Temperature Below 30 mK Achieved by Adiabatic Demagnetization Refrigeration.

Adiabatic demagnetization refrigeration (ADR) is the only technique capable of reaching ultralow temperatures without helium-3 and plays a crucial role at the forefront of both fundamental and applied science. However, progress in ADR is constrained by the limited magnetic entropy change (-Δ) of existing refrigerants at ultralow temperatures. This limitation primarily stems from the inherent contradiction of simultaneously attaining a large -Δ and a low magnetic ordering temperature () in magnetic refrigerant design. Here, we show that a magnetic refrigerant exhibiting both a large -Δ and a low can be simultaneously achieved by incorporating weak magnetic exchange and dipolar interactions into the dense frustrated magnet KYbF (). Notably, the average -Δ of in the 0.05-1.0 K range surpasses those of commercial refrigerants (NH)Fe(SO)·12HO and CrK(SO)·12HO by 146 and 219%, respectively, while its is lower than 50 mK. Practical ADR testing confirms that a minimum temperature of 27.2 mK can be reached under a magnetic field of 6 T using as the refrigerant. Thus, this work not only presents a high-performance ultralow-temperature refrigerant but also addresses the long-standing challenge of simultaneously achieving a large -Δ and a low in the design of magnetic refrigerants.