Effect of magnetizing field on the martensitic transformations in a melt spun NiMnGa alloy

The investigation addresses the effect of magnetizing field on the magnetic properties of melt spun Ni_52.84Mn_19.6Ga_27.56 (at%) alloy ribbons. Magnetization behaviour at different fields was observed using a superconducting quantum interference device magnetometer for heating and cooling cycles. The plots showed distinct changes in magnetization around the characteristic temperatures at austenitic start and finish (A_S, A_F), martensitic start and finish (M_S, M_F). With increasing field A_S, M_F were unaffected. In the range of martensitic start and its finish temperature, the zero field cooled and field cooled measurements indicated magnetization drops indicating antiferromagnetic interactions, which is characteristic of the martensitic phase formation. It was shown from x-ray diffraction analysis that the low martensitic fraction in the majority austenite phase induced the splitting in the L2₁ austenitic ordering. This was further corroborated by the evidence of a few martensitic plates around grain boundaries at room temperature which is close to martensitic start temperature.