## 3 Results and Discussion
### Effect of Absorbing Agent Composition on the Property of Composite Absorbers  
The reflection loss of composite absorbers with different absorbing agent composition is presented in Figure 8. It can be seen that as the frequency increases, the variation of the reflection loss of the three composite absorbers first decreases, and then rises. Among them, the reflection loss of graphene/nano-Fe\({}_{3}\)O\({}_{4}\)/PLA composite absorber is significantly lower than that of graphene/PLA and nano-Fe\({}_{3}\)O\({}_{4}\)/PLA composite absorbers, and the effective absorption bandwidth (RL \(\leq-10\) dB) of the graphene/nano-Fe\({}_{3}\)O\({}_{4}\)/PLA composite absorber is 3.6 GHz (12.4 to 16.0 GHz). Figure 8: Reflection loss of composite absorbers with different absorbing agent composition. Figure 7: SEM cross-section of pure PLA and nano-Fe\({}_{3}\)O\({}_{4}\)/PLA composite absorbers. (**a**) Pure PLA absorber; (**b**) 10 wt\(\%\) nano-Fe\({}_{3}\)O\({}_{4}\)/PLA composite absorber; (**c**) 20 wt\(\%\) nano-Fe\({}_{3}\)O\({}_{4}\)/PLA composite absorber; and (**d**) 30 wt\(\%\) nano-Fe\({}_{3}\)O\({}_{4}\)/PLA composite absorber. The complex permittivity of composite absorbers with different absorbing agent composition are presented in Figure 9. Their variation law is gradually decreasing with increasing frequency, which is due to the relaxation phenomenon of the dielectric polarization in the composite absorber which cannot keep up with the changing speed of the electric field [18]. It can be seen from Figure 9a that the real part of the complex permittivity of the graphene/nano-Fe\({}_{3}\)O\({}_{4}\)/PLA composite absorber is higher than that of the graphene/PLA and nano-Fe\({}_{3}\)O\({}_{4}\)/PLA composite absorbers, because the real part of the complex permittivity of the material mainly depends on the interfacial polarization between the absorbing agent and the substrate. The graphene/nano-Fe\({}_{3}\)O\({}_{4}\)/PLA composite absorber increases the graphene/PLA interface and the graphene/nano-Fe\({}_{3}\)O\({}_{4}\) interface with respect to the nano-Fe\({}_{3}\)O\({}_{4}\)/PLA composite absorber; the graphene/nano-Fe\({}_{3}\)O\({}_{4}\)/PLA composite absorber increases the nano-Fe\({}_{3}\)O\({}_{4}\)/PLA interface and the graphene/nano-Fe\({}_{3}\)O\({}_{4}\) interface relative to graphite/PLA composite absorber. The increase of the interface leads to the highest real part of the complex permittivity. Secondly, since the dielectric property of graphene is significantly better than those of nano-Fe\({}_{3}\)O\({}_{4}\), the polarization of graphene/PLA interface is stronger than that of nano-Fe\({}_{3}\)O\({}_{4}\)/PLA interface. Therefore, the real part of the complex permittivity of graphene/PLA composite absorber is higher than that of nano-Fe\({}_{3}\)O\({}_{4}\)/PLA composite absorber. It can be seen from Figure 9b that the imaginary part of the complex permittivity of graphene/nano-Fe\({}_{3}\)O\({}_{4}\)/PLA composite absorber is higher than graphene/PLA and nano-Fe\({}_{3}\)O\({}_{4}\)/PLA composite absorbers, because in the microwave frequency band, the inherent electric dipole orientation polarization and interfacial polarization have the characteristics of large damping and small restoring force, which becomes the main factor affecting \"\(\varepsilon\)\". In addition, since graphene itself is superior in conductivity to nano-Fe\({}_{3}\)O\({}_{4}\), the imaginary part of complex permittivity of graphene/PLA composite absorber is higher than that of nano-Fe\({}_{3}\)O\({}_{4}\)/PLA composite absorber. The complex permeability of composite absorbers with different absorbing agent compositions are shown in Figure 10. Since the graphene/PLA composite absorber does not have magnetism, it does not participate in the comparison.