Magnetic exchange mechanisms in magnesium¿manganese ferrites

Reported values of saturation magnetization in iron-deficient magnesium¿manganese ferrites show significant variations from those expected by the simple theory of the ferrimagnetism of spinel ferrites. These variations have been qualitatively explained by the occurrence of angles between the spin directions of groups of ions, either in octahedral (B) lattice sites or in both these and tetrahedral (A) sites, associated with the presence of trivalent manganese ions in these positions. Since the assumed grouping of ions is not necessarily the only possibility, the problem is re-examined in the paper from the point of view of semicovalent exchange. It is shown that this mechanism can account for the magnetic properties of simple ferrites quite as well as superexchange, and in terms of A¿B interactions alone, while it provides a clearer picture of the electron transfer constituting double exchange than that given by the original mathematical theory. In the more complicated magnesium¿manganese ferrites the same principles are shown to be compatible with the observed results, but mathematical treatment of the probable relative positions of magnetic and non-magnetic ions is needed for closer correlation. The existence of angles between ordered spins is probably a mathematical fiction, the reduction in net magnetic moment of each sub-lattice being caused by a degree of semicovalent bonding of trivalent manganese ions in B sites, which deflects the oxygen orbitals from directly collinear A and B cations.