ions, while a volatile precursor of iron ͓i.e., Fe͑CO͒ 5 ] is dosed onto the substrate surface. In the latter case, Ar atoms are incorporated within the iron oxide lattice. Atomic force microscopy, Rutherford backscattering spectroscopy, and x-ray photoelectron spectroscopy were utilized to characterize the films' surface morphology, stoichiometry and chemical state. The film structure has been analyzed by grazing angle x-ray diffraction ͑XRD͒ and infrared spectroscopies. In particular, infrared spectroscopy has permitted a thorough structural characterization of the films, even in the cases where XRD does not provide information about the structure. Thus, when O 2 ϩ ions are used for the synthesis, iron oxide thin films grow with a hematite structure with the c axis of the crystallites perpendicular to the film surface. However, when an Ar ϩ /O 2 ϩ ion mixture is used, the thin films have a hematite structure with the c axis of the crystallites oriented parallel to the film surface.