Magnetic and structural evolution in Ni-rich off-stoichiometric NiMnGa melt spun ribbons

The investigation is focussed on the magnetic and structural behaviour in a Ni-rich off-stoichiometric NiMnGa alloy. The Ni₅₅Mn₂₂Ga₂₅ (at %) alloy prepared in the form of melt-spun ribbon showed martensitic structure in its as-cast state. Differential scanning calorimetric (DSC) heating and cooling cycles revealed austenitic (A_S=406K) and martensitic (M_S=415K) transformation temperatures respectively which were above room temperature. The Curie temperatures obtained from thermo-magnetic experiments were found to be below these transformation temperatures. The as-spun ribbons revealed low fraction of L2₁ of austenitic phase with a predominant tetragonal (5M) martensite in combination with martensite of higher modulations (7M, orthorhombic). Magnetic coercivity and saturation magnetisation also evidenced the magnetic transitions during ferromagnetic martensite/austenite to paramagnetic austenite.