Martensitic transformations in Ni-Mn-Ga system affected by external fields

The influence of hydrostatic pressure, uniaxial stress and magnetic field on the martensitic transformation temperatures for the ferromagnetic single crystalline Ni-Mn-Ga alloys is studied. It is shown that the experimental results are satisfactorily described by the Landau theory. Ni-Mn-Ga L2 1 -type ordered alloys exhibit a number of the first order and weak first order structural transformations in a ferromagnetic or paramagnetic parent phase depending on the alloy composition and being either thermally or stress activated [1,2]. Most of these phase transformations are of the martensitic type, i.e., they are accompanied by the spontaneous elastic strains forming a multicomponent order parameter in the Landau expansion for the Gibbs potential (see [3] and references therein). In this work we analyze the influence of the external fields (mechanical and magnetic) on the martensitic transformation (MT) from cubic parent phase (P) to five-layered martensitic one (5M-martensite) usually exhibited by the ferromagnetic ordered Ni-Mn-Ga alloys. In accordance with [4], we treat the 5M-martensite as a twinned tetragonal phase and, so, describe the experimental results in the framework of the theory of cubic-tetragonal MT [5]. The original experimental data of high magnetic field influence on MT in near stoichiometric Ni 2 MnGa compound are presented to compare with the theoretical estimations.