Martensitic transition, ferromagnetic transition, and their interplay in the shape memory alloys Ni₂ Mn_1-x Cu_x Ga

Magnetization and initial permeability measurements were done on the ferromagnetic shape memory alloys Ni₂ Mn_1-x Cu_x Ga (0≤x≤0.4). On the basis of the results, the phase diagram in the temperature-concentration plane was determined for this alloy system. The determined phase diagram is spanned by the paramagnetic austenite phase (I), paramagnetic martensite phase (II), ferromagnetic austenite phase (III), ferromagnetic martensite phase (IV), and the premartensite phase (V). It was found that the magnetostructural transitions between the phases I and IV can occur in the concentration region 0.23≤x≤0.30 and that Ni₂ Mn_1-x Cu_x Ga has the characteristics of the phase diagram closely similar to those of the phase diagram of Ni_2+x Mn _1-x Ga. In order to understand the phase diagram, the phenomenological free energy as a function of the martensitic distortion and the magnetization was constructed and analyzed, where couplings between these order parameters in existing theories were improved. Satisfactory agreements between the theory and experiments were obtained except for the appearance of the premartensite phase. The analyses show that the biquadratic coupling term, together with a higher order coupling term, of the martensitic distortion to the magnetization, plays an important role in the interplay between the martensite phase and the ferromagnetic phase.