Process for producing cobalt-containing ferromagnetic iron oxide powder

A process for producing a cobalt-containing ferromagnetic iron oxide powder suitable for magnetic recording which consists essentially of the sequential steps of

• (a) mixing cobalt ion, ferrous ion and alkali with an aqueous dispersion of acicular ferromagnetic iron oxide nuclei under non-oxidizing conditions of nitrogen gas or inert gas which completely prevent oxidation at a temperature of about 50.degree. C. or less;
• (b) heating the dispersion to a temperature of at least about 60.degree. C. but below the boiling point of the dispersion under non-oxidizing conditions of nitrogen gas or inert gas which completely prevent oxidation to cause a reaction;
• and
• (c) treating the dispersion at a temperature of at least about 60.degree. C. but below the boiling point of the dispersion under the following conditions:
• (1) initially treating the dispersion under non-oxidizing conditions of nitrogen gas or inert gas which completely prevent oxidation for about 5 minutes or more:
• (2) subsequentially treating the dispersion under oxidizing conditions for about 5 minutes or more prior to the termination of the reaction;
• and
• (3) a total treating time under non-oxidizing conditions and oxidizing conditions of at least about 30 minutes, said non-oxidizing conditions which completely prevent oxidation being maintained by bubbling nitrogen or inert gas through said dispersion and said non-oxidizing conditions being maintained continuously from said mixing of said (a) through said initial treating of step (c) (1).