Unidirectional magnetoresistance and spin-orbit torque in NiMnSb

TL;DR

This study combines theory and experiments to analyze spin-orbit torque and unidirectional magnetoresistance in NiMnSb, revealing complex dependencies on crystal orientation and challenging common interpretative assumptions.

Contribution

It provides a comprehensive analysis of spin-orbit phenomena in NiMnSb, highlighting the influence of crystal symmetry and spin-orbit coupling types, and questions standard interpretation methods.

Findings

Unidirectional magnetoresistance has both longitudinal and transverse components.

The effects depend on the crystal direction of the applied electric field.

There is no direct link between spin polarization and unidirectional magnetoresistance.

Abstract

Spin-dependent transport phenomena due to relativistic spin-orbit coupling and broken space-inversion symmetry are often difficult to interpret microscopically, in particular when occurring at surfaces or interfaces. Here we present a theoretical and experimental study of spin-orbit torque and unidirectional magnetoresistance in a model room-temperature ferromagnet NiMnSb with inversion asymmetry in the bulk of this half-heusler crystal. Besides the angular dependence on magnetization, the competition of Rashba and Dresselhaus-like spin-orbit couplings results in the dependence of these effects on the crystal direction of the applied electric field. The phenomenology that we observe highlights potential inapplicability of commonly considered approaches for interpreting experiments. We point out that, in general, there is no direct link between the current-induced non-equilibrium spin polarization inferred from the measured spin-orbit torque and the unidirectional magnetiresistance. We also emphasize that the unidirectional magnetoresistance has not only longitudinal but also transverse components in the electric field -- current indices which complicates its separation from the thermoelectric contributions to the detected signals in common experimental techniques. We use the theoretical results to analyze our measurements of the on-resonance and off-resonance mixing signals in microbar devices fabricated from an epitaxial NiMnSb film along different crystal directions. Based on the analysis we extract an experimental estimate of the unidirectional magnetoresistance in NiMnSb.