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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 Móssbauer spectroscopy. The reactive deposition of Fe led to the island growth of Fe3O4, 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 wiistite. Unusual long-range magnetic order appeared at room temperature for thicknesses between 3 and 10 monolayers, the appearance of which requires severe structural modification from the rock-salt structure. [ABSTRACT FROM AUTHOR]