PURPOSE: To industrially provide magnetically and chemically stable grain powder of black color having high coercive force.
CONSTITUTION: The objective grain powder having Fe3O4 grains as nuclear crystal grains and consisting of such spinel-type multiple grains that CoFe2O4 are being in crystal growth on the surface of the nuclear crystal grains. The grain powder can be obtained by the following method: (A) an alkaline dispersion of pH≥10 with Fe3O4 grains dispersed therein is incorporated with (B) a mixed solution containing Co2+ and Fe2+ at the molar ratio Fe2+/Co2+ of 2 to 3 or a suspension containing Co2+ hydroxide and Fe2+ hydroxide so that the total concentration for the Co2+ and Fe2+ in the alkaline dispersion fall within the range from 0.005M to 0.5M, while aerating an oxygen-contg. gas in said alkaline dispersion at a temperature within the range of 50°C to the boiling point.
TSUNODA HIROSHI
HORIISHI NANAO
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