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Ultrathin (111)-oriented polar iron oxide films were grown on a Pt(111) single crystal either by the reactive deposition of iron or oxidation of metallic iron monolayers. These films were characterized using low energy electron diffraction, scanning tunneling microscopy and conversion electron Mossbauer spectroscopy. The reactive deposition of Fe led to the island growth of Fe 3 O 4 , in which the electronic and magnetic properties of the bulk material were modulated by superparamagnetic size effects for thicknesses below 2 nm, revealing specific surface and interface features. In contrast, the oxide films with FeO stoichiometry, which could be stabilized as thick as 4 nm under special preparation conditions, had electronic and magnetic properties that were very different from their bulk counterpart, wüstite. Unusual long range magnetic order appeared at room temperature for thicknesses between three and ten monolayers, the appearance of which requires severe structural modification from the rock-salt structure. PACS: 68.47.Gh Oxide surfaces, 68.55.-a Thin film structure and morphology, 75.70.Ak Magnetic properties of monolayers and thin films, 81.15.-z Methods of deposition of films and coatings; film growth and epitaxy, 82.80.Ej X-ray, Mössbauer, and other γ-ray spectroscopic analysis methods

The structural and magnetic properties of ultrathin FeO(111) films on Pt(111) with thicknesses from 1 to 16 monolayers (MLs) were studied using the nuclear inelastic scattering of synchrotron radiation. A distinct evolution of vibrational characteristics with thickness, revealed in the phonon density of states (PDOS), shows a textbook transition from 2D to 3D lattice dynamics. For the thinnest films of 1 and 2 ML, the low-energy part of the PDOS followed a linear ∝E dependence in energy that is characteristic for two-dimensional systems. This dependence gradually transforms with thickness to the bulk ∝E^{2} relationship. Density-functional theory phonon calculations perfectly reproduced the measured 1-ML PDOS within a simple model of a pseudomorphic FeO/Pt(111) interface. The calculations show that the 2D PDOS character is due to a weak coupling of the FeO film to the Pt(111) substrate. The evolution of the vibrational properties with an increasing thickness is closely related to a transient long-range magnetic order and stabilization of an unusual structural phase.

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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 57 Fe 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.