Suggestion Concerning Magnetic Interactions in Spinels

The indirect-exchange mechanisms which produce spontaneous magnetization in metal oxides are an optimum if two interacting cations are located on opposite sides of an anion. The coupling rules which have been developed for this case are not applicable to spinels in which the cation-anion-cation angles differ appreciably from 180°. An examination of the $d$-orbital symmetry of various cations in respect to the crystal lattice leads to several suggestions concerning the magnetic interactions in spinels. Of particular interest are those spinels containing cations with three or fewer $d$ electrons in the octahedral sites, or four if the symmetry is tetragonal. The properties of several spinels of this kind are considered in the light of the suggestions offered. Direct interactions between octahedral-site cations appear to be possible.