Martensitic Transformation of Ni-Mn-Ga Alloys under Magnetic Field and Hydrostatic Pressure

The effects of magnetic field and hydrostatic pressure on martensitic transformation have been systematically investigated by using Ni2MnGa, Ni2.14Mn0.84Ga1.02, and Ni2.14Mn0.92Ga0.94, which exhibit P(parent phase)-I(intermediate phase)-10M, P-14M-2M, and P-2M transformation, respectively. The following results were obtained. (i) The P-I transformation temperature does not change by magnetic field. (ii) The I-10M and the P-14M transformation temperatures decrease under applied magnetic field up to 0.8 MA/m and 0.4 MA/m, respectively, and then increase with increasing applied magnetic field higher than those fields. (iii) The 14M-2M transformation temperature increases under a magnetic field up to 0.4 MA/m and decreases under magnetic field up to 0.8 MA/m and then increases again when the magnetic field becomes higher than 0.8 MA/m. (iv) The P-2M transformation temperature increases linearly with increasing applied magnetic field. (v) All transformation temperatures increase linearly with increasing hydrostatic pressure. The experimental results mentioned above (i)~(iv) under magnetic field can be well explained by using the Clausius-Clapeyron equation.