Nanoscale diffusion in amorphous Fe₇₅Zr₂₅ films

Self-diffusion of Fe in amorphous Fe₇₅Zr₅ films has been studied over a wide temperature range by combining secondary-ion-mass spectrometry and nuclear-resonance reflectivity measurements. Subnanometer accuracy of nuclear-resonance reflectivity in diffusion length measurement allows quantitative determination of time-dependent diffusivity of Fe during structural relaxation. A clear correlation between diffusivity and different types of structural relaxations is observed. It is found that in both structurally relaxed and unrelaxed states, diffusive jumps occur via a collective motion of a group of atoms. However, the presence of excess free volume in unrelaxed amorphous films causes the activation energy as well as the diffusion entropy to decrease, suggesting that the average number of atoms participating in a diffusive jump is significantly less as compared to that in the fully relaxed state. The typical diffusion length involved in annihilation of free volume is 0.7 nm, which agrees with the length scale of structural fluctuations as seen in neutron- and x-ray-scattering experiments.