Surface magnetism in ZnO/Co3O4 mixtures

TL;DR

This paper investigates the unexpected room temperature ferromagnetism in ZnO/Co3O4 mixtures, attributing it to surface reduction of Co ions and electrostatic interactions, supported by spectroscopic evidence.

Contribution

It provides a detailed electronic structure analysis explaining the ferromagnetism in ZnO/Co3O4 mixtures, a phenomenon not previously understood.

Findings

Surface reduction of Co+3 to Co2+ at particle surfaces.

Electrostatic interactions cause dispersion of Co3O4 particles.

Reduction explains the observed ferromagnetic signal.

Abstract

We recently reported the observation of room temperature ferromagnetism in mixtures of ZnO and Co3O4 despite the diamagnetic and antiferromagnetic character of these oxides respectively. Here we present a detailed study on the electronic structure of this material in order to account for this unexpected ferromagnetism. Electrostatic interactions between both oxides lead to a dispersion of Co3O4 particles over the surface of ZnO larger ones. As a consequence, the reduction of Co+3 to Co2+ at the particle surface takes place as evidenced by XAS measurements and optical spectrocopy. This reduction allows to xplain the observed ferromagnetic signal within the well established theories of magnetism.