Magnetocaloric properties of Ni50Mn28Ga22 melt-spun ribbons

ABSTRACT NiMnGa-based Heusler alloys are known for their shape-memory effect. However, in this study, the focus will be on the magneto-thermal (magnetocaloric) properties of rapidly solidified Ni50Mn28Ga22 ribbons melt spun at 1300 and 1600 RPM as well as annealed bulk specimens. The ribbons, both as-spun and annealed, and the annealed bulk specimens were tested in a SQUID to determine the magnetic properties at a field of 50 kOe with a temperature step of 3 K in the temperature range 355–385 K. The magnetic data from the isotherms were used to achieve Arrott plots. Second-order transitions were observed in the materials at the TC temperature. The values of magnetic entropy ΔSm were calculated from the magnetic data and these were further used to calculate the values of refrigeration capacity RC. The highest RC value of 273 J/kg was obtained for 1300NMG5 800 where 1300, 800 and 5, represent the melt spinning rate in RPM, the annealing temperature in Celsius and the duration of annealing in hours, respectively. The values of ΔSm were similar for all the ribbons. To confirm the order of the magnetic transition, universal curves were plotted which led to the conclusion that the transformation at the Curie temperature is a second-order ferromagnetic to paramagnetic transition.