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Abstract
A phenomenological theory of phase transitions in the systems with several ordering parameters has been elaborated. The analysis is carried out in the example of ferroelectric-ferromagnetic solid solutions with an account of the entire set of the system thermodynamic parameters (temperature, pressure, composition, linear size, etc.) and the analytical description of the main macroscopic characteristics (polarization, magnetization, dielectric and magnetic susceptibility, etc.) near a phase transition point is obtained. Proposed and analysed is a fluctuon mechanism of formation of a system phase inhomogeneity in the region of reconstruction of its internal structure.