Observation of iron diffusion in the near-surface region of magnetite at 470 K

Experiments are reported, which allow us to quantify the near-surface cation diffusion in (001) oriented Fe$_3$O$_4$ single crystals at temperatures between 470 and 770 K. Thin homoepitaxial films of magnetite, grown using isotopically labeled $^{57}$Fe, were investigated by neutron reflectivity and time-of-flight secondary ion mass spectrometry. By heating the thin films in high vacuum to different temperatures for a well-defined time and determining the ${57}$ Fe distribution along the surface normal, the diffusion lengths are obtained. For the investigated temperature range, diffusion constants of the order of 10$^{−20}$ m$^2$/s are deduced. These results are important in view of near-surface mass transport induced by oxygen chemical potential differences occurring when magnetite is exposed to different gas atmospheres or by adsorbates