Exchange biasing of single-domain Ni nanoparticles spontaneously grown in an antiferromagnetic MnO matrix

TL;DR

This study demonstrates the spontaneous formation of Ni nanoparticles within MnO, resulting in significant exchange bias effects that enhance magnetic hardness, with potential implications for magnetic device applications.

Contribution

It introduces a novel method of creating Ni/MnO composites with spontaneous Ni nanoparticle formation and high exchange bias, advancing magnetic nanomaterial design.

Findings

Exchange bias fields up to 100 Oe observed.

Coercive field scales nearly linearly with exchange bias.

Ni nanoparticles are 15-30 nm and fully encased in MnO.

Abstract

Exchange biased composites of ferromagnetic single-domain Ni nanoparticles embedded within large grains of MnO have been prepared by reduction of Ni$_x$Mn$_{1-x}$O$_4$ phases in flowing hydrogen. The Ni precipitates are 15-30 nm in extent, and the majority are completely encased within the MnO matrix. The manner in which the Ni nanoparticles are spontaneously formed imparts a high ferromagnetic- antiferromagnetic interface/volume ratio, which results in substantial exchange bias effects. Exchange bias fields of up to 100 Oe are observed, in cases where the starting Ni content $x$ in the precursor Ni$_x$Mn$_{1-x}$O$_4$ phase is small. For particles of approximately the same size, the exchange bias leads to significant hardening of the magnetization, with the coercive field scaling nearly linearly with the exchange bias field.