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Ferromagnetic semiconductor materials and spintronic transistors. Tom as Jungwirth. Universit y of Nottingham Bryan Gallagher, Tom Foxon, Richard Campion, Kevin Edmonds, Andrew Rushforth, Chris King et al. Institute of Physics ASCR - PowerPoint PPT Presentation
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Ferromagnetic semiconductor materials and spintronic transistors
Tomas Jungwirth
University of Nottingham Bryan Gallagher, Tom Foxon,
Richard Campion, Kevin Edmonds, Andrew Rushforth, Chris King 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&MJairo Sinova, et al.
Electric field controlled spintronics
HDD, MRAMcontrolled by Magnetic field
Spintronic Transistorcontrol by
electric gates
Low-voltage controlled magnetization and magnetotransport
STT MRAMspin-polarized charge current
From storage to logic
Magnetic race track memory
OutlineOutline
1) 1) Sensitivity to electric fields via magnetic anisotropiesSensitivity to electric fields via magnetic anisotropies generic to both metals and semiconductors with spin-orbit couplinggeneric to both metals and semiconductors with spin-orbit coupling - Tunneling AMR device- Tunneling AMR device - Coulomb blockade AMR spintronic SET- Coulomb blockade AMR spintronic SET
2) 2) Direct charge depletion effects on electric&magnetic proprtiesDirect charge depletion effects on electric&magnetic proprties ferromagnetic semiconductors are the favorable systems here ferromagnetic semiconductors are the favorable systems here
- GaMnAs and related dilute-moment ferromagnetic semiconductors- GaMnAs and related dilute-moment ferromagnetic semiconductors - GaMnAs-based p-n junction spintronic FET - GaMnAs-based p-n junction spintronic FET
AMRAMR TMRTMR
TAMRTAMR
) vs.( ~ IMvg
M
FM exchange int.:
Spin-orbit int.:
FM exchange int.:
)()( TDOSTDOS
)(MTDOS
Au
Discovered in GaMnAs Gould et al. PRL’04
ab intio theoryTAMR is generic to SO-coupled systems including room-Tc 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
GMMGG0
20
C
C
e
)M(V&)]M(VV[CQ&
C2
)QQ(U
electric && magneticmagnetic
control of Coulomb blockade oscillations
Q
0
'D
'
e
)M(Q)Q(VdQU
Source Drain
GateVG
VDQ
Devices utilizing M-dependent electro-chemical potentials: FM SET
SO-coupling (M)
[010] M[110]
[100]
[110][010]
(Ga,Mn)As nano-constriction SET
CB oscillations shifted by changing M
(CBAMR)
Electric-gate controlled magnitude and sign of magnetoresistance spintronic transistor
or
Magnetization controlled transistor characteristic (p or n-type) programmable logic
Wunderlich et al, PRL '06
OutlineOutline
1) 1) Sensitivity to electric fields via magnetic anisotropiesSensitivity to electric fields via magnetic anisotropies generic to both metals and semiconductors with spin-orbit couplinggeneric to both metals and semiconductors with spin-orbit coupling - Tunneling AMR device- Tunneling AMR device - Coulomb blockade AMR spintronic SET- Coulomb blockade AMR spintronic SET
2) 2) Direct charge depletion effects on electric&magnetic proprtiesDirect charge depletion effects on electric&magnetic proprties ferromagnetic semiconductors are the favorable systems here ferromagnetic semiconductors are the favorable systems here
- GaMnAs and related dilute-moment ferromagnetic semiconductors- GaMnAs and related dilute-moment ferromagnetic semiconductors - GaMnAs-based p-n junction spintronic FET - GaMnAs-based p-n junction spintronic FET
Mn-d-like localmoments
As-p-like holes
Mn
Ga
AsMn
EF
DO
S
Energy
spin
spin
Ferromagnetic semiconductor GaAs:Mn
FM due to p-d hybridization
(Zener kinetic-exchange)
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
(Ga,Mn)As: - heavily-doped SC difficult to grow and gate- dilute moment FM difficult to achieve high Tc
Jungwirth et al, RMP '06
(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 (Tc can increase by more than 100K)
Tc up to 187 K at 12% Mn doping
0 1 2 3 4 5 6 7 8 9 100
20
40
60
80
100
120
140
160
180
TC(K
)
Mntotal
(%)
No indication for reaching technological or physical Tc limit in (Ga,Mn)As yet
Novak et al. PRL ‘08
1998
2005Growth & post-growth optimized GaMnAs films
Weak hybrid.Delocalized holeslong-range coupl.
Strong hybrid.Impurity-band holesshort-range coupl.
InSb
GaP
d5
GaAs seems close to the optimal III-V host
Other (III,Mn)V’s DMSs
Mean-field butlow Tc
MF
Large TcMF but
low stiffness
Kudrnovsky et al. PRB 07
Magnetism in systems with coupled dilute moments and delocalized band electrons
cou
pli
ng
str
eng
th /
Fer
mi
ener
gy
band-electron density / local-moment density
Jungwirth et al, RMP '06
III = I + II Ga = Li + Zn
Other DMS candidates
Masek et al. PRL 07But 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
Towards spintronics in (Ga,Mn)As: FM & transport
Ordered magnetic semiconductors
Eu - chalcogenides
Disordered DMSs
Sharp critical contribution to resistivity at Tc ~ magnetic susceptibility
Broad peak near Tc and disappeares with annealing (higher uniformity)???
Udkk F ~)/1~~(
2~ Suncor
~)0~/1~( dkk F
smalluncor
vcdTdUdTd /~/
Tc
Tc
Eu0.95Cd0.05S Ni, Fe
][~),(~)( 002 SSSSJTRT iipdi
0k1kd
1~kd
Sharp d/dT singularity in GaMnAs at Tc – consistent with F~d-
Novak, et al. PRL‘08
Optimized GaMnAs materials with x~4-12% and Tc~80-185K: well behaved FMs
Annealing sequence
t=(Tc-T)/Tc
4.03.0~ tM
As-p-like holes
Strong spin-orbit coupling favorable for spintronics
LSdr
rdV
err
mc
p
mc
SeBH effSO
)(1
Strong SO due to the As p-shell (L=1) character of the top of the valence band
V
BBeffeff
pss
Mn
Ga
AsMn
Low-voltage gating of the highly doped (Ga,Mn)As
p-n junction depletion simulations
~25-50% depletion feasible at low voltages
2x 1019 cm-3
Owen, et al. arXiv:0807.0906
10’s-100’s Volts in conventional MOS FETs Ohno et al. Nature ’00, APL ‘06
p-n junction FET
Basic charcteristics of the device
can deplete charge at low Vg
can “deplete” magnetization at low Vg
low Vg dependent competition of uniaxial and cubic anisotropies
30% AMR tuneable by low Vg
Magnetization switching by 10ms low-Vg pulses
ConclusionConclusion
1) 1) Studies in GaMnAs suggest new generic approaches to Studies in GaMnAs suggest new generic approaches to electric field controlled spintronics via magnetic anisotropieselectric field controlled spintronics via magnetic anisotropies - TAMR- TAMR - CBAMR - CBAMR
2) 2) Direct charge depletion effects on electric&magnetic properties Direct charge depletion effects on electric&magnetic properties of GaMnAs demonstrated at low gate voltagesof GaMnAs demonstrated at low gate voltages - GaMnAs junction FET - GaMnAs junction FET
Mn
GaAs
Mn