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.