Superstructures on Epitaxial Fe3O4(111) Films: Biphase Formation versus the Degree of Reduction

We analyzed the formation of biphase superstructures on the (111)-oriented epitaxial magnetite films on Pt(111) as a function of controlled film stoichiometry. The stoichiometry of the films several nanometers thick was changed by ultra-high vacuum in situ deposition of a few monolayers of metallic iron onto the stoichiometric film, followed by annealing. The samples were characterized in situ. The surface structure was determined using low-energy electron diffraction and scanning tunneling microscopy, whereas the phase composition and electronic structure were verified using X-ray photoemission spectroscopy and conversion electron Mossbauer spectroscopy (CEMS) with isotopic Fe-57 probe layers. As a function of the added-Fe (ad-Fe) dose and annealing temperature, we identified four types of superstructures that were homogeneously distributed over the entire surface, and we associated them with an increasing degree of surface reduction. We have proposed a coherent atomic-scale model of the observed superstructures that explains them in terms of the modifications of the two outermost Fe atomic layers. CEMS also allowed us to follow in-depth changes accompanying the biphase occurrence. An excess of ad-Fe, which stabilizes a given surface superstructure, migrates toward the substrates, partially dissolves in Pt, and partially forms an interfacial layer with FeO stoichiometry.