2011 Volume 52 Issue 6 Pages 1142-1147
Thermal expansion, permeability, and magnetization measurements of ferromagnetic shape memory alloys, Ni52Mn12.5Fe12.5Ga23 and Ni2Mn0.75Cu0.25Ga, were performed across the martensitic transformation temperature TM and the reverse martensitic transformation temperature TR. 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 Ni2Mn0.75Cu0.25Ga, the region above TM or TR is the paramagnetic-austenite (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 Ni52Mn12.5Fe12.5Ga23 and Ni2Mn0.75Cu0.25Ga, 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.