Giant and reversible room-temperature magnetocaloric effect in Ti-doped Ni-Co-Mn-Sn magnetic shape memory alloys

The practical applications of magnetocaloric materials for magnetic refrigeration require not only large magnitude but also reversibility, as well as room-temperature operating temperature, of the magneto caloric effect. However, it is a complex problem to simultaneously meet these demands in the Heusler-type Ni-(Co)-Mn-X (X = In, Sn, Sb, Ga) magnetic shape memory alloys which are a promising candidate for magnetocaloric materials, owing to the interdependence of magnetostructural transformation parameters. Here, through synergic tuning of magnetostructural transformation parameters via alloying with Ti in Ni42-xTixCo9Mn39Sn10 alloys, we achieved a large reversible room-temperature magnetocaloric effect. By alloying with a proper amount of Ti, the martensitic transformation temperature was brought down to room temperature and the sensitivity of transformation temperature to magnetic field change was greatly enhanced with the transformation entropy change still remaining a large value, while the thermal hysteresis and the transformation interval only slightly increased. Thus, the field required to induce the complete and reversible transformation was significantly reduced. As a result, a large reversible room-temperature magnetic entropy change Delta S-m of 18.7 J kg(-1) K-1 under 5 T was achieved in the Ni41Ti1Co9Mn39Sn10 alloy. This is the first report on reversible Delta S-m in Ni-Mn-Sn-based Heusler alloys and the Delta S-m we achieved represents the highest reversible Delta S-m under 5 T reported heretofore in Ni-Mn-based Heusler alloys. Furthermore, the Ni41Ti1Co9Mn39Sn10 alloy shows good compressive properties and stable martensitic transformation during thermal cycling, beneficial for potential magnetocaloric applications. This study is instructive for the development of high-performance magnetocaloric materials for room-temperature magnetic refrigeration. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.