A1258609015iBYzT

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Ferromagnetic and non-magnetic spintronic devices
based on spin-orbit coupling
Tomas Jungwirth
University of Nottingham
Bryan Gallagher, Tom Foxon,
Richard Campion, Kevin Edmonds, Andrew
Rushforth, Devin Giddings et al.
Institute of Physics ASCR Alexander
Shick
Hitachi Cambridge
University of Texas and
Texas AM Jorg
Wunderlich, Bernd Kaestner
Allan MacDonald, Jairo Sinova
David Williams


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SO-coupling and electric field controlled
spintronics
1. Coulomb-blockade anisotropic
magnetoresistance 2. Spin-Hall effect
Spintronic SET in thin-film GaMnAs
Electric-field induced edge spin polarization in
GaAs 2DHG
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1. Coulomb blockade AMR
Spintronic transistor - magnetoresistance
controlled by gate voltage
Strong dependence on field angle ?hints to AMR
origin
Huge hysteretic low-field MR Sign
magnitude tunable by small gate valtages
Wunderlich, Jungwirth, Kaestner et al.,
cond-mat/0602608
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AMR nature of the effect
Coulomb blockade AMR
normal AMR
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Single electron transistor
Narrow channel SET dots due to disorder
potential fluctuations (similar to non-magnetic
narrow-channel GaAs or Si SETs)
CB oscillations low Vsd ? blocked due to SE
charging
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CB oscillation shifts by magnetication rotations
At fixed Vg peak ? valley or valley ? peak ? MR
comparable to CB negative or positive MR(Vg)
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Coulomb blockade AMR
SO-coupling ? ?(M)
magnetic
electric
control of Coulomb blockade oscillations
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Different doping expected in leads an dots in
narrow channel GaMnAs SETs

• CBAMR if change of
• ??(M) e2/2C? 10Kelvin from exp.
• ? consistent
• In room-T ferromagnet change of ??(M)100Kelvin
• CBAMR works with dot both ferro
• or paramegnetic

Calculated doping dependence of ?(M1)-?(M2)
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CBAMR SET

• Huge, hysteretic, low-field MR tunable
• by small gate voltage changes
• Combines electrical transistor action
• with permanent storage

Other FERRO SETs

• Non-hysteretic MR and large B -
• chemical potential shifts due to Zeeman effect
• Ono et al. '97, Deshmukh et al. '02
• Small MR - subtle effects of spin-coherent and
• resonant tunneling through quantum dots
• Ono et al. '97, Sahoo '05

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2. Spin Hall effect
Spin-orbit only electric fields only
induced transverse spin accummulation
Detection through circularly polarized
electroluminescence
x
applied electrical current
y
z
spin (magnetization) component
Wunderlich, Kaestner, Sinova, Jungwirth, Phys.
Rev. Lett. '05 Nomura, Wunderlich, Sinova,
Kaestner, MacDonald, Jungwirth, Phys. Rev. B '05
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Testing the co-planar spin LED only first
p-n junction current only (no SHE driving
current)
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EL
EL peak
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Bz0

• Can detect edge polarization
• Zero perp-to-plane component
• of polarization at Bz0 and Ip0

0
Circ. polarization
-10
-20
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1.5?m channel
SHE experiments
10?m channel
- show the SHE symmetries - edge polarizations
can be separated over large distances with no
significant effect on the magnitude
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Szedge Lso jzbulk tso
Theory 8 over 10nm accum. length for the GaAs
2DHG Consistent with experimental 1-2
polarization over detection length of 100nm
Murakami et al. '03, Sinova et al.'04, Nomura et
al. '05, ...
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Other SHE experiments
Spin injection from SHE GaAs channel
Electrical measurement of SHE in Al
Valenzuela, Tinkham '06
Kato et al. '04, Sih et al. '06
100's of theory papers transport with SO-coupling
intrinsic vs.
extrinsic
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Microscopic origin
Q
VD
Source
Drain

• Vg 0

Gate
VG
? Coulomb blockade

• Vg ? 0

Qne - discrete Q0CgVg - continuous
Q0-ne ? blocked Q0-(n1/2)e ? open
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Sub GaAs gap spectra analysis PL vs EL


--
X bulk GaAs excitons I recombination with
impurity states B (A,C) 3D electron 2D
hole recombination
Bias dependent emission wavelength for 3D
electron 2D hole recombination A. Y. Silov et
al., APL 85, 5929 (2004)
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Circularly polarized EL
In-plane detection angle
Perp.-to plane detection angle
? NO perp.-to-plane component of polarization at
B0 ? B?0 behavior consistent with SO-split HH
subband
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1. Introduction
Non-relativistic many-body
Pauli exclusion principle Coulomb repulsion ?
Ferromagnetism

• Robust (can be as strong as bonding in solids)
• Strong coupling to magnetic field
• (weak fields anisotropy fields needed
• only to reorient macroscopic moment)

