Theory of spin Hall magnetoresistance (SMR) and related phenomena

TL;DR

This paper reviews the theoretical understanding of spin Hall magnetoresistance (SMR), explaining its origin in bilayers of magnetic insulators and metals, and discusses its applications and limitations in measuring magnetization and spin torques.

Contribution

It introduces a minimal theoretical framework for calculating SMR using spin diffusion theory and boundary conditions, and discusses related mechanisms and recent developments.

Findings

SMR is modulated by the angle between current and magnetization.

A minimal set of parameters can fit experimental data.

Theoretical limitations and alternative mechanisms are discussed.

Abstract

We review the recently discovered spin Hall magnetoresistance (SMR) and related effects from a theoretical point of view. The SMR is observed in bilayers of a magnetic insulator and a metal, in which spin currents aregenerated in the normal metal due to the spin Hall effect. The associated angular momentum transfer to the ferromagnetic layer and thereby the electrical resistance is modulated by the angle between the applied current and the magnetization direction. The SMR provides a convenient tool to non-invasively measure the magnetization direction and spin-transfer torque to an insulator. We introduce the minimal theoretical instruments to calculate the SMR, i.e. spin diffusion theory and quantum mechanical boundary conditions. This leads to a small set of parameters that can be fitted to experiments. We discuss the limitations of the theory as well as alternative mechanisms such as the ferromagnetic proximity effect and Rashba spin-orbit torques, and point out new developments related to the SMR.