Coexistence of conventional and inverse magnetocaloric effects and critical behaviors in Ni50Mn50-xSnx (x = 13 and 14) alloy ribbons

A systematic study of the conventional and inverse magnetocaloric effects and critical behaviors in Ni50Mn50-xSnx (x = 13 and 14) alloy ribbons has been performed. We show that although the magnetic entropy change around the second-order ferromagnetic-paramagnetic (FM-PM) transition ( Delta Sm approximately -4 J/kg K) in the austenitic phase is about five times smaller than that around the first-order martensitic-austenitic (M-A) transformation ( Delta Sm approximately 22 J/kg K), the refrigerant capacity (RC) - an important figure of merit - is about two times larger for the former case (RC approximately 160 J/kg) than for the latter case (RC approximately 75 J/kg). This finding points to an important fact that to assess the usefulness of a magnetocaloric material, one should not only consider Delta Sm but also must evaluate both Delta Sm and RC. Our critical analysis near the second-order FM-PM transition reveals that Sn addition tends to drive the system, in the austenitic FM phase, from the short-range (x = 13) to long-range (x = 14) FM order.