Formation and magnetic properties of ultrafine spinel ferrites

Investigations of the formation and magnetic properties of MnδCo1-δFe2O4formed in alkaline aqueous solution by the coprecipitation method are covered. The lattice constant of these ferrites apparently reduced with decreasing average particle size below about 500 Å. The coercive force ratio of MnFe2O4and Co-Fe2O4is 1 to 50 and equal to that of the crystal magnetic anisotropy coefficient K1of sintered ferrites. As the average particle size became larger, the coercive force and the apparent remanence increased linearly, and superparamagnetic critical size was determined. The sizes range, for example, from 50 to 100 Å for CoFe2O4and from 150 to 200 Å. for MnFe2O4. From the correlation between the average particle size and their magnetization, the limit size of ferromagnetic critical particle was also determined. Results obtained in this experiment are approximately in accord with Néel´s Version of the theory on thermal fluctuation aftereffect of magnetic fine particles.