Magnetic phase diagram of the ferromagnetic shape memory alloys Ni ₂MnGa_1-xCu_x

X-ray powder diffraction, permeability, magnetization and differential scanning calorimetry measurements were carried out on the magnetic shape memory alloys Ni₂MnGa_1-xCu_x (0 ≤ × ≤ 0.25). On the basis of the experimental 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 (Para-A), paramagnetic martensite phase (Para-M), ferromagnetic austenite phase (Ferro-A), ferromagnetic martensite phase (Ferro-M) and the premartensite phase. It was found that the magnetostructural transition between the phases Para-A and Ferro-M can occur in the concentration region 0.12 < × ≤ 0.14 and that Ni₂MnGa_1-xCu_x has the characteristics of the phase diagram similar to those of the phase diagrams of Ni _2+xMn_1-xGa and Ni₂Mn_1-xCu _xGa. In order to understand the phase diagram, the phenomenological free energy as a function of the martensitic distortion and magnetization was constructed and analyzed.