Transcript Slide 1
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 MaDonald, et al. Electric field controlled spintronics From storage to logic HDD, MRAM STT MRAM Spintronic Transistor controlled by Magnetic field spin-polarized charge current 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 - GaMnAs and related dilute-moment ferromagnetic semiconductors - GaMnAs-based p-n junction spintronic FET AMR FM exchange int.: Spin-orbit int.: TMR M ~ vg (M vs. I ) 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 systems including room-Tc FMs Park et al PRL '08 experiment Devices utilizing M-dependent electro-chemical potentials: FM SET SO-coupling (M) Q VD Source Drain Gate VG [110] [010] Q( M ) ' ' U dQ VD ( Q ) e 0 Q [100] [110] M [010] ( Q Q0 ) ( M ) C U & Q0 CG [ VG VM ( M )] &VM 2C e CG 2 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 - GaMnAs and related dilute-moment ferromagnetic semiconductors - GaMnAs-based p-n junction spintronic FET Ferromagnetic semiconductor GaAs: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 As-p-like holes 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 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) 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)??? 2 (T ) ~ ( Ri , T ) ~ J pd [ Si S0 Si S0 ] uncor small Tc uncor 2 ~ S Eu0.95Cd0.05S k 0 kd 1 kd ~ 1 Ni, Fe (k ~ kF ~ 1/ d ) ~ U d / dT ~ dU / dT cv (k ~ kF 1/ d ~ 0) ~ Tc Sharp d/dT singularity in GaMnAs at Tc – consistent with F~d- Novak, et al. PRL‘08 Optimized GaMnAs materials with x~412% and Tc~80-185K: well behaved FMs Annealing sequence M ~t t=(Tc-T)/Tc 0.3 0.4 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 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 Basic charcteristics of the device can “deplete” magnetization at low Vg can deplete charge at low Vg 30% AMR tuneable by low Vg low Vg dependent competition of uniaxial and cubic anisotropies Magnetization switching by 10ms low-Vg pulses Conclusion 1) Studies in GaMnAs suggest new generic approaches to electric field controlled spintronics via magnetic anisotropies - TAMR - CBAMR 2) Direct charge depletion effects on electric&magnetic properties of GaMnAs demonstrated at low gate voltages Ga As - GaMnAs junction FET Mn Mn