Martensitic transformation and magnetic properties of Heusler alloy Ni-Fe-Ga ribbon

The martensitic transformation and magnetic properties of ferromagnetic shape memory alloy Ni-50 (+ x) Fe25 - x Ga-25 (x = -1, 0, 1, 2, 3, 4) ribbons have been systematically studied. It has been found that with the increase of Ni concentration, the martensitic transformation temperature increases, but the Curie temperature decreases. Both the two-step thermally induced structural transformation and the one-step transition have been observed in NiFeGa alloys with different compositions. It is found that the two-step transition became the one-step transition after the ribbon being heat treated at 873 K or higher. X-ray diffraction patterns show that only L2(1) --> B2 transition occurs in the samples treated at 873 K, while the gamma phase will form in the samples treated at higher temperature. Transmission electron microscopy (TEM) studies show that the alloys with martensitic transformation temperature above the room temperature are non-modulated martensite with the large domain size, being different from the stoichiometric Ni2FeGa alloy that is a modulated martensite with small domain size. The influences of Fe substitution for Ni in Ni2FeGa on the saturation magnetization and exchange interaction are also discussed. (C) 2004 Elsevier B.V. All rights reserved.