Monte Carlo Simulations of Thermal Hysteresis in Ni‐Mn‐Based Heusler Alloys

We present a study of the thermal hysteresis that takes place across the magnetostructural phase transition of the Ni‐Mn‐based Heusler alloys. The results have been obtained by means of first‐principles calculations and Monte Carlo simulations based on the proposed Potts‐Blume‐Emery‐Griffiths Hamiltonian involving magnetic and elastic degrees of freedom. First, separating the structural and magnetic contributions, we calculate the phase diagram including the martensitic transformation as a function of ratio between exchange structural constants in martensite and austenite (K/J). It is shown that the width of the thermal hysteresis depends on K/J and it is found to increase at a ratio of K/J between 0.20 and 0.37. A similar trend is also observed for the Landau free energy expansion. The evolutions of the magnetization and deformation order parameter of alloys studied under various applied magnetic fields are simulated as functions of temperature upon heating and cooling cycles. Furthermore, under high applied fields the nonlinear shift of dTm/dHext and change of hysteresis width according to the relation between magnetostructural interaction constants (U1 and U2) are observed. The results obtained with the Monte Carlo routine are discussed and demonstrated a good agreement with experimental data. The possibility of thermal hysteresis in the BEG (Blume‐Emery‐Griffiths) Hamiltonian in dependence on the ratio between exchange constants (K/J) is shown in the figure. Here the martensitic transformation temperature is displayed for heating and cooling protocols. An increase in K/J ratio leads to a change of the second by first order phase transition and to an appearance of thermal hysteresis.