Sakai Chemical Industry Co., Ltd.

Shigeharu Higuchi

Seiji Matsumoto

Hajime Uno

Yasuo Araki

A method of producing cobalt-modified magnetic particles is provided. The method comprises heating an aqueous suspension of ferric hydroxide at temperatures of 100.degree.-250.degree. C. at an alkaline pH in the presence of a water soluble organic or inorganic compound capable of forming complexes with iron as a growth regulating agent, and preferably also in the presence of .alpha.-ferric oxide seed crystals of minor axes not larger than 0.4 microns in average in amounts of 0.1-25 mole % in terms of the Fe content thereof in relation to the ferric hydroxide, converting the obtained .alpha.-ferric oxide into .gamma.-ferric oxide by reduction thereof followed by oxidation, maintaining an aqueous suspension of the .gamma.-ferric oxide at temperatures of 30.degree.-50.degree. C. at an alkaline pH in the presence of ferrous hydroxide and cobaltous hydroxide in amounts of 0.5-50 mole %, respectively, in relation to the Fe content of the .gamma.-ferric oxide in the suspension for a length of time sufficient to modify the .gamma.-ferric oxide particles.

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Sakai, Japan

Shigeharu Higuchi