Effect of intermartensitic reaction on the co-occurrence of the magnetic and structural transition in Ni–Mn–Ga alloys

The different magnetic transition temperatures during heating and cooling in certain Ni–Mn–Ga alloys have recently gained interest, since they may be beneficial for the giant magneto-calorimetric effect. Khovailo et al. and Chernenko et al. have suggested that this dissimilarity is connected to the different Curie points of the cubic parent phase and the non-layered tetragonal martensite as the magnetic transition is delayed due to the structural transformation. The present work extends this approach to the alloys exhibiting the intermartensitic transformation with the seven-layered orthorhombic martensite (7M) in the co-occurrent magnetic and structural transition region. We confirm the previous results that the Curie point of the tetragonal martensite is higher than the one of the parent cubic phase. It is also confirmed that in certain alloys the magnetic transition occurs in the 7M structure and, consequently, the magnetic transition temperature should be the Curie point of the 7M martensite. We verify that in the same alloy the Curie point of the 7M martensite is lower than the one of the tetragonal martensite.