The modeling of the magnetocaloric effect in the shape memory Heusler Ni-Mn-Ga alloys by Monte-Carlo simulations

The magnetocaloric effect (MCE) has a significant technical importance since magnetic materials with large MCE values can be used as refrigerants in the devices working on principles of magnetic refrigeration. Recent researches have shown that Heusler Ni-Mn-Ga alloys are also the perspective materials as refrigerants in cooling devices. These alloys have approximately the same properties as the best MCE materials. The aim of the present work is the modeling of the magnetic entropy change and other magnetic properties of Ni-Mn-Ga alloys by classical Monte-Carlo simulations. In the proposed model we use the three-dimensional cubic lattice with periodic boundary conditions and consider interactions only between nearest sites of the lattice. The whole system can be representing as two interacting parts – magnetic and structural subsystems. The magnetic part is describes by the “q-state” Potts model for the first order magnetic phase transition from ferromagnetic to paramagnetic phases. The structural part is describes by the degenerated three – state Blume – Emery – Griffiths model for the structural transformation from cubic (austenitic) phase to tetragonal (martensitic) one. In our model we considered the alloys with compositions at which the first order coupled magnetostructural phase transition takes place. For the modeling lattice we use the standard Metropolis algorithm. The isothermal magnetic entropy change upon variation of the magnetic field from 0 to 5 T and other magnetic properties, such as the structural and the magnetic order parameters, the magnetic specified heat, and the magnetic part entropy were theoretically calculated. It is shown that the theoretical results are in good agreement with available experimental ones.