## 3 Results and Discussion
### Effect of Absorbing Agent Composition on the Property of Composite Absorbers
As shown in Figure 10a, as the frequency increases, the real part of the complex permeability gradually decreases, because the magnetization speed of the nano-Fe\({}_{3}\)O\({}_{4}\) in the composite absorber cannot keep up with the speed of the magnetic field change, resulting in the magnetization relaxation phenomenon [19]. The complex permeability of the graphene/nano-Fe\({}_{3}\)O\({}_{4}\)/PLA composite absorber is higher than that of the nano-Fe\({}_{3}\)O\({}_{4}\)/PLA composite absorber, because in addition to the magnetic effect of nano-Fe\({}_{3}\)O\({}_{4}\), the graphene/nano-Fe\({}_{3}\)O\({}_{4}\)/PLA composite absorber has a synergistic effect between graphene and nano-Fe\({}_{3}\)O\({}_{4}\)[20]. Therefore, the addition of graphene improves the magnetic storage capacity of the material. It can be seen from Figure 10b that as the frequency increases, the imaginary part of the complex permeability first rises and then falls, and a resonance peak appears at 16 GHz, which is mainly caused by the natural  
Figure 9: Complex permittivity of composite absorbers with different absorbing agent composition. (**a**) The real part of the complex permittivity; (**b**) the imaginary part of the complex permittivity. resonance of the nano-Fe\({}_{3}\)O\({}_{4}\) in the composite absorber. It is worth noting that the imaginary part of the composite permeability of the graphene/nano-Fe\({}_{3}\)O\({}_{4}\)/PLA composite absorber is higher than that of the nano-Fe\({}_{3}\)O\({}_{4}\)/PLA composite absorber, and the resonance peak is obviously improved. This is due to the fact that the addition of graphene leads to an increase in the anisotropy effect of the nano-Fe\({}_{3}\)O\({}_{4}\) in the composite absorber, resulting in an increase in the magnetic loss of the electromagnetic wave. The composite absorbing agent can realize the synergy of various loss mechanisms because it easily generates interfacial polarization [20]. The absorbing mechanism is further revealed by the SEM cross section. Figure 11 shows the SEM cross section of pure PLA and the composite absorbers with different absorbing agent composition. Figure 11a is a pure PLA absorber, the surface of which is relatively flat without holes. Figure 11b is a graphene/PLA composite absorber and it can be clearly observed that the graphene sheet is coated on the surface of the PLA substrate. Figure 11c is a nano-Fe\({}_{3}\)O\({}_{4}\)/PLA composite absorber and it can be clearly seen that the nano-Fe\({}_{3}\)O\({}_{4}\) magnetic microspheres are adsorbed on the surface of the PLA substrate with holes. Figure 11d shows the graphene/nano-Fe\({}_{3}\)O\({}_{4}\)/PLA composite absorber and it can be clearly observed that the nano-Fe\({}_{3}\)O\({}_{4}\) magnetic microspheres are adsorbed on the graphene sheets with a uniform distribution. This is because the graphene sheet has a large specific surface area and sufficient active adsorption point, which can effectively prevent the agglomeration of the nano-Fe\({}_{3}\)O\({}_{4}\) magnetic microspheres and also contributes to the adsorption of nano-Fe\({}_{3}\)O\({}_{4}\) magnetic microspheres on the graphene sheets. At the same time, the nano-Fe\({}_{3}\)O\({}_{4}\) magnetic microspheres increase the distance between the graphene sheets and avoid the agglomeration of graphene. When the electromagnetic wave enters the inside of the graphene/nano-Fe\({}_{3}\)O\({}_{4}\)/PLA composite absorber, the \"protrusion\" formed by the nano-Fe\({}_{3}\)O\({}_{4}\) magnetic microspheres and the graphene sheets have a reflection effect on the electromagnetic wave, which generates multiple reflections inside the material, resulting in the propagation path of electromagnetic waves becoming longer, increasing the loss of electromagnetic waves. This shows that the absorbing property of material can be effectively improved by compounding different loss types of absorbing agents. Figure 10: Complex permeability of composite absorbers with different absorbing agent composition.