Structural Change in Cobalt-Adsorbed [gamma]-Fe2O3 Particles by Heat Treatment

The surface state of the cobalt-adsorbed γ-Fe2O3 particles and their structural changes caused by heat treatment are studied by chemical analysis and magnetic measurements. As shown in Fig.1, the coercive force of γ-Fe2O3 increases with increasing amount of cobalt adsorbed. The increased coercivity decreases to the level of starting γ-Fe2O3 by removing the cobalt-absorbed layer in a diluted acid solution (Fig.2). The distribution of cobalt across the surfa ce layer of a particle is estimated from the results of dissolution experiments (Fig.2), assuming that the number of cations per unit volume of the oxide is approximately equal to 5×10-2/Å3 (Fig.3). As shown in Fig.3 and 4, the surface anisotropy, responsible for the increased coercivity of the particles, is in less than 3Å depth from the original particle surface before cobalt adsorption and is strongest at the original interface. The, cobalt concentration in the surface region decreases when the particles are heated to about 650°Cin air. A remarkable decrease in cobalt content was observed in the temperature range between 250°C and 400°C with X-ray photoelectron spectroscopy (Fig.10). The observed decrease in cobalt content is due to the inward diffusion of cobalt, and the surface anisotropy disappears with the formation of a solid solid solution of cobalt iron(III) ferrite. The cobalt concentration in the surface region, however, increases again above 650°Caccompanied by the precipitation of α-Fe2O3. The growth of α-Fe2O3 is presumed to begin near the center of a particle because of the lower cobalt content, resulting in the decrease in iron content and the increase in cobalt content in a spinel phase of the inner part of the particle. Consequently, interdiffusion of iron to the inner part and cobalt to the outer part is considered to occur in the particle. Cobalt is finally concentrated to a stoichiometric cobalt ferrite thus showing a maximum peak value of coercivity (Fig.5), and particles are subject to phase separation into α-Fe2O3 and CoFe2O4.