Martensitic and magnetic transformation behaviors in Heusler-type NiMnln and NiCoMnln metamagnetic shape memory alloys

Martensitic and magnetic transformation behaviors of Ni50MnIn, Ni45Co5MnIn, and Ni42.5Co7.5MnIn Heusler alloys were investigated by differential scanning calorimetry (DSC), vibrating sample magnetometry (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.