Cobalt-containing iron oxide pigments, process for producing the same and magnetic recording medium containing the same

A process for producing cobalt-containing iron oxide pigments comprising

• a cobalt-containing iron oxide core, the average concentration of cobalt distributed evenly in the innermost 50% by weight of the cobalt-containing iron oxide core being from 0.1 to 1.0% by weight, calculated as Co, based on iron oxide contained therein, and the content of Fe.sup.2+ in the core being from 16 to 22% by weight based on the iron oxide contained in the cobalt-containing iron oxide core;
• and
• a cobalt containing iron oxide surface layer, the average concentration of cobalt in the surface layer of the outermost 10% by weight of the pigment being on average at least five times as high as the average concentration of cobalt in the innermost 50% by weight of the cobalt-containing iron oxide core
• said process comprising the steps of coating a cobalt-containing berthollide in which the evenly distributed cobalt average concentration is 0.1 to 1.0% by weight, calculated as Co, based on the iron oxide contained therein, with a cobalt compound or a cobalt compound and an iron compound in an aqueous phase.

Cobalt-containing iron oxide pigment comprising

• a cobalt-containing iron oxide core, the average concentration of cobalt distributed evenly in the innermost 50% by weight of the cobalt-containing iron oxide core being from 0.1 to 1.0% by weight, calculated as Co, based on iron oxide contained therein, and the content of Fe.sup.2+ in the core being from 16 to 22% by weight based on the iron oxide contained in the cobalt-containing iron oxide core;
• and
• a cobalt containing iron oxide surface layer, the average concentration of cobalt in the surface layer of the outermost 10% by weight of the pigment being on average at least five times as high as the average concentration of cobalt in the innermost 50% by weight of the cobalt-containing iron oxide core
• produced by a process comprising coating a cobalt-containing berthollide in which the evenly distributed cobalt average concentration is 0.1 to 1.0% by weight, calculated as Co, based on the iron oxide contained therein, with a cobalt compound or a cobalt compound and an iron compound in an aqueous phase.

A magnetic recording medium comprising a support and coated thereon a coating composition composed of

• cobalt-containing iron oxide pigments comprising a cobalt-containing iron oxide core, the average concentration of cobalt distributed evenly in the innermost 50% by weight of the cobalt-containing iron oxide core being from 0.1 to 1.0% by weight, calculated as Co, based on iron oxide contained therein, and the content of Fe.sup.2+ in the core being from 16 to 22% by weight based on the iron oxide contained in the cobalt-containing iron oxide core;
• a cobalt-containing iron oxide surface layer, the average concentration of cobalt in the surface layer of the outermost 10% by weight of the pigment being on average at least five times as high as the average concentration of cobalt in the innermost 50% by weight of the cobalt-containing iron oxide core;
• and
• a binder resin.

A cobalt-containing iron oxide pigment comprising

• a cobalt-containing iron oxide core, the average concentration of cobalt distributed evenly in the innermost 50% by weight of the cobalt-containing iron oxide core being from 0.1 to 1.0% by weight, calculated as Co, based on the iron oxide contained therein, and the content of Fe.sup.2+ in the core being from 16 to 22% by weight based on the iron oxide contained in the cobalt-containing iron oxide core;
• and
• a cobalt-containing iron oxide surface layer, the average concentration of cobalt in the surface layer of the outermost 10% by weight of the pigment being on average at least five times as high as the average concentration of cobalt in the innermost 50% by weight of the cobalt-containing iron oxide core, wherein the core is represented by the formula:
• Co.sub.a' Fe.sup.2+.sub.b'.Fe.sup.3+.sub.[2+2/3.times.(1-a'-b')] O.sub.4
• wherein 0.0189.ltoreq.a'.ltoreq.0.1974, and 0.498.ltoreq.b'.ltoreq.0.834.

A cobalt-containing iron oxide pigment comprising

• a cobalt-containing iron oxide core, the average concentration of cobalt distributed evenly in the innermost 50% by weight of the cobalt-containing iron oxide core being from 0.1 to 1.0% by weight, calculated as Co, based on iron oxide contained therein, and the content of Fe.sup.2+ in the core being from 16 to 22% by weight based on the iron oxide contained in the cobalt-containing iron oxide core;
• and
• a cobalt-containing iron oxide surface layer, the average concentration of cobalt in the surface layer of the outermost 10% by weight of the pigment being on average at least five times as high as the average concentration of cobalt in the innermost 50% by weight of the cobalt-containing iron oxide core, wherein the cobalt-containing iron oxide core is represented by the formula:
• Co.sub.a' Fe.sup.2+.sub.b'.Fe.sup.3+.sub.[2+2/3.times.(1-a'-b')] O.sub.4
• wherein 0.00382<a<0.0392, and 0.6448<b<0.9087.