Coexistence of Antiferromagnetic and Ferromagnetic Interactions in Dimer‐Like Arranged EuAl 2 O 4 Systems: Regulatory Strategy to Giant Low Field Cryogenic Magnetocaloric Effects

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In the realm of rare‐earth‐based magnetic refrigeration materials, the precise tuning of magnetic exchange interactions among rare‐earth ions is the key challenge to achieve large magnetocaloric effect (MCE) over a wide temperature range in the sub‐Kelvin region (< 1 K) under low magnetic fields. Here, this work demonstrates that modulating the magnetic interactions within a system exhibiting coexisting antiferromagnetic and ferromagnetic interactions represents an ideal strategy. EuAl 2 O 4 features a unique dimer‐like structure, wherein every four Eu 2+ ions form a monomer. Within each monomer, the Eu 2+ ions are ferromagnetically coupled, whereas antiferromagnetic coupling exists between the monomer. This weak coexisting interaction facilitates a significant MCE under low fields: the magnetic ordering temperature ( T ord ) is 0.9 K, and a maximum magnetic entropy change is 28.2 J kg −1 K −1 at 10 kOe, representing one of the best magnetic refrigerants reported with T ord < 1.5 K. Furthermore, by substituting Eu 2+ ions with non‐magnetic Sr 2+ ions, the local magnetic moment effect promotes a rebalancing of the magnetic exchange interactions, thereby further reducing the T ord to 0.73 K and achieving an extended wide temperature range with a large MCE under low fields. This provides a new strategy for breaking the performance bottleneck of ultra‐low temperature magnetic refrigeration materials.