Thermodynamical Properties of Isotropic Ferromagnets with Biquadratic Interactions

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Abstract
The thermodynamical properties of a S = 1 system described by \documentclass{article}\pagestyle{empty}$ H = - \frac{1}{2}I\mathop \Sigma \limits_{i,j} {\rm S}_i {\rm S}_j - \frac{1}{2}K\mathop \Sigma \limits_{i,j} ({\rm S}_i {\rm S}_i)^2 $ are investigated by means of the Green functions method. Using the RPA decoupling the temperature dependence of the two order parameters are determined in the low‐temperature region and in the neighbourhood of the transition temperatures. The value of K for the tricritical point is found. The possibility of the first order transition from the quadrupolar to the disordered phase is analysed.