Tunable magnetocaloric effect towards cryogenic range by varying Mn:Ni ratio in all-d-metal Ni(Co)-Mn-Ti Heusler alloys

Cryogenic magnetic refrigeration (CMR) is a highly efficient and environmentally friendly technique for gas liquefaction. However, refrigerant materials undergoing large magnetocaloric response at the interesting cryogenic range are dominated by critical elements (mainly rare-earth elements) which impedes practical applicability of such refrigeration systems. Therefore, there is a need for dedicated investigations on optimization of magnetocaloric response at cryogenic range by utilizing compositions that are rare-earth free. In this work, we synthesize the mechanically stable and rare-earth free, all-d-metal Ni35Co15Mn35Ti15 Heusler alloys and investigate the role of varying Mn:Ni ratio on the magnetostructural and magnetocaloric properties of the alloy system. The results of the microstructural characterization indicate homogenous composition for the investigated alloy series. As the Mn:Ni ratio increases from 1.01 to 1.10, the martensitic transition shifts from near-room temperature down to cryogenic region (120-140 K) while the magnetization of the austenitic phase remains unaltered. Isothermal entropy change as high as ~ 13 J kg-1 K-1 at 1.5 T is achieved for the sample with the highest Mn:Ni ratio at the temperature region for natural gas liquefaction, which significantly surpasses the values previously reported in the literature for similar alloys. In addition to large magnetocaloric response, the martensitic transformation falls in an interesting temperature of the cryogenic region, paving the way for various low-temperature magnetocaloric applications.