Isotope-periodic multilayer method for short self-diffusion paths – a comparative neutron and synchrotron Mössbauer reflectometric study of FePd alloys

FePt, FePd, CoPt, and CoPd in equilibrium exhibit the L1₀ structure with high perpendicular magnetic anisotropy making them attractive candidates for high-density magnetic recording. Magnetic properties of these films depend on the distribution and orientation of the L1₀ fraction controlled by diffusion on atomic scale. Epitaxial isotope-periodic natFePd/⁵⁷FePd alloy films were prepared by molecular beam epitaxy and heat treated at 500°C for various retention times. Isotope-sensitive non-destructive methods, neutron reflectometry and synchrotron Mössbauer reflectometry were applied to follow very short diffusion paths normal to the film plane. Squared diffusion lengths and diffusion profiles were obtained from the fitting of experimental reflectivity curves for each annealing treatment steps. The somewhat different diffusion lengths obtained for the neutron and synchrotronMössbauer reflectograms of the same samples are explained by the larger footprint of the sample in the neutron experiment for which interface inhomogeneities are to be averaged. Diffusion in the microscopically different local environments were modelled by piecewise constant diffusion coefficients in the regions identified as different species by conversion electron Mössbauer spectroscopy.