Study On The Physical Properties Of NiMn-Based Ferromagnetic Shape Memory Alloys

Since the phase transformation in NiMn-based ferromagnetic shape memory alloys can be tuned by both the temperature and the magnetic field, a lot of new related properties, such as magnetic resistance, magnetostrain, magnetocaloric effect and so on, have been reported. NiMnGa and NiMnIn alloys are two representative materials. The large magnetostrain effect has been reported much in NiMnGa alloys. However, its magnetic power output is relatively small. From the perspective of exploring new materials, NiMnIn alloys are more attractive because of its larger magnetic power output and many interesting properties. In this paper, NiMnIn alloys are prepared by the arc-melting and melt-spun method. Also, a varied of physical properties are measured by X-ray diffraction (XRD), scanning electron microscope and superconducting quantum interference device (SQUID). The main content is as following:(1) The structure and strain behavior associated with martensitic transformation for polycrystalline Ni5oMn33Ini7-xGax (x=4,5,7,8) has been investigated. All samples show a fine martensitic transformation. A large positive transformation strain parallel the solidification direction and negative strain perpendicular to this direction are observed upon martensitic transformation. Due to the magnetic field induced reverse martensitic transformation, the large value of reversible magnetostrain is found in a magnetic field of 70 kOe. The largest value of -0.28% and 0.5% along this two directions were observed for x=4 sample, which is much larger than that in Co-doped NiMnIn polycrystalline. The strain value shows a little decrease with Ga content, indicating a weakening of the preferred orientation.(2) The martensitic transformation and related properties for Ni55Mn25In20 alloy have been studied. Also, the first principles calculation is used for the theoretical research. The samples are prepared by the melt-spun method and the ribbons show an ordered L21 structure at room temperature. From the results about the measurement of magnetization and electrical resistance, we found that martensitic transformation occurring at about 156K, with both phase showing ferromagnetic ordering. In addition, a negative magnetoresistance of 17% is observed at a magnetic field of 50 kOe due to the magnetic field induced reverse martensitic transformation. Electronic structure calculations indicate that the 3d states of Ni occupied in In site strongly hybridize with the Ni:3d states. Such hybridization plays an important role in driving the martensitic transformation.(3) A series of Ni-rich Ni50+xMn25In25-x(x=0-9) alloys have been prepared. The structure and magnetic properties have been investigated in detail. We found that the lattice constant decreases with reducing In content. The martensitic transformation began to appear when x≥5 and the martensitic transformation temperature increases with decreasing In content. A pure L21 structure cannot be obtained when x≥9 and the martensitic transformation disappeared. Further investigation about the origin of martensitic transformation is ongoing.