Absence of Orbital Hall Magnetoresistance in Nonmagnet/Ferromagnet Bilayers with Large Orbital Torque

TL;DR

This study demonstrates that orbital Hall magnetoresistance is absent in certain bilayers despite large orbital torques, highlighting fundamental differences between orbital and spin transport mechanisms.

Contribution

The paper reveals that orbital currents do not produce OMR in nonmagnet/ferromagnet bilayers, contrasting with spin transport, and clarifies how to distinguish orbital from spin currents.

Findings

Orbital Hall magnetoresistance is not observed despite large orbital torques.

Orbital currents are absorbed isotropically in the ferromagnet, unlike spin currents.

Texture-induced effects can mimic signals, requiring careful interpretation.

Abstract

We report the absence of orbital Hall magnetoresistance (OMR) in nonmagnet/ferromagnet bilayers, challenging the general assumption that orbital transport mimics spin transport. Despite the observation of giant orbital torques, confirming the generation of orbital currents, thickness-dependent magnetoresistance measurements reveal that the signal is dominated by the intrinsic magnetoresistance of the ferromagnet and current shunting, with no discernible OMR contribution. We attribute this contradiction to the distinct transport properties of orbital compared with spin. Orbital currents undergo isotropic bulk absorption in the ferromagnet rather than anisotropic interfacial reflection required for OMR. Furthermore, we find that texture-induced magnetoresistance and self-torques in Ni-based bilayers can generate misleading signals, suggesting that caution is required when employing Ni in orbitronic studies. These findings clarify the distinct physical rules governing orbital transport and provide a simple method to distinguish spin and orbital currents.