Structural transition in Fe ultrathin epitaxial films grown on Ni(111)

A structural study of Fe ultrathin epitaxial films, grown at room temperature on Ni(111), has been performed in the 1.5-18 ML coverage range by angle-scanned photoelectron diffraction. Both backscattering and forward-scattering energy regimes have been employed, in order to enhance the structural sensitivity at lower and higher film thicknesses, respectively. Modeling of the experimental data has been performed with multiple scattering calculations. We found indications that Fe atoms in the first layer occupy fcc hollow sites and stack with a pseudomorphic fcc structure up to 2 ML. Concerning the growth mode at these early stages, data suggest that a good substrate wetting and a sharp Fe/Ni interface take place. Between 3 and 6 ML, transition to a bcc(110) phase develops. By quantitative R-factor analysis, we found that Nishiyama-Wassermann (NW) in-plane orientation of the bcc(110) cell (<001>(bcc)parallel to<110>(fcc)) is favored over the Kurdjumov-Sachs (<1 (1) over bar1>(bcc)parallel to<1 (1) over bar0>(fcc)) orientation. The best-fit vertical interlayer distance between bcc(110) planes is d(NW)=2.11 Angstrom (+3.9% expansion) at 6 ML and relaxes to d(NW)=2.05 Angstrom (+1.0%) at 18 ML, in agreement with the angular shift observed for the forward-focusing features. In the same coverage range, the angle between bcc(110) surface basis vectors changes from 67.7degrees to 69.0degrees, corresponding to -1.7% and -1.0% contractions of the surface cell area, respectively.