| Summary: | The microstructure and magnetocaloric properties of the melt-spun and annealed Mn50Ni40.5In9.5 ribbons were studied. It is shown that the post-annealing results in a considerable increase of the grain size for the initial austenite, where the columnar-shaped austenite grains almost run through the whole ribbon. Both the melt-spun and annealed ribbons consist of the mixture of austenite and martensite at room temperature, where a 8-layered modulated (8M) martensite structure was identified through selected area electron diffraction (SAED). Further High-angle Annular Dark-field (HAADF) characterizations reveal that the modulation period of 8M martensite is not homogeneous in one martensite plate. Due to strong magneto-structural coupling, the inverse martensitic transformation from a weak magnetic martensite to a strong magnetic austenite can be induced by the magnetic field, resulting in the inverse magnetocaloric effect around room temperature. For a field change of 5 T, the magnetic entropy change ΔSM of 3.7 J·kg−1·K−1 and 6.1 J·kg−1·K−1, and the effective refrigerant capacity RCeff of 52.91 J·kg−1 and 99.08 J·kg−1 were obtained for melt-spun and annealed ribbons, respectively. The improvement of the magnetocaloric properties after annealing should be attributed to the enhanced atomic ordering and magnetization difference between two phases, as well as the reduced hysteresis loss. In addition, both the melt-spun and annealed ribbons can work at a relatively wide temperature range, i.e., δTFWHM = 34 K for melt-spun ribbons and δTFWHM = 28 K for annealed ribbons.
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