Coercivity and Mössbauer spectroscopy studies of cobalt-adsorbed γ-Fe2O3

The room-temperature coercivity of acicular γ-Fe2O3 particles having cobalt ions adsorbed in the pH range from 5 to 13.5 and in the temperature range from 283 to 373 K was measured. Emission Mössbauer spectroscopy at room temperature was used to study cobalt adsorbed on the surface of acicular γ-Fe2O3 particles. The coercivity of cobalt-adsorbed γ-Fe2O3 particles increased with increasing cobalt content up to about 2 wt %, above which it leveled off. The saturated values of coercivity increased with increasing adsorption temperature from 283 to 303 K, decreased with further increasing temperature up to 343 K, and again increased remarkably at 373 K. It was found from Mössbauer spectroscopy that the sample with high coercivity shows a strong magnetic interaction between the absorbed cobalt ions and the ordered magnetic ions of the substrate. Nonmagnetic components that do not contribute to the coercivity are probably due to the oxides and hydroxides of divalent and trivalent cobalts. The effect of pretreatment with Mg2+, Ca2+, and Sr2+ ions on the adsorption of cobalt on the surface of γ-Fe2O3 at 373 K was also studied by emission Mössbauer spectroscopy. It was found that the average internal magnetic field of γ-Fe2O3 with both Mg2+ and Co2+ ions adsorbed was larger than that for Co2+ ions alone, whereas in the case of Ca2+ and Sr2+ ions, the average internal magnetic field did not change. These phenomena are discussed on the basis of the site preference of cations in ferrites.