Broadband electromagnetic wave absorbers with Ni-Co substituted Ba-W hexaferrites synthesized via citric acid-assisted sol-gel reaction

While there is a huge demand for a thin and broadband electromagnetic wave absorber, it has been challenging to achieve such absorber in Ku-band (12.4–18 GHz) or millimeter-wave (mmWave, 30–300 GHz) frequency bands, which are employed for 5 G and 6 G communication. Though effective magnetic materials are necessary for electromagnetic wave absorption, most magnetic materials exhibit Ferromagnetic Resonance (FMR) at frequencies below several GHz and lose their magnetic characteristics in high frequency bands. To address this problem, an effective electromagnetic wave absorber is proposed in this study with Ni-Co-substituted Ba-W hexaferrites synthesized via citric acid-assisted sol-gel reaction. The single-phase hexaferrite powder was synthesized via a citric acid-assisted sol-gel method and molten salt heat treatment at a reduced calcination temperature of 1250 °C. By modulating nickel and cobalt concentrations, the ferrite’s FMR frequency and operational frequency of the absorber were tuned from 41 GHz to 10 GHz, making it suitable for diverse high-frequency applications. An optimized Ku-band absorber based on BaNi₁.₂Co₀.₈Fe₁₆O₂₇ demonstrated more than 90 % absorption within the 12.04–19.78 GHz range with a thickness of only 1.793 mm. The results suggest the high potential of Ni-Co-substituted Ba-W hexaferrites for millimeter-wave applications and electromagnetic interference reduction in advanced communication systems. ● Single phase Ba-W hexaferrites were synthesized at a reduced calcination temperature of 1250 ℃. ● Their ferromagnetic resonance frequencies were tuned with different Ni-Co substitution levels. ● Effective absorption bandwidth of 10.16–19.78 GHz was obtained, covering the entire Ku-band (12.4–18 GHz).