Equation of state of a ferromagnetic crystal

The equation of state for a ferromagnetic anharmonic crystal is derived in the weak coupling approximation between phonon and spin subsystems. The phonon frequency in the pseudoharmonic approximation with the account of the magnetization of the crystal and the energies of spin excitations in the random phase approximation with the account of the thermal motions of atoms are obtained. The relation between the mechanical, thermal and magnetic properties of the crystal is discussed.