Synthesis and characterisation of Fe-substituted Ni50Mn25Fe Ga25- single crystals—Development of the phase transformations with Fe content

Despite promising initial properties, the industrial applicability of magnetic shape memory alloys of the Ni-Mn-Ga family is distinctly limited by the temperature range it can operate in. Here we investigate the effects of minor substitutions of Fe for Ga on the thermodynamic, structural, and magnetic properties of Ni50Mn25FexGa25-x (3.0 < x < 7.5). Single crystals are essential for most intended applications, the studied compositions were prepared in single crystalline form. With increasing iron content, the melting point of the alloy decreases slightly from 1407(2) to 1394(2) K, while the structural B2’→L21 ordering transition increases in temperature from 1066(2) to 1082(4) K. Magnetisation measurements find that both the Curie temperature and the martensitic transformation temperature increase approximately linearly with Fe concentration, with Curie temperature increasing from 398(1) to 427(2) K, and martensitic transformation temperature increasing from 274(1) to 372(1) K when going from x = 3.0–7.5. The martensitic transformations above room temperature were confirmed by temperature-dependent single-crystal X-ray diffraction. These observations are compared with polycrystalline materials of the same series and support results of the previous studies. The shrinking gap between Curie and martensitic transformation temperatures, as well as the reduction in the low temperature saturated moment from 3.52(2) to 2.78(3) µB/f.u. make this substitution less industrially attractive but provide essential structural and property evolution details needed to improve and tune theoretical modelling of this important material family.