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Topological proximity effect
Spin Pumping and Spin-Transfer Torque in Magnetic Tunnel
Junctions and Topological
Insulators: Theory and Experiments
Branislav K. NikolićDepartment of Physics & Astronomy,
University of Delaware, Newark, DE 19716, U.S.A.
1638–1655
NQS 2017, Kyoto
https://wiki.physics.udel.edu/qttg
Topological Proximity Effect in Ferromagnetic Metals:
Fundamentals and Spintronic Applications
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Topological proximity effectNQS 2017, Kyoto
Collaborators Theory
Dr. Po-Hao Chang
Experiment
Prof. J. Q. Xiao Dr. Farzad Mahfouzi
Prof. N. NagaosaProf. J.-P. Wang Dr. Kurt StokbroProf. Nicholas
Kioussis
J. M. Marmolejo-Tejada
Computation
Dr. Kapildeb Dolui
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Topological proximity effect
Spin-Transfer Torque: Fundamentals and Applications
NQS 2017, Kyoto
{FundamentalsA
pplic
atio
ns{
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Topological proximity effect
Quantum Transport Theory is Needed to Describe STT
NQS 2017, Kyoto
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Topological proximity effect
Spin-Orbit Torque (SOT): Fundamentals and Applications
NQS 2017, Kyoto
Fund
amen
tals
App
licat
ions{
{PRL 119, 077702 (2017)
T=300 K
Nat. Mater. 12, 240 (2013)
solid-state nonvolatile analogue memory with infinite read-write
endurance
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Topological proximity effect
Current-Driven Nonequilibrium Spin Density as the Origin of
Fieldlike SOT
NQS 2017, Kyoto
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Topological proximity effect
Quantum Transport Theory (Which One?) is Needed to Describe
Interfacially-Driven Antidamping SOT
NQS 2017, Kyoto
Nature 511, 449 (2014)PRB 93, 125303 (2016)
PRB 90, 174423 (2014); PRB 96, 014408 (2017) PRB 91, 134402
(2015); Nat. Nanotech. 9, 211 (2014)
SHE antidamping torque
PRB 84, 113407 (2011)PRB 86, 161406(R)
(2012)arXiv:1604.07885
Berry curvature antidamping torque
MISSING INGREDIENTS: 3D geometry and switching at the
boundaries
PRB 86, 014416 (2012)
PRB 95, 094401 (2017)
antidamping torque is zero in the absence of
spin-dependent scattering
missing diagrams
PRB 94, 104419 (2016) PRB 94, 104420 (2016)
Nat. Nanotech. 12, 980 (2017)
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Topological proximity effect
Trouble with Simplistic Hamiltonians for Describing SOT
Experiments
NQS 2017, Kyoto
Nature 511, 449 (2014)
“Our findings have potential importance for technology, in that
the spin torque ratio for Bi2Se3
at room temperature is larger than that forany previously
measured spin current source
material. However, as noted above, for practical applications
the specific layer structure of our
devices (topological insulator/metallic magnet) does not make
good use of this high intrinsic efficiency
because most of the applied current is shuntedthrough the
metallic magnet and does not contribute
to spin current generation within the topological insulator.
Applications will probably require coupling
topological insulators to insulating (or high-resistivity)
magnets so that the majority of the
current will flow in the topological insulator.”
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Topological proximity effect
Trouble with Simplistic Hamiltonians for Describing
Spin-to-Charge Conversion Experiments
NQS 2017, Kyoto
Nat. Comm. 4, 2944 (2013)
Nano Lett. 15, 7126 (2015)
PRL 113, 196601 (2014)
“possibly due to inhomogeneity of kF and/or instability of the
helical spin structure”
Nature Phys. 12, 1027 (2016)
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Topological proximity effectNQS 2017, Kyoto
This Talk in a Nutshell: ΨTM + ΨFM
What is the electronic and spin structure of interfacial
states
and how they affect SOT?
