Thermal Expansion and Magnetization Studies of Novel Ferromagnetic Shape Memory Alloys Ni₅₂Mn_12.5Fe_12.5Ga₂₃ and Ni₂Mn_0.75Cu_0.25Ga

Abstract

Thermal expansion, permeability, and magnetization measurements of ferromagnetic shape memory alloys, Ni₅₂Mn_12.5Fe_12.5Ga₂₃ and Ni₂Mn_0.75Cu_0.25Ga, were performed across the martensitic transformation temperature T_M and the reverse martensitic transformation temperature T_R. 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_0.75Cu_0.25Ga, the region above T_M or T_R is the paramagnetic-austenite (Para-A) phase and the region below T_M or T_R is the ferromagnetic-martensite (Ferro-M) phase. Magnetic phase diagrams were constructed based on the results of the temperature dependence of thermal expansion. T_M and T_R increased gradually with increasing magnetic field. For Ni₅₂Mn_12.5Fe_12.5Ga₂₃ and Ni₂Mn_0.75Cu_0.25Ga, the shifts of T_M in magnetic fields (B) were estimated as dT_M⁄dB≈0.5 K/T and 1.2 T/K, respectively. The shifts of T_M indicate that the that magnetization influences martensitic transition and the increase of T_M in accordance with the magnetic fields is proportional to the difference between the magnetization of austenite phase with that of martensitic phase.