Phase transitions in Ni2+xMn1-xGa with a high Ni excess

Ferromagnetic shape memory alloys Ni2+xMn1-xGa were studied in the range of compositions 0.16≤x≤0.36. The experimental phase diagram, constructed from differential scanning calorimetry, transport, and magnetic measurements, exhibits distinctive feature in a compositional interval 0.18≤x≤0.27, where martensitic and magnetic transitions merge in a first-order magnetostructural phase transition ferromagnetic martensite aramagnetic austenite. Observed in this interval of compositions a nonmonotonous behavior of the magnetostructural phase transition temperature was ascribed to the difference in the exchange interactions of martensitic and austenitic phase and to the competition between increasing number of valence electron and progressive dilution of the magnetic subsystem which occur in the presence of a strong magnetoelastic interaction. Based on the experimental phase diagram, the difference between Curie temperature of martensite TCM and Curie temperature of austenite TCA was estimated. Influence of volume magnetostriction was considered in theoretical modeling in order to account for the existence of the magnetostructural phase transition over a wide range of compositions.