Investigation of magnetic field-induced phase transition in Ni50Mn28Ga22, a ferromagnetic shape memory alloy

Ferromagnetic shape memory alloys (FSMAs) exhibit magnetic-field-induced strain (MFIS) through a reversible martensitic transformation. Among these, Nickel–Manganese–Gallium (Ni–Mn-Ga) alloys have drawn significant attention due to their large MFIS (up to ∼10% in single crystals) under moderate magnetic fields, far surpassing even the best magnetostrictive materials such as Terfenol. Such a large axial strain change in single-crystal Ni–Mn-Ga arises from a coupled structural and magnetic phase transition, enablingrapid, reversible deformation without mechanical loading, making these materials promising for compact actuators, sensors, and energy harvesters. Their transition temperatures and magnetic properties are tunable via composition, offering flexibility for several device applications. Our research focuses on the investigation of Ni50Mn28Ga22 FSMA for the development of a core-shell structure with the MFIS materials as a core and piezoelectric material as the shell for potential biomedical applications such as imaging and stimulation. Transformation behavior reveals a first-order-like phase transition near 300 K under fields from 0 to 0.6 T, which broadens into a second-order-like transition between 0.6 and 9 T. A structural transformation occurs between the high-temperature austenite and low-temperature martensite phases around 300 K; however, both phases exhibit ferromagnetic hysteresis, indicating that the transition involves changes in magnetic permeability rather than a magnetic phase change. Significant strain variations are observed across the transformation during both heating and cooling, along with magnetostrictive-like strain under applied fields, though no field-induced phase transition was detected in either samples.