Magnetoresistance of heavy and light metal/ferromagnet bilayers

TL;DR

This study investigates the magnetoresistance in heavy and light metal/ferromagnet bilayers, revealing a large anisotropic SMR in heavy metals and a current-dependent unidirectional SMR linked to spin-orbit torques.

Contribution

It demonstrates the presence of a large anisotropic SMR in heavy metal/Co bilayers and identifies a current-dependent unidirectional SMR correlated with spin-orbit torque.

Findings

Heavy NM/Co bilayers exhibit large anisotropic SMR (~0.5%)

Light NM/Co bilayers show standard AMR without SMR

Unidirectional SMR correlates with spin-orbit torque amplitude

Abstract

We studied the magnetoresistance of normal metal (NM)/ferromagnet (FM) bilayers in the linear and nonlinear (current-dependent) regimes and compared it with the amplitude of the spin-orbit torques and thermally induced electric fields. Our experiments reveal that the magnetoresistance of the heavy NM/Co bilayers (NM = Ta, W, Pt) is phenomenologically similar to the spin Hall magnetoresistance (SMR) of YIG/Pt, but has a much larger anisotropy, of the order of 0.5%, which increases with the atomic number of the NM. This SMR-like behavior is absent in light NM/Co bilayers (NM = Ti, Cu), which present the standard AMR expected of polycrystalline FM layers. In the Ta, W, Pt/Co bilayers we find an additional magnetoresistance, directly proportional to the current and to the transverse component of the magnetization. This so-called unidirectional SMR, of the order of 0.005%, is largest in W and correlates with the amplitude of the antidamping spin-orbit torque. The unidirectional SMR is below the accuracy of our measurements in YIG/Pt.