Ferromagnetic semiconductor materials and spintronic transistors

1
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.
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
From storage to logic
HDD, MRAM controlled by Magnetic field
Spintronic Transistor control by electric gates
STT MRAM spin-polarized charge current
Low-voltage controlled magnetization and
magnetotransport
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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 electricmagnetic proprties
ferromagnetic semiconductors are the
favorable systems here - GaMnAs and
related dilute-moment ferromagnetic
semiconductors - GaMnAs-based p-n
junction spintronic FET
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AMR
TMR
FM exchange int.
Spin-orbit int.
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 systems including
room-Tc FMs
experiment
Park et al PRL '08
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Devices utilizing M-dependent electro-chemical
potentials FM SET
magnetic
electric
control of Coulomb blockade oscillations
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(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 Mag
netization controlled transistor characteristic
(p or n-type) ? programmable logic
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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 electricmagnetic proprties
ferromagnetic semiconductors are the
favorable systems here - GaMnAs and
related dilute-moment ferromagnetic
semiconductors - GaMnAs-based p-n
junction spintronic FET
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Ferromagnetic semiconductor GaAsMn
Jungwirth et al, RMP '06
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
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-d-like local moments
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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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Towards spintronics in (Ga,Mn)As FM transport
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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Tc
Ni, Fe
Eu0.95Cd0.05S
Tc
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Sharp d?/dT singularity in GaMnAs at Tc
consistent with ?Fd?-?
Novak, et al. PRL08
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Annealing sequence
Optimized GaMnAs materials with x4-12 and
Tc80-185K well behaved FMs
t(Tc-T)/Tc
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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
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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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Basic charcteristics of the device
can deplete magnetization at low Vg
can deplete charge at low Vg
low Vg dependent competition of uniaxial and
cubic anisotropies
30 AMR tuneable by low Vg
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Magnetization switching by 10ms low-Vg pulses
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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 electricmagnetic properties
of GaMnAs demonstrated at low gate voltages
- GaMnAs junction FET