Surface coordination engineering of Fe₃O₄ with carboxylic acids: Tuning magnetic permeability for wideband C-band microwave absorption

The widespread deployment of 5G/IoT devices in the 4–8 GHz frequency range has intensified electromagnetic pollution, creating an urgent demand for mid/low-frequency magnetic loss absorption materials with high magnetic permeability and favorable impedance matching. Conventional Fe₃O₄ is constrained by the Snoek limit, resulting in restricted intrinsic magnetic loss (μ″) in the mid/low-frequency band. Existing strategies such as cation doping or structural design often involve complex processing or compromised impedance matching. Herein, we propose a surface coordination engineering approach to construct a series of functionalized composites by grafting monocarboxylic acids with different chain lengths (acetic acid, n-octanoic acid and myristic acid) onto the Fe₃O₄ surface. The results demonstrate that precise regulation of the interfacial magnetic environment and magnetic domain structure of Fe₃O₄ through surface coordination chemistry can effectively pin surface spins and induce perpendicular magnetic anisotropy, leading to a maximum enhancements of 18% and 41% in μ′ and μ″, respectively, for the modified Fe₃O₄ in the 4–8 GHz range. The acetic acid-modified composite (Fe@HA) achieves a minimum reflection loss of −49 dB with an effective absorption bandwidth of 6.32 GHz (4.62-10.94 GHz) at a thickness of only 3.6 mm, which almost covers the entire C-band (4-8 GHz). This study reveals that carboxylic acid coordination serves as the critical mechanism for tuning the magnetic permeability of Fe₃O₄, offering a novel surface chemistry route toward high-performance and scalable electromagnetic compatibility materials for 5G frequency bands.