Magnetic properties of normal spinels with only a-a interactions

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Abstract
The theory of ferrimagnetism accounts for the magnetic properties of the ferrites by strong antiferromagnetic A-B interactions and weak antiferromagnetic A-A and B-B interactions. It was recently discovered that the A-A interaction in Co3O 4 was much stronger than anticipated. To ascertain whether the anomalous strong interaction was uniquely a property of Co+3 in an octahedral environment, the study of the A-A interaction has been extended to CoAl 2O4, FeAl2O4 and MnAl2O 4. Neutron diffraction and magnetic susceptibility measurements have shown that MnAl2O 4, and probably CoAl2O4, develop the same antiferromagnetic structure as Co3O4 at low temperature. FeAl2O 4 does not develop long range magnetic order, and the difference is attributed to competing A-B interactions due to inversion. The A-A interaction in the aluminates is an order of magnitude weaker than that observed in Co 3O4. It now appears that the anomalous strong interaction in Co3O4 is due to indirect exchange coupling through the octahedral Co+3 complex