K-Band Electromagnetic Wave Absorption of Ni-Co-Substituted Ba-W Hexaferrites Prepared via Sol-Gel Method with Molten Salt

The demand for thin and broadband high-frequency electromagnetic wave absorbers has grown rapidly with the advancement of communication technologies. However, most conventional magnetic materials exhibit ferromagnetic resonance (FMR) at low frequencies, restricting their applicability in high-frequency ranges such as the K-band, which is widely used in 5G communication. In this study, Ni-Co-substituted Ba-W hexaferrites (BaNi_2-xCo_xFe₁₆O₂₇) were synthesized via a citric acid-assisted sol–gel method combined with molten salt calcination to achieve single-phase formation at a reduced temperature of 1250 °C. By adjusting the Ni and Co substitution levels, the FMR frequency was effectively tuned within the 18–40 GHz range. The composite absorber with the optimized composition, BaNi_1.4Co_0.6Fe₁₆O₂₇, exhibited over 90% absorption efficiency across the entire K-band with a minimal thickness of 1.2 mm. This absorber outperforms many previously reported materials by offering wider bandwidth and thinner thickness. These findings highlight the suitability of Ni-Co- substituted Ba-W hexaferrites for next-generation communications.