Structural and magnetic phase transitions in shape-memory alloys Ni_2+xMn_1-xGa

The Heusler-type alloy Ni_2+xMn_1-xGa exhibits well defined shape memory properties in a ferromagnetic state, which means that the martensitic transition temperature is lower than the Curie point of this material. The change of composition makes these characteristic temperatures approach each other. To study this behavior, the measurements of specific heat, ac magnetic susceptibility, and dc resistivity were performed. The phase diagram of the cubic ferromagnet describing possible structural and magnetic transitions is obtained theoretically. This diagram is compared with experimental data on Ni_2+xMn_1-xGa. An estimate is given of the magnetic-field influence on the temperature of martensitic transformation in the studied alloys.