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.