Magnetic and lattice entropy change across martensite transition of Ni-Mn-Sn melt spun ribbons: Key factors in magnetic refrigeration

Ni45Mn44Sn11 melt spun ribbons have been prepared by melt spinning technique. A single austenite L2(1) phase with lattice constant 5.945 angstrom is confirmed at room temperature. The first order martensite transition is observed from calorimetric, electronic transport and magnetic measurements in our Ni45Mn44Sn11 melt spun ribbon sample. The change in lattice entropy due to martensite transformation is found to be nearly 15 J kg(-1)K(-1) from calorimetric data, both for heating and cooling cycle. A noticeable shift (- 7.5 K) of reverse martensite transition for the application of 8 T magnetic field has been clearly observed from temperature dependent resistivity measurement. Field induced transformation of martensite phase to austenite phase has also been noticed directly from the field dependent isothermal resistivity data inside martensite transition regime. As a result large negative magnetoresistance of 38% has been observed at 265 K. Sharp increase of magnetization, on heating, near reverse martensite transition is also observed due to transformation of weak magnetic martensite phase to ferromagnetic austenite phase. Isothermal magnetization data show magnetic hysteresis near the martensite transition regime due to difference in austenite and martensite phase fraction between increasing and decreasing field. A maximum positive magnetic entropy change of 7.2 J kg(-1) K-1 is observed for the field change of 5 T. Magnetic hysteresis and refrigerant capacity are the important parameters for magnetic refrigerant material. The maximum value of magnetic hysteresis loss is 16.6 J kg(-1). However, the net refrigerant capacity is found to be 30.4 J kg(-1) for Ni45Mn44Sn11 melt spun ribbon sample. The adiabatic temperature change is found to be of - 2.68 K for application of 5 T magnetic field at 276 K.