Complementary-Ligand Pair Strategy toward High-Performance Sub-Kelvin Magnetic Refrigerants

Adiabatic demagnetization refrigeration (ADR) provides a 3He-free approach to sub-Kelvin cooling, yet its efficiency is fundamentally limited by an intrinsic structural paradox in magnetic refrigerants: minimizing the magnetic ordering temperature (T0) and maximizing the magnetic entropy change (−ΔSm) typically impose mutually exclusive structural demands. To address this challenge, we propose a complementary-ligand pair strategy and synthesize three coordination polymers: GdC4O4F (1), GdSO4F(H2O) (2), and Gd2(oda)2F2(H2O)2 (3). Structural analysis reveals that pairing short fluoride bridges with diverse extended linkers (C4O42–, SO42–, and oda2–) modulates the framework dimensionality and topology, thereby regulating the magnetic connectivity of the Gd3+ centers. Magnetic susceptibility measurements further show that all three compounds exhibit weak net magnetic interactions. Ultralow-temperature heat capacity studies reveal that they all exhibit T0 values below 1 K while delivering −ΔSm significantly larger than that of the commercial refrigerant Gd3Ga5O12 (GGG). Specifically, compound 3 exhibits a T0 of 0.3 K, approaching the lowest value reported to date for Gd-based materials. Under quasi-adiabatic demagnetization experiments utilizing a 2 K heat sink, a pelletized sample of 3 achieves a record-low minimum temperature of 115 mK among reported Gd- and Eu-based refrigerants relevant to the 0.25–4 K regime. These findings demonstrate the complementary-ligand pair strategy as an effective approach for developing next-generation sub-Kelvin refrigerants.