Toda Kogyo Corporation

Toshitada Shigemura

Katsumi Yamashita

Hideaki Sadamura

Minoru Yamasaki

Katsunori Fujimoto

Kazushi Takama

Kazuhiro Fujita

Mitsuaki Hatatani

Tomohisa Moriya

Takanobu Uemoto

Osamu Okimoto

Kazuaki Kouziro

Yoshifumi Fukuhara

Hiroshi Oka

Cobalt-coated acicular magnetic iron oxide particles of the present invention have an average major axial diameter of 0.15 to 0.50 .mu.m, and comprise:

Cobalt-coated acicular magnetic iron oxide particles

Acicular magnetic iron oxide particles comprise:

Acicular magnetic iron oxide particles and process for producing the same

Disclosed herein is a process for producing acicular magnetic iron oxide particles comprising the steps of: mixing an aqueous dispersion of acicular magnetic iron oxide particles with an aqueous solution of a Co salt and an alkaline aqueous solution, or an aqueous solution of a Co salt, an aqueous solution of an Fe(II) salt and and an alkaline aqueous solution; and heat-treating the alkaline suspension solution obtained at 50.degree. to 100.degree. C.; wherein an Sr salt, a Ba salt or an Sr salt and a Ba salt is added to one selected from the group consisting of said aqueous dispersion of acicular magnetic iron oxide particles, said aqueous solution of a Co salt, said aqueous solution of an Fe(II) salt, said alkaline aqueous solution and said alkaline suspension solution before heat-treatment, and at least one selected from the group consisting of Si compound, Al compound, Zn compound and Mg compound is added to one selected from the group consisting of said aqueous dispersion of acicular magnetic iron oxide particles, said aqueous solution of a Co salt, said aqueous solution of an Fe(II) salt, said alkaline aqueous solution, said alkaline suspension solution before heat-treatment and said alkaline suspension solution during heat-treatment.

Acicular magnetic iron oxide particles

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Yamaguchi-ken, Japan

Toshitada Shigemura