Migration and annihilation of non-equilibrium point defects in sputter deposited nanocrystalline alpha-Fe films

Thin metallic films produced by sputtering methods show a considerable amount of non-equilibrium point defects, which annihilate during annealing close to room temperature. In order to identify type and migration enthalpy of these defects, volume self-diffusion is studied in ion-beam sputtered nanocrystalline Fe films on short time scales (<20 min) and for short diffusion lengths (<2 nm) between 250 and 510 °C. Neutron reflectometry is applied on [natFe (7 nm)/57Fe(3 nm)]15 isotope multilayers in order to derive the diffusivities. The diffusivities decrease strongly as a function of annealing time as a result of an annihilation process of non-equilibrium point defects. The initial diffusivities present at the beginning of annealing follow the Arrhenius law with an activation enthalpy of 0.8 eV, which is identified as the migration enthalpy of single vacancies by comparison to the literature. The motion of these non-equilibrium vacancies seems to be the dominating mechanism in the annihilation procedure. From the pre-exponential factor an initial vacancy concentration of about 10−12 (mole fraction) can be assessed, which is significantly higher than the corresponding thermal equilibrium value.