Crystal Structure and Magnetic Properties of Cobalt Ferrite at Low Temperatures

Abstract

COBALT ferrite (CoO.Fe₂O₃) and magnetite (FeO.Fe₂O₃) both crystallize at room temperature with the inversed-spinel-type of structure¹. A magnetic transition consisting of an abrupt decrease of saturation magnetization takes place in magnetite² at 116° K., and is accompanied by a slight deformation of the structure cell from cubic symmetry above the transition to approximately rhombohedral symmetry below³. As cobalt ferrite⁴ undergoes a similar magnetic transition at 90° K., we might expect that, like magnetite, it is no longer cubic below its magnetic transition temperature. We have shown by means of X-ray powder photography that this is indeed true, but the departure from cubic symmetry is even smaller than in the case of magnetite. Our results indicate that the structure cell of cobalt ferrite at low temperatures is approximately tetragonal.