Modeling and Simulation of Spin Transfer Torque Generated at Topological Insulator/Ferromagnetic Heterostructure

TL;DR

This paper develops a physics-based simulation framework for topological insulator/ferromagnet heterostructures, accurately modeling their electronic band structure and spin transport properties, aligning well with experimental data.

Contribution

It introduces a novel 3D band structure-aware model for TI/FM heterostructures using NEGF and LLGS, improving upon previous models by capturing quantum confinement and exchange effects.

Findings

Successfully models TI surface states and quantum confinement effects.

Achieves good agreement with experimental spin transport data.

Simulates magnetization dynamics incorporating TI surface effects.

Abstract

Topological Insulator (TI) has recently emerged as an attractive candidate for possible application to spintronic circuits because of its strong spin orbit coupling. TIs are unique materials that have an insulating bulk but conducting surface states due to band inversion and these surface states are protected by time reversal symmetry. In this paper, we propose a physics-based spin dynamics simulation framework for TI/Ferromagnet (TI/FM) bilayer heterostructures that is able to capture the electronic band structure of a TI while calculating the electron and spin transport properties. Our model differs from TI/FM models proposed in the literature in that it is able to account for the 3D band structure of TIs and the effect of exchange coupling and external magnetic field on the band structure. Our proposed approach uses 2D surface Hamiltonian for TIs that includes all necessary features for spin transport calculations so as to properly model the characteristics of a TI/FM heterostructure. Using this Hamiltonian and appropriate parameters, we show that the effect of quantum confinement and exchange coupling are successfully captured in the calculated surface band structure compared with the quantum well band diagram of a 3D TI, and matches well with experimental data reported in the literature. We then show how this calibrated Hamiltonian is used with the self-consistent non equilibrium Green's functions (NEGF) formalism to determine the charge and spin transport in TI/FM bilayer heterostructures. Our calculations agree well with experimental data and capture the unique features of a TI/FM heterostructure such as high spin Hall angle, high spin conductivity etc. Finally, we show how the results obtained from NEGF calculations may be incorporated into the Landau-Lifshitz-Gilbert-Slonczewski (LLGS) formulation to simulate the magnetization dynamics of an FM layer sitting on top of a TI.