Theory of Magnetism in CoCl₂·2H₂O

Crystal field theory developed by Abragam and Pryce has been used to formulate the anisotropic exchange interactions in CoCl 2 ·2H 2 O and CoBr 2 ·2H 2 O. These crystals are antiferromagnets in which Co ++ ions form ferromagnetic linear chains coupling antiferromagnetically to adjacent chains. There are two abrupt jumps in the high-field magnetization in these crystals. These phenomena are explained quantitatively based on the model proposed by Oguchi and Takano and by Narath. The model, which involves several competing exchange interactions and the Zeeman energy, is reduced to the Ising model of a linear chain. Thus the problem is solved by the eigenvalue-method. The Néel point is obtained in terms of the anisotropic exchange parameters. In this way we estimate the numerical values of the exchange integrals. The frequencies of magnetic resonance absorption are also estimated.