Magnetic and structural phase transitions in the shape-memory ferromagnetic alloys Ni2+xMn1-xGa

The results of an experimental investigation of the temperature dependences of the magnetic susceptibility and resistivity in the shape-memory ferromagnetic alloys Ni2+xMn1-xGa (x=0-0.20) are reported. A T-x phase diagram is constructed on the basis of these data. It is shown that partial substitution of Ni for Mn causes the temperatures of the structural (martensitic) T-M and magnetic T-C (Curie point) phase transitions to converge. In the region where T-C=T-M the transition temperature increases linearly with magnetic field in the range from 0 to 10 kOe. The kinetics of a magnetic-field-induced martensitic phase transition is investigated, and the velocities of the martensite-austenite interphase boundary during direct and reverse transitions are measured. A theoretical model is proposed and the T-x phase diagram is calculated. It is shown that there exist concentration ranges where the magnetic and martensitic transitions merge into a first-order phase transition. The theoretical results are in qualitative agreement with experiment. (C) 1999 American Institute of Physics. [S1063-7761(99)01605-4].