Abstract
The cluster variational method in pair approximation is applied to a study of the Ising ferromagnet having S = 1, 3/2, 2 and 5/2, with the inclusion of biquadratic exchange and single-ion anisotropy. Attention is focused on the anomalous behaviour of the magnetization curves that is predicted when the ratio of the nearest-neighbor bilinear and biquadratic exchange interactions is negative. The critical temperature and magnetization curves are studied in detail in this regime and comparison is made with the recently reported results derived within the Oguchi pair approximation. A marked qualitative difference in the results of the two theories is found to exist for integer spin values.
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