Enhanced iron self-diffusion in the near-surface region investigated by nuclear resonant scattering

The access to X-rays of third generation synchrotron radiation sources enables studies of dynamics in metallic systems in grazing incidence geometry. Combining grazing incidence reflection of X-rays with nuclear resonant scattering of synchrotron radiation allows depth-selective investigations of hyperfine parameters and diffusion phenomena of iron and iron compounds. The unique feature of this method is its sensitivity to near-surface motions of atoms and not exclusively to the atoms on the surface. The depth sensitivity can be varied between about two and more than 10 nm. A 300 nm thick 57Fe sample grown by molecular beam epitaxy on a cleaved MgO(0 0 1) substrate was investigated. The diffusion coefficient of iron in the near-surface layer (thickness about 2 nm) is almost two orders of magnitude larger than in bulk bcc iron at the same temperature.