Ultralow-Temperature Magnetic Refrigeration Inorganic Materials: From Designed Synthesis to Adiabatic Demagnetization Refrigeration

ConspectusAdiabatic demagnetization refrigeration (ADR), which exploits the magnetocaloric effect (MCE), remains the only helium-free refrigeration technology capable of reaching temperatures below 1 K. With the rapid growth of quantum computing and astronomical observation, there is a pressing need for large-capacity ADR systems─underscoring the critical demand for magnetic refrigerants capable of generating substantial magnetic entropy changes (-Δ<i>S</i>_m) at millikelvin temperatures. However, a long-standing challenge persists: achieving both large -Δ<i>S</i>_m values and low magnetic ordering temperatures (<i>T</i>₀). This trade-off has limited the performance of existing refrigerants in the sub-Kelvin regime, thereby hindering ADR advancement. To address this, our work focuses on the rational design of next-generation refrigerants. Rather than relying on readily available materials, we emphasize the importance of tuning key magnetic parameters, <i>T</i>₀, exchange and dipolar interactions. This approach began with the discovery that incorporating fluoride (F^-) bridges into antiferromagnetic frameworks transforms their magnetic behavior, shifting it from antiferromagnetic to weak ferromagnetic and lowering <i>T</i>₀ from 1.4 K to 1.0 K while yielding large -Δ<i>S</i>_m values. Building on this concept, we extended the strategy to the Gd(OH)_3-<i>x</i>F_<i>x</i>. In particular, Gd(OH)F₂ combines weak magnetic interactions with high magnetic density, achieving record -Δ<i>S</i>_m values and demonstrating a robust route to simultaneously enhance -Δ<i>S</i>_m and suppress <i>T</i>₀. By integrating the mean-field approximation with quantum Monte Carlo (QMC) simulations, we accurately predicted <i>T</i>₀ values in systems with weak exchange and low anisotropy, such as GdCO₃F, Gd(HCOO)F₂, Gd₂(SO₄)₃·8H₂O, GdF₃ and Gd(HCOO)₃, revealing the dominant role of dipolar interactions in determining <i>T</i>₀. Expanding this framework, we introduced Gd³⁺ into Yb-based compounds characterized by intrinsically weak exchange and dipolar interactions. This doping strategy enabled the realization of low <i>T</i>₀ and large -Δ<i>S</i>_m values at ultralow temperatures, demonstrating that a balance between competing magnetic interactions and chemical disorder can achieve the coexistence of high -Δ<i>S</i>_m and low <i>T</i>₀. To translate these insights into practical ADR systems, we synthesized LiGd_0.1Yb_0.9F₄ and investigated its performance in an ADR setup. Notably, LiGd_0.1Yb_0.9F₄ cooled a test sample to 160 mK and delivered a specific cooling capacity more than twice that of the commercial refrigerant CrK(SO₄)₂·12H₂O. Additionally, by integrating high magnetic density with weak exchange and dipolar interactions in a frustrated magnet, we developed KYb₃F₁₀, which achieved significant -Δ<i>S</i>_m with a low <i>T</i>₀. Quasi-adiabatic demagnetization experiments with KYb₃F₁₀ reached a minimum temperature of 27.2 mK, highlighting its promise as a next-generation ADR refrigerant. In summary, we proposed and validated rational design strategies for high-performance ADR refrigerants, achieving enhanced -Δ<i>S</i>_m across the cryogenic temperatures below 4 K. From theoretical modeling to experimental realization, our work lays a solid foundation for advancing ADR technologies in both fundamental research and applied low-temperature systems.