PURPOSE: To obtain the iron oxide magnetic powder with the reduced change of the corrective force with time and excellent in coating dispersibility by adding colloidal silica to an aq. suspension at specified pH in which γ-Fe2O3-based magnetic powder is dispersed, cleaning and then dehydrating the powder to coat the surface of the powder grain with a specified amt. of silicon oxide.
CONSTITUTION: The γ-Fe2O3-based powder, e.g. Co-γ-Fe2O3 powder, is dispersed in water to obtain a suspension. The suspension is held at ≤pH11 or preferably at pH9.5-10, colloidal silica is added, and the powder is cleaned and then dehydrated to coat the surface of powder with 1000-10,000ppm silicon oxide expressed in terms of Si. In this case, the grain diameter of colloidal silica is preferably controlled to 5-10nm. By this method, the Hc is not significantly reduced by virtue of the surface treatment, the dispersibility in coating formulation process is improved, and the iron oxide magnetic powder coated with silicon oxide capable of reducing the change of the Hc of a magnetic tape with lapse of time is obtained.
OMORI MASAHIRO
NISHIKAWA SHUICHI
