The optimization of coercivity in granular recording material using rapid thermal processing

Granular cobalt films about 500 nm thick and ranging in composition from 0.25-0.85 fraction by volume of cobalt have been prepared by co-sputtering cobalt and silicon dioxide from a mosaic target. At a given sputtering power and pressure the volume fraction (q) of cobalt in the films is determined by the areal fraction of the SiO ₂ base target covered by the cobalt mosaic. Transmission electron microscopy indicates that the samples are composed of particles of cobalt with diameters in the range 2.5 nm-6.5 nm dispersed in the SiO ₂ matrix. Magnetic properties of samples with q in the range 0.25-0.85 are measured using a vibrating sample magnetometer. Samples with q below 0.52 are generally superparamagnetic in their as-deposited state. Subsequent rapid thermal processing (RTP) is shown to initiate ferromagnetic behaviour with a coercivity that increases with processing temperature. Optical constants of the films, measured by ellipsometry, are found to be in good agreement with Maxwell Garnett theory. Some films exhibit a saturation polar Kerr rotation that is significantly greater than that of pore cobalt films