Domain Structures Across the Martensitic Transformation in Ni_(2+x)Mn_(1-x)Ga

摘要

Evolution of domain structures across the martensitic transition (T_m) in the ferromagnetic shape memory alloy system Ni-Mn-Ga is studied using an optical microscope with a temperature variation. Compositions chosen have T_m < T_c, T_m = T_c and T_m > T_c, (T_c=Curie temperature) so that one can compare the nature of martensitic domains. There are no appreciable domain structures when T_m < T_c as compared to the one with T_m > T_c. However, giant morphological changes in the form of appearance of well-developed domains that are propagating with different directions are seen for the composition in which T_m=T_c. The results are discussed in the light of Magnetic Force Microscopy observations as well as giant entropy changes known to occur on samples with cooccurrence of T_m and T_c.