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 (-ΔS_m) of existing refrigerants at ultralow temperatures. This limitation primarily stems from the inherent contradiction of simultaneously attaining a large -ΔS_m and a low magnetic ordering temperature (T₀) in magnetic refrigerant design. Here, we show that a magnetic refrigerant exhibiting both a large -ΔS_m and a low T₀ can be simultaneously achieved by incorporating weak magnetic exchange and dipolar interactions into the dense frustrated magnet KYb₃F₁₀ (1). Notably, the average -ΔS_m of 1 in the 0.05-1.0 K range surpasses those of commercial refrigerants (NH₄)Fe(SO₄)₂·12H₂O and CrK(SO₄)₂·12H₂O by 146 and 219%, respectively, while its T₀ 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 1 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 -ΔS_m and a low T₀ in the design of magnetic refrigerants.