Water atomized Fe16Ni82Mo2 (wt. %) alloy powder with spherical-like form was processed in a planetary mill for 2 h and 4 h, respectively. The complex permittivity (ε′−jε″) and permeability (μ′−jμ″) are measured using the transmission/reflection method in the frequency range of 2–18 GHz for FeNiMo/epoxy composites with 70 wt. % of magnetic fillers. The results show that all ε′, ε″, μ′, μ″ substantially increase due to the increase of aspect ratios. The magnetic susceptibility (μ′−1) of FeNiMo/epoxy absorber is negative above 5 GHz due to the natural magnetic resonance. The frequency dispersion of μ″ of flake-shaped FeNiMo composite shows a higher resonance frequency which exceeds the Snoek's limit in the gigahertz range as a result of large shape anisotropy. For composite absorbers with 2 mm thickness, the calculated reflection loss reaches −12.1 dB at 14.6 GHz without milling, −20.4 dB at 13.0 GHz for 2 h milling, and −13.1 dB at 11.0 GHz for 4 h milling.

Radar absorbing materials (RAMs) also known as microwave absorbers, which can absorb and dissipate incident electromagnetic wave, are widely used in the fields of radar-cross section reduction, electromagnetic interference (EMI) reduction and human health protection. In this study, the synthesis of functionally graded material (FGM) (CI/Polyurethane composites), which is fabricated with semi-sequentially varied composition along the thickness, is implemented with a genetic algorithm (GA) to optimize the microwave absorption efficiency and bandwidth of FGM. For impedance matching and broad-band design, the original 8-layered FGM was obtained by the GA method to calculate the thickness of each layer for a sequential stacking of FGM from 20, 30, 40, 50, 60, 65, 70 and 75 wt% of CI fillers. The reflection loss of the original 8-layered FGM below –10 dB can be obtained in the frequency range of 5.12∼18 GHz with a total thickness of 9.66 mm. Further optimization reduces the number of the layers and the stacking sequence of the optimized 4-layered FGM is 20, 30, 65, 75 wt% with thickness of 0.8, 1.6, 0.6 and 1.0 mm, respectively. The synthesis and measurement of the optimized 4-layered FGM with a thickness of 4 mm reveal a minimum reflection loss of –25.2 dB at 6.64 GHz and its bandwidth below – 10 dB is larger than 12.8 GHz.