Magnetic-Field-Induced Isothermal Entropy Change Across the Magnetostructural Transition in Ni-Mn-Ga Melt-Spun Ribbons

The phase transformation and the magnetocaloric effect of Ni53Mn22Ga25 and Ni52Mn26Ga22 ribbons were studied. It is shown that in melt-spun Ni53Mn22Ga25 ribbons the magnetic transition and the martensitic transformation are well separated. For this sample, the austenite grains are columnar in shape with a typical preferential orientation of {001}(A)//ribbon plane. For a magnetic field change of 5 T, the maximum entropy change Delta S-T(max) of -4.7 Jkg(-1)K(-1) is achieved at the vicinity of the martensitic transformation. For the Ni52Mn26Ga22 ribbons, the structural transformation and the magnetic transition occurred simultaneously in both melt-spun and annealed ribbons. Owing to the coupled magnetostructural transformation, a Delta S-T(max) of -11.4 Jkg(-1)K(-1) is reached for a magnetic field change of 5 T in melt-spun Ni52Mn26Ga22 ribbons. After thermal annealing, Delta S-T(max) increases to -30.0 Jkg(-1)K(-1), which is almost three times higher as that of the initial melt-spun ribbons, due to the improved atomic ordering and the coupling of the intermartensitic transformation with the magnetostructural transformation.