EXAFS analysis of ultrathin Fe films grown on Ni(100)

We performed an extended X-ray absorption fine structure (EXAFS) investigation of ultrathin Fe films grown in situ. on Ni(1 0 0) surface. From the first shell analysis of the EXAFS oscillations we obtained values of the bond length, of the effective coordination number and of the Debye-Waller factor that are compatible with a model where there is coexistence of face centred cubic phase (with tetragonal distortion) and of body centred cubic (bcc) phase in the films. We also performed theoretical simulations of the EXAFS data including signals from higher order shells with multiple scattering effects. From these simulations we obtained that the experimental data can be well reproduced with a linear combination of spectra, one reproducing the bulk bcc Fe structure and one obtained by a face centred tetragonal (fct) lattice structure with an interlayer distance d = 1.9 A. In the model the relative weights of the two spectra change with an increasing value for the weight corresponding to the bcc phase. At 20 ML, the weight for the fct phase (30-40%) is still considerable. (C) 2001 Elsevier Science B.V. All rights reserved