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

Abstract 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.