Magnetocaloric properties of Ni_(2+x)Mn_(1-x)Ga with coupled magnetostructural phase transition

摘要

Systematic studies of magnetic entropy change △S_m and adiabatic temperature change △T_(ad) have been performed for ferromagnetic shape memory alloys Ni_(2+x)Mn_(1-x)Ga (0.18 ≤ x ≤ 0.27) undergoing coupled magnetostructural phase transition from ferromagnetic martensite ⇄ paramagnetic austenite. The magnetic entropy change calculated from isothermal magnetization measurements has the highest value (for the magnetic field change of △H = 5 T), △S_m = -29 J/kg K, in a Ni_(2.20)Mn_(0.80)Ga composition. The decrease in △S_m observed in the alloys with the larger Ni excess (0.22 ≤ x ≤ 0.27) is attributed to the progressive reduction in both the saturation magnetization and the total entropy change at the martensitic transition temperature. The adiabatic temperature change △T_(ad) measured upon heating in the Ni_(2+x)Mn_(1-x)Ga (0.18 ≤x≤ 0.27) alloys does not exceed 0.8 K (for the magnetic field change of △H =1.85 T). A higher value of △T_(ad) measured upon cooling has been explained as caused by the contribution of the structural subsystem to △T_(ad), i.e., to a partial magnetic field-induced structural transformation that has, for the given magnetic field change, an irreversible character in the alloys studied.