Realization of Large Magnetocaloric Effect in the Kagome Antiferromagnet Gd₃BWO₉ for Sub-Kelvin Cryogenic Refrigeration

Rare-earth based frustrated magnets have attracted great attention as excellent candidates for magnetic refrigeration at sub-Kelvin temperatures, while the experimental identification of systems exhibiting both large volumetric cooling capacity and reduced working temperatures far below 1 K remains a challenge. Here, through ultra-low temperature magnetism and thermodynamic characterizations, we unveil the large magnetocaloric effect (MCE) realized at sub-Kelvin temperatures in the frustrated Kagome antiferromagnet Gd_3BWO_9 with T_\rm N \sim 1.0 K. The isothermal magnetization curves indicate the existence of field (B) induced anisotropic magnetic phase diagrams, where four distinct magnetic phases for B \| c-axis and five magnetic phases for B \| ab-plane are identified at T < T_\rm N. The analysis of magnetic entropy S(B,T) data and direct adiabatic demagnetization tests reveal remarkable cooling performance at sub-Kelvin temperatures featured by a large volumetric entropy density of 502.2 mJ/K/cm^3 and a low attainable minimal temperature T_\rm min \sim 168 mK from the initial cooling condition of 2 K and 6 T, surpassing most Gd-based refrigerants previously documented in temperature ranges of 0.25-4 K. The realized T_\rm min \sim 168 mK far below T_\rm N \sim 1.0 K in Gd_3BWO_9 is related to the combined effects of magnetic frustration and criticality-enhanced MCE, which together leave substantial magnetic entropy at reduced temperatures by enhancing spin fluctuations.