Unusual nature of the martensite and ferromagnetic transitions in Ni2Mn0.4-XFeXCr0.6Ga Heusler alloys

An experimental study has been performed on a series of Ni2Mn0.4-xFexCr0.6Ga Heusler alloys. At room temperature, the alloys crystallize in either the tetragonal martensite (x < 0.1) or cubic L21 structure (x ≥ 0.1). Additionally, a Cr-Fe based face-centered cubic γ-Fe type secondary phase was found to co-exist in the samples. Magnetization and transport measurements revealed that the Curie and martensitic transition temperatures decrease as Mn is replaced with Fe. Atypical to other Ni2MnGa-derivative Heusler alloys, the transition temperatures decrease at the same rate with respect to x for x ≥ 0.1. Thus, the two transitions do not couple in to a single magnetostructural transition at any composition. Transport measurements revealed that all samples exhibit a sharp drop in resistivity during the martensitic phase transition (13 - 17 %), with the magnitude of this drop remaining relatively constant over the entire series. The possible origins of the observed experimental behavior are discussed.