Mössbauer study of ultrathin FeAl multilayer films

Multilayers of Fe (between 0.3 and 2.0 nm thickness) separated by a 3.0 nm thick Al spacer were prepared by vacuum evaporation and were then investigated by Fe57 Mössbauer spectroscopy measurements between 4.2 and 300 K and in various external magnetic fields. Mixing of the components at the interface was studied by transmission electron microscopy. The formation of a nonmagnetic Al-Fe interface alloy is verified by a detailed analysis of the low temperature Mössbauer spectra. The effective thickness of the Fe layers was deduced from the amount of the nonmagnetic component and it was found to be correlated with the shape of the Fe hyperfine field distribution. A marked change of the temperature and of the external magnetic field dependence of the Fe hyperfine fields were observed as a function of the effective layer thickness. The hyperfine field component attributed to two monolayer thick Fe regions decreases linearly with increasing temperature; it disappears at well below room temperature and it is hardly influenced by external fields up to 7 T. The formation of three and more monolayer thick regions with increasing effective thickness results in an approach to the bulk behavior, T32 -temperature dependence, and smaller magnetic anisotropy.