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To overcome the physical limitations of traditional paramagnetic salts in sub-100 mK cooling, this work evaluates the geometrically frustrated $\text{Gd}^{3+}$ double perovskites $\text{Ba}_2\text{GdSbO}_6$ and $\text{Sr}_2\text{GdSbO}_6$ for adiabatic demagnetization refrigeration (ADR). The materials exploit frustrated lattice geometries and competing dipolar-exchange interactions to suppress magnetic ordering down to 100–190 mK while preserving high entropy densities up to $201\text{ mJ K}^{-1}\text{ cm}^{-3}$. Demagnetization from 2 K and 5 T yields record-low temperatures of 67 mK and 68 mK for $\text{Gd}^{3+}$ systems, with $\text{Ba}_2\text{GdSbO}_6$ maintaining sub-100 mK performance even under a persistent 0.5 T magnetic field.
Record-low temperatures down to 67 mK prove that geometrically frustrated double perovskites can replace fragile paramagnetic salts for robust, solid-state sub-kelvin quantum refrigeration.
Achieving temperatures below 100 mK is essential for advancing quantum technologies and exploring fundamental quantum phenomena. While paramagnetic salts have traditionally enabled adiabatic demagnetization refrigeration (ADR), their limitations have driven the search for more effective alternatives. In this work, we present Gd3+-based double perovskites, Ba2GdSbO6 and Sr2GdSbO6, as high-performance magnetocaloric materials. Starting ADR from 2 K and 5 T, these compounds reach 67 mK and 68 mK in small finite magnetic fields, and 83 mK and 78 mK in zero field, respectively, which are the lowest reported ADR temperatures for Gd3+ magnets (S = 7/2) under these conditions. The frustrated geometry and the complex interplay of exchange and dipolar interactions suppress their antiferromagnetic ordering temperatures to 100 and 166 mK for Ba2GdSbO6 and 190 mK for Sr2GdSbO6, while maintaining an outstanding entropy density of 189 mJ K-1 cm-3 and 201 mJ K-1 cm-3, respectively. Notably, Ba2GdSbO6 sustains sub-100 mK cooling even at finite fields of 0.5 T, making it particularly promising for practical ADR applications.