Unveiling the structural and magnetic properties of RENaGeO4 (RE = Gd, Dy, and Ho) oxides and remarkable low-temperature magnetocaloric responses in GdNaGeO4 oxide

Abstract

Low-temperature magnetocaloric (MC) responses in various types of solid-state magnets have been extensively determined, with the aim of developing high-performing MC materials for magnetic refrigeration applications and deepening our understanding of their underlying intrinsic magneto-physical characteristics. Herein, we fabricated a family of single-phase rare-earth (RE)-dominated oxides, namely, RENaGeO₄ (RE = Gd, Dy, and Ho), by applying the solid phase reaction method and unveiled their structural and magnetic properties, specifically to low-temperature MC responses through experimental determination and theoretical calculation. All RENaGeO₄ oxides crystallize in an orthorhombic olivine-type structure with the space group Pnma (No. 62) and order magnetically at temperatures of 0.70, 2.28, and 2.15 K for GdNaGeO₄, DyNaGeO₄, and HoNaGeO₄ oxides, respectively. The consistent elements in these RENaGeO₄ oxides are distributed uniformly and present as RE³⁺, Na¹⁺, Ge⁴⁺, and O²⁻ valence states. The low-temperature MC responses in these RENaGeO₄ oxides are identified by the MC parameters of maximum magnetic entropy change and relative cooling power. These MC parameters under magnetic field changes (Δμ₀H) of 0–2/0–5 T are as follows: 34.98/47.30 J (kg K)⁻¹ and 107.56/320.70 J kg⁻¹ for GdNaGeO₄, 11.23/14.82 J (kg K)⁻¹ and 77.70/236.82 J kg⁻¹ for DyNaGeO₄, and 12.21/15.37 J (kg K)⁻¹ and 81.18/239.47 J kg⁻¹ for HoNaGeO₄. Evidently, these determined MC parameters for GdNaGeO₄ oxide, especially under relatively low Δμ₀H, are much larger than those for the commercial MC material of Gd₃Ga₅O₁₂ oxide and surpass those of most updated benchmarked low-temperature MC materials, making the GdNaGeO₄ oxide an excellent candidate for low-temperature magnetic refrigeration application.