Phase diagram of the ferromagnetic shape memory alloys Ni ₂MnGa _1-xCo _x

Permeability, magnetization, and differential scanning calorimetry measurements are carried out on the ferromagnetic shape memory alloys Ni ₂MnGa _1-xCo _x (0≤x≤ 0.20). On the basis of the experimental results, the phase diagram in the temperature-concentration plane is determined. The determined phase diagram is spanned by a paramagnetic austenite (Para-A) phase, paramagnetic martensite phase, ferromagnetic austenite phase, ferromagnetic martensite (Ferro-M) phase, and a premartensite phase. The measurements show that a magnetostructural transition between the Para-A and Ferro-M phases does not appear in this alloy system. The phase diagram is discussed using phenomenological Landau theory.