Sub-kelvin refrigeration is essential for emerging technologies such as quantum science and space exploration. Compared with conventional low-temperature cooling approaches that rely heavily on scarce 3He, adiabatic demagnetization refrigeration (ADR) offers a compact, gravity-independent, and 3He-free route to sub-kelvin temperatures. For ADR materials, a large magnetic entropy change and a very low magnetic ordering temperature are both required, making highly isotropic Gd3+ ions with S = 7/2 particularly attractive for cryogenic magnetic refrigeration.
A long-standing challenge in this field is the intrinsic trade-off between high magnetic-ion density and weak magnetic interactions. Lightweight ligands such as F−、OH−、HCOO−, and CO32− can maximize the magnetic mass fraction and magnetic entropy density, but the resulting short Gd···Gd separations often strengthen exchange and dipolar interactions, thereby increasing the magnetic ordering temperature. In contrast, bulky bridging ligands can weaken magnetic coupling, but at the cost of increased molecular weight and reduced mass-specific magnetic entropy.
To address this dilemma, we developed a complementary-ligand pair strategy, in which lightweight short fluoride bridges are combined with different extended bridging ligands within the same framework. The short F− bridges help preserve a high density of magnetic centers, while the extended ligands regulate framework dimensionality, topology, magnetic connectivity, and interunit separation. Using this concept, three Gd-based coordination polymers, GdC4O4F (1), GdSO4F(H2O) (2), and Gd2(oda)2F2(H2O)2 (3), were constructed.
Among them, compound 3 achieves the best overall balance, with a magnetic ordering temperature as low as 0.30 K and large low-field magnetic entropy changes of 27.2 and 38.7 J kg−1 K−1 under field changes of 1 and 2 T, respectively.
Its practical cooling performance was further confirmed by quasi-adiabatic demagnetization experiments. Starting from 2.0 K, demagnetization from 6 T cooled the sample to 115 mK, while the temperature remained below 200 mK for more than 2 h.
This work demonstrates that assigning complementary structural roles to different ligands provides an effective way to tune magnetic connectivity and interaction scales in Gd-based frameworks, offering a new route to overcome the conventional trade-off between large magnetic entropy and low magnetic ordering temperature. The strategy may provide a useful design principle for next-generation sub-kelvin magnetic refrigerants and compact ADR systems.
Article link: https://doi.org/10.1021/jacs.6c13704