Fluctuations of the Molecular Field in the Two‐Ion Model for Temperature‐Induced Metamagnets

A special quartic spin model of spontaneous metamagnetism, proposed by Onyszkiewicz is considered involving dipole‐octopole interaction between two neighbouring ions. The thermodynamic properties of this model are investigated, using the high‐density expansion method in the approximation including Gaussian fluctuations of the molecular field. The free energy, the magnetization, the octopole parameter as well as mean squares of fluctuations are calculated for A → F metamagnets. The results are then compared with those obtained by the molecular field approximation. Moreover, a comparison of the results with those obtained earlier for the so called four‐ion model confirm effectiveness of the two‐ion model also in such refined procedure as the Gaussian fluctuation approximation.