Effect of Fe2+content on the instability of coercivity of cobalt-substituted acicular iron oxides

The variation in coercivity was examined by annealing at 60°C cobalt-substituted acicular iron oxides with various Fe
²⁺
and cobalt contents. Except for samples containing less than 1 wt% Fe
²⁺
, the coercivity increased by the annealing at 60°C, and samples containing more Fe
²⁺
showed higher coercivity. The increase of coercivity was attributed to the magnetic anisotropy induced in cobalt-substituted acicular iron oxides, and the existence of Fe
²⁺
was considered to facilitate the migration of cobalt ions. The dependence of coercivity on Fe
²⁺
content was similar for the samples with 0.5, 1.0, 1.6, and 2.4 wt% cobalt. The coercivity of the samples containing 22-23 wt% Fe
²⁺
increased linearly with cobalt content. The coercivity of the samples containing less than 1 wt% Fe
²⁺
increased slightly with cobalt content.