Theoretical approach to investigation of the magnetic and magnetocaloric properties of Heusler Ni–Mn–Ga alloys

The magnetic and magnetocaloric properties of Heusler Ni$_{2+x}$Mn$_{1-x}$Ga ($x$ = 0.16, 0.18, and 0.3) have been studied using a model based on the Malygin theory of smeared phase transitions, the Bean–Rodbell theory of first-order phase transitions, and the mean-field theory. The temperature dependences of strain, magnetization, and isothermal change in entropy of these alloys have been analyzed. It is shown that the largest change in the magnetic entropy is observed in a Ni$_{2.18}$Mn$_{0.82}$Ga alloy, in which the martensitic transition is accompanied by a change in the magnetic ordering. The smallest change in the entropy is observed in a Ni$_{2.3}$Mn$_{0.7}$Ga alloy, which exhibits the magnetocaloric effect in a martensitic phase with a change in the magnetic ordering. However, the refrigeration capacity of this alloy is twice as high as that of the other considered compositions.