A Unified Theory of Unusual Anisotropic Magnetoresistance and Unidirectional Magnetoresistance in Nanoscale Bilayers

TL;DR

This paper presents a unified theoretical framework explaining the universal behaviors of unusual anisotropic and unidirectional magnetoresistance in nanoscale magnetic bilayers, linking various experimental observations through fundamental electron transport principles.

Contribution

The authors develop a comprehensive theory that unifies the understanding of UAMR and UMR in magnetic bilayers based on electron transport influenced by magnetization and interfacial potentials.

Findings

Unified explanation for UAMR and UMR phenomena.

Dependence of effects on current direction and magnetization.

Framework captures experimental features like film thickness and magnetic field effects.

Abstract

Nanoscale bilayers containing at least one magnetic layer exhibit universal unusual anisotropic magnetoresistance (UAMR) and unidirectional magnetoresistance (UMR). They are currently understood through various mechanisms related to the interconversion of charge current and spin current, giant magnetoresistance, thermal magnonic effects, thermoelectric effects, and diverse spindependent scattering processes. This raises a fundamental question: do the universal behaviors observed in a wide range of systems stem from underlying general principles? We demonstrate here that both UAMR and UMR arise from electron transport influenced by the magnetization vector present in the magnetic material and the interfacial potential inherent in heterostructures. Specifically, UAMR represents current-independent resistance (resistivity) of bilayers. UMR is the resistance proportional to the current although electron transports of the bilayers are the linear response to high current densities and their induced thermal gradients. Our theory introduces a novel approach that considers the interplay between the magnetization vector, thermal gradients, and the effective internal electric field at the interface. This framework provides a unified explanation for both UMR and UAMR, effectively capturing key experimental features such as dependence on current direction, magnetization orientation, film thickness, and magnetic field strength. Furthermore, it offers a universal perspective that bridges UMR and UAMR effects, enhancing our understanding of spin-dependent transport phenomena in bilayers.