Nano Lett. 17, 5626 (2017)
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Topological proximity effect
Crash Course on Rashba SO Coupling
1D:
2D:G/WS2
NQS 2017, Kyoto
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Topological proximity effect
Spin Density and Torque from Nonequilibrium Green Function
(NEGF) Formalism
Fundamental quantities of NEGF formalism:
NEGF for steady-state transport:
density of available quantum states: how are those states
occupied:
NEGF-based expression for spin-transfer torque:
NQS 2017, Kyoto
PRB 90, 045115 (2014)
SPIN 3, 1330002 (2013)
Learn more about NEGF from:
Most general torque formula valid in the presence of SOC and
other
spin-nonconserving processes
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Topological proximity effect
Current-Driven Nonequilibrium Spin Texture on the Surface and in
the Bulk of Bi2Se3
PRB 92, 201406(R) (2015)
NQS 2017, Kyoto
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Topological proximity effect
Spectral Function and Spin Textures on the TI Side of TI/FM
Heterostructures
NQS 2017, Kyoto
Nano Lett. 17, 5626 (2017)
PRB 82, 195417 (2010)arXiv:1707.06319
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Topological proximity effect
Spectral Function and Spin Textures on the TI Side of NM/TI/FM
Heterostructures
NQS 2017, Kyoto
Nano Lett. 17, 5626 (2017)
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Topological proximity effect
Spectral Function and Spin Textures on the FM Side of TI/FM
Heterostructures
NQS 2017, Kyoto
Nano Lett. 17, 5626 (2017)
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Topological proximity effect
Tunneling Anisotropic Magnetoresistance (TAMR) as a Probe of
Interfacial Spin Texture
NQS 2017, KyotoNano Lett. 17, 5626 (2017)
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Spin-orbit proximity effect
Adiabatic Expansion of NEGF Spits Out Expressions for Torque,
Pumping and Gilbert Damping
NQS 2017, Kyoto
PRB 95, 113419 (2016)
charge current
charge pumping
spin torque
Gilbert damping
gives antidamping STT or field-like SOT
gives field-like STT or antidamping SOT+
+
+{ contains both equilibrium (should be
subtracted) and nonequilibrium contributions SPIN 3, 1330002
(2013)
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Topological proximity effect
Spatial Profile of Antidamping SOT in TI/FI Heterostructures and
the Role of Evanescent States
NQS 2017, Kyoto
PRB 95, 113419 (2016)
PRB
89, 1
9541
8 (2
014)
antidamping field-like
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Topological proximity effect
xy
z
LLG Simulations of Magnetization Reversal and Switching Phase
Diagram for TI/FI Bilayer
NQS 2017, Kyoto
PRB 92, 144424 (2015)APL 105, 212402 (2014)
PRB
95, 1
1341
9 (2
016)
SOT induced magnetization reversal by domain nucleation and
propagation
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Topological proximity effect
Exact Rotating Frame Approach to Spin Pumping in the Absence of
Spin Flips
t tt
Δω
Lead Leadt
. . .. . .t
Ub
Sample
PRB 79, 054424 (2009)
NQS 2017, Kyoto
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Topological proximity effect
Quantized Spin and Charge Pumping Due to Spin-Momentum Locking
in 2D TIs
NQS 2017, Kyoto
PRB 82, 195440 (2010)
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Topological proximity effect
How to Create 2D TI with Exposed SurfacePRB 93, 155104
(2016)
NQS 2017, Kyoto
Nano Lett. 14, 3779 (2014)
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Topological proximity effect
Spin Pumping-to-Charge Conversion in TI/FM Heterostructures
PRB 90, 115432 (2014) Floquet-NEGF with ~10 exchanged photons:
PRB 85, 054406 (2012)
Nan
o Le
tt. 1
5, 7
126
(201
5)Ambipolar electronic transport on the surface of an insulating
bulk
Nat
ure
Phys
. 8, 4
59 (2
012)
NQS 2017, Kyoto
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Topological proximity effect
Conclusions and Open Questions in Pictures
0 30 60 90 120 150 1800
1
2
HgTe: dTI=400, Ny=100, dFI=200
HgTe: dTI=200, Ny=50, dFI=100
GNR γSO=0 GNR γSO=0.03γ
Spin
Cur
rent
eIS z/Ñ
ω (e
/4π)
Precession Cone Angle θ (o)
NQS 2017, Kyoto
Spin-to-chargeconversion
Open questions: computationally
efficient ab initio calculations of SOT in
arbitrary geometry
SO torque
Spin Pumping and Spin-Transfer Torque in Magnetic Tunnel
Junctions and Topological Insulators: Theory and
ExperimentsCollaborators Spin-Transfer Torque: �Fundamentals and
ApplicationsQuantum Transport Theory �is Needed to Describe
STTSpin-Orbit Torque (SOT): �Fundamentals and
ApplicationsCurrent-Driven Nonequilibrium Spin Density as the
Origin of Fieldlike SOTQuantum Transport Theory (Which One?) is
Needed to Describe Interfacially-Driven Antidamping SOTTrouble with
Simplistic Hamiltonians �for Describing SOT ExperimentsTrouble with
Simplistic Hamiltonians for Describing Spin-to-Charge Conversion
Experiments This Talk in a Nutshell: ΨTM + ΨFM Crash Course on
Rashba SO CouplingSpin Density and Torque from Nonequilibrium Green
Function (NEGF) FormalismCurrent-Driven Nonequilibrium Spin Texture
on the Surface and in the Bulk of Bi2Se3Spectral Function and Spin
Textures on �the TI Side of TI/FM HeterostructuresSpectral Function
and Spin Textures �on the TI Side of NM/TI/FM
HeterostructuresSpectral Function and Spin Textures �on the FM Side
of TI/FM HeterostructuresTunneling Anisotropic Magnetoresistance
(TAMR) as a Probe of Interfacial Spin TextureAdiabatic Expansion of
NEGF Spits Out Expressions for Torque, Pumping and Gilbert Damping
�Spatial Profile of Antidamping SOT in TI/FI Heterostructures and
the Role of Evanescent StatesLLG Simulations of Magnetization
Reversal �and Switching Phase Diagram for TI/FI BilayerExact
Rotating Frame Approach to �Spin Pumping in the Absence of Spin
FlipsQuantized Spin and Charge Pumping �Due to Spin-Momentum
Locking in 2D TIs How to Create 2D TI with Exposed SurfaceSpin
Pumping-to-Charge Conversion �in TI/FM HeterostructuresConclusions
and Open Questions �in Pictures