Magnetic Properties of Some Ferrite Micropowders

Abstract

Micropowders of stoichiometric copper, nickel, and cobalt ferrites were prepared by low temperature (<800°C) treatment of the coprecipitated metal oxalates. The magnetic properties of each ferrite were determined as functions of particle or crystallite size in the range from 70 to >2000 A. Particle sizes were determined from x-ray line broadening and from electron micrographs. The micropowders exhibited superparamagnetic, single domain, or multidomain behavior depending on the particle size. The critical size for single domain behavior, depending on composition, was between 300 and 700 A. In this size range, high coercive force and remanence were observed in spite of appreciable sintering of the particles in some cases. The remanence values and the magnitude and temperature dependence of the coercive force indicated that magnetocrystalline anisotropy determined the properties of the single domain particles. The magnetic properties of the very small particle size samples showed the large temperature dependence characteristic of superparamagnetism. Quenched and slowly cooled copper ferrite micropowders were prepared with similar particle sizes. The remanence and coercive force of the quenched samples were much lower than those of the slowly cooled samples. This indicated different critical sizes in these two types of copper ferrite. This could be due to the difference in either the moments or the magnetocrystalline anisotropy.