Engineering the cryogenic magnetocaloric effect of Tm2O3 by oxygen vacancies

Defect engineering is an effective means to improve the electronic structure and physicochemical properties of materials. In this work, the cryogenic magnetocaloric effect (MCE) of commercial Tm₂O₃ was significantly improved by the introduction of oxygen vacancy (OV) defects. A series of monocrystalline Tm₂O₃ powders (cubic structure, Ia $\overline{3}$ space group) with high-level OV content was obtained by ball-milling (BM) the commercial counterpart for 3–12 h. Of these samples, the 9 h-ball milled Tm₂O₃ sample (Tm₂O₃-9) has the highest ratio value of oxygen atoms near OV (O_{near OV}) of 15.79 % and largest effective magnetic moment (μ_eff) of Tm³⁺ ion of 6.8 μ_B compared with the corresponding values of 8.06 % and 6.32 μ_B for the commercial Tm₂O₃. Consistently, Tm₂O₃-9 has the best MCE performance with the maximal magnetic-entropy change (ΔS_M) and the refrigerant capacity (RC) of 7.0 J⋅kg⁻¹⋅K⁻¹ and 96.8 J⋅kg⁻¹ in 0–5 T, respectively as compared to 4.2J⋅kg⁻¹⋅K⁻¹ and 66.9 J⋅kg⁻¹ for the commercial Tm₂O₃. Further increasing the BM time to 12h, the OV content almost remains unchanged, accompanied by a minor reduction of ΔS_M and RC due to the decrease of grain size. This work provides a novel approach for enhancing the MCE of rare earth-based oxides.