Spintronic transistors: magnetic anisotropy and direct charge depletion concepts

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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.
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.

• Texas AM
• Jairo Sinova, et al.

University of Texas Allan MaDonald, et al.

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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
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AMR
TMR
parallel state
FM exchange int.
Spin-orbit int.
antiparallel state
TAMR
FM exchange int.
Discovered in GaMnAs Gould et al. PRL04
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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
experiment
Park et al PRL '08
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Optimizing TAMR in transition-metal structures
Consider uncommon TM combinations e.g. Mn/W ?
voltage-dependent upto 100 TAMR
Shick, et al PRB 08
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Devices utilizing M-dependent electro-chemical
potentials FM SET
SO-coupling ? ??(M)
magnetic
mV in GaMnAs 10mV in FePt
electric
control of CB oscillations
Wunderlich et al, PRL '06
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CB oscillations shifted by changing M(CBAMR)
(Ga,Mn)As nano-constriction SET
Electric-gate controlled magnitude and sign of
magnetoresistance ? spintronic transistor Magn
etization controlled transistor characteristic (p
or n-type) ? programmable logic
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Ferromagnetic semiconductor GaAsMn
Exchange-split, SO-coupled, itinerant holes
EF
spin ?
1 Mn
ltlt 1 Mn
gt2 Mn
DOS
Energy
spin ?
onset of ferromagnetism near MIT
As-p-like holes localized on Mn acceptors
valence band As-p-like holes
As-p-like holes

• - random dilute moment FM ?
• difficult to achieve high Tc
• intrinsically very disordered
• system
• - heavily-doped SC ?
• difficult to grow and gate

Mn-d-like local moments
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FM transport in the disordered GaMnAs DMS
Ordered magnetic semiconductors
Disordered DMSs
Eu? - chalcogenides
Broad peak near Tc and disappeares with annealing
(higher uniformity)
Sharp critical contribution to resistivity at Tc
magnetic susceptibility
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Scattering off correlated spin-fluctuations
FisherLanger, PRL68
singular
singular
Ni, Fe
Eu0.95Cd0.05S
Tc
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In GaMnAs ?Fd?-? ? sharp singularity at Tc in
d?/dT
Annealing sequence
Optimized GaMnAs materials with x4-12 and
Tc80-185K very well behaved FMs
Novak et al., PRL 08
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Low-voltage gating of the highly doped (Ga,Mn)As
10s-100s Volts in conventional MOS FETs Ohno
et al. Nature 00, APL 06
p-n junction FET
p-n junction depletion simulations
2x 1019 cm-3
25-50 depletion feasible at low voltages
Owen, et al. arXiv0807.0906
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Complete spintronic FET characteristics
Tc
Tc
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Magnetization switching by short low-Vg pulses
Due to voltage-controlled Kc and Ku anisotropies
-1V
3 V
semiquantitative microscpic theory understanding
? depletion/accumulation high-frequency studies
of DMS materials and spintronics
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Conclusion 1) Studies in GaMnAs suggest new
generic approaches to electric field
controlled spintronics via magnetic anisotropies
- TAMR - CBAMR
2) Optimized GaMnAs is excellent itinerant
FM low-voltage charge depletion
effects on electricmagnetic properties
demonstrated in all-semiconductor p-n junction
transistor - d?/dT singularity at
Tc - GaMnAs junction FET
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(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
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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 GaMnAs films
1998
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Other (III,Mn)Vs DMSs
Kudrnovsky et al. PRB 07
Delocalized holes long-range coupl.
Weak hybrid.
Mean-field but low TcMF
InSb
d5
Impurity-band holes short-range coupl.
Strong hybrid.
Large TcMF but low stiffness
GaP
GaAs seems close to the optimal III-V host
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Magnetism in systems with coupled dilute moments
and delocalized band electrons
Jungwirth et al, RMP '06
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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
• no Mn concentration limit and self-compensation
• possibly both p-type and n-type ferromagnetic SC
• (Li / Zn stoichiometry)

Masek et al. PRL 07
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Sharp d?/dT singularity in GaMnAs at Tc
consistent with ?Fd?-?
Novak, et al. PRL08
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Strong spin-orbit coupling ? favorable for
spintronics
As-p-like holes
Strong SO due to the As p-shell (L1) character
of the top of the valence band