Coercivity of γ‐Fe2O3 particles growing iron‐cobalt ferrite

Acicular γ‐Fe2O3 particles were heated at 90°C in alkali solution containing Co2+ and Fe2+. The coercivity of the resultant particles remarkably increased with the increasing Co2+/Fe2+ ratio, and in the neighborhood of the Co2+/Fe2+ ratio of 0.4, reached a maximum value. When the particles with a Co2+/Fe2+ ratio of 0.5 were dissolved in hydrochloric acid, the cobalt content and the coercivity were rapidly decreased with increasing dissolved weight. Iron‐cobalt ferrite was expected to grow on the surface of γ‐Fe2O3 particles. The increase of coercivity was attributed to the iron‐cobalt ferrite.