Enhancing the Magnetocaloric Effect of Trigonal Eu ₃ P ₂ O ₈ Phosphate Near Liquid Helium Temperature via Ionic Substitution Strategy

Magnetic refrigeration at liquid helium temperatures (4.2 K) serves as a critical technology for frontier scientific applications, yet its advancement is constrained by performance limitations of magnetocaloric materials (MCMs). Therefore, it is imperative to develop MCMs with large magnetocaloric effects (MCE) driven by a low magnetic field near liquid helium temperature. In this study, Sr 2+ ‐doped Eu 3 −x Sr x P 2 O 8 ( x = 0, 1/3, 2/3, and 1) series compounds were synthesized via solid‐state reaction and their magnetization characteristics were investigated. As a result, we demonstrate for the first time that nonmagnetic Sr 2+ substitution in ferromagnetic Eu 3 P 2 O 8 enables effective regulation of the magnetic ordering temperature from 6.4 K to 3.5 K. Remarkably, the maximum magnetic entropy change () remains almost constant between 10.0 and 11.1 J·kg −1 ·K −1 at μ 0 Δ H = 0–1 T despite the reduced Eu 2+ concentration upon Sr 2+ substitution; the fine‐tuning of the crystal structure and reconstruction of the electronic structure compensate for the negative impact of mere magnetic dilution on magnetocaloric performance. These compounds exhibit giant reversible MCEs near liquid helium temperature, positioning them as promising candidates for helium‐free cryogenic refrigeration. This work establishes a novel paradigm for synergistic optimization of magnetocaloric performance and phase transition temperature through single‐elementsubstituting.