Exceptional Cooling Capacity of LiGd_0.1Yb_0.9F₄ at Sub-Kelvin Temperatures.

Abstract

Adiabatic demagnetization refrigeration, which utilizes the magnetocaloric effect of magnetic refrigerants, stands as the sole cooling technology capable of achieving sub-Kelvin temperatures efficiently and reliably without relying on scarce ³He resources or gravity. However, current sub-Kelvin magnetic refrigerants encounter challenges such as structural instability in vacuum or under mild heating, along with small magnetic entropy change (-ΔS_m) values, which significantly limit their practical applications. Here a water-free magnetic refrigerant, LiGd_0.1Yb_0.9F₄ is reported, prepared by introducing Li⁺ ions to reduce the dipolar interactions between Gd³⁺ ions and/or Yb³⁺ ions. Notably, this refrigerant possesses a magnetic ordering temperature of 85 mK, while its experimental -ΔS_m reaches up to 136 mJ cm^-3 K^-1 (0.68 K and 2 T), more than three times the theoretical value of CrK(SO₄)₂·12H₂O. Significantly, this refrigerant not only cools the test sample to temperatures as low as 160 mK but also achieves a specific cooling capacity of 46.943 mJ cm^-3 T^-1 at 300 mK. Remarkably, the specific cooling capacity at 300 mK is more than double that of commercial CrK(SO₄)₂·12H₂O, representing one of the most notable values reported among all known magnetic refrigerants operating at sub-Kelvin temperatures.