Large low-field cryogenic magnetocaloric effect in ferromagnetic NaNd1-xEuxO2 (x=0, 0.05 and 0.1) compounds

Heavy rare-earth (RE)-based oxides possessing large magnetocaloric effect (MCE) have great application prospects in ultra-low-temperature magnetocaloric cooling. However, their large MCE based on the antiferromagnetic (AFM) ordering of heavy RE ions were generally obtained under high magnetic field changes. In this work, light RE-based tetragonal NaNd_1-xEu_xO₂ (x = 0, 0.05, 0.1) compounds (I4₁/amd space group) were prepared by a facile ball-milling assisted sintering method. These oxides exhibit a second-order ferromagnetic (FM) to paramagnetic transition (PM) transition at the Curie temperature (T_C) of 2.4 K to below 2 K, which is accompanied by an excellent low-field magnetocaloric cooling performance. In 0–1 T, a large magnetic entropy change ΔS_M of 7.7, 7.6 and 7 J kg⁻¹ K⁻¹ was obtained at 2.5 K in NaNd_1-xEu_xO₂ for x = 0, 0.05 and 0.1, respectively. These values are approximately twice as much as that for the commercial gallium gadolinium garnet (GGG) material (3.9 J kg⁻¹ K⁻¹, at 2 K). More favorably, these compounds show negligible coercivity and hysteretic loss, indicating reversibility of the large MCE. Eu³⁺ doping leads to a decrease of both the lattice constants and lengths of the Nd-O and Na-O bonds, accompanied by a lowering of T_C and a decrease of effective magnetic moments of the RE³⁺. The excellent low-field MCE in the light RE-based FM oxides provides a novel insight for exploring suitable cryogenic magnetic refrigeration materials.