PHASE AND MAGNETIC-PROPERTIES OF IRON-OXIDE CLUSTERS IN 20CAO-CENTER-DOT-20SIO(2)CENTER-DOT-7FE(2)O(3)CENTER-DOT-6FEO GLASSES

The annealing of 20CaO . 20SiO(2) . 7Fe(2)O(3) . 6FeO glasses at 973K in vacuo produced clusters of iron oxide, the shape of which was nearly spherical and the diameter distributed in the narrow range 25-115 Angstrom. The phase of clusters was identified to be Fe3+(Fe-1.30(3+) Fe-0.55(2+) V-0.15). O-4 in the inverse spinel structure based upon the Mossbauer spectra and x-ray diffraction profiles. The clusters exhibited superparamagnetism and their effective anisotropy energy constant was inversely proportional to the cluster diameter. The magnetization of the glasses measured by a vibrating sample magnetometer was 7.2 x 10(-6) Wbmkg(-1) at 10 kOe at room temperature and smaller than the value calculated assuming that the whole clusters have superparamagnetism. These results suggest the pinning of spins near the cluster surface.