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 in the several nanometer thickness range was changed by UHV 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 57Fe probe layers. As a function of the added-Fe (ad-Fe) dose and annealing temperature, we identified four types of super-structures that were homogenously 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 supers...