The structure and magnetic properties of Fe-based shape memory alloys

An experimental investigation of the magnetic and structural properties of Ni2FeGa, Ni51Fe22Ga27 and Ni2MnGa0.95Sn0.05 is reported. The structure of the compounds has been characterised using X-ray diffraction at Loughborough University and neutron diffraction at the Institut Laue-Langevin (ILL), Grenoble. At room temperature Ni51Fe22Ga27 and Ni2MnGa0.95Sn0.05 show a single phase Heusler structure with space group Fm3m which transforms to an orthorhombic martensitic phase below the transition temperature. In contrast Ni2FeGa exhibits an additional f.c.c. phase at high temperature. The low temperature structure of Ni51Fe22Ga27 was found to be completely different to the structure of other shape memory alloys investigated so far, e.g. Ni2MnGa. The results for the Ni–Fe–Ga compounds presented in this thesis are in contrast to recently published investigations. Magnetisation, specific heat and resistivity measurements have been carried out to determine the martensitic phase transition temperatures. All compounds investigated exhibit a first order transition accompanied by a hysteresis far below room temperature. A martensitic phase transition is essential for shape memory behaviour.