Abstract: Cobalt-containing magnetic iron oxide particles extremely suitable for magnetic recording mediums, especially for image recording mediums and a process for producing the same from the precursor, .gamma.-FeOOH which is advantageous in industry are disclosed. Cobalt-containing magnetic iron oxide particles satisfying the following equations (1), (2) and (3):20.ltoreq.SSA.ltoreq.50 (1)-16.70.ltoreq.log [V].ltoreq.-3.05 log [SSA]-11.35 (2)0.10.ltoreq.Fe.sup.2+ /total Fe.ltoreq.0.30 (3)where SSA is a BET specific surface area (m.sup.2 /g), and V is a magnetic switching unit (cm.sup.3), can be produced by producing iron oxide particles having a concentration of Fe.sup.2+ of 0.10 to 0.36 as expressed by Fe.sup.2+ /total Fe from the precursor, .gamma.-FeOOH, and coating a cobalt compound or a combination of a cobalt compound and other metal compounds on the surfaces of the iron oxide particles.

Abstract: A process for producing magnetic ion oxide particles for magnetic rcording which comprises heat-treating a magnetic iron oxide particles of magnetite, berthollide or maghemite as a starting materials, in the presence of a zinc component or both a zinc component and a silicon component at a temperature of 400.degree. to 700.degree. C. in a specific atmosphere, and if necessary, subjecting the heat-treated particles to wet treatment and/or dry treatment, thereby doping the particles with zinc ions resulting in magnetic iron oxide particles having an improved saturation magnetization, a proportion of said zinc ions being 2 to 9 at. % by weight relative to the total amount of iron ions.