Array of cobalt nanoparticles in silica: Synthesis and effects of thermal annealing
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abstract
The synthesis, characterization, and the effects of isochronal annealing of nearly monodispersive three-dimensional arrays of Co nanoparticles are presented. Samples were obtained by alternating electron beam evaporation of Co and SiO2 layers 0.5 and 20nm thick, respectively, onto fused silica. Transmission electron microscopy (TEM) revealed the presence of 3.2nm diameter Co particles with a size dispersion standard deviation of 16%. Annealing at temperatures up to 800C was carried out in vacuum for 5min. Field and zero-field cooled measurements of the magnetic susceptibility monitored the effects of annealing on the magnetic behavior. Two temperature regimes were observed. One, up to 700C with an activation energy of 0.062eV where no significant particle size change was observed, was tentatively related to bonding ordering within the particles. The other regime, with an activation energy of 0.67eV, was attributed to Co diffusion and reflected particle growth as observed by TEM. These results were supported by exchange bias anisotropy and x-ray photoelectron spectroscopy measurements.
