The antiferromagnetic properties of mixed cobalt and manganese oxides

The paramagnetic susceptibilities of oxides of cobalt and manganese intimately mixed in various proportions have been measured as a function of temperature in the range 90 to 600° K. The mixtures are antiferromagnetic, and it is shown in particular that the observed Néel temperatures are linearly related to the atomic composition of the mixture. Neutron diffraction experiments on a sample of composition 50% MnO, 50% CoO show that the mixture is random and that the magnetic ordering below the Néel temperature is of the second kind. Anderson’s molecular field theory of face-centred cubic antiferromagnetics is extended to the case of mixed systems, and it is shown that, by making reasonable assumptions, super-exchange can account for the observed variation of Néel temperature with composition. In other respects there is poor agreement between the theory and experiment, and it is suggested that the disagreement arises from the use of a molecular field treatment.