Magnetocaloric Properties Of Ni2+Xmn1-Xga With Coupled Magnetostructural Phase Transition

Systematic studies of magnetic entropy change Delta S-m and adiabatic temperature change Delta T-ad 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 Delta H=5T), Delta S-m=-29J/kgK, in a Ni2.20Mn0.80Ga composition. The decrease in Delta 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 Delta T-ad measured upon heating in the Ni2+xMn1-xGa (0.18 <= x <= 0.27) alloys does not exceed 0.8K (for the magnetic field change of Delta H=1.85T). A higher value of Delta T-ad measured upon cooling has been explained as caused by the contribution of the structural subsystem to Delta 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.