First-principles and Monte Carlo study of magnetostructural transition and magnetocaloric properties of Ni2+x Mn1-x Ga

First-principles calculations of magnetic exchange parameters of the austenitic and martensitic phases of Ni-Mn-Ga allow one to characterize this Heusler system across the phase diagram in agreement with experimental trends. The ab initio investigations have been combined with Monte Carlo simulations for a detailed description of the magnetic, martensitic, and magnetocaloric properties of ${\text{Ni}}_{2+x}{\text{Mn}}_{1\ensuremath{-}x}\text{Ga}$ $(0.18\ensuremath{\le}x\ensuremath{\le}0.27)$ Heusler alloys, which undergo a first-order magnetostructural phase transition. For these alloys, the calculated temperature dependence of the magnetic and lattice contributions to the total specific heat as well as the evaluation of the isothermal magnetic entropy $\ensuremath{\Delta}{S}_{mag}(T,{H}_{ext})$ and adiabatic temperature $\ensuremath{\Delta}T(T,{H}_{ext})$ changes around the magnetostructural transition in an external magnetic field agree fairly well with the experimental data. In particular, results for $\ensuremath{\Delta}{S}_{mag}(T,{H}_{ext})$ and $\ensuremath{\Delta}T(T,{H}_{ext})$ may be used to speculate about designing new magnetic Heusler alloys with better magnetocaloric properties, i.e., larger $\ensuremath{\Delta}T(T,{H}_{ext})$ values.