Spin Hall magnetoresistance as a probe for surface magnetization in Pt/CoFe$_2$O$_4$ bilayers

TL;DR

This study demonstrates that spin Hall magnetoresistance (SMR) can effectively probe surface magnetization in Pt/CoFe$_2$O$_4$ bilayers, revealing surface-specific magnetic properties and behaviors distinct from bulk magnetization.

Contribution

The paper introduces the use of SMR as a surface-sensitive tool to map surface magnetization, highlighting differences from bulk magnetization and identifying contributions from surface effects and ordinary magnetoresistance.

Findings

Surface magnetization is independent of CFO thickness.

Surface magnetization does not saturate at high fields.

Surface magnetization lacks zero-field steps present in the bulk.

Abstract

We study the spin Hall magnetoresistance (SMR) in Pt grown $\textit{in situ}$ on CoFe$_2$O$_4$ (CFO) ferrimagnetic insulating (FMI) films. A careful analysis of the angle-dependent and field-dependent longitudinal magnetoresistance indicates that the SMR contains a contribution that does not follow the bulk magnetization of CFO but it is a fingerprint of the complex magnetism at the surface of the CFO layer, thus signaling SMR as a tool for mapping surface magnetization. A systematic study of the SMR for different temperatures and CFO thicknesses gives us information impossible to obtain with any standard magnetometry technique. On one hand, surface magnetization behaves independently of the CFO thickness and does not saturate up to high fields, evidencing that the surface has its own anisotropy. On the other hand, characteristic zero-field magnetization steps are not present at the surface while they are relevant in the bulk, strongly suggesting that antiphase boundaries are the responsible of such intriguing features. In addition, a contribution from ordinary magnetoresistance of Pt is identified, which is only distinguishable due to the low resistivity of the $\textit{in-situ}$ grown Pt.