Core–shell structure cobalt–cobalt oxide nanocomposites were directly synthesized via annealing Co nanocrystals in air at 300 °C. Their microstructure and magnetic properties were characterized by XRD, TEM, XPS and VSM, respectively. The microwave absorbing properties of the nanocomposite powders by dispersing them in wax were investigated in the 2–18 GHz frequency range. The sample that was annealed for 1 h exhibits the maximum reflection loss of −30.5 dB and a bandwidth of less than −10 dB covering the 12.6–17.3 GHz range with the coating thickness of only 1.7 mm. At the same thickness, the sample annealed for 3 h exhibits the maximum reflection loss of −24 dB and a bandwidth that almost covers the whole X-band (8–11.5 GHz). With increase in the insulating cobalt oxide shell, the enhanced permeability could contribute to the decrease of eddy current loss, and the permittivity could be easily adjusted; thus, the microwave absorption properties of the cobalt oxide nanocrystals could be easily adjusted.

•Cobalt microflakes are assemblies structure composed of cobalt nanocrystallines.•Cobalt microflakes/wax exhibits an excellent microwave absorption in 2–16 GHz.•Cobalt microflakes/wax composite has a relatively high permittivity in 2–16 GHz.•Cobalt microflakes/wax composite exhibits antiresonance permeability in 2–16 GHz.The high real part of permittivity (34 ～ 63), resonant permittivity and antiresonant permeability were observed simultaneously in flake-shaped cobalt microcrystal powders embedded in the wax composite in 2–16 GHz frequency range. These cobalt flakes are assembled structure composed of cobalt nanocrystalline. The estimated crystallite size of cobalt nanocrystalline is 25 nm. The scattering of complex permittivity at microwave frequency was thoroughly discussed in this paper and the antiresonant permeability behavior was explained using Mie theory and electromagnetic induction principle. The magnetic hysteresis loops of the pressed disk using cobalt powders revealed that the sample exhibited ferromagnetic characteristics with a saturation magnetization of 149 emu/g and a coercivity of 263 Oe at room temperature. The reflection loss of cobalt/wax composite with 60 wt % cobalt reaches a minimum of −12.2 dB at 11.2 GHz and the bandwidth less than −6 dB covering 9.28–13.04 GHz range with the coating thickness only 1 mm. In addition, the absorption peak sites can be adjusted easily by varying the coating thickness. The excellent microwave absorption properties in 2–16 GHz range can attribute to strong dielectric loss and relative high permeability coming from strong magnetic anisotropy.

•Highly uniform flowers-like Ni spheres was synthesized by hydrothermal method.•Flowers-like Ni sample exhibits an excellent microwave absorption in 2–18 GHz.•Fowers-like Ni sample has a relatively low eddy current in 2–18 GHz.•Fowers-like sample has a larger electric resistivity due to particle shape effect.The Ni microcrystals with hierarchical branch-like and flowers-like shapes have been fabricated by hydrothermal method. The flowers-like microspheres with an average diameter of 500 nm are built of intercrossed flakes. Magnetic study of the Ni microcrystals indicates that the flowers-like microspheres sample has a little higher coercive force (276 Oe) than that of the branch-like sample (161 Oe). An important application of these hierarchical Ni microcrystals is used as high-performance microwave absorbers in 2–8 GHz range. Compared to the branch-like sample, flowers-like microspheres sample exhibits more excellent microwave absorbing performance and the reflection loss reaches a minimum of −17 dB at 13 GHz with −6 dB bandwidth covering the 6.5–14 GHz range. The further analysis of electromagnetic parameters indicates the branch-like sample has a significant eddy current effect which leads to the μ′μ′ value below 1.0, the μ″μ″ value below zero and the poor microwave absorption in 11.5–14 GHz range. However, flowers-like microspheres sample has relatively low eddy current effects coming from the particle shape effects, which results in relatively high permeability and more suitable impedance matching between permittivity and permeability in 2–18 GHz range.The highly uniform flowers-like Ni spheres with an average diameter of 500 nm are successfully prepared by hydrothermal method. The flowers-like Ni sample exhibits a strong microwave absorption in 11.5–14 GHz range besides the absorption peak in 2–8 GHz range, moreover, the absorption frequency ranges of sample can be tuned easily by varying the coating layer thickness. Compared to the branch-like sample, flowers-like spheres sample exhibits relatively low permittivity, relatively high permeability and more excellent microwave absorption in the whole 2–18 GHz due to the relatively low eddy current coming from the particle shape effects because of flowers-like sample exhibits a higher electric resistivity.

•Ni-Zn ferrite nanocrystals can use as matching layer in double-layer absorbers.•Co67Ni33 microspheres with high dielectric loss can use as absorption layer.•Double-layer absorbers exhibits an excellent microwave absorption in 2–18 GHz.Co67Ni33 microspheres and Ni0.6Zn0.4Fe2O4 nanocrystals were synthesized by hydrothermal method. The complex permeability and complex permittivity of the as-prepared powders dispersing in wax (60 wt% powder) were measured using a vector network analyzer in 2–18 GHz frequency range. The calculated microwave absorption of single-layer and double-layer absorbers based on Co67Ni33 microspheres and Ni0.6Zn0.4Fe2O4 nanocrystals were analyzed in 2–18 GHz frequency range. The results show that the Ni0.6Zn0.4Fe2O4 nanocrystals with the relatively low permittivity and Co67Ni33 microspheres with the relatively high dielectric loss and magnetic loss can be used as proper matching layer and excellent absorption layer, respectively. The double-layer absorber with a coating thickness of 2.1 mm exhibits a maximum reflection loss of −43.8 dB as well as a bandwidth (reflection loss less than −10 dB) of 5 GHz. Moreover, their absorption peak and the absorption intensity can be adjusted easily through changing the stacking order and each layer thickness.

Core–shell structure cobalt–cobalt oxide nanocomposites were directly synthesized via annealing Co nanocrystals in air at 300 °C. Their microstructure and magnetic properties were characterized by XRD, TEM, XPS and VSM, respectively. The microwave absorbing properties of the nanocomposite powders by dispersing them in wax were investigated in the 2–18 GHz frequency range. The sample that was annealed for 1 h exhibits the maximum reflection loss of −30.5 dB and a bandwidth of less than −10 dB covering the 12.6–17.3 GHz range with the coating thickness of only 1.7 mm. At the same thickness, the sample annealed for 3 h exhibits the maximum reflection loss of −24 dB and a bandwidth that almost covers the whole X-band (8–11.5 GHz). With increase in the insulating cobalt oxide shell, the enhanced permeability could contribute to the decrease of eddy current loss, and the permittivity could be easily adjusted; thus, the microwave absorption properties of the cobalt oxide nanocrystals could be easily adjusted.