Multi-layered nano-hollow spheres for efficient electromagnetic wave absorption

Ferrite nano-hollow spheres (NHS) are of great significance to improve electromagnetic (EM) wave absorption performance. Herein, the deposition of dielectric SiO₂and ferrimagnetic CoFe₂O₄(CFO) layers on MnFe₂O₄(MnFO) NHS are found as an effective strategy to enhance EM wave attenuation. EM wave absorption properties of as-synthesized bare and bi-layered samples are investigated within a widely-used frequency range of 1-17 GHz. MnFO@CFO bi-layered NHSs exhibit an excellent reflection loss (RL) of -47.0 dB at only 20 wt% filler content with an effective broad bandwidth (BW) of ∼2.2 GHz (frequency region for RL < -10 dB). The attenuation constant is observed to increase from 191.6 Np m^-1to 457.8 Np m^-1for bare MnFO and MnFO@CFO NHSs respectively. Larger interfacial area, additional pairs of dipole, higher magnetic anisotropy, internal reflections and scattering from NHSs are responsible for superior absorption properties of MnFO@CFO NHSs. Moreover, the best impedance matching,∣Z/Z₀∣ ∼ 1, promotes the optimum RL in MnFO@CFO at 5.96 GHz. MnFO@SiO₂bi-layered NHSs result in a sufficiently high RL ∼ -30.0 dB with a composite absorber of a thickness of only 3 mm. Analysis from theλ/4 model for best matching thickness (t) displays a good agreement between experimental and simulatedtvalues. This study demonstrates optimized MnFO@CFO NHS as a highly promising low-cost and lightweight EM wave absorber suitable for practical high-frequency applications.