Martensitic transformation and physics characterization of ferromagnetic shape memory alloy Ni_52.2Mn_23.8Ga₂₄

We have measured the transformation temperature of Ni_52.2Mn_23.8Ga₂₄ single crystalline sample by various methods, such as DSC, magnetization, ac magnetic susceptibility, dc resistance, and transformation strain. Comparing the experimental results between each other, we found that, due to the different physical mechanism, various measuring methods will give different martensitic transformation temperatures. Based on these observations, we suggest a multi-step lattice deformation for the transformation of NiMnGa, which is similar to the G-T relation. The whole process of transformation can be observed by DSC and R-T measurements, while the magnetic measurements is only sensitive to the formation of martensitic lattices. The measurement on transformation strain is not so reliable for the transformation temperature as the other measurements, but it really reflects the situation of the sub-structures, such as twin boundaries, variants and defects in the material.