Using the Green function method there has been presented the mathematical description of dilute two sub-lattice metamagnets with ferro- and antiferromagnetic exchange interactions. The Green function averaging procedure is based on the Cumulant Expansion Technique and includes any localized Green function irrespectively of the decoupling algorithm making the chain system of equations self-consistent. The derived form of the averaged Green function is the base for the analysis of magnetization processes and phase transitions occurring in the system under the influence of external magnetic field. It has been found in particular that the tricritical point and the second-order phase transition vanish when concentrations of magnetic atoms coupled with different sub-lattice points become different.
The results of study of Pb influence on high temperature magnetic and thermal properties of the chalcogenides (Pb_ySn_{1-y})_2P_2S_6 are presented. The increasing Pb content shifts phase transition to the ferroelectric state at about 337 K towards lower temperatures while magnetic field till 3 T has no influence on this transition. The measured susceptibility and magnetisation data are discussed.
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