Control of magnetism in cobalt nanoparticles by oxygen passivation

TL;DR

This paper demonstrates how oxygen passivation modifies the magnetic properties of cobalt nanoparticles by forming oxide shells, leading to enhanced coercivity and exchange anisotropy, with potential implications for magnetic nanomaterials.

Contribution

It reports a novel method for controlling magnetism in cobalt nanoparticles through oxide passivation, specifically by growing CoO and Co3O4 shells and analyzing their effects.

Findings

Co-CoO core-shell structures show strong exchange anisotropy.

Enhanced coercive field observed in Co-CoO nanoparticles.

Low temperature paramagnetic behavior linked to oxide shell defects.

Abstract

We report on the preparation of ferromagnetic cobalt nanospheres with antiferromagnetic oxide capping layer and its implication for the variation in magnetic property. The hcp cobalt nanospheres were prepared by thermal decomposition of cobalt carbonyl in the presence of organic surfactants. The spherical nanoparticles thus prepared were oxidized to grow antiferromagnetic layers of varying composition and thickness on top of cobalt spheres. High resolution transmission electron microscopy confirmed growth of Co3O4 in one case and CoO in another case. Strong exchange anisotropy and enhanced coercive field was observed due to the core-shell structure in Co-CoO system. On the other hand only a marginal improvement was seen in Co-Co3O4 system. A low temperature paramagnetic behavior was also observed that is interpreted in the framework of crystal defects in the oxide shell.