Plate-like composite ferrite fine particles for magnetic recording and a process for producing the same

Claim 1

Plate-like composite Ba-containing ferrite fine particles for magnetic recording, containing Ni, Co and Ti in the molar ratios defined by 0.02≦Ni/Fe≦0.10, 0.01≦Co/Fe≦0.10 and 0.01≦Ti/Fe≦0.10, provided that 0.03≦(Ni+Co)/Fe≦0.14 and Ti/(Ni+Co)<1, and having 0.2 to 5.0 wt % of Zn as a solid solution close to the particle surface, said particles showing a temperature dependence of coercive force in the range of -0.5 to +2.0 Oe/°C. in the temperature range of -20° to 120° C., and an anisotropy field of not less than 4.0 KOe, and prepared by a process comprising: (a) autoclaving a suspension of an alkaline iron (III) hydroxide containing Ba ions as well as a Ni compound, a Co compound and a Ti compound in the molar ratios defined by 0.02≦Ni/Fe≦0.10, 0.01≦Co/Fe≦0.10 and 0.01≦Ti/Fe≦0.10, but 0.03≦(Ni+Co)/Fe≦0.14 and Ti/(Ni+Co)<1, at a temperature of 90° to 300° C., thereby producing plate-like composite Ba-containing ferrite fine particles which contain Ni, Co and Ti; (b) suspending the thus obtained particles in an aqueous solution containing zinc and having a pH of 4.0 to 12.0 to obtain plate-like composite Ba-containing ferrite fine particles having a Zn hydroxide precipitated on the particle surfaces; filtering out the resultant particles; and, after washing with water and drying, (c) heat-treating the thus obtained particles at a temperature in the range of 600° to 900° C.