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The magnetic properties of antiferromagnetics, FeO and CoO, are investigated from the standpoint of the one-ion approximation. In their crystalline field of cubic symmetry the orbital degeneracies of Fe++ and Co++ are not completely removed and the r<siduaJ orbital angular momenta play an important role through the presence of the spin-orbit coupling and through the direct effect of the orbital magnetic moments. After deriving the effective Hamiltonian for these degenerate cases, which corresponds to the Hamiltonian derived by Pryce for the non-degenerate case, we discuss the paramagnetic susceptibility, the Nee! temperature and the state of each ion at absolute zero. We can give a reasonable quantitative interpretation of the susceptibility of CoO in paramagnetic state and obtain a good agreement with experiment for the magnetic moment of CoO in antiferromagnetic state. For FeO, sufficiently reliable data to compare with theory are not available at present, but a preliminary comparison is made. Further, in connection with the investigation of the validity of our theory, the origins of the crystalline field are discussed and it is pointed out that the covalent effect may make an appreciable contribution to the crystalline field.
Junjiro Kanamori (Fri,) studied this question.