Electromagnetic Wave Absorption Properties of Cation-Substituted Ba0.5Sr0.5Zn2−xMexFe16O27 (Me = Fe, Ni, Co, Cu, Mn) W-Type Hexagonal Ferrites

W-type hexaferrites with compositions Ba0.5Sr0.5Zn2-xMexFe16O27 (Me = Fe, Ni, Co, Cu, Mn; x = 1) and Ba0.5Sr0.5Zn2−xMnxFe16O27 (x = 0–2.0) were synthesized via solid-state reaction and optimized using a two-step calcination process to obtain single-phase or nearly single-phase structures. Their electromagnetic (EM) wave absorption properties were investigated by fabricating composites with 10 wt% epoxy and measuring the complex permittivity and permeability across two frequency bands: 0.1–18 GHz and 26.5–40 GHz. Reflection loss (RL) was calculated and visualized as two-dimensional (2D) maps with respect to frequency and sample thickness. In the 0.1–18 GHz range, only the Co-substituted sample exhibited strong ferromagnetic resonance (FMR) and broadband absorption, achieving a minimum RL of −41.5 dB at 4.84 GHz and a −10 dB bandwidth of 11.8 GHz. In contrast, the other Ba0.5Sr0.5Zn2-xMexFe16O27 samples (Me = Fe, Mn, Ni, Cu) showed no significant absorption in this range due to the absence of FMR. However, all these samples clearly exhibited FMR characteristics and distinct absorption peaks in the 26.5–40 GHz range, particularly the Mn-substituted series, which demonstrated RL values below −10 dB over the 32.0–40 GHz range with absorber thicknesses below 1 mm. The FMR frequency varied depending on the substitution type and amount. In the Mn-substituted series, the FMR frequency was lowest at x = 1.0 and increased as x deviated from this composition. This study confirms the potential of Co-free W-type hexaferrites as efficient, cost-effective, and broadband EM wave absorbers in the 26.5–40 GHz range.