Magnetic field and stress induced martensite reorientation in NiMnGa ferromagnetic shape memory alloy single crystals

Magnetic shape memory properties of a single crystal Ni₂MnGa alloy were characterized through monitoring magnetic field induced strain (MFIS) as a function of compressive stress, and applied stress induced strain as a function of magnetic field. Compressive stress and magnetic field were applied perpendicular to each other along the [1 0 0] and [0 1 1] axes, respectively. The critical magnetic fields for variant reorientation, first cycle effect and cyclic evolution of MFIS are reported as a function of stress level. It was revealed that increasing constant magnetic field level significantly increases the stress required for the reorientation, i.e., magnetostress and leads to superelasticity in martensite. Possible microstructural mechanisms, considering the interplay between stress and magnetic field favored martensite variants, magnetic domains and magnetization rotation, are proposed. Moreover, it was observed that the MFIS evolution is field rate dependent as was evidenced by a rate dependent two-stage reorientation where the maximum MFIS magnitude increases as the field rate increases. This effect was attributed to the difference between the nucleation and propagation barrier strength for twin boundary motion in NiMnGa alloys. The magnetostress (5.7 MPa), blocking stress (5 MPa) and maximum MFIS (5.8%) combination observed in this study is the highest reported to date in NiMnGa alloys. The high blocking and magnetostresses are a consequence of the low test temperature (-95 °C) where the magnetocrystalline anisotropy energy is high and detwinning stress is low. Thus, for magnetic shape memory alloys, the selection of the operating temperature with respect to martensite start and Curie temperatures is critical in optimizing actuator performance since both magnetocrystalline anisotropy energy and detwinning stress are a strong function of temperature below the characteristic temperatures.