FERROMAGNETIC SHAPE MEMORY ALLOYS Ni2+xMn1−x Ga

Abstract

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 TC to decrease and of the martensitic transformation temperature Tm to increase upon sub- stitution of Mn for Ni. This tendency leads to the occurrence of three characteristic compositional intervals in which (i) a martensitic transfor- mation occurs in the ferromagnetic matrix (Tm TC). Magnetic properties of Ni2+xMn1−xGa alloys were studied system- atically in a compositional interval 0 ≤ x ≤ 0.19. As the results of these studies, compositional dependencies of the spontaneous magnetization Ms(T), the jump of magnetization ∆M at the martensitic transforma- tion, the paramagnetic Curie temperature Θ, the effective paramagnetic moment µeff, and the influence of a magnetic field on the martensitic transformation temperature Tm were clarified. Based on the experimen- tal results obtained, the degree of delocalization of magnetic moments in the Ni and Mn magnetic subsystems was evaluated