Unidirectional orbital magnetoresistance in light metal/ferromagnet bilayers

TL;DR

This paper reports the discovery of a unidirectional orbital magnetoresistance in Cu/Co bilayers caused by orbital momentum effects, revealing new insights into orbital-to-spin transfer processes relevant for spintronic device optimization.

Contribution

It demonstrates the existence of orbital-UMR in light metal/ferromagnet bilayers and elucidates its dependence on orbital and magnon interactions, advancing understanding of orbital transport in spintronics.

Findings

Orbital-UMR scales with torque efficiency and orbital Rashba-Edelstein effect.

Magnon contribution to UMR appears in Co layers thicker than 5 nm.

Orbital-to-spin conversion length is comparable to the thickness where magnon effects emerge.

Abstract

We report the observation of a unidirectional magnetoresistance (UMR) that originates from the nonequilibrium orbital momentum induced by an electric current in a naturally oxidized Cu/Co bilayer. The orbital-UMR scales with the torque efficiency due to the orbital Rashba-Edelstein effect upon changing the Co thickness and temperature, reflecting their common origin. We attribute the UMR to orbital-dependent electron scattering and orbital-to-spin conversion in the ferromagnetic layer. In contrast to the spin-current induced UMR, the magnon contribution to the orbital-UMR is absent in thin Co layers, which we ascribe to the lack of coupling between low energy magnons and orbital current. The magnon contribution to the UMR emerges in Co layers thicker than about 5 nm, which is comparable to the orbital-to-spin conversion length. Our results provide insight into orbital-to-spin momentum transfer processes relevant for the optimization of spintronic devices based on light metals and orbital transport.