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Branislav K. Nikoli Department of Physics & Astronomy, - - PowerPoint PPT Presentation

Jan 25, 2023 160 likes •414 views

Topological Proximity Effect in Spin Pumping and Spin-Transfer Torque in Ferromagnetic Metals: Magnetic Tunnel Junctions and Topological Fundamentals and Spintronic Applications Insulators: Theory and Experiments Branislav K. Nikoli

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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. Nikolić

Department 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 effect NQS 2017, Kyoto

Collaborators

Theory

  • Dr. Po-Hao Chang

Experiment

  • Prof. J. Q. Xiao
  • Dr. Farzad Mahfouzi
  • Prof. N. Nagaosa
  • Prof. J.-P. Wang
  • Dr. Kurt Stokbro
  • Prof. 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

{

Fundamentals Applications

{

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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

Fundamentals Applications

{

{

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 for any 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 shunted through 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 effect NQS 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

  • n 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

  • n 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, Kyoto Nano 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

  • r 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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Spatial Profile of Antidamping SOT in TI/FI Heterostructures and the Role of Evanescent States

NQS 2017, Kyoto PRB 95, 113419 (2016) PRB 89, 195418 (2014) antidamping field-like

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Topological proximity effect

x y 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, 113419 (2016) SOT induced magnetization reversal by domain nucleation and propagation

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Exact Rotating Frame Approach to Spin Pumping in the Absence of Spin Flips

t t t Δ ω

Lead Lead

t

... ...

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 Surface

PRB 93, 155104 (2016) NQS 2017, Kyoto Nano Lett. 14, 3779 (2014)

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Spin Pumping-to-Charge Conversion in TI/FM Heterostructures

PRB 90, 115432 (2014) Floquet-NEGF with ~10 exchanged photons: PRB 85, 054406 (2012)

Nano Lett. 15, 7126 (2015)

Ambipolar electronic transport

  • n the surface of an insulating bulk

Nature Phys. 8, 459 (2012)

NQS 2017, Kyoto

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Conclusions and Open Questions in Pictures

30 60 90 120 150 180 1 2

HgTe: dTI=400, Ny=100, dFI=200 HgTe: dTI=200, Ny=50, dFI=100

GNR γSO=0 GNR γSO=0.03γ

Spin Current eI

Sz/Ñ

ω (e/4π) Precession Cone Angle θ (

  • )

NQS 2017, Kyoto

Spin-to-charge conversion Open questions:

  • computationally

efficient ab initio calculations of SOT in arbitrary geometry

SO torque