Toda Kogyo Corporation

Hiroyuki Kondo

Hiroshi Sumita

Akira Mukaizaka

Nanao Horiishi

Tosiharu Harada

Kohji Mori

Akio Mishima

Hirofumi Kawasaki

Masaru Isoai

Goro Matsui

Yosiro Okuda

Atushi Takedoi

Atsushi Takedoi

Yoshitaka Yoshinaga

Disclosed herein are magnetic iron oxide particles comprising spindle-like magnetite or maghemite particles having an aspect ratio(major:minor) of less than 4:1 and containing 0.1 to 20 atomic % of Si to Fe, and a process for producing the same.

Spindle-like magnetic iron oxide particles and process for producing the

Fine magnetic particles with high saturation magnetization (over 140 emu/g) as well as high stability against oxidation in air, are produced by a process which comprises the first stage of oxidizing the surfaces of metallic particles consisting mainly of iron in an atmosphere containing both H.sub.2 O and H.sub.2 gases at an oxidizing temperature below 900.degree. C., and the second stage of further oxidizing the surfaces of the particles produced at the first stage in an atmosphere containing O.sub.2 gas.

Process for preparing magnetic particles with metallic region therein,

A process for treating acicular magnetite containing Co to stabilize the magnetic properties thereof comprises the steps of heating acicular magnetite containing 0.1 to 2.5% by atom of Co with respect to Fe as starting material at a temperature in the range of 300 - 1000.degree. C in an inert gas atmosphere in which the partial pressure of oxygen is controlled at a value in the range of 0.001 - 1% by weight and for the time sufficient to homogenize the crystal structure of the acicular magnetite by thermal-diffusion of oxygen thereinto, and thereafter saturating the resultant acicular magnetite with oxygen by heating it at a temperature in the range of 40 - 100.degree. C in an oxygen rich atmosphere such as air.

Process for treating acicular magnetite containing Co to stabilize the

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Hiroshima, Japan

Hiroyuki Kondo