## 4 Summary  
The W-type hexaferrite BaZn\({}_{2-x}\)Co\({}_{x}\)W (\(x\) = 0.5, 0.6, 0.75, 1.0, 1.25, 1.5) was synthesized via a solid-state method. For compositions of \(x\) = 0.75, 1.0, and 1.25, hexaferrite powders were additionally synthesized using the M-S method, and their EM wave absorption properties were compared with those of samples prepared by the conventional solid-state method. Magnetic powder prepared by the conventional solid-state method exhibited changes in M\({}_{\rm S}\), H\({}_{\rm C}\), and H\({}_{\rm anti}\) depending on the \(x\) value. H\({}_{\rm anti}\) was minimized at \(x\) = 0.75, and H\({}_{\rm anti}\) gradually increased as the \(x\) value increased or decreased from \(x\) = 0.75. BaZn\({}_{2-x}\)Co\({}_{x}\)W exhibited c-axis anisotropy for \(x\) \(\leq\) 0.75 and c-plane anisotropy for \(x\) \(\geq\) 0.75. This change in H\({}_{\rm anti}\) led to a shift in the \(f_{FMR}\), which is the primary mechanism of EM wave absorption in BaZn\({}_{2-x}\)Co\({}_{x}\)W, actively controlling the absorption frequency range by varying \(x\). The variations in the EM wave absorption regions depending on \(x\) and processing methods were presented on an RL map, derived from the high-frequency complex permittivity and permeability spectra, as a function of the frequency (0.1 \(\leq\)\(f\)\(\leq\) 18 GHz) and absorber thickness (0 \(\leq\)\(d\)\(\leq\) 10 mm). The \(x\) = 1.25 sample exhibited a maximum absorption of RL\({}_{\rm min}\) = \(-\)69 dB near \(f\) = 8 GHz with a thickness of \(d\) = 2.57 mm, and it achieved a maximum bandwidth for 90% energy absorption, \(\Delta\)\(f\) > 11.7 GHz, ranging from 6.3 GHz to beyond 18 GHz, with a thickness of \(d\) = 2.37 mm. When the same composition was fabricated using the M-S method and the epoxy composite, the \(FMR\) frequency slightly decreased, and the absorption bandwidth shifted to 5.48 GHz-17.85 GHz. The M-S process caused significant grain growth, shifting the permeability spectra towards lower frequencies, and the \(f_{FMR}\) also decreased slightly, resulting in a corresponding EM absorption band shift to a lower-frequency region. The BaZn\({}_{2-x}\)Co\({}_{x}\)W powder-epoxy (10 wt%) composites prepared by the solid-state method demonstrated outstanding EM wave absorption properties compared to other absorbing materials. Therefore, BaZn\({}_{2-x}\)Co\({}_{x}\)W is a highly promising material for broad-band absorbers in the C-Ku bands and even beyond, with tunable absorption frequency ranges depending on the Co substitution composition.  
Conceptualization, Y.-M.K. and J.-H.H.; methodology, J.-H.H.; software, J.-H.H.; validation, Y.-M.K. and J.-H.H.; formal analysis, Y.-M.K. and [PERSON]-[PERSON]; investigation, [PERSON]-[PERSON] and [PERSON]-[PERSON]; writing--original draft preparation, [PERSON]-H.H.; writing--review and editing, [PERSON]-H.H.; visualization, [PERSON]-M.K.; supervision, [PERSON]-M.K.; project administration, [PERSON]-M.K.; funding acquisition, Y.-M.K. All authors have read and agreed to the published version of the manuscript.  
This study was supported by the Research Fund of the Basic Science Research Program through the National Research Foundation of Korea, funded by the Ministry of Science and ICT (Grant No. 2022R1F1A1062933), and by the Basic Science Research Capacity Enhancement Project (National Research Facilities and Equipment Center) through the Korea Basic Science Institute, funded by the Ministry of Education (Grant No. 2019R1A6C1010047).  
Not applicable.  
The data presented in this study are available on request from the corresponding author. The data are not publicly available due to privacy.  
The authors declare no conflict of interest.