NEW PHASES AND CHEMICAL SHORT-RANGE ORDER IN CO-DEPOSITED COFE THIN-FILMS WITH BCC STRUCTURE - AN NMR-STUDY

A Co-59 NMR study has been carried out on several series of co-evapora ted Co1-xFex thin-film alloys prepared on MgO (001), GaAs (100), and G aAs (110) substrates at deposition temperatures between 175 degrees C and 500 degrees C. The sample thicknesses varied between 100 Angstrom and 1000 Angstrom and the alloy concentrations were in the range 0.1 < x < 0.3, X-ray diffraction and NMR show that the stability limits of the bcc phase in CoFe alloys is shifted from the x = 0.25 observed in the bulk alloys down to about x = 0.11 in thin films. For x = 0.27 and at the deposition temperature of 500 degrees C, a new ordered phase h as been stabilised where Co has two Fe atoms only in its first coordin ation shell, Other samples, grown at lower temperatures, also exhibit an exotic chemical short range order (CSRO) where Co coordinations wit h zero and two Fe neighbours dominate. A mixture of bcc Co (and not fe e Co as in the bulk alloys) and unknown ordered bcc intermetallics can account for the observed CSRO. Theoretical ground-state phases for th e bcc lattice are considered in order to explain the observations.