Martensitic and Magnetic Transformation Behaviors in Heusler-Type NiMnln and NiCoMnln Metamagnetic Shape Memory Alloys

摘要

Martensitic and magnetic transformation behaviors of Ni_(50)MnIn, Ni_(45)Co_5MnIn, and Ni_(42.5)Co_(7.5)MnIn Heusler alloys were investigated by differential scanning calorimetry (DSC), vibrating sample mag-netometry (VSM), and transmission electron microscopy (TEM). The martensitic transformation starting temperature (M_s) decreases with increasing In composition, while the Curie temperatures (T_c) of the parent phase are almost independent in each alloy series. On the other hand, the addition of Co resulted in a decrease of the M_s and an increase of the T_c, and the degree of the decline of M_s was accelerated by magnetic transformation of the parent phase. The M_s temperature change induced by the magnetic field was also confirmed. It was found that the degree of M_s change is strongly related to the entropy change by the martensitic transformation, which shows a correlation with T_c-M_s. These behaviors can be qualitatively explained on the basis of thermodynamic considerations.