Effect of magnetic field on martensitic transformation temperature in Ni-Mn-Ga ferromagnetic shape memory alloys

The magnetic field dependence of martensitic transformation temperature has been investigated using single crystals of 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 transformations, respectively. The following results are obtained: (i) the P-I transformation temperature does not change with magnetic field; (ii) the I-10M and P-14M transformation temperatures decrease under a magnetic field up to 0.8 and 0.4 MA/m, respectively, and then increase with increasing magnetic fields higher than those values; (iii) the 14M-2M transformation temperature increases under a magnetic field up to 0.4 MA/m, decreases up to 0.8 MA/m and then increases again when the magnetic field is higher than 0.8 MA/m; (iv) the P-2M transformation temperature increases linearly with increasing magnetic field. These results are well explained by the Clausius-Clapeyron equation.