A thermodynamic theory of “weak” ferromagnetism of antiferromagnetics

A thermodynamic theory of “weak” ferromagnetism of α-Fe2O3, MnCO3 and CoCO3 is developed on the basis of landau's theory of phase transitions of the second kind. It is shown that the “weak” ferromagnetism is due to the relativistic spin-lattice and the magnetic dipole interactions. A strong dependence of the properties of “weak” ferromagnetics on the magnetic crystalline symmetry is noted and the behaviour of these ferromagnetics in a magnetic field is studied.