Free-Spin Dominated Magnetocaloric Effect in Dense Gd³⁺ Double Perovskites

Frustrated lanthanide oxides with dense magnetic lattices are of fundamental interest for their potential in cryogenic refrigeration due to a large ground state entropy and suppressed ordering temperatures but can often be limited by short-range correlations. Here, we present examples of frustrated fcc oxides, Ba₂GdSbO₆ and Sr₂GdSbO₆, and the new site-disordered analogue Ca₂GdSbO₆ ([CaGd] [CaSb] O₆), in which the magnetocaloric effect is influenced by minimal superexchange (J ₁ ∼ 10 mK). We report on the crystal structures using powder X-ray diffraction and the bulk magnetic properties through low-field susceptibility and isothermal magnetization measurements. The Gd compounds exhibit a magnetic entropy change of up to -15.8 J/K/mol_Gd in a field of 7 T at 2 K, a 20% excess compared to the value of -13.0 J/K/mol_Gd for a standard in magnetic refrigeration, Gd₃Ga₅O₁₂. Heat capacity measurements indicate a lack of magnetic ordering down to 0.4 K for Ba₂GdSbO₆ and Sr₂GdSbO₆, suggesting cooling down through the liquid 4-He regime. A mean-field model is used to elucidate the role of primarily free-spin behavior in the magnetocaloric performance of these compounds in comparison to other top-performing Gd-based oxides. The chemical flexibility of the double perovskites raises the possibility of further enhancement of the magnetocaloric effect in the Gd³⁺ fcc lattices.