Influence of the nature of the magnetic phase transition on the associated magnetocaloric effect in the Ni-Mn-Ga system

A summary of extensive measurements of magnetisation isotherms and of the field- and temperature-dependent AC susceptibility for Ni-Mn-Ga alloys is presented. Detailed analysis of these data provide a quantitative appraisal of the magnetic phase transitions in this system through the establishment of metamagnetic boundaries at first-order magnetic/martensitic structural transitions and crossover lines at continuous magnetic transitions. At the “tuned” composition Ni55.2Mn18.6Ga26.2 a giant magnetocaloric effect with ΔSm=−20.4×104 ergs g−1 K−1 (−20.4 J kg−1 K−1) (at 317 K in a 50 kOe (5 T) field) is achieved, and this is shown to result from the essential coincidence of two transitions exhibiting both first- and second-order characteristics simultaneously.