Absence of Spin Hall Magnetoresistance in Pt/(CoNi)n multilayers

TL;DR

This study investigates the magnetoresistance in Pt/(CoNi)n multilayers, finding no detectable spin Hall magnetoresistance (SMR) despite theoretical expectations, due to spin relaxation effects in the ferromagnetic metal.

Contribution

It provides the first systematic experimental analysis of SMR in Pt/(CoNi)n multilayers and explains the absence of SMR through spin relaxation mechanisms.

Findings

SMR magnitude in Pt/(CoNi)n is below 10^{-4}

Experimental spin Hall angle in Pt is ≥ 0.07

Spin relaxation suppresses SMR in ferromagnetic metals

Abstract

We systematically studied the magnetoresistance effect in a Pt/(CoNi)n multilayer system with perpendicular magnetic anisotropy and the fcc (111) texture. The angular dependence of magnetoresistance, including high-order cosine terms, was observed in a plane perpendicular to the electrical current; this was attributed to the geometrical-size effects caused by crystal symmetry, the ordered arrangement of grains, and the anisotropic interface magnetoresistance effect caused by the breaking of the symmetry at interfaces. Based on the accuracy of our experimental results, the magnitude of spin Hall magnetoresistance (SMR) in Pt/(CoNi)n was expected to be below $1\times10^{-4}$. However, on evaluating the spin Hall angle of $\geq$ 0.07 for Pt using spin-torque ferromagnetic resonance measurements, the theoretical magnitude of SMR in our samples was estimated to exceed $7\times10^{-4}$. This absence of SMR in the experimental results can be explained by the Elliott-Yafet spin relaxation of itinerant electrons in the ferromagnetic metal, which indicates that the boundary conditions of the spin current in the heavy metal/ferromagnetic insulator may not be applicable to all-metallic heterostructures.