Wheel speed-dependent martensitic transformation and magnetocaloric effect in Ni-Co-Mn-Sn ferromagnetic shape memory alloy ribbons

This study investigated systematically the effects of different wheel speeds (10-30 m/s) on the structural, microstructural, magnetic, and magnetocaloric properties of as-spun and annealed Ni-Co-Mn-Sn ribbons. The ribbons comprised a mixture of austenite with L21 cubic structure and non-modulated martensite with L10 tetragonal structure. Both as-spun and annealed ribbons showed an increase in the reverse martensitic transformation temperature (Tm) and a decrease in the thermal hysteresis (Thys) when the wheel speed was increased from 10 m/s to 25 m/s. Unexpectedly, the Tm decreased and the Thys increased in the ribbons prepared with a wheel speed of 30 m/s. This finding may be attributed to the appearance of the γ-precipitated phase, which was lacking in the other samples. The martensitic transformation with a large magnetocaloric effect can be tailored in a wide temperature range, i.e., 30 K for the as-spun ribbons and 33 K for the annealed ribbons. The as-spun ribbons prepared at 15 m/s and annealed ribbons prepared at 25 m/s achieved large effective refrigerating capacities of 48.8 and 47.8 J/kg, respectively, in the field change of 0-1 T. These results provided significant insights into the effects of wheel speed on the magnetic and functional properties, as well as into the underlying mechanisms, of Ni-Mn-based multifunctional ribbon alloys.