Patent ID: 7399523

Claim:
Spinel ferrimagnetic particles, a composition formula of which when prepared is (CoO) 0.5-x (NiO) 0.5-y (MO) x+y •n/2(Fe 2 O 3 ) (M is a bivalent metal except Co and Ni), where, a value of n (molar ratio)=Fe/(Co+Ni+M) is 2.0<n <3.0, which is larger than stoichiometric amount (n=2) of a spinel ferrite and less than that of 1.5 times, and, values of said x, y satisfy 0≦x<0.5, 0≦y<0.5, 0<x+y<0.5, wherein, also, superparamagnetic fine particles contained in said spinel ferrimagnetic particles produced by coprecipitation is 5% by mass or less.