Competing charge orders in magnetite ultrathin films

Abstract

Magnetite (Fe3O4) is a half-metallic ferrimagnet with high critical temperature and large magnetic moment. The strong electron correlations lead to a rich scenario of phase transitions at low temperatures where charge order plays a crucial role: the Verwey transition at 120 K, with the opening of an insulating gap accompanied by a structural distortion, or the development of a ferroelectric polarization at 40 K. Modification of the Verwey transition has been pursued in thin films, under pressure or strain, or introducing defects, always causing a decrease of the Verwey temperature. However, the existence of an insulating gap at different Fe3O4(001) terminations at ambient conditions could translate the rich Verwey physics to higher temperatures. By means of ab-initio calculations, we will demonstrate that low dimensionality introduces a competition between surface and bulk charge orders, while preserving the magnetic properties. This is accompanied by a structural distortion involving the surface and subsurface layers, that ultimately manifests in a phase diagram distinct from the bulk one. We will also evidence the existence of a threshold thickness for the emergence of the two Fe valence states required for charge order and half-metallicity.