).We have grown 22 ML thick Fe films on a Cu(100) single crystal. The films were stabilized in the face-centered cubic (fcc) gamma phase by adsorption of carbon monoxide during growth, preventing the transformation to the body-centered cubic (bcc) alpha phase. A structural transformation of these films from fcc to bcc can be induced by Ar+ ion irradiation. Scanning tunneling microscopy images show the nucleation of bcc crystallites, which grow with increasing Ar+ ion dose and eventually result in complete transformation of the film to bcc. Surface magneto-optic Kerr effect measurements confirm the transformation of the Fe film from paramagnetic (fcc) to ferromagnetic (bcc) with an in-plane easy axis. The transformation can also be observed by low energy electron diffraction. We find only very few nucleation sites of the bcc phase and argue that nucleation of the bcc phase happens under special circumstances during resolidification of the molten iron in the thermal spike after ion impact. Intermixing with the Cu substrate impedes the transformation. We also demonstrate the transformation of films coated with Au to protect them from oxidation at ambient conditions.
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