The part played by magnetoelastic interaction in the forming of thermodynamic functions of ferromagnets: Thermodynamic potential and its first thermodynamic derivatives

The “construction of thermodynamics” of ferromagnet is performed in the spirit of Landau’s phenomenological approach but in view of magnetoelastic interaction (MEI) between magnetic subsystem and crystal lattice, which is not considered in the traditional interpretation of the theory. It is demonstrated that the inclusion of MEI reflected in the magnetization dependence of Debye temperature may be reduced to renormalization of the thermodynamic potential (TDP) and thermodynamic coefficients of Landau. Within the developed model, expressions are obtained for renormalized magnetic components of TDP and its first thermodynamic derivatives, in particular, the variation of molar enthalpy, molar volume, density, specific magnetization, and bulk magnetostriction. Model calculations demonstrating the importance of taking the TDP into account are performed on the real scale of temperatures and observed values of thermophysical functions in the absence of magnetic field.