| 摘要 |
PURPOSE: To obtain the subject particulate powder, having great saturation magnetization, excellent in blackness degree and having a high coercive force with a small amount of cobalt and excellent magnetic stability with time by forming a coating layer containing a magnetitic layer in the lower layer and a cobalt-containing oxide layer containing a prescribed amount of the cobalt in the upper layer on the particle surface and further maghemite present in the interfacial part between both the layers. CONSTITUTION: An aqueous solution of a ferrous salt is added to an alkaline suspension containing needlelike magnetitic particles having 15-24wt.% content of ferrous salts at 40-60 deg.C in a nonoxidizing atmosphere to produce a ferrous hydroxide colloid. A gas containing oxygen is then aerated to oxidize the colloid and a magnetitic layer is then formed on the surfaces of the needlelike magnetitic particles, which are then oxidized to form maghemite. An aqueous solution of a cobalt salt is added thereto in a nonoxidizing atmosphere and the resultant mixture is stirred at 60-100 deg.C to form an oxide layer containing the cobalt on the surface of the maghemite. |
