Geometry‑Guided Ferromagnetic Correlations Between Eu (II) and Gd (III) in Eu ₅ Gd(BO ₃ ) ₄ Cl Orthoborate: Low‑Field Magnetocaloric Response and Ligand Involvement at Liquid‑Helium Temperatures

ABSTRACT As an effective route to surpass the Gifford–McMahon (GM) refrigeration limit, developing low‑temperature magnetic refrigerants that combine ferromagnetic interactions with high magnetic density remains the central challenge and current bottleneck. Using a rigid orthoborate lattice as the platform, we propose and validate a geometry‐based strategy that combines the ordered occupation of distinct crystallographic sites by Eu(II) and Gd(III), with Eu–O–(Eu/Gd) linkages constrained near 90°. This structural arrangement enhances the effective overlap between the rare‐earth 5 d and O 2 p states, promoting short‐range ferromagnetic correlations through both Eu‐O‐Eu and Eu‐O‐Gd interactions, which together contribute to the overall magnetic behavior. Spin‑resolved PDOS and spin‑density maps reveal pronounced Eu/Gd‑5 d— O‑2 p hybridization together with measurable ligand polarization. These observations are consistent with a short‑range ligand‑mediated pathway: localized 4 f moments polarize the more extended 5 d states, which hybridize with O‑2 p and transmit spin to neighboring rare‑earth ions. Experimentally, the compound exhibits ferromagnetism with a low ordering temperature and a significant magnetocaloric effect, with a maximum magnetic entropy change of 35.0 J·kg −1 ·K −1 at 2 T. The demonstrated geometry‑to‑exchange design paradigm provides a general route to engineer efficient low‑field MCE at liquid‑helium temperatures in f ‑electron oxides.