Thermal Expansion and Magnetization Studies of Novel Ferromagnetic Shape Memory Alloys Ni<SUB>52</SUB>Mn<SUB>12.5</SUB>Fe<SUB>12.5</SUB>Ga<SUB>23</SUB> and Ni<SUB>2</SUB>Mn<SUB>0.75</SUB>Cu<SUB>0.25</SUB>Ga

Abstract

Thermal expansion, permeability, and magnetization measurements of ferromagnetic shape memory alloys, Ni 52 Mn 12:5 Fe 12:5 Ga 23 and Ni 2 Mn 0:75 Cu 0:25 Ga, 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 Ni 2 Mn 0:75 Cu 0:25 Ga, the region above T M or T R is the paramagneticaustenite (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 Ni 52 Mn 12:5 Fe 12:5 Ga 23 and Ni 2 Mn 0:75 Cu 0:25 Ga, 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.

Thermal expansion and magnetization studies of the novel ferromagnetic shape memory alloy Ni2MnGa0.88Cu0.12in a magnetic field

“…Ni 2 Mn 0.75 Cu 0.25 Ga also shows the difference in magnetization between 302 and 305 K, which is a little lower than T C = 307 K or T M = 308 K [10]. It is notable that the Arrott plots of Ni 2 MnGa 0.88 Cu 0.12 left a space between 333 and 336 K, and those of Ni 2 Mn 0.75 Cu 0.25 Ga left a space between 302 and 303 K. The spontaneous magnetizations of Ni 2 MnGa 0.88 Cu 0.12 are 33.4 J (µ 0 kg T) −1 at 333 K and 16.7 J (µ 0 kg T) −1 at 336 K, which were obtained by the Arrott plot shown in figure 6.…”

“…The contraction in T M under zero fields is about 1.3 × 10 −3 (0.13%). As for Ni 52 Mn 12.5 Fe 12.5 Ga 23 and Ni 2 Mn 0.75 Cu 0.25 Ga, the contraction occurs at the martensite temperature [10]. The strain at T M of polycrystalline Ni 52 Mn 12.5 Fe 12.5 Ga 23 was estimated as 0.14% contraction.…”

“…Ni 2 MnGa 0.88 Cu 0.12 is an alloy for which the martensitic transition occurs around T C , where T C is the Curie temperature of the martensite phase. Our previous paper revealed that the steep shrinking due to the martensitic transition was obtained at linear thermal expansion measurements of polycrystalline alloys, Ni 52 Mn 12.5 Fe 12.5 Ga 23 and Ni 2 Mn 0.75 Cu 0.25 Ga [10]. T M and T R increase gradually with increasing magnetic field.…”

“…Therefore they are candidates for smart materials. For Ni 2 Mn 0.75 Cu 0.25 Ga, which shows a Para-A-to-Ferro-M phase transition at T M ≈ 308 K [9,10], it was observed that the ferromagnetic transition and the martensitic transition occur around the same temperature.…”