CLUSTER APPROXIMATION FOR FERROMAGNETS WITH FIRST AND SECOND NEIGHBOR EXCHANGE, WITH APPLICATION TO THE EUROPIUM CHALCOGENIDES.
Technical rept. no. 16,
PENNSYLVANIA UNIV PHILADELPHIA
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The two-particle cluster approximation of Strieb, Callen and Horwitz is extended to ferromagnets and paramagnets with nearest and next-nearest neighbor exchange, with particular reference to EuO, EuS, EuSe and EuTe. Curie temperatures, magnetization curves for general values of the applied field, susceptibility, spin correlation functions, energy, and specific heat curves are calculated. To explore the sensitivity of the theory to the exchange constants the results of the cluster approximation are given for various ratios of the first and second neighbor exchange constants, including the special case of vanishing second neighbor exchange in this latter case the approximation generalizes the constant coupling approximation to arbitrary spin. All the presently measured magnetic properties are rather insensitive to the particular choice of exchange constants provided that these are chosen to be consistent with the observed Curie temperature measurement of the spin correlation functions, however, would provide a sensitive criterion for the choice of exchange constants. Author