Abstract
Magnesium manganese ferrites containing less than 50% (mol) Fe2O3 must contain tri- or tetravalent manganese if there is to be no oxygen deficiency. For some compositions this entails electronic conduction and ferromagnetic coupling between manganese ions of different valencies in the same type of lattice site. Reported magnetic data on a number of such ferrites show significant departures from the results to be expected with probable ionic distributions in the spinel lattice if all A sub-lattice spins were anti-parallel to all B sub-lattice spins. Existing theoretical work on the occurrence of angles between the spin directions of two different kinds of magnetic ions is applied to two groups of ions in one or both sub-lattices. The Mn3+ and exchanging Mn2+ ions are treated as one group, the other group consisting of the Fe3+ ions and any excess Mn2+ ions. In view of the necessary averaging assumptions the degree of agreement found between observed trends and those predicted by the theoretical treatment is satisfactory. The angles concept is here considered as a mathematical expression of average relative spin directions rather than as indicating a physical ordering of spins. Thus other groupings might possibly show the same trends, the basic causes being unaltered.
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