The magnetic structure of Co3O4

Many of the magnetic properties of ferrites are explained by the theory of ferrimagnetism which shows that the predominant magnetic interaction is antiferromagnetic exchange between ions occupying tetrahedral (A) sites and octahedral (B) sites. The A-A and B-B interactions are also antiferromagnetic, but weak relative to the A-B interaction. The present paper describes the observation by magnetic susceptibility and neutron diffraction of an anomalously large magnetic interaction between ions in the A sites of Co3O4. Co3O4 is not ferrimagnetic although it crystallizes in the magnetite structure; it is a normal spinel with Co3+ ions occupying the B sites. The Co3+ have zero permanent moment as a consequence of the splitting of the 3d levels by the octahedral cubic field. At 40°K, Co3O4 undergoes a transition to form an ordered antiferromagnetic state. The magnetic structure is due to antiferromagnetic ordering of spins in the A sites; each Co2+ ion in an A site is surrounded by four nearest neighbors with oppositely directed spins. The Co2+ moment in the antiferromagnetic state is 3.02 microB. This A-A interaction is an order of magnitude stronger than expected. It is suggested that the exchange mechanism involves indirect exchange through the tightly bound octahedral complex about the Co3+ ion.