  In the present work a detailed thermodynamic consideration for the magnetic
free energy of the composite material consisting of the ferromagnetic powder
particles embedded into a polymer matrix is given. We estimate their
magnetostatic interaction energy and its dependence on the microscopic
distribution of the magnetization and the magnetic field in the composite
material. We also define the hydrostatic component of the mechanical force
developed and the volume change effect caused by the magnetostatic interactions
in such composites.
