Co/C nanoparticles (NPs) with a size of 45 nm have been prepared by an arc plasma method under a mixed methane and argon atmosphere. They display a uniform carbon shell of 3 nm in thickness and have a low graphitization degree. The saturation magnetization (M_S) and coercivity (H_C) of the NPs are 159.9 emu g⁻¹ and 275.7 Oe, respectively. The microwave absorption properties of the Co/C NPs mixed with paraffin wax (PW) at 25, 50 and 75 wt% NPs have been investigated in the range of 2–18 GHz. The Co/C-PW composite with 50 wt% NPs exhibits the best microwave absorption performance. It has a minimum reflection loss (RL) value of −43.4 dB at a thickness of 2.3 mm, and the absorption bandwidth for RL ≤ −20 dB is as large as 7.2 GHz, which is the highest value among the reported Co-based absorbers. The high microwave absorption performance is interpreted in terms of impedance matching, core/shell structure and nanosize effects. The carbon-coated magnetic metal NPs prepared by methane plasma arc are potential microwave absorbers with strong absorption intensity and wide absorption bandwidth.

Nanoporous metal materials with many potential applications have been synthesized by a chemical dealloying approach. The fabrication of nanoporous metal nanoparticles (NPs), however, is still challenging due to the difficulties in producing suitable nanoscale precursors. Herein, nanoporous Co NPs of 31 nm have been successfully prepared by dealloying Co–Al NPs, and surprisingly they possess micropores in a range from 0.7 to 1.7 nm and a large surface area of 50 m² g⁻¹. The crystalline size of the microporous NPs is 2–5 nm. Through the passivation process, the microporous Co NPs covered with CoO (Co@CoO) are generated as a result of the surface oxidation of Co. They exhibit better microwave absorption properties than their nonporous counterpart. An enhanced reflection loss (RL) value of −90.2 dB is obtained for the microporous Co@CoO NPs with a thickness of merely 1.3 mm. The absorption bandwidth corresponding to the RL below −10 dB reaches 7.2 GHz. The microwave absorption mechanism is discussed in terms of micropore morphology, core@shell structure and nanostructure. This novel microporous material may open new routes for designing high performance microwave absorbers.