Abstract
Acicular γ-Fe2O3 particles were coated with an oxide layer composed of almost all the Fe3O4 by the treatment in alkali solution containing Fe2+ at 45°C. The coercivity of the obtained particles decreased linealy with increasing Fe2+ content in the coated oxide layer. When the coated oxide layer was removed by soaking the particles in hydrochloric acid, the coercivity of γ-Fe2O3 particles was recovered to their normal value. Furthermore the coercivity of the coated particles was greatly enhanced by the heat-treatment in evacuated glass capsule at 50 to 300°C, and higher values were obtained by the heating at higher temperatures. The increase of coercivity was considered to be due to the formation of γ-Fe2O3-Fe3O4 solid solution by the heat-treatment. The variation of coercivity with the lapse of time observed in the particles annealed at 60°C was small compared with that of γ-Fe2O3-Fe3O4 solid solution. When the particles were heated at 300°C, however, a remarkable variation of coercivity as shown in γ-Fe2O3-Fe3O4 solid solution was observed.
