EPITAXY OF CO ON FEAL(001) .1. 1X1 FILMS

Ultrathin films of Co have been grown by slow vapor deposition on an FeAl{001} substrate (lattice constant a0 = 2.904 angstrom). Up to a thickness of about 13 angstrom the films are found to be pseudomorphic with the substrate, but contain 5-10 at. % Al. Quantitative low-energy electron diffraction (LEED) finds the best-fit model for the films to be pure Co with body-centered-tetragonal structure, bulk interlayer spacing 1.40+/-0.03 angstrom, and first interlayer spacing 1.25+/-0.03 angstrom, suggesting that the Al impurities were disordered and contributed only to the background of the LEED pattern. Strain analysis gives ambiguous results with regard to the equilibrium phase, as both fcc and bcc Co (metastable phases) have strain ratios within acceptable limits. The strains themselves are unusually large for fcc Co and fairly small for bcc Co, but the Poisson ratio has normal values for fcc Co and unusually small values for bcc Co. The contraction of the first interlayer spacing is abnormally large (approximately 11%) for magnetic materials.