Excellent cryogenic magnetocaloric properties in heavy rare-earth based HRENiGa2 (HRE = Dy, Ho, or Er) compounds

RENiX 2 compounds, where RE = rare-earth element and X = p -block element, have been highly regarded for cryogenic magnetocaloric applications. Depending on the elements, they can crystallize in CeNiSi 2 -type, NdNiGa 2 -type, or MgCuAl 2 -type crystal structures, showing different types of magnetic ordering and thus affect their magnetic properties. Regarding the magnetocaloric effect, MgCuAl 2 -type aluminides show larger values than those of the CeNiSi 2 -type silicides and the NdNiGa 2 -type gallides due to the favored ferromagnetic ground state. However, RENiGa 2 gallides can crystallize in either NdNiGa 2 - or MgCuAl 2 -type structures depending on the RE element. In this work, we select heavy RE (HRE) elements for exploring the microstructure, magnetic ordering and magnetocaloric performance of HRENiGa 2 (HRE = Dy, Ho or Er) gallides. They all crystallize in the desired MgCuAl 2 -type crystal structure which undergoes a second-order transition from ferro- to para-magnetic state with increasing temperature. The maximum isothermal entropy change (∣∆ S iso max ∣) values are 6.2, 10.4, and 11.4 J kg −1 K −1 (0–5 T) for DyNiGa 2 , HoNiGa 2 , and ErNiGa 2 , respectively, which are comparable to many recently reported cryogenic magnetocaloric materials. Particularly, the excellent magnetocaloric properties of HoNiGa 2 and ErNiGa 2 compounds, including their composite, fall in the temperature range that enables them for the in-demand hydrogen liquefaction systems.