The electromagnetic ͑EM͒ characteristics of ZnO-coated Fe nanocapsules synthesized by arc discharging were studied at 2-18 GHz. A reflection loss ͑RL͒ exceeding −20 dB was obtained in the frequency range of 6.1-15.7 GHz for an absorber thickness of 1.5-5 mm. An optimal RL of −57.1 dB was found at 7.8 GHz for an absorber thickness of 3.00 mm. The excellent microwave-absorption properties are a consequence of a proper EM match in the nano-microstructure, a strong natural resonance, as well as multipolarization mechanisms. ZnO-coated Fe nanocapsules may be attractive candidates for EM-wave-absorption materials.

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The electromagnetic characteristics of Ni/ZnO nanocapsules were studied at 2-18 GHz. The dual nonlinear dielectric resonance and strong natural resonance at 16.6 GHz contribute to excellent electromagnetic absorption. A reflection loss ͑RL͒ exceeding Ϫ20 dB was calculated in 14-18 GHz for an absorber thickness of 2.05 mm, and RL exceeds Ϫ10 dB in the whole X-band ͑10-12.4 GHz͒ and the whole Ku-band ͑12.4-18 GHz͒ for a thickness of 2.50 mm. The equivalent circuit model was used to explain the dual nonlinear dielectric resonance, which is ascribed to a cooperative consequence of the core/shell interfaces and the dielectric ZnO shells.