Hierarchical CoS/CoFe₂O₄@polypyrrole heterostructures for thin-layer microwave absorption

Developing microwave absorbers with low matching thickness and an adequate effective absorption bandwidth requires a balance between impedance matching and electromagnetic attenuation. Herein, a ternary CoS/CoFe₂O₄@PPy (CCFP) hierarchical heterostructure was constructed through ultrasonication-assisted interfacial assembly and in situ polymerization. The flower-like CoS framework accommodates CoFe₂O₄ particles and a PPy coating, integrating dielectric, magnetic, and conductive components within a hierarchical architecture. At a filler loading of 30 wt% in paraffin, the optimized CCFP-2 composite achieves a minimum reflection loss (RL_min) of -46.07 dB at 17.22 GHz with a matching thickness of 1.65 mm, and a maximum effective absorption bandwidth (EAB) of 5.46 GHz at 2.25 mm. Compared with CoS@PPy (CP-2), CCFP-2 exhibits a lower matching thickness at RL_min and a wider maximum EAB. The electromagnetic parameter analysis indicates that PPy incorporation and the introduction of CoFe₂O₄ regulate the dielectric and magnetic responses, providing a favorable combination of attenuation and input-impedance matching. Charge transport and dielectric relaxation are proposed to contribute to dielectric attenuation, while CoFe₂O₄ introduces an additional magnetic response. Furthermore, CST simulations at 17.22 GHz under normal incidence show a radar cross-section (RCS) reduction of 28.07 dB for the 1.65 mm coating in the θ = 0° observation direction relative to the bare perfect electric conductor (PEC) plate. This work provides a strategy for tuning the composition and interfacial structure of CoS-based microwave absorbers with reduced matching thickness.