Self-diffusion and defect annihilation in nanocrystalline Fe films probed by neutron reflectometry

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