Stability, magnetic order, and electronic properties of ultrathin Fe3O4 nanosheets

We study the stability, magnetic order, charge segregation, and electronic properties of a novel three-layered ${\mathrm{Fe}}₃{\mathrm{O}}₄$ film by means of Hubbard-corrected density functional theory calculations. The stable film is predicted to consist of close-packed iron and oxygen layers, comprising a center layer with octahedrally coordinated Fe sandwiched between two layers with tetrahedrally coordinated Fe. The film exhibits an antiferromagnetic type I spin order. A charge analysis confirms that the stable structure has distinct charge segregation, with ${\mathrm{Fe}}²⁺$ ions in the center layer and ${\mathrm{Fe}}³⁺$ in the tetrahedral surface layers. Examination of the electronic band structures and densities of states shows that the bandgap is substantially reduced, from 2.4 eV for the bulk rocksalt to 0.3 eV for the film. The reduction in the bandgap is a consequence of the $2+$ to $3+$ change in oxidation state of Fe in the surface layers.