High precision LEED structure analysis of ultra-thin epitaxial fcc Fe films on Cu(100)

Abstract

We present structure determinations of ultra-thin epitaxial fcc Fe films on Cu(100) by low energy electron diffraction in the coverage regime of 5–10 monolayers. Special emphasis is on the precision of both the measurement of diffraction intensities and their full dynamical analysis. We demonstrate that it is possible to achieve an almost perfect fit between theory and experiment mirrored by R-factors as low as R P = 0.090, R ZJ= 0.026 and R 2 = 0.028 which to our knowledge are the best reported for non-trivial structures. The structure of the films, which stays practically constant in the coverage regime considered, is characterized by a substantial surface reconstruction, an expanded spacing between the first two iron layers and an unstrained in-plane lattice constant a P(Fe) = 2.52 Å rather than that for the substrate ( a P(Cu) = 2.55 Å). The surface reconstruction, which always moves to the top layer when the film thickness is made to grow, can be interpreted as a precursor for the transition to bcc Fe which takes place at higher coverages. The expansion of the top layer spacing, which is consistent with the reconstruction, is fully in line with published surface magnetic measurements which exhibit ferromagnetic coupling in the first two layers.