LIMITS OF METASTABLE EPITAXY - THE STRUCTURE OF ULTRATHIN FE FILMS ONCU3AU(100)

The influence of epitaxial strain on the structure of thin films was i nvestigated using a model system: Fe on Cu3Au(100). Low-energy electro n diffraction (LEED) I(V) measurements and full dynamical I(V) calcula tions were used to determine the atomic structures of Fe films at diff erent thicknesses (3.3, 4.8, 18, and 53 monolayers (ML). LEED measurem ents show that for thicknesses above approximately 1.5 ML iron films a re no longer pseudomorphic. Between 3.3 and 4.8 ML, the Fe films were determined to have a strained bcc structure, rather than the previousl y proposed fee structure. The existence of the strained bcc phase at l ow coverage is consistent with estimates of the total energy, which su ggests that the energy gained by stabilizing the bulk ground state of Fe overcompensates the higher strain energy of the bcc phase. At highe r film thicknesses, 18-53 ML, dynamical LEED I(V) analysis confirm tha t the film structure has evolved into the expected unstrained bcc stru cture of Fe.