The diffusion of Fe atoms on clean W(100) and W(110) surfaces and along surface steps, and the diffusion of Fe adatoms and vacancies on Fe/W(100) and Fe/W(110) films was investigated by using ab initio DFT methods. The results demonstrated that even a single Fe adatom on the W(100) surface locally induced partial de-reconstruction of the surface, leading to an activation energy for hopping diffusion of 1.2eV, which was lower on the reconstructed than on the ideal surface. On W(110), diffusion occurred via elementary jumps along close-packed directions. The calculated activation energies of 0.7eV were in quantitative agreement with experimental estimates. The exchange diffusion of Fe was unfavorable on both surfaces. Investigations of adatom diffusion on 1ML Fe films revealed an interplay between structural and magnetic effects. For non-magnetic Fe/W(100) films, and coverages below 0.4ML, adatoms did not propagate the pseudomorphic structure, but occupied bridge instead of hollow sites. The site preference changed to hollows at higher coverages. Correspondingly, the potential energy surface was quite smooth; leading to low activation energies for hopping diffusion of 0.4 and 0.5eV for jumps to nearest- and next-nearest neighbor sites, respectively. Exchange diffusion required a larger activation energy (0.7eV). Antiferromagnetic ordering of the film completely changed the situation. Magnetic interactions around the adsorbate were frustrated, and the adatom induced the formation of a ferromagnetic defect. As a result of the frustration, the potential energy was flatter than for the non-magnetic case; with an activation energy of 0.3eV. This led to the prediction of faster diffusion below the Néel temperature of the film. The Fe adatoms on Fe/W(110) induced a local transition from a pseudomorphic to a close-packed arrangement. The adatom was incorporated into the film to form a Fe-Fe dumb-bell occupying a lattice-site. Investigations were also made of vacancy diffusion in Fe/W films and adatom diffusion along step edges. Vacancy diffusion required a higher activation energy than did adatom hopping; especially in Fe/W(100) films. Diffusion along steps was studied on a vicinal (110) surface with <100>-type steps. The minimal activation energy was, at 1.3eV, considerably higher than that for diffusion on the terraces. Decoration of the steps with a row of Fe atoms lowered all of the activation energies, so that the diffusion rates on terraces and along Fe-decorated steps were comparable.

Diffusion of Fe Atoms on W Surfaces and Fe/W Films and along Surface Steps. D.Spišák, J.Hafner: Physical Review B, 2004, 70[19], 195426 (13pp)