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 ΔS_T^max of -4.7 Jkg^-1K^-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 ΔS_T^max of -11.4 Jkg^-1K^-1 is reached for a magnetic field change of 5 T in melt-spun Ni₅₂Mn₂₆Ga₂₂ ribbons. After thermal annealing, ΔS_T^max increases to -30.0 Jkg^-1K^-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.