Magnetomechanical properties of melt-spun off-stoichiometric Ni50Mn28Ga22 ribbons

Abstract

A large magnetic field induced strain is indicative of an ideal ferromagnetic shape memory alloy. To achieve high strain values melt-spun ribbons of NiMnGa based Heusler alloy were prepared and annealed at different temperatures over different time periods. Thermal analysis was done to obtain the transformation temperatures of the annealed ribbons. Besides the magnetic measurements, magnetic field induced strains were measured using strain gauges and strain indicator. The dependence of magnetic moment on the angle of alignment of the sample surface with respect to the field direction has been reported. The transformation temperatures were closer to the room temperature in all the ribbons with an incremental dependence on the annealing temperature and annealing time. The martensitic transformation temperatures obtained from the thermomagnetic curves were almost similar to those obtained from thermal analysis. There was an improvement in the saturation magnetization values after annealing whereas the Curie temperature was roughly similar for all the specimens. Presence of precipitates along the grain boundaries had no significant role to play in determining the properties. The annealed ribbons showed higher magnetic field induced strain (MFIS) than the as-spun ribbons; ribbon prepared at 1300 RPM and subsequently annealed at 900 °C for 5 h showed the highest value of MFIS ~0.15%. From the texture analysis, fraction of 〈001〉 oriented grains decreased and that of 〈110〉 increased with the progression of angle of alignment from 0° to 90°. Hence, the moment was higher when the angle of alignment was 90° in the annealed specimens.