## 3 Results and Discussion
### Effect of Graphene Content on the Property of Graphene/PLA Composite Absorbers
Therefore, the greater the content of nano-Fe\({}_{3}\)O\({}_{4}\), the higher the value of the complex permeability real part. It can be seen from Figure 6b that the imaginary part of the complex permeability increases first, and then decreases; there exists a drastic change at a frequency of 16 GHz, because nano-Fe\({}_{3}\)O\({}_{4}\) produces the ferromagnetic resonance phenomenon at higher frequencies. When the content of nano-Fe\({}_{3}\)O\({}_{4}\) in nano-Fe\({}_{3}\)O\({}_{4}\)/PLA composite absorber is 20 wt%, the imaginary part of complex permeability is the highest, followed by when the content is 30 wt%, and the lowest when the content is 10 wt%. When the content of nano-Fe\({}_{3}\)O\({}_{4}\) in the nano-Fe\({}_{3}\)O\({}_{4}\)/PLA composite absorber is 10 wt%, the polarization of Fe\({}^{2+}\) ions in nano-Fe\({}_{3}\)O\({}_{4}\) is weak. When the addition amount of nano-Fe\({}_{3}\)O\({}_{4}\) is 20 wt%, the polarization of Fe\({}^{2+}\) ions in nano-Fe\({}_{3}\)O\({}_{4}\) is  
Figure 5: Complex permittivity of nano-Fe\({}_{3}\)O\({}_{4}\)/PLA composite absorbers with different nano-Fe\({}_{3}\)O\({}_{4}\) content. (**a**) The real part of the complex permittivity; (**b**) the imaginary part of the complex permittivity. enhanced, which causes the improvement of magnetic loss. When the content of nano-Fe\({}_{3}\)O\({}_{4}\) is 30 wt%, a large amount of nano-Fe\({}_{3}\)O\({}_{4}\) is agglomerated together, the magnetic loss of the electromagnetic wave is weakened, and therefore the imaginary part of the complex permeability has dropped. One of the earliest ferrites that people have come into contact with is Fe\({}_{3}\)O\({}_{4}\). It has high dielectric and magnetic properties, etc., widely used in the field of absorbing waves [17]. Figure 7 shows a SEM cross-section of pure PLA and nano-Fe\({}_{3}\)O\({}_{4}\)/PLA composite absorbers. As can be seen from Figure 7a, the pure PLA section has no holes. When the content of nano-Fe\({}_{3}\)O\({}_{4}\) is 10 wt%, as shown in Figure 7b, the nano-Fe\({}_{3}\)O\({}_{4}\) magnetic microspheres begin to adsorb on the surface of PLA, forming \"protrusions\" with pores. These \"protrusions\" promote multiple reflection losses of electromagnetic waves inside the material, while holes can act as polarization centers, generating polarization and relaxation under varying electromagnetic fields, thereby consuming electromagnetic energy. When the addition amount of nano-Fe\({}_{3}\)O\({}_{4}\) is 20 wt%, as shown in Figure 7c, more nano-Fe\({}_{3}\)O\({}_{4}\) magnetic microspheres are adsorbed on the PLA surface, which promotes the loss and absorption of electromagnetic waves. When the amount of nano-Fe\({}_{3}\)O\({}_{4}\) added reaches 30 wt%, as shown in Figure 7d, the nano-Fe\({}_{3}\)O\({}_{4}\) magnetic microspheres are agglomerated and the pores become larger. Agglomeration affects the ability of electromagnetic wave loss, since large holes allow electromagnetic waves to pass through the material directly, which weakens the absorption performance. Figure 6: Complex permeability of nano-Fe\({}_{3}\)O\({}_{4}\)/PLA composite absorbers with different nano-Fe\({}_{3}\)O\({}_{4}\) content. (**a**) The real part of the complex permeability; (**b**) the imaginary part of complex permeability.