Ferromagnetic shape memory alloys Ni2+xMn1-xGa

We present results of experimental studies of magnetic properties and phase transitions in a new class of smart materials, ferromagnetic shape memory alloys Ni2+xMn1-xGa. The phase diagram of these alloys was determined from magnetic, transport and calorimetric measurements in the concentration interval 0 <= x <= 0.36. These measurements revealed a general tendency of the Curie temperature T-C to decrease and of the martensitic transformation temperature T(m)to increase upon substitution of Mn for Ni. This tendency leads to the occurrence of three characteristic compositional intervals in which (i) a martensitic transformation occurs in the ferromagnetic matrix (T-m < T-C), (ii) a first-order magnetostructural phase transition from ferromagnetic martensite <-> o paramagnetic austenite takes place at T-m approximate to T-C, and (iii) a martensitic transformation occurs in the paramagnetic state when (T-m > TC). Magnetic properties of Ni2+xMn1-xGa alloys were studied systematically in a compositional interval 0 <= x <= 0.19. As the results of these studies, compositional dependencies of the spontaneous magnetization M-s (T), the jump of magnetization Delta M at the martensitic transformation, the paramagnetic Curie temperature Theta, the effective paramagnetic moment mu(eff), and the influence of a magnetic field on the martensitic transformation temperature T-m were clarified. Based on the experimental results obtained, the degree of delocalization of magnetic moments in the Ni and Mn magnetic subsystems was evaluated.