Structural analysis of Fe/Ni(001) films by photoelectron diffraction

The structure of Fe films, epitaxially grown on Ni(001), has been studied in the 0--14 ML coverage range by means of photoelectron diffraction (PD) in the forward scattering regime. Quantitative analysis by a multiple scattering approach has been performed on Fe films at a coverage of 3 and 7 ML. Analysis of the 3-ML data showed that growth was not layer-by-layer but rather occurred through islands nucleation and that transition from the pseudomorphic fcc to the bcc phase was located in this early stage of growth. In fact, best fit was obtained by calculations on a 2 ML bcc(110)/3 ML fcc(001) Fe film with the $\mathrm{bcc}〈111〉\ensuremath{\parallel}\mathrm{fcc}〈110〉$ in-plane orientation. Interlayer spacings of $2.05\ifmmode\pm\else\textpm\fi{}0.06\AA{},$ $2.01\ifmmode\pm\else\textpm\fi{}0.03\AA{},$ and $1.85\ifmmode\pm\else\textpm\fi{}0.03\AA{}$ were found in the bcc region, between bcc and fcc layers and in the fcc region, respectively. Best-fit in-plane nearest-neighbors (n-n) distance was $2.49\ifmmode\pm\else\textpm\fi{}0.02\AA{},$ in registry with that of the Ni substrate. To analyze the 7-ML data a 4 ML bcc(110)/3 ML fcc(001) film was employed, varying the fitting parameters in the bcc region only. Best fit was obtained for an interlayer spacing of $2.04\ifmmode\pm\else\textpm\fi{}0.04\AA{}$ and in plane n-n distance of $2.47\ifmmode\pm\else\textpm\fi{}0.01\AA{}.$ At 14 ML the PD pattern collected over a 94\ifmmode^\circ\else\textdegree\fi{} azimuthal range displayed symmetry around the [110] substrate direction, which was explained by the equipopulation of the 4 bcc(110) domains satisfying the $\mathrm{bcc}〈111〉\ensuremath{\parallel}\mathrm{fcc}〈110〉$ alignment.