Martensitic Transformation And Magnetic Properties Of Ni-Mn-Sn(Co) Magnetocaloric Ribbons

Heusler Ni–Mn based magnetocaloric materials attracted much attention aspromising magnetic referigation materials. The martensitic transformation canproduce magnetocaloric effect together with the magnetic transition of austenite.Meanwhile, both the phase transition temperatures can be adjusted in the vicinity ofroom temperature. In this present work, martensitic transformation and the crystalstructure of Ni–Mn–Sn(Co) materials was investigated, and rapid solidificationprocess, alloying effect of ferrogmagnetic elements and heat treatment wereperformed. The main results and conclusions are listed as follows：(1) Modulated four-and five-layered orthorhombic, seven-layered monoclinic(4O,10M and14M), and unmodulated double tetragonal (L10) martensites arecharacterized in Heusler Ni–Mn–Sn alloys. All modulated layered martensitesexhibit nanotwins and stacking faults. With increasing transition temperatures, theresultant martensitic crystal structure evolves in a sequence of4O10M14M L10in Ni–Mn–Sn alloys. The compositional dependence of the martensitictransition temperatures is mainly attributed to the valence electron concentration (e/a)and the unit-cell volume of austenite phase.(2) It is proposed that besides grain refinement effect, highly-orientedmicrostructure of Ni50Mn41Sn9melt-spun ribbons would induce strong internal stress,which leads to the decrease of the transition temperature, and as a result, the refinedmarternsite plate and the dense broken martensitic variants formed.(3) Co atomic can enhances the magnetization of austenite and result in themagnetization discrepancy strikingly intensified between the martensite and theaustenite across the martensitic transformation, and thus increase the Curietemperature of austenite. In (Ni49Mn39Sn12)100xCox(at.%, x=0,2,4) melt-spun ribbons,the magnetic entropy change (ΔSM) around MT and TACall enhance by Co-dopingand refrigerant capacities of these ribbons increase from77.3Jkg-1to110.7Jkg-1undera magnetic field of18kOe.(4) The L21â†'B2ordering transition of Ni49Mn39Sn12ribbons takes place ataround900K. The magnetic entropy change (ΔSM) of ribbons annealed at973K is30%higher than that annealed at1173K owing to smaller thermal hysteresis andsub-grain microstructure.