  The continuous need towards improving the capacity of magnetic storage
devices requires materials with strong perpendicular magnetic anisotropy. FePd,
CoPd and their Co(Fe)Pt counterparts very attractive candidate for such
purposes. The magnetic properties of these films are largely dependent on the
orientation and local distribution of the L10 FePd phase fraction which are
mainly controlled by diffusion processes and involve diffusion paths of a few
angstroms. Highly ordered as well as disordered epitaxial isotope-periodic
57FePd/natFePd (001) thin films were prepared by molecular-beam epitaxy on
MgO(001) substrate. Short range diffusion of different phases in FePd thin film
induced by room temperature 130 keV He+ irradiation was investigated at
fluences up to 30\times1015 ions/cm2. Conversion electron M\"ossbauer
spectroscopy and synchrotron M\"ossbauer reflectivity experiments showed that
the inter-atomic diffusion across 57FePd/natFePd interface occur mainly via the
iron rich regions. The ratio of the diffusion length in the L10, fcc and iron
rich structure are 1:1.4:5.6 respectively. Assuming that the diffusion
coefficient in the fcc phase is between the diffusion coefficient of the L10
phase in the crystallographic c direction and perpendicular to it, the
diffusion coefficient in the c-direction of the L10 phase is found more than
1.9 times lower than in the a-b plane.
