Confined Diffusion Strategy for Customizing Magnetic Coupling Spaces to Enhance Low‐frequency Electromagnetic Wave Absorption

Abstract The rational design of magnetic composites has great potential for electromagnetic (EM) absorption, particularly in the low‐frequency range of 2–8 GHz. However, the scalable synthesis of such magnetic absorbers with both high magnetic content and good dispersity remains challenging. In this study, a confined diffusion strategy is proposed to fabricate functional magnetic‐carbon hollow microspheres. Driven by the ferromagnetic enhanced Kirkendall diffusion effect, the in situ alloying of FeCo nanoparticles is tightly confined in carbon shells, effectively inhibiting magnetic agglomeration. Moreover, the core–shell FeCo–carbon nano‐units further assemble into dispersive microscale magnetic‐carbon Janus bulges on both the inner and outer surfaces of the hollow microsphere. The optimized hollow FeCo@C microspheres exhibit excellent low‐frequency EM wave absorption performance: the minimum reflection loss ( RL min ) is −35.9 dB, and the absorption bandwidth covers almost the entire C‐band. Systematic investigation reveals that the large size of the magnetic‐carbon integration, high–density confined magnetic units, and strong magnetic coupling are essential for enhancing the magnetic loss dissipation of low‐frequency EM waves. This study provides a novel strategy for fabricating advanced EM wave absorbers and significant inspiration for investigating the magnetic attenuation mechanism at low frequency.