ИСТИНА

The commercial implementation of solid-state magnetic cooling technology encounters significant challenges, including limited adiabatic temperature change values within a restricted magnetic field range (up to 1T), a narrow operating temperature range for materials exhibiting first-order phase transitions, and accompanying temperature and field hysteresis phenomena that further reduce the cooling capacity of magnetic refrigerators. Various approaches have been explored to overcome these challenges. Firstly, one approach involves utilizing active magnetic regenerators, which can expand the temperature range but require a set of working bodies. Moreover, the cooling capacity achieved with such regenerators remains considerably lower compared to classical refrigeration cycles. Secondly, the manipulation of materials' hysteresis characteristics through microstructural and compositional adjustments has reached a critical point. Here, the material exhibits sufficiently high magnetic entropy change (ΔSM) values, while the hysteresis behavior vanishes. The most auspicious way lies in employing multi-stimulus materials and constructing novel cooling cycles based on them. This approach implies the simultaneous application of several generalized forces, including temperature, external magnetic field, and pressure, upon the material in proximity to the phase transition between two magnetic states. The realization of magnetic cooling cycles employing multi-stimulus materials holds the promise of not only expanding the operating temperature range by shifting the Curie point through field and pressure influences. It can also enhance the cooling capacity of magnetic refrigerators by effectively utilizing the interpolar volume of the magnetic system. Moreover, the application of external pressure serves to amplify the adiabatic temperature change in these materials by intensifying the sharpness of the transition. Several approximate models [1, 2] have been developed to predict the behavior of magnetocaloric materials near phase transitions under the simultaneous influence of temperature, an external magnetic field, and pressure. However, the existing phenomenological models have limitations as they do not fully consider the impact of the crystal lattice on free energy, leading to insufficient accuracy in describing and predicting the material's behavior under pressure. Therefore, the objective of this study was to incorporate the Debye entropy of the crystal lattice into the Helmholtz free energy within the framework of the L.D. Landau approximation. This work includes estimates of the internal hysteresis for the La(Fe,Si)13 alloy and the prediction of the optimal combination of external magnetic field and pressure values to achieve maximum efficiency of the multi-stimulus cycle and of the dynamic effects during the first-order magnetic phase transition