Formation and magnetic properties of γ-Fe2O3particles surface modified with crystallized cobalt-ferrite

Acicular γ-Fe
2
O
3
particles were heated at 90°C in alkali solution containing Co
²⁺
and Fe
²⁺
with Co
²⁺
/Fe
²⁺
ratio of 0.5. The coercivity of resultant particles increased linearly with increasing the Co
²⁺
content, and the coercivity of 900 Oe was obtained for the particles with Co
²⁺
content of 7 wt%. The shape of the particles is acicular, and an appreciable variation of morphology by the treatment in alkali solution was not observed. Cobalt-ferrite was expected to crystallize epitaxially on the surface of γ-Fe
2
O
3
particles, and the increase of coercivity was attributed to the magnetic anisotropy of the cobalt-ferrite. A variation of coercivity by annealing at 60°C and print-through were small compared with those of the particles in which iron were homogeneously substituted by cobalt ions. Such stability was explained by considering that a very high concentration of cobalt ions exist only on the surface of γ-Fe
2
O
3
particles, and the migration of cobalt ions is extremely difficult.