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 magnetocaloric 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 Ni_42-xTi_xCo₉Mn₃₉Sn₁₀ 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 ΔS_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 ΔS_m in Ni-Mn-Sn-based Heusler alloys and the ΔS_m we achieved represents the highest reversible ΔS_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.