Formation and magnetic properties of ultrafine spinel ferrites

Investigations of the formation and magnetic properties of Mn
δ
Co
1-δ
Fe
2
O
4
formed 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 MnFe
2
O
4
and Co-Fe
2
O
4
is 1 to 50 and equal to that of the crystal magnetic anisotropy coefficient K
1
of 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 CoFe
2
O
4
and from 150 to 200 Å. for MnFe
2
O
4
. 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.