Enhanced Electromagnetic Wave Absorption in W-Type Ba-Hexaferrites: Role of Composition and Grain Growth

W-type Ba-hexaferrite, BaZn2−xCoxFe16O27, was synthesized via both conventional solid-state (x = 0.4, 0.5, 0.6, 0.75, 1.0, 1.25, 1.5) and molten-salt methods (x = 0.75, 1.0, 1.25). The structure, electromagnetic (EM) properties, and EM wave absorption characteristics were examined across a frequency range of 0.1–18 GHz, focusing on the influence of varying x. As x increased from 0.4 to 0.75, the magnetic anisotropy field (Hani) decreased, reaching its minimum at x = 0.75, before rising again as x continued to increase up to 1.5. Hani was found to be proportional to the ferromagnetic resonance (FMR) frequency, allowing for the tuning of the EM wave absorption frequency range. W-type hexaferrite–epoxy composites (10 wt%) with x values between 0.6 and 1.5 exhibited outstanding wideband EM wave absorption, with a maximum absorption (RLmin < −50 dB) and a wide absorption bandwidth where RL < −10 dB extended beyond 10 GHz (Δfwb > 10 GHz). The x = 1.25 sample with a thickness of 2.37 mm achieved RLmin = −69 dB at 7.8 GHz, while the x = 1.0 sample with a thickness of 2.33 mm delivered Δfwb = 12.5 GHz (5.1–17.6 GHz). Samples synthesized via the molten-salt method showed larger plate-like grain growth compared to those produced by the solid-state method, with permeability spectra shifting to lower frequencies, consequently lowering the EM wave absorption band.