## 2 Materials and Methods  
W-type hexaferrite powders with the chemical composition BaZn\({}_{2-x}\)Co\({}_{x}\)Fe\({}_{16}\)O\({}_{27}\) (\(x\) = 0.4, 0.5, 0.6, 0.75, 1.0, 1.25, 1.5) were synthesized using a conventional solid-state reaction method. Additionally, for compositions \(x\) = 0.75, 1.0, and 1.25, the molten-salt (M-S) method was employed to synthesize the powders. The precursor materials, BaCO\({}_{3}\) (99+%, Kojundo Chemical, Saitama, Japan), ZnO (99.9%, Kojundo Chemical), CoO (99%, Sigma-Aldrich, St. Louis, MO, USA), and Fe\({}_{2}\)O\({}_{3}\) (99.9%, Kojundo Chemical), were weighed according to the cation composition ratio and ball-milled with ZnO\({}_{2}\) balls in deionized (DI) water. After the mixture was dried in an electric oven, it was placed in an alumina crucible and subjected to an initial calcination in air at 900 \({}^{\circ}\)C for 6 h. The calcined powder was then crushed and returned to the crucible for a second calcination at 1250 \({}^{\circ}\)C for 6 h, with the furnace left to cool naturally.  
For the M-S process, the same procedures were followed, from powder weighing to the first calcination, as in the solid-state method. After the initial calcination, portions of the BaZn\({}_{2-x}\)Co\({}_{x}\)Fe\({}_{16}\)O\({}_{27}\) (\(x\) = 0.75, 1.0, 1.25) samples were separated, mixed with NaCl (99%, Alfa Aesar, Haverhill, MA, USA) powder at a 1:1 weight ratio, and subjected to a second calcination under the same conditions as the solid-state method. The calcined powders were then crushed to obtain W-type hexaferrite powders for each composition and synthesis method. The hexaferrite powders were mixed with solid epoxy resin (YD-014, Kuk-do Chemical, Seoul, Republic of Korea) at a 9:1 weight ratio and placed into a toroidal mold (inner diameter = 3.03 mm, outer diameter = 7.00 mm). The samples were shaped under a pressure of 0.2 tons and cured at 180 \({}^{\circ}\)C for 20 min. Post-processing ensured that the samples conformed to the desired dimensions of an inner diameter of 3.03 mm and an outer diameter of 7.00 mm.  
Phase analysis was carried out using an X-ray diffractometer (XRD, D8 Advance, Bruker, Billerica, MA, USA) with Cu K\(\alpha\) radiation (\(\lambda\) = 0.154056 nm). Microstructural observations were performed using field emission scanning electron microscopy (FE-SEM, JSM-7610F, JEOL, Seoul, Republic of Korea). The magnetization curves of the powder samples were measured at room temperature using a vibrating sample magnetometer (VSM, LakeShore 7410, Westerville, OH, USA) within a magnetic field range of \(-\)25 kOe \(\leq\) H \(\leq\) 25 kOe, with the hexaferrite powders embedded in epoxy resin for the VSM measurements. The complex permittivity (\(\epsilon^{\prime}\), \(\epsilon^{\prime\prime}\)) and complex permeability (\(\mu^{\prime}\), \(\mu^{\prime\prime}\)) spectra were obtained across the frequency range of 0.1 to 18 GHz using a vector network analyzer (VNA, E50356A, Keysight, Santa Rosa, CA, USA) in conjunction with an airline kit (85052 BR03) and software (N1500A) integrated into the VNA. The reflection loss (RL) of the composites was calculated based on the \(\epsilon^{\prime}\), \(\epsilon^{\prime\prime}\), \(\mu^{\prime}\), and \(\mu^{\prime\prime}\) spectra.