Transcript talk-prague-Lanna-Namaste-08

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.
64-node high-performance computer cluster
State of the art
molecular-bean epitaxy
& electron-beam lithography systems
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
Materials

Semiconductor 2D electron and hole systems
with spin-orbit coupled bands
Ga As

Dilute-moment ferromagnetic
semiconductors
Mn
 Transition metal ferro and antiferromagnets
Research goal: Electric field controlled spintronics
HDD, MRAM
STT MRAM
Spintronic Transistors
controlled by
Magnetic field
spin-polarized
charge current
Low-V 3-terminal
devices
& Opto-spintronics
Paradigms
1. Exchange & spin-orbit coupling & direct
link to spintronics (magnetotransport)
2. Semiconducting multiferroic systems
3. Spin dynamics in non-magnetic spinorbit coupled channels
AMR
TMR
Exchange & spin-orbit coupling;
complex link to transport
TAMR

TDOS (M )
Au
Exchange & spin-orbit coupling;
direct link to transport
Exchange only;
direct link to transport
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
Park et al PRL '08
experiment
TAMR in TM structures
Consider uncommon TM combinations
Mn/W  ~100% TAMR Shick, et al,
unpublished
spontaneous moment
Consider both Mn-TM FMs & AFMs
exchange-spring rotation of the AFM
Scholl et al. PRL ‘04
Proposal for AFM-TAMR: first microelectronic device with active AFM component
Shick, et al,
unpublished
Devices utilizing M-dependent electro-chemical potentials: FM SET
[110]
[010]
M
Source
Drain
Gate
VG
[100]
[110]
Q VD
[010]
SO-coupling  (M)
~ mV in GaMnAs
~ 10mV in FePt

(Q  Q0 ) 2
U
& Q0  CG [VG  VM ( M )]
2C

 ( M ) C
& VM 
e
CG electric & magnetic
control of CB oscillations
Wunderlich et al, PRL '06
(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
Magnitude and
sensitivity to electric
fields of the MR
Complexity of the relation between
SO & exchange-split bands and
transport
Complexity of the
device design
Chemical potential
 CBAMR
SET
Tunneling DOS
 TAMR
Tunneling device
Group velocity & lifetime
 AMR
Resistor
Paradigms
1. Exchange & spin-orbit coupling & direct
link to spintronics (magneotransport)
2. Semiconducting multiferroic systems
3. Spin dynamics in non-magnetic spinorbit coupled channels
Semiconducting multiferroic spintronics
Control via (non-volatile) charge depletion and/or strain effects
Magnetic materials
spintronic magnetosensors, memories
Ferroelectrics/piezoelectrics
electro-mechanical
transducors,
large & persistent el. fields
Semiconductors
transistors, logic,
sensitive to doping
and electrical gating
Ferromagnetic semiconductors
Need true FSs not FM inclusions in SCs
Ga
Mn
As
GaAs - standard III-V semiconductor
Group-II Mn - dilute magnetic moments
& holes
(Ga,Mn)As - ferromagnetic
semiconductor
Mn
GaAs:Mn – extrinsic p-type semiconductor
DOS
spin 
EF
<< 1% Mn
~1% Mn
>2% Mn
Energy
spin 
onset of ferromagnetism near MIT
As-p-like holes localized on Mn acceptors
valence band As-p-like holes
Ga
As-p-like holes
FM due to p-d hybridization
(Zener local-itinerant kinetic-exchange)
Mn
Mn-d-like local
moments
Mn
As
Ferromagnetism & strong spin-orbit coupling
Ga
Mn
As
Mn
As-p-like holes
H SO


 
 eS   p  1 dV (r ) 
 r
    Beff  
   S  L

 mc   mc  er dr 
V
s
Beff
Strong SO due to the As p-shell (L=1) character of the top of the valence band
Beff
Bex + Beff
p
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
Low-voltage gating (charge depletion) of ferromagnetic semiconductors
Magnetization
Switching by short low-voltage pulses
Owen, et al. arXiv:0807.0906
Paradigms
1. Exchange & spin-orbit coupling & direct
link to spintronics (magnetotransport)
2. Semiconducting multiferroic systems
3. Spin dynamics in non-magnetic spinorbit coupled channels
Spin dynamics in non-magnetic spin-orbit coupled channels
Datta-Das transistor
Datta and Das, APL ‘99
Spin-injection Hall effect transistor and spin-photovoltaic cell
Non-destructive detection of spin-dynamics
along the channel
Compatible with optical and electrical spininjection and tunable by electrical gates