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Prague IoP group and theoretical studies of ferromagnetic materials and nanostructure with strong spin-orbit coupling
Institute of Physics ASCR Tomas Jungwirth, Alexander Shick, Karel Výborný, Jan Zemen,
Jan Masek, Jairo Sinova, Vít Novák, Kamil Olejník, et al.
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64-node high-performance computer cluster
State of the art
molecular-bean epitaxy
& electron-beam lithography systems
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Theoretical methods
Electronic structure
Analytical models (Rashba, Dresselhaus, spherical-Luttinger)
k.p semiphenomenological modelling (typical for semiconductors) extensive library of home-made routines
spd-tight-binding modelling (half way between phenomenological and ab initio) home-made relativistic codes
Full ab initio heavy numerics (transition metals based structures) standard full-potential libraries, home-made relativistic ab-initio codes
Observables micromagnetic parameters from total energy, thermodunamics, and linear response theories
Boltzmann and Kubo equations for extraordinary, anisotropic, and coherent transport
Device specific modeling Finite-element methods, Schrodinger-Poisson solvers, Monte-Carlo semiclassical methods, Landauer-Buttiker formalism
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Semiconductor 2D electron and hole systems with spin-orbit coupled bands
Dilute-moment ferromagnetic semiconductors
AsAsGaGa
MnMn
Transition metal ferro and antiferromagnets
Materials
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Research goal: Electric field controlled spintronics
HDD, MRAMcontrolled by Magnetic field
Spintronic TransistorsLow-V 3-terminal
devices
STT MRAMspin-polarized charge current
& Opto-spintronics
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1. Exchange & spin-orbit coupling & direct link to spintronics (magnetotransport)
2. Semiconducting multiferroic systems
3. Spin dynamics in non-magnetic spin-orbit coupled channels
Paradigms
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AMRAMR TMRTMR
TAMRTAMR
Exchange & spin-orbit coupling;complex link to transport
Exchange only; direct link to transport
)(MTDOS
Au
Exchange & spin-orbit coupling; direct link to transport
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ab intio theoryTAMR is generic to SO-coupled 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
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spontaneous momentmag
netic su
sceptib
ility
Consider uncommon TM combinationsMn/W ~100% TAMR
Consider both Mn-TM FMs & AFMs
exchange-spring rotation of the AFMScholl et al. PRL ‘04
Proposal for AFM-TAMR: first microelectronic device with active AFM component
spin
-orb
it cou
plin
g
TAMR in TM structures
Shick, et al,unpublished
Shick, et al,unpublished
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GM
MGG
C
C
e
MV
MVVCQC
QQU
)(&
)]([&2
)(0
20
electric && magneticmagnetic
control of CB oscillations
Source Drain
GateVG
VDQ
Devices utilizing M-dependent electro-chemical potentials: FM SET
SO-coupling (M)
[010] M[110]
[100]
[110][010]
~ mV in GaMnAs~ 10mV in FePt
Wunderlich et al, PRL '06
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(Ga,Mn)As nano-constriction SET CB oscillations shifted by changing M(CBAMR)
Electric-gate controlled magnitude and sign of magnetoresistance spintronic transistor
&
Magnetization controlled transistor characteristic (p or n-type) programmable logic
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Complexity of the relation between SO & exchange-split bands and
transport
SET
Resistor
Tunneling device
Chemical potential CBAMR
Tunneling DOS TAMR
Group velocity & lifetime AMR
Complexity of the device design
Magnitude and sensitivity to electric
fields of the MR
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1. Exchange & spin-orbit coupling & direct link to spintronics (magneotransport)
2. Semiconducting multiferroic systems
3. Spin dynamics in non-magnetic spin-orbit coupled channels
Paradigms
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Magnetic materials
Ferroelectrics/piezoelectrics Semiconductors
spintronic magneto-sensors, memories
electro-mechanical transducors, large & persistent el. fields
transistors, logic,sensitive to doping and electrical gating
Semiconducting multiferroic spintronics
Control via (non-volatile) charge depletion and/or strain effects
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Ferromagnetic semiconductors
GaAs - GaAs - standard III-V semiconductorstandard III-V semiconductor
Group-II Group-II Mn - Mn - dilute dilute magneticmagnetic moments moments & holes& holes
(Ga,Mn)As - fe(Ga,Mn)As - ferrromagneticromagnetic semiconductorsemiconductor
Need true FSs not FM inclusions in SCs
Mn
Ga
AsMn
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Mn-d-like localmoments
As-p-like holes
Mn
Ga
AsMn
EF
DO
S
Energy
spin
spin
GaAs:Mn – extrinsic p-type semiconductor
FM due to p-d hybridization
(Zener local-itinerant 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
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As-p-like holes
Ferromagnetism & strong spin-orbit coupling
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
Beff Bex + Beff
Mn
Ga
AsMn
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Rushforth et al., ‘08
Strain & SO
Electric field control of ferromagnetism
k.p kinetic exchange model predicst sensitivity to strains ~10-4
and hole-density variations of ~1019-1020 cm-3
slow and requires ~100V
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Low-voltage gating (charge depletion) of ferromagnetic semiconductors
Owen, et al. arXiv:0807.0906
Switching by short low-voltage pulses
Mag
neti
zati
on
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1. Exchange & spin-orbit coupling & direct link to spintronics (magnetotransport)
2. Semiconducting multiferroic systems
3. Spin dynamics in non-magnetic spin-orbit coupled channels
Paradigms
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Datta-Das transistor
Spin dynamics in non-magnetic spin-orbit coupled channels
Datta and Das, APL ‘99
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Spin-injection Hall effect transistor and spin-photovoltaic cell
Non-destructive detection of spin-dynamics along the channel
Compatible with optical and electrical spin-injection and tunable by electrical gates