Magnetic refrigeration, utilizing the magnetocaloric effect in solid-state magnets, has emerged as a promising cooling technology. The development of practical ultra-low-temperature magnetic refrigeration applications has been limited by the absence of high-performance magnetic refrigerants. Herein, we provide a frustrated Gadolinium-dominated magnet, Gd₂B₂MoO₉ oxide, which exhibits remarkable ultra-low temperature magnetocaloric performances. The maximum magnetic entropy changes reach 13.3 (76.6), 33.7 (194.0) and 45.1 (259.8) J/kgK (mJ/cm³K) under relatively low magnetic field changes of 0-1, 0-2 and 0-3 T, respectively. A minimum temperature of 0.16 K has been achieved by a custom-built quasi-adiabatic demagnetization apparatus. These magnetocaloric performances surpass those of the commercial ultra-low temperature magnetic refrigerant Gd₃Ga₅O₁₂ and outperform most of recently reported materials. Combined with its relatively high density and environmental stability, the geometrically frustrated Gd₂B₂MoO₉ oxide is established as an attractive ultra-low temperature magnetic refrigerant.

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 ₃ BWO ₉ with T _N ∼ 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 _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 _min ∼ 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 _min ∼ 168 mK far below T _N ∼ 1.0 K in Gd ₃ BWO 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.

Adiabatic demagnetization refrigeration (ADR) has attracted considerable attention as an effective approach to reach ultra-low temperatures required for fundamental physics and quantum technologies. Here we directly characterize the cryogenic magnetocaloric performance of the rare-earth-based double-perovskite oxide Gd₂CuTiO₆ (GCTO) through quasi-adiabatic demagnetization measurements. Magnetization measurements show no long-range magnetic transition above 1.8 K and indicate dominant antiferromagnetic (AFM) interactions, consistent with an AFM ordering temperature of $T_{\mathrm{N}}\approx 1.15$ K reported previously. Notably, the isothermal magnetization $M\left(H\right)$ at 1.8 K deviates from an ideal single-ion Brillouin response and is better described by a molecular-field correction for the Gd sublattice, suggesting correlated paramagnetism persisting above $T_{\mathrm{N}}$. In contrast to previous studies that inferred cooling performance from thermodynamic estimates, we directly validate the achievable sub-Kelvin cooling in GCTO through quasi-adiabatic measurements. In the quasi-ADR process starting from $T_0$∼2 K, demagnetization fields of 4, 6, and 9 T yield minimum temperatures of $T_{\min }=761.5$, 452.4, and 289.2 mK, respectively, well below $T_{\mathrm{N}}$. After complete removal of the magnetic field, the sample temperature remains highly stable for at least several tens of minutes, demonstrating a long hold time under quasi-adiabatic conditions. Moreover, the $T\left(H\right)$ curves reveal a characteristic field scale around $H_{\mathrm{c}}$∼1 T, implying a field-induced modification of the low-temperature magnetic-entropy landscape that is relevant to the cooling behavior during demagnetization. These results highlight GCTO as a promising magnetic refrigerant for sub-Kelvin ADR applications and underscore the role of correlated magnetism in optimizing cryogenic magnetocaloric performance.