Electrical and magnetic properties of the MgMn2O4-MgAl2O4 system

Solid solutions of tetragonal MgMn2O4 and cubic MgAl2O4 have been synthesized and characterized using the x-ray diffraction technique. Electrical conductivity (σ) and magnetic susceptibility (χ) have been measured over the temperature ranges 25-1000 °C and - 195-60 °C respectively. All the compounds were found to be monophasic and crystallize, with either tetragonal (c/a> 1) or cubic symmetry. The critical concentration of MgAl2O4, required to stabilize the cubic structure, was found to be only 12·5%. This may be explained as due to migration of a considerable amount of Mn3+ to A sites. The values of energy of activation and free energy of activation obtained from ln σ-1/T and ln (σT)-1/T plots were found to be almost identical. All the samples exhibited paramagnetism at T> - 195 °C and indications of antiferromagnetic ordering at lower temperatures. Further, an increase in χ values with increasing concentration of MgAl2O4 was observed. An explanation is offered for this behaviour.