Cobalt-containing iron oxide magnetic particles and method for the preparation of the same

摘要

Cobalt-containing iron oxide magnetic particles having a high coercive force and a small surface electrical resistance which are useful as a recording element for a magnetic recording medium, and a method for the preparation of the same comprising reducing acicular gamma -Fe2O3 particles in a reducing gas or liquid until the iron oxide reaches an intermediate oxidation state between gamma -Fe2O3 and Fe3O4, dispersing the resulting iron oxide particles into an aqueous solution containing a cobalt salt and optionally a ferrous salt and an alkali of one equivalent or more to the metal salt(s), and heating the mixture at a temperature of higher than room temperature up to a boiling point of the mixture under an atmospheric condition such that the divalent iron is substantially not oxidized. The cobalt is thereby predominantly incorporated into the most outer layer of the acicular iron oxide particles.