Spintronics in ferromagnetic semiconductor (Ga,Mn)As

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Spintronics in ferromagnetic semiconductor
(Ga,Mn)As
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 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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Outline 1. Curie temperature and critical
transport anomaly 2. Low-voltage control of
ferromagnetism in a p-n junction 3.
Coulomb-blockade AMR single electron transistor
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Electric field controlled spintronics
From storage to logic
Magnetic Transistor control by Electric field
HDD, MRAM controlled by Magnetic field
STT MRAM, spin-polarized charge current
Low-voltage controlled magnetization and MR
effects
FS spintronic transitor
J. Wunderlich, et al. 06
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Ferromagnetic semiconductor GaAsMn
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
local-itinerant kinetic-exchange)
Mn-d-like local moments
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(Ga,Mn)As growth
high-T growth

• Low-T MBE to avoid precipitation
• High enough T to maintain 2D growth
• need to optimize T stoichiometry
• for each Mn-doping
• Inevitable formation of interstitial Mn-donors
• compensating holes and moments
• ? need to anneal out

optimal-T growth
Optimizing annealing time temperature
(removing int. Mn keeping MnGa in place)
again for each Mn-doping is essential
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Olejnik et al., 08
Tc limit in (Ga,Mn)As remains open
Nottingham Prague (08) Tc up to 188 K so far
Mack et al. 08 Tc 150-165 K independent of
xMngt10 contradicting Zener kinetic exchange ...
?
Mack et al. 08
Combinatorial approach to growth with fixed
growth and annealing conditions
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Towards spintronics in (Ga,Mn)As FM transport
Dilute-moment MS ?F d?-?
Dense-moment MS ?Fltlt d?-?
Eu? - chalcogenides
Broad peak near Tc disappeares with annealing
(higher uniformity)???
Critical contribution to resistivity at Tc
magnetic susceptibility
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Tc
Ni
EuCdSe
Tc
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d?/dT singularity at Tc consistent with kFd?-?
Novak, et al.08
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Annealing sequence
Optimized materials with x4-12.5 and Tc80-185K
Remarkably universal both below and above Tc
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Ferromagnetism strong spin-orbit coupling
As-p-like holes
Strong SO due to the As p-shell (L1) character
of the top of the valence band
Beff
Bex Beff
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Electric field control of ferromagnetism
k.p kinetic exchange model predicst sensitivity
to strains 10-4
Strain SO ?
Rushforth et al., 08
slow and requires 100V
and hole-density variations of 1019-1020 cm-3
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Gating of the highly doped (Ga,Mn)As p-n
junction
p-n junction depletion estimates
2x 1019 cm-3
25 depletion feasible at low voltages
Olejnik et al., 08
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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
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Magnetization switching by 10ms low-Vg pulses
Due to the Vg-dependent Stoner-Wolfarth diamond
(tuneable uniaxial and cubic anisotropy terms)
dRc/dH
normalized dRc/dH
-1V
-1V
3V
3V
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(Ga,Mn)As spintronic single-electron transistor
Wunderlich et al. PRL 06
Huge, gatable, and hysteretic MR
Single-electron transistor
Two "gates" electric and magnetic
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AMR nature of the effect
Coulomb blockade AMR
normal AMR
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magnetic
electric
SO-coupling ? ?(M)
control of Coulomb blockade oscillations
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Theory confirms chemical potential anisotropies
in (Ga,Mn)As predicts CBAMR in SO-coupled
room-Tc metal FMs

• CBAMR if change of ??(M) e2/2C?
• In our (Ga,Mn)As meV ( 10 Kelvin)
• In room-T ferromagnet change of ??(M)100K
• Room-T conventional SET
• (e2/2C? gt300K) possible

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Nonvolatile programmable logic
Variant p- or n-type FET-like transistor in one
single nano-sized CBAMR device
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Nonvolatile programmable logic
Variant p- or n-type FET-like transistor in one
single nano-sized CBAMR device
OR