Spinel-type ferrimagnetic particles, process for producing the same, and magnetic recording medium using the same

A process for producing coated spinel-type ferrimagnetic particles having a composition represented by the formula:

• (MO)·n/2(Fe2O3)
• wherein M is a divalent metal;
• and n is a molar ratio of Fe to M, n=Fe/M,
• which is from more than 2.05 to less than 2.5, 2.05<n<2.5, the particles containing a superparamagnetic component in an amount of not more than 2% by mass, and having an average particle diameter of 5 to 30 nm,
• said spinel-type ferrimagnetic particles being respectively coated on a surface thereof with an hydroxide of at least one metal selected from the group consisting of Si, Al, P and Zn,
• the metal hydroxide coated on the spinel-type ferrimagnetic particles is present in an amount of not more than 10% by mass, calculated as the metal based on the weight of ferrimagnetic particles, the process comprising the steps of:
• a preparation step for a raw aqueous solution of mixing an aqueous iron (Fe)-containing solution and an aqueous divalent metal (M)-containing solution with each other such that a molar ratio of Fe to M is in the range of from more than 2.05 to less than 2.5, 2.05<Fe/M<2.5;
• a production step for a co-precipitate-containing solution of adding an aqueous alkali solution to the raw aqueous solution at a temperature of from 60° C. to less than 110° C. so as to adjust a concentration of alkali in the raw aqueous solution to 0.05 to 10 M;
• a production step for black particles of heat-treating the co-precipitate-containing solution at a temperature of 80 to 120° C.;
• an etching step for the black particles of washing the black particles to remove the alkali therefrom, and adding an acid thereto to remove a superparamagnetic component therefrom;
• and
• a coating step for a hydroxide of the metal of subjecting the resultant solution containing the etched black particles to filtration and washing, and then adding an aqueous solution containing at least one metal selected from the group consisting of Si, Al, P and Zn to the resultant solution containing the washed black particles.