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.