Linear and nonlinear magnetoelastic interactions in the molecular field theory and invar anomalies

In terms of the molecular field theory for the Heisenberg model the dependence of the exchange integral upon homogeneous deformations in the first- and second-order is taken into account. The correlated equations of state for magnetic and elastic ferromagnetic subsystems are obtained. The calculated and experimental values for magnetic susceptibility, spontaneous and induced magnetostriction, linear expansion coefficient, elastic constants and T c vs. pressure are compared. The model of localised moments is shown to explain the anomalies in the main thermodynamic properties of Invars. To obtain better agreement between the calculated and theoretical data more real models are needed.