Synergistic S-Band Microwave Attenuation in Protonated Polyaniline-Coated (Co _0.8 Ni _0.2 )Fe ₂ O ₄ Spinel Ferrite Nanocomposites

Efficient low-frequency (S-band) microwave absorption remains a challenge due to the limited availability of materials that combine suitable magnetic and dielectric properties. In this work, we address this gap by designing a series of (Co0.8Ni0.2)Fe2O4/polyaniline (PANI) nanocomposites via in situ emulsion polymerization with varied aniline-to-ferrite mass ratios (0, 0.5, 1, 1.5). X-ray diffraction confirmed a single cubic spinel phase (Fd3̅m), and FTIR spectra displayed both metal–oxygen vibrations of the ferrite and characteristic PANI bands. FE-SEM and TEM revealed that polyhedral ferrite aggregates become progressively embedded in a continuous PANI network, while elemental mapping verified uniform distribution of all constituents. This tailored morphology correlates with a systematic decrease in specific surface area (from 14.7 to 6.4 m2 g–1) and pore diameter (from 50.6 to 21.9 nm), indicating the formation of a more compact, interfacially rich composite. Magnetic measurements showed a reduction in saturation magnetization (from 67 to 18 emu g–1) along with a slight increase in coercivity, reflecting magnetic-phase dilution and modified interparticle coupling. The electromagnetic analysis demonstrates that an intermediate PANI content (P0.5) achieves an optimal balance between magnetic and dielectric losses, leading to a minimum reflection loss of −39 dB at 2.6 GHz and an effective absorption bandwidth of 3.06 GHz (at 10 mm thickness). Our study underscores the critical role of controlled polymer loading in constructing uniform ferrite/PANI architectures and provides a viable strategy for designing high-performance, low-frequency microwave absorbers.