Low temperature-induced intermartensitic phase transformations in Ni-Mn-Ga single crystal

Single crystalline Ni52.6Mn23.6Ga23.8 alloy has been found to undergo a sequence of temperature-induced, reversible, intermartensitic transformations (IMTs). Such transformations are accompanied by anomalies in the calorimetric, mechanical, magnetic and electrical resistivity measurements, both on cooling and heating the samples. TEM observations have confirmed the transformation sequence austenite → 10M → 14M → 2M. The low-temperature forward IMTs are controlled by the internal stresses and elastic energy stored in the martensitic state after the performed transformation sequence; while this causes a notable variability of the forward transformation temperatures for different specimens, the corresponding reverse transformations are much more reproducible. Application of an external magnetic field of 50 kOe is observed to produce shifts of ∼5 K in all the transformation temperatures. Theoretical considerations suggest that in the intermartensitic transformations, this shift is predominantly produced by the anisotropic magnetoelastic interactions, while for the martensitic transformation, the jump in the absolute value of magnetization and magnetoelastic contribution are equally important.