Thermodynamic model of quasi-first-order paramagnetic–ferromagnetic phase transition and giant magnetocaloric effect

Recent experiments showed that the ferromagnetic ordering of solids can demonstrate the features of the first-order phase transition even if this process is accompanied by the unusually small volume change and narrow thermal hysteresis. In the present communication, a conception of the quasi-first-order phase transition is proposed. The conception is based on the assumption that the spin-exchange energy density of magnetic solid sharply depends on the temperature in the vicinity of the Curie point. It is argued that such dependence can be caused by the relative displacement of magnetic atoms occupying different crystallographic sites. The computations based on the general thermodynamic equations interrelating the entropy and Gibbs free energy have been carried out for the estimation of the magnetic-field-induced entropy change. The agreement between the obtained results and existing experimental data has been achieved.