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Spin-orbit coupling (Dirac eq. in external field
?V(r) 2nd-order in v /c around
non-relativistic limit)
Beff
Bex Beff
Bex
FM without SO-coupling
GaMnAs valence band tunable FM large SO
GaAs valence band As p-orbitals ? large SO
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AMR (anisotropic magnetoresistance)
Ferromagnetism sensitivity to magnetic
field SO-coupling anisotropies in Ohmic
transport characteristics
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TMR (tunneling magnetoresistance)
Based on ferromagnetism only
spin-valve
no (few) spin-up DOS available at EF
large spin-up DOS available at EF
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Tunneling AMR anisotropic tunneling DOS due to
SO-coupling
MRAM
(Ga,Mn)As
Au
Au
- no exchange-bias needed - spin-valve with
ritcher phenomenology than TMR
Gould, Ruster, Jungwirth, et al., PRL '04, '05
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Wavevector dependent tunnelling probabilityT (ky,
kz) in GaMnAs Red high T blue low T.
thin film
Magnetization perp. to plane Magnetization
in-plane
Magnetisation in plane
constriction
Giddings, Khalid, Jungwirth, Wunderlich et al.,
PRL '05
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TAMR in metals
ab-initio calculations
Shick, Maca, Masek, Jungwirth, PRB '06
NiFe
TAMR
TMR
Bolotin,Kemmeth, Ralph, cond-mat/0602251
TMR TAMR gtgtAMR
Viret et al., cond-mat/0602298
Fe, Co break junctions TAMR gtTMR
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EXPERIMENT Spin Hall Effect
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Single Electron Transistor
Q
VD
Source
Drain

• Vg 0

Gate
VG
? Coulomb blockade

• Vg ? 0

Qne - discrete Q0CgVg - continuous
Q0-ne ? blocked Q0-(n1/2)e ? open
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Coulomb blockade anisotropic magnetoresistance
Spin-orbit coupling
Band structure (group velocities, scattering
rates, chemical potential) depend on
If lead and dot different (different carrier
concentrations in our (Ga,Mn)As SET)
magnetic
electric
control of Coulomb blockade oscillations
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Wunderlich, Jungwirth, Kaestner, Shick, et al.,
preprint

• 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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CBAMR ? new device concepts
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Electrically generated spin polarization in
normal semiconductors SPIN HALL EFFECT
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Ordinary Hall effect
Lorentz force deflect charged-particles towards
the edge
B
_ _ _ _ _ _ _ _ _ _
_
FL

I
V
Detected by measuring transverse voltage
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Spin Hall effect
Spin-orbit coupling force deflects like-spin
particles
_
_
_
FSO
_
non-magnetic
FSO
I
Spin-current generation in non-magnetic systems
without applying external magnetic fields
Spin accumulation without charge
accumulation excludes simple electrical detection
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Microscopic theory and some interpretation
experimentally detected
spin velocity
non-conserving (ambiguous) theoretical quantity
- weak dependence on impurity scattering time -
Szedge jzbulk / vF tsoh/?so
(intrinsic) spin-precession time LsovF tso
spin-precession length Szedge Lso jzbulk tso
Nomura, Wunderlich, Sinova, Kaestner,
MacDonald, Jungwirth, Phys. Rev. B '05
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SHE experiment in GaAs/AlGaAs 2DHG
Wunderlich, Kaestner, Sinova, Jungwirth, Phys.
Rev. Lett. '05
10?m channel
- shows the basic SHE symmetries - edge
polarizations can be separated over large
distances with no significant effect on the
magnitude - 1-2 polarization over detection
length of 100nm consistent with theory
prediction (8 over 10nm accumulation length)
Nomura, Wunderlich, Sinova, Kaestner,
MacDonald, Jungwirth, Phys. Rev. B '05
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Conventionally generated spin polarization in
non-magnetic semiconductors spin injection from
ferromagnets, circular polarized light sources,
external magnetic fields
SHE small electrical currents in simple
semiconductor microchips
SHE microchip, 100?A
high-field lab. equipment, 100 A
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Spin and Anomalous Hall effects
Spin-orbit coupling force deflects like-spin
particles
InMnAs
Simple electrical measurement of magnetization
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Skew scattering off impurity potential (Extrinsic
SHE/AHE)
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SO-coupling from host atoms (Intrinsic SHE/AHE)
bands from l0 atomic orbitals ? weak
SO (electrons in GaAs) bands from lgt0 atomic
orbitals ? strong SO (holes in GaAs)
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Intrinsic AHE approach explains many experiments

• (Ga,Mn)As systems Jungwirth et al. PRL 02, APL
  03
• Fe Yao, Kleinman, Macdonald, Sinova,
• Jungwirth et al PRL 04
• Co Kotzler and Gil PRB 05
• Layered 2D ferromagnets such as SrRuO3 and
  pyrochlore ferromagnets Onoda and Nagaosa, J.
  Phys. Soc. Jap. 01,Taguchi et al., Science 01,
  Fang et al Science 03, Shindou and Nagaosa, PRL
  01
• Ferromagnetic spinel CuCrSeBr Lee et al. Science
  04

Experiment sAH ? 1000 (W cm)-1 Theroy sAH ? 750
(W cm)-1
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Hall effects family

• Ordinary carrier density and charge magnetic
  field sensing
• Quantum text-book example a strongly correlated
  many-electron system with e.g. fractionally
  charged quasiparticles universal, material
  independent resistance
• Spin and Anomalous relativistic effects in solid
  state
• spin and magnetization generation and
  detection

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