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, Chris King et al. Texas A&M Jairo Sinova, et al. University of Texas Allan Ma. Donald, et al.

Electric field controlled spintronics From storage to logic HDD, MRAM controlled by Magnetic field STT MRAM spin-polarized charge current Spintronic Transistor control by electric gates Low-voltage controlled magnetization and magnetotransport Magnetic race track memory

Outline 1) Sensitivity to electric fields via magnetic anisotropies generic to both metals and semiconductors with spin-orbit coupling - Tunneling AMR device - Coulomb blockade AMR spintronic SET 2) Direct charge depletion effects on electric&magnetic proprties ferromagnetic semiconductors are the favorable systems here - Ga. Mn. As and related dilute-moment ferromagnetic semiconductors - Ga. Mn. As-based p-n junction spintronic FET

AMR TMR FM exchange int. : Spin-orbit int. : 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 systems including room-Tc FMs Park et al PRL '08 experiment

Devices utilizing M-dependent electro-chemical potentials: FM SET SO-coupling (M) Source Q VD Drain Gate VG [110] [010] M [100] [110] [010] electric & magnetic control of Coulomb blockade oscillations

(Ga, Mn)As nano-constriction SET CB oscillations shifted by changing M (CBAMR) Wunderlich et al, PRL '06 Electric-gate controlled magnitude and sign of magnetoresistance spintronic transistor or Magnetization controlled transistor characteristic (p or n-type) programmable logic

Outline 1) Sensitivity to electric fields via magnetic anisotropies generic to both metals and semiconductors with spin-orbit coupling - Tunneling AMR device - Coulomb blockade AMR spintronic SET 2) Direct charge depletion effects on electric&magnetic proprties ferromagnetic semiconductors are the favorable systems here - Ga. Mn. As and related dilute-moment ferromagnetic semiconductors - Ga. Mn. As-based p-n junction spintronic FET

Ferromagnetic semiconductor Ga. As: Mn DOS spin Jungwirth et al, RMP '06 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 Ga FM due to p-d hybridization (Zener kinetic-exchange) (Ga, Mn)As: - heavily-doped SC difficult to grow and gate - dilute moment FM difficult to achieve high Tc Mn Mn-d-like local moments Mn As

(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)

Towards spintronics in (Ga, Mn)As: FM & transport 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)? ? ?

Tc Eu 0. 95 Cd 0. 05 S Ni, Fe Tc

Sharp d /d. T singularity in Ga. Mn. As at Tc – consistent with F~d - Novak, et al. PRL‘ 08

Optimized Ga. Mn. As materials with x~412% and Tc~80 -185 K: well behaved FMs Annealing sequence t=(Tc-T)/Tc

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

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

Basic charcteristics of the device can “deplete” magnetization at low Vg can deplete charge at low Vg 30% AMR tuneable by low Vg dependent competition of uniaxial and cubic anisotropies

Magnetization switching by 10 ms low-Vg pulses

Conclusion 1) Studies in Ga. Mn. As suggest new generic approaches to electric field controlled spintronics via magnetic anisotropies - TAMR - CBAMR 2) Direct charge depletion effects on electric&magnetic properties of Ga. Mn. As demonstrated at low gate voltages Ga As - Ga. Mn. As junction FET Mn Mn