GROWTH MODE AND ATOMIC-STRUCTURE OF ULTRATHIN FE FILMS ON RH(001) DETERMINED BY QUANTITATIVE LOW-ENERGY-ELECTRON DIFFRACTION

The epitaxial growth of Fe on Rh{001} at room temperature is studied by means of quantitative low-energy electron diffraction and Auger electron spectroscopy. The Fe films are pseudomorphic to the substrate and grow in the layer-by-layer mode for at least three layers-no attempts were made to determine the growth mode above this thickness. The spacing between Fe and Rh at the substrate-film interface remains approximately the same (about 1.75 angstrom), within experimental error, when the Fe films grow from one to two and three layers. The Fe-Fe interlayer spacing in the bilayer films is also about the same (1.73 angstrom), but in the three-layer film the first two interlayer spacings collapse to about 1.65 angstrom. Thicker (eight- to ten-layer) Fe films have bulk spacings of 1.56 angstrom and a 5.8%-expanded surface interlayer spacing (1.65 angstrom). These films have a compressive strain in the film plane (the misfit to Rh{001} is - 6.3%) and have a body-centered-tetragonal structure. Elastic strain analysis shows that the equilibrium (i.e., the unstrained) phase is bcc Fe; the bulk interlayer spacing in the films is expanded by 8.7% over the equilibrium value of bcc Fe (1.43 angstrom) as a consequence of the epitaxial strain in the plane of the layers, and the atomic volume is reduced by 4.5%.