Decomposition in Mn50Ni40In10 melt-spun ribbons and its influence on magnetic properties

The phase stability, decomposition and its influence on the martensitic transformation and magnetic properties of magnetic shape memory alloy (MSMA) Mn50Ni40In10 melt-spun ribbons were investigated experimentally and theoretically. As-spun Mn50Ni40In10 was fine single Heusler phase with pronounced martensitic and magnetic transition below room temperature. However, in differential scanning calorimetry measurement an exothermic peak at 690 K and an endothermic peak at 765 K is observed during the heating of as-spun ribbons. The two peaks correspond to the growing of the punctiform precipitates in the grain interiors and the formation of a large amount of grain boundary segregation, respectively. After annealing at 870 K, the ribbon samples decompose into a cubic In-rich phase at grain boundaries and In-poor 7 M-modulate martensitic phase in the matrix. After annealing at 690 K and above, the martensitic transformation below room temperature is strongly weakened. Annealing temperature also has obvious influence on the 5 and 300 K saturation magnetization Ms of Mn50Ni40In10. Theoretical calculations on Mn2Ni1.5In0.5 (Mn50Ni37.5In12.5) give a negative formation energy (-0.11 eV/ f.u.) and a positive decomposition energy (0.22 eV/f.u.), indicating that this alloy can be metastable and tends to decompose into Mn2NiIn cubic phase and NiMn martensite, agreeing with the experimental results qualitatively.