Manufacture of cobalt-modified .gamma.-iron(III)oxides

A magnetic recording medium of high coercivity and improved thermal and mechanical stability comprising:

• a base and a magnetic coating comprising magnetic pigment dispersed in a resin binder on said base, said magnetic coating containing as the magnetic pigment, a cobalt-modified gamma-iron (III) oxide which has been prepared by applying a cobalt compound which at the temperature used decomposes into cobalt oxide or cobalt to an acicular gamma-iron (III) oxide or an acicular magnetite, and subsequently heating the gamma-iron (III) oxide to which said cobalt compound has been applied or heating the magnetite to which said cobalt compound has been applied under oxidation conditions to form gamma-iron (III) oxide, said gamma-iron (III) oxide or magnetite having been formed from the magnetite precursor acicular iron oxide or its hydrate which prior to reducing the precursor to form magnetite was substantially encapsulated with from about 0.1 to 6% by weight based on the weight of the iron oxide or its hydrate of an inorganic protective coating to prevent the needles of the precursor from sintering together during the reduction reaction, said reduction reaction taking place at a temperature of from 350.degree. to 650.degree. C, and wherein the gamma-iron (III) oxide or magnetite to which the cobalt compound was applied is heated at a temperature of between 80.degree. and 300.degree. C.