Abstract: A magnetic iron oxide powder having high coercive force is produced by dispersing a magnetic iron oxide comprising a silicon component and a phosphorus component in an aqueous solution of a cobalt ion and adding a base to the dispersion to be an alkaline condition and heat-treating the dispersion whereby the cobalt ion is adsorbed and diffused in the surface layer of the magnetic iron oxide. A zinc component can be also incorporated in the crystal. The silicon component and the phosphorus component are preferably incorporated in the crystalline structure.

Abstract: A magnetic recording medium having more than 550 Oersted of a coercive force, less than 1.8 of a ratio of a coercive force at -196.degree. C. to a coercive force at 25.degree. C., less than 1.0 wt. % of a content of Fe.sup.2+ is prepared by incorporating a magnetic powder obtained by dispersing acicular .gamma.-Fe.sub.2 O.sub.3 in an aqueous solution of a cobalt salt and reducing it with a reducing agent to deposit a cobalt compound on the acicular .gamma.-Fe.sub.2 O.sub.3.