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Poly­(3,4-ethylenedioxythiophene) (PEDOT) is widely used in several fields because of its excellent conductivity, strong adhesion, easy synthesis, and good flexibility. However, liquid-phase polymerization often encounters vast organic solvents and complex postprocessing, while vapor-phase polymerization suffers from agglomeration of precursors. Interestingly, when an external magnetic field was carried out and adjusted, numerous magnetic particles were assembled into different arrangements, which could effectively overcome particles’ agglomeration. Herein, magnetic-field-induced vapor-phase polymerization has been explored to make nanoscale PEDOT layers decorate porous Fe3O4 particles. Tailoring the magnetic field forces can control the PEDOT loading mass together with their doped levels. Then, the absorption performance of electromagnetic waves of PEDOT-decorated porous Fe3O4 particles was optimized by changing the PEDOT loadings and doped levels. Results indicate that the minimum reflection loss value can reach −43.4 dB (14.0 GHz) and the maximum effective absorption bandwidth can extend to 6.49 GHz, which is broader than that of similar absorbers. Related electromagnetic parameters reveal that dielectric loss mechanisms mainly include conductive loss from PEDOT layers, interfacial polarizations from Fe3O4–PEDOT and PEDOT–air/paraffin interfaces, dipole polarizations between doped counterpart anions and positive sulfur ions in the PEDOT skeletons, and relaxation loss. Besides, multiple reflections among numerous particles, abundant scatterings in the porous structures, and magnetic loss involving natural resonance, exchange resonance, and eddy current effect also account for the electromagnetic energy attenuation. Magnetic-field-induced vapor-phase polymerization is a novel and effective method for preparing PEDOT-decorated magnetic materials.

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