Ultrathin films of cobalt on Fe{001} and the effect of oxygen

Deposition of Co on a clean Fe{001} surface in ultrahigh vacuum yields initially films with 1×{}1 low-energy electron-diffraction (LEED) patterns up to thicknesses of about 15 {}, then films with increasingly high-background c(2×{}2) LEED patterns for thicknesses up to about 30 {}, and finally disordered films for larger thicknesses. Quantitative LEED analysis finds the 1×{}1 films to have a body-centered-tetragonal structure with in-plane lattice constant a=2.87 {} (pseudomorphic with the Fe{001} substrate) and c=2.792 {}; the first interlayer spacing is contracted by about 11%. Strain analysis of the bulk identifies this structure as a tetragonal distortion of metastable bcc Co. The c(2×{}2) films are found to be pseudomorphic distortions of hcp Co(112{}0), which has a rectangular unit mesh that requires a 6.6% compression in one and a 0.4% compression in the other direction in the surface plane; the film has a bulk interlayer spacing of 1.29 {} (expanded 2.9% over the equilibrium value 1.2535 {}) and about 10% contraction of the first interlayer spacing. It was found that a monolayer of oxygen chemisorbed on the Fe{001} surface before deposition of Co extends the stability range of the strained bcc phase considerably. The oxygen ``floats'' on the surface of the Co film and a 30-{} film still has the 1×{}1 distorted bcc structure, but with a monolayer of oxygen on top. Thicker Co films no longer produce measurable LEED patterns, so that the onset of the c(2×{}2)-distorted hcp (112{}0) structure cannot be observed. {} 1996 The American Physical Society.