Spin anomalous-Hall unidirectional magnetoresistance

TL;DR

This paper predicts a new type of unidirectional magnetoresistance caused by the spin anomalous Hall effect in ferromagnetic bilayers, which does not depend on the spin Hall effect in the nonmagnetic layer, offering a novel way to analyze spin-related phenomena.

Contribution

It introduces the concept of spin anomalous-Hall unidirectional magnetoresistance and analyzes its dependence on material and geometric parameters, providing a new method to quantify spin Hall angles.

Findings

The spin AH-UMR arises from charge-spin conversion via the spin anomalous Hall effect.

The effect depends on material properties and layer thicknesses.

Sign change in UMR can occur when layer thicknesses are varied, enabling measurement of spin Hall angles.

Abstract

We predict a spin anomalous-Hall unidirectional magnetoresistance (AH-UMR) in conducting bilayers composed of a ferromagnetic layer and a nonmagnetic layer, which does $\textit{not}$ rely on the spin Hall effect in the normal metal layer$-$in stark contrast to the well-studied unidirectional spin-Hall magnetoresistance$-$but, instead, arises from the spin anomalous Hall effect in the ferromagnetic layer. Physically, it is the charge-spin conversion induced by the spin anomalous Hall effect that conspires with the structural inversion asymmetry to generate a net nonequilibrium spin density in the ferromagnetic layer, which, in turn, modulates the resistance of the bilayer when the direction of the applied current or the magnetization is reversed. The dependences of the spin AH-UMR effect on materials and geometric parameters are analyzed and compared with other nonlinear magnetoresistances. In particular, we show that, in magnetic bilayers where spin anomalous Hall and spin Hall effects are comparable, the overall UMR may undergo a sign change when the thickness of either layer is varied, suggesting a scheme to quantify the spin Hall or spin anomalous Hall angle via a nonlinear transport measurement.