Thermal expansion, permeability, and magnetization measurements of ferromagnetic shape memory alloys, Ni52Mn12.5Fe 12.5Ga23 and Ni2Mn0.75Cu 0.25Ga, were performed across the martensitic transformation temperature TM and the reverse martensitic transformation temperature 7r. When cooling from austenite phase, a steep decrease in the thermal expansion due to the martensitic transformation was found for both alloys. Considering the permeability and magnetization results of Ni2Mn 0.75Cu0.25Ga, the region above TM or T R is the paramagneticaustenite (Para-A) phase and the region below TM or TR is the ferromagnetic-martensite (Ferro-M) phase. Magnetic phase diagrams were constructed based on the results of the temperature dependence of thermal expansion. TM and TR increased gradually with increasing magnetic field. For Ni52Mn 12.5Fe12.5 Ga23 and Ni2Mn 0.75Cu0.25Ga, the shifts of TM in magnetic fields (B) were estimated as dTM/dB ≈ 0.5 K/T and 1.2 T/K, respectively. The shifts of TM indicate that the that magnetization influences martensitic transition and the increase of TM in accordance with the magnetic fields is proportional to the difference between the magnetization of austenite phase with that of martensitic phase.
ASJC Scopus subject areas