Self-diffusion in ion-beam-sputtered nanocrystalline Fe was studied between 310 and 510C, using neutron reflectometry on [natFe(7nm)/57Fe(3nm)]15 isotope multilayers. Neutron reflectometry has the advantage over other methods of diffusivity determination, that diffusion lengths on the order of 1 nm and below could be determined. This enables diffusion experiments in a nanostructure which was not significantly modified by grain growth during annealing. The determined diffusivities were time depended and decrease by more than two orders of magnitude during isothermal annealing. In early stages, diffusion was controlled by frozen-in non-equilibrium point defects (interstitials or vacancies) present after deposition. The decrease in the diffusivities could be attributed to the annihilation of these point defects. For very long annealing times the diffusivities above 400C were in good agreement with volume diffusivities measured in single crystals given in literature. However, at a temperature of 400C and below the diffusivities were still higher than extrapolated literature data also after more than 8 days of annealing, indicating that defect annihilation was still going on.

Self-Diffusion and Defect Annihilation in Nanocrystalline Fe Films Probed by Neutron Reflectometry. S.Chakravarty, H.Schmidt, U.Tietze, D.Lott, N.P.Lalla, A.Gupta: Physical Review B, 2009, 80[1], 014111