Negative spin Hall magnetoresistance of normal metal/ferromagnet bilayers

Min-Gu Kang,Gyungchoon Go,Byong-Guk Park,Kyoung-Whan Kim,Jong-Guk Choi,Kyung-Jin Lee

doi:10.1038/s41467-020-17463-3

Abstract

Interconversion between charge and spin through spin-orbit coupling lies at the heart of condensed-matter physics. In normal metal/ferromagnet bilayers, a concerted action of the interconversions, the spin Hall effect and its inverse effect of normal metals, results in spin Hall magnetoresistance, whose sign is always positive regardless of the sign of spin Hall conductivity of normal metals. Here we report that the spin Hall magnetoresistance of Ta/NiFe bilayers is negative, necessitating an additional interconversion process. Our theory shows that the interconversion owing to interfacial spin-orbit coupling at normal metal/ferromagnet interfaces can give rise to negative spin Hall magnetoresistance. Given that recent studies found the conversion from charge currents to spin currents at normal metal/ferromagnet interfaces, our work provides a missing proof of its reciprocal spin-current-to-charge-current conversion at same interface. Our result suggests that interfacial spin-orbit coupling effect can dominate over bulk effects, thereby demanding interface engineering for advanced spintronics devices.

Highlights

Interconversion between charge and spin through spin-orbit coupling lies at the heart of condensed-matter physics
Our results demonstrate that the interconversion between charge and spin in normal metal (NM)/FM bilayers can be dominated by the interfacial spin–orbit coupling effect, thereby requiring interface engineering for the development of high-efficiency spintronic devices
We obtain a good fitting with our model calculation, but quantitative understanding of the magnetoresistance demands further studies. Based on these results combined with the extended spin drift-diffusion model including the interfacial spin–orbit coupling (ISOC), we suggest approaches to observe the negative spin Hall magnetoresistance (SMR) in NM/FM bilayers at the end of Supplementary Note 8

Summary

Introduction

Interconversion between charge and spin through spin-orbit coupling lies at the heart of condensed-matter physics. Our theory shows that the interconversion owing to interfacial spin-orbit coupling at normal metal/ ferromagnet interfaces can give rise to negative spin Hall magnetoresistance. A normal metal (NM)/ferromagnet (FM) bilayer is a system of extensive research nowadays as it offers a framework to investigate various spin–orbit coupling (SOC) physics, in particular, the interconversion between charge and spin currents. From the physics point of view, identifying its dominant mechanism is of crucial importance for fundamental understanding of spin and charge transport coupled through spin–orbit coupling[1]. A combined action of the SHE and inverse SHE in NM/FM bilayers causes spin Hall magnetoresistance (SMR); the resistivity changes with the y component of magnetization, where the y-axis is perpendicular to both directions of charge-current flow (x) and thickness (z)[8,9]. In metallic NM/FM bilayers[10,11], the resistivity as a function of magnetization is described as ρ 1⁄4 ρ0 þ Δρ1m2x À Δρ2m2y; ð1Þ where ρ0 is the magnetization-independent resistivity, Δρ1 and Δρ2 are ones for the anisotropic magnetoresistance and SMR, respectively, and mx (my) is the normalized magnetization along the x (y) direction

Methods

Results

Conclusion