Lavanov H. Thermodynamic description of phase transitions in the strongly non- Heisenberg magnets with single-iron anisotropy

The dissertation is devoted to the thermodynamic description of phase transitions in magnets with single-ion anisotropy, which are of the type of non-Heisenberg ones. The influence of the external magnetic field, temperature, single -ion anisotropy, and parameters of the exchange on the thermodynamics of phase transitions in magnets in the conditions of their competition is considered. Using the expression for the nonequilibrium free energy, the phase diagrams of the states are calculated (numerically) and analyzed H-T (D-T), and the caloric properties (magnetocaloric effect) of the first- and second-order phase transitions are investigated. The peculiarity of magnetic ordering of an Ising antiferromagnet in the case of competition of the interaction constant with the single-ion anisotropy constant is investigated. The magnetocaloric effect of the Ising two-sublattice antiferromagnet in competition between sublattice exchange and the external magnetic field in the isostructural phase transition is considered. The ordering and magnetocaloric effect in van - Fleck paramagnet with spin S = 1 are considered under the condition of competition of exchange, magnetic field and single - ion anisotropy for two temperature limits.