Giant Low-Field Magnetocaloric Effect at Sub-Kelvin Temperatures in Ferromagnetic NH₄GdF₄

The development of materials for helium-free sub-Kelvin adiabatic demagnetization refrigeration (ADR) is crucial for advancing research in condensed matter physics, quantum computing, astrophysical measurements in space, and other related fields. Among such materials, Gd₃Ga₅O₁₂ (GGG) has been widely regarded as a benchmark cryogenic refrigerant for ADR systems. In this article, we report on a rare-earth fluoride, NH₄GdF_4, whose low-field magnetocaloric effect is significantly stronger than that of GGG. NH₄GdF₄ orders at T_c = 0.85 K and exhibits a ferromagnetic ground state at low temperatures as proved by the nearest neighbor exchange interaction model and the specific heat analysis. It has excellent low-field magnetization properties, as evidenced by approaching its saturation magnetization of 160 emu/g below a magnetic field of 10 kOe. Furthermore, the magnetic entropy change of up to 38.2 J·kg^-1·K^-1 and its measured temperature change of 1.1 K at an initial temperature of 1.8 K under the magnetic field change of 0-10 kOe are greater than all reported results and much greater than the entropy change and temperature change of GGG under the same conditions. Thus, NH₄GdF₄ is a competitive refrigerant candidate for low-field-driven ADR applications at sub-Kelvin temperatures.