# Negative spin Hall magnetoresistance of Pt on the bulk easy-plane antiferromagnet NiO

## TL;DR

This study investigates the unusual negative spin Hall magnetoresistance in Pt on NiO, revealing how magnetic moment alignment in the easy-plane antiferromagnet influences SMR signals across various temperatures and magnetic fields.

## Contribution

It demonstrates that the negative SMR arises from magnetic moments aligning perpendicular to the external field in the easy-plane NiO without ferromagnetic coupling, and shows how surface treatment enhances spin-mixing conductance.

## Key findings

- Negative SMR observed over wide temperature and field ranges.
- SMR sign linked to magnetic moments perpendicular to external field.
- Surface treatment doubles the spin-mixing conductance.

## Abstract

We report on spin Hall magnetoresistance (SMR) measurements of Pt Hall bars on the antiferromagnetic NiO(111) single crystal. An SMR with a sign opposite of conventional SMR is observed over a wide range of temperatures as well as magnetic fields stronger than 0.25T. The negative sign of the SMR can be explained by the alignment of magnetic moments being almost perpendicular to the external magnetic field within the easy plane (111) of the antiferromagnet. This correlation of magnetic moment alignment and external magnetic field direction is realized just by the easy-plane nature of the material without the need of any exchange coupling to an additional ferromagnet. The SMR signal strength decreases with increasing temperature, primarily due to the decrease in N\'eel order by including fluctuations. An increasing magnetic field increases the SMR signal strength as there are less domains and the magnetic moments are more strongly manipulated at high magnetic fields. The SMR is saturated at an applied magnetic field of $6$~T resulting in a spin-mixing conductance of $\sim10^{18}~ \Omega^{-1}$m$^{-2}$, which is comparable to that of Pt on insulating ferrimagnets such as yttrium iron garnet. An argon plasma treatment doubles the spin-mixing conductance.