THICKNESS-DEPENDENT COERCIVITY OF ULTRATHIN CO FILMS ON A ROUGH SUBSTRATE - CU-BUFFERED SI(111)

The hysteresis loops, of Co films with thicknesses ranging from 12- to 80-monolayer-equivalent (MLE) coverages grown by thermal evaporation on a Cu-covered Si(lll) surface, were measured in situ by the surface magneto-optic Kerr effect (SMOKE) technique. The hysteresis loops were measured as a function of Co coverage under an external sinusoidal ma gnetic field at fixed driving frequency. The coercivity H-c of the Co him versus thickness t followed a power law t(-n) with n = 0.4 +/- 0.1 between 12 and 44 MLE, and stabilized after 44 MLE, up to the 80 MLE studied. The surface morphology of the 80-MLE Co film was imaged ex si tu by atomic force microscopy (AFM) and scanning tunneling microscopy (STM), revealing cauliflower-like islands that were rough both in the short and long range. Analysis of the height-height correlation functi on for the largest image gave measurements of the effective roughness exponent alpha (similar to 0.8), the vertical interface width w(simila r to 2500 Angstrom), and the lateral correlation length xi(similar to 10 000 Angstrom). We suggest that the coercivity changed in part due t o changes in roughness of the Co films, deposited on a rough substrate : the spatial roughness would create an additional surface anisotropy. contributing to a fluctuation in the domain wall energy, resulting in a roughness-dependent coercivity. (C) 1998 Elsevier Science B.V.