Iron-based oxide magnetic powder and method for producing same

A method for producing an iron-based oxide magnetic powder, which has an average particle diameter measured with a transmission electron microscope of 10 nm or more and 20 nm or less, and is composed of particles of ε-iron oxide or ε-iron oxide in which Fe sites are partially substituted by other metal elements, comprising:

• a raw material solution preparation step of preparing an aqueous solution containing trivalent iron ions, or trivalent iron ions and ions of a metal element that partially substitutes the Fe sites (hereinafter referred to as a raw material solution), and an alkaline aqueous solution for neutralizing the raw material solution;
• a first neutralization step comprising providing a reaction system, and then either continuously or intermittently adding each of the raw material solution and the alkaline aqueous solution to the reaction system and mixing so as to adjust the pH of the reaction system to 1.0 or higher and 3.0 or lower;
• a step of adding a hydroxycarboxylic acid to the aqueous solution after the first neutralization step;
• a second neutralization step of neutralizing the pH to 7.0 or higher and 10.0 or lower by adding an alkali to the aqueous solution to which the hydroxycarboxylic acid is added, thereby obtaining a slurry containing a precipitate of an iron oxyhydroxide or a substituent metal element-containing iron oxyhydroxide;
• a step of adding a silicon compound having a hydrolyzable group to the slurry containing the iron oxyhydroxide or the substituent metal element-containing iron oxyhydroxide, thereby coating the iron oxyhydroxide or the substituent metal element-containing iron oxyhydroxide with a hydrolysate of the silicon compound;
• and
• a step of heating the iron oxyhydroxide or the substituent metal element-containing iron oxyhydroxide coated with the hydrolysate of the silicon compound, thereby forming ε-iron oxide or ε-iron oxide, in which Fe sites are partially substituted by other metal elements, coated with silicon oxide,
• wherein the average diameter measured with a dynamic light scattering particle size distribution analyzer of the slurry containing the iron oxyhydroxide or the substituent metal element-containing iron oxyhydroxide obtained in the second neutralization step is 300 nm or less.