Thermal stability of fcc structure in FeNi films of various thicknesses

Thin films of a polymorphous iron-nickel (FeNi) alloy with a nickel content of 25–30% and a thickness from 0.1 to 1.3 μm have been obtained by thermal deposition in vacuum in a setup with turbomolecular pumps. The phase transitions in these FeNi films have been studied by X-ray diffraction and magnetic phase analysis. The results of these measurements show that the crystallographic stability of a face-centered cubic (fcc) structure in a temperature interval from 70 to −196°C depends on the film thickness. It is suggested that this dependence is determined by crystal structure defects, the density of which is proportional to the film thickness and grain size. It is also established that substrate type does not influence the temperature interval for the existence of stable crystallographic modifications in the sample studied. The results can be used for determining the optimum composition and thickness of a working FeNi layer in thermovoltaic recording media, ensuring a wide operating temperature range.