Metastability of ferromagnetic Ni–Mn–Sn Heusler alloys

We previously showed that extended 4-week anneals at 1223 K were required to achieve structural and chemical homogeneity in the off-stoichiometric Ni–Mn–Sn Heusler alloy solid solution. To insure that the transition from the high temperature B2 to the room temperature L2 1 phase was complete, we performed low temperature (773 K) anneals, ∼150 K below the reported B2 → L2 1 transition temperature, on the 1223 K homogenized samples. Drastic changes in the magnetization measurements of the annealed samples suggested that the intermediate compositions of the L2 1 phase were metastable. TEM and XRD analyses confirmed the decomposition of the single-phase Heusler alloys into two phases with compositions close to Ni 54Mn 45Sn 1 and Ni 50Mn 30Sn 20. The observed phase decomposition indicated that the off-stoichiometric Ni–Mn–Sn Heusler alloys, that feature martensitic transformations, are metastable at 773 K.