Magnetocaloric properties of Ni2+xMn1−xGa with coupled magnetostructural phase transition

Systematic studies of magnetic entropy change ΔSm and adiabatic temperature change ΔTad have been performed for ferromagnetic shape memory alloys Ni2 + xMn1−xGa (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 ΔH = 5 T), ΔSm = −29 J/kg K, in a Ni2.20Mn0.80Ga composition. The decrease in ΔSm 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 ΔTad measured upon heating in the Ni2 + xMn1−xGa (0.18 ≤ x ≤ 0.27) alloys does not exceed 0.8 K (for the magnetic field change of ΔH = 1.85 T). A higher value of ΔTad measured upon cooling has been explained as caused by the contribution of the structural subsystem to ΔTad, 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.