Development of the phenomenological model of metamagnetic phase transition in La(Fe,Si)13 compounds

The phenomenological model that predicts the behavior of a magnetic material near a first-order phase transition under the simultaneous action of several generalized forces: magnetic field, temperature, and external pressure, is presented in this article. Today, there is a variety of such models, based on the classical thermodynamic approach developed by Bean and Rodbell in 1962. One of their main disadvantages is the use of a linear approximation of the Curie temperature dependence on the crystal lattice volume the change. As consequence, this limits applicability of the previous models in a narrow range of temperatures and external pressures. According to the emerging paradigm, when creating a basically new generation of magnetic refrigerators, it is supposed to adjust the phase transition temperature of the working bodies in a wide temperature range down to cryogenic. The latter will require the application of high pressures up to 1 GPa. The prediction of material behavior under such extreme conditions required refinement of the constructed models: taking into account the phonon contribution to the Gibbs potential of the system, and processing the influence of external pressure on the behavior of the material. The constructed model made it possible to predict changes in the Curie temperature under the simultaneous applying of external pressure and magnetic field, to estimate the value of internal hysteresis within phase transition, and to determine the required value of external pressure to minimize the field hysteresis during the isothermal process of magnetization - demagnetization.