## 3 Results and Discussion
### Effect of Graphene Content on the Property of Graphene/PLA Composite Absorbers
Effect of Nano-Fe\({}_{3}\)O\({}_{4}\) Content on the Property of Nano-Fe\({}_{3}\)O\({}_{4}\)/PLA Composite Absorbers  
Comparisons of the reflection loss of the composite absorbers are shown as Figures 1 and 4, respectively. The variation is similar to that of graphene/PLA composite absorbers. The difference is that the minimum reflection loss value is shifted to the high frequency band. In the band of 8.2 to 18.0 GHz, when the content of nano-Fe\({}_{3}\)O\({}_{4}\) in the nano-Fe\({}_{3}\)O\({}_{4}\)/PLA composite absorber is 20 wt%, the reflection loss curve is the lowest, followed by 30 wt%, and finally 10 wt%. The minimum reflection loss values achieved \(-\)4.8 dB, \(-\)3.9 dB, \(-\)3.3 dB, respectively. Figure 4: Reflection loss of nano-Fe\({}_{3}\)O\({}_{4}\)/PLA composite absorbers with different nano-Fe\({}_{3}\)O\({}_{4}\) content. Figure 3: SEM cross section of pure PLA absorber and graphene/PLA composite absorbers. (**a**) Pure PLA absorber; (**b**) 5 wt% graphene/PLA composite absorber; (**c**) 7 wt% graphene/PLA composite absorber; and (**d**) 9 wt% graphene/PLA composite absorber. As shown in Figure 5a,b, the complex permittivity of the nano-Fe\({}_{3}\)O\({}_{4}\)/PLA composite absorber decreases with increasing frequency. The complex permittivity can be expressed as \(\varepsilon=\varepsilon_{\infty}+(\varepsilon_{0}-\varepsilon_{\infty})/(1+i \omega\tau)\), where \(\varepsilon_{0}\) and \(\varepsilon_{\infty}\) are the complex dielectric constants of \(\omega\to 0\) and \(\omega\to\infty\), respectively, \(\tau\) is the relaxation time, and \(\omega\) is the circular frequency [12; 13; 14; 15]. It can be seen that the complex permittivity \(\varepsilon\) decreases as the frequency \(f\) (relative to the circular frequency \(\omega\)) increases. In Figure 5a, with an increase of the mass fraction of nano-Fe\({}_{3}\)O\({}_{4}\), the real part of the complex permittivity is improved. When the content of nano-Fe\({}_{3}\)O\({}_{4}\) is 30 wt%, the complex permittivity is the highest, it is the second highest when the content is 20 wt%, and it is the lowest when the content is 10 wt%. This is explained by the fact that as the mass fraction of nano-Fe\({}_{3}\)O\({}_{4}\) increases, the number of multi-interfaces formed by nano-Fe\({}_{3}\)O\({}_{4}\) adsorbed on the surface of PLA and the interface polarization are increased, which leads to the increase in the real part of the complex permittivity. As shown in Figure 5b, when the content of nano-Fe\({}_{3}\)O\({}_{4}\) in the nano-Fe\({}_{3}\)O\({}_{4}\)/PLA composite absorber is 20 wt%, the complex permittivity imaginary part is the highest, followed by when the content is 30 wt%, and it is the lowest when the content is 10 wt%. The reason for this is that as the amount of nano-Fe\({}_{3}\)O\({}_{4}\) increased, the number of dipoles in the composite absorber increased, which caused the dipole polarization effect of the material when the addition amount of nano-Fe\({}_{3}\)O\({}_{4}\) reached 30 wt%. Due to the enhanced adsorption between nano-Fe\({}_{3}\)O\({}_{4}\), the dispersion state of nano-Fe\({}_{3}\)O\({}_{4}\) in the matrix is affected, which weakens the ability of the dipole to lose electromagnetic waves. Figure 6 is a graph showing the complex permeability of nano-Fe\({}_{3}\)O\({}_{4}\)/PLA composite absorbers with different nano-Fe\({}_{3}\)O\({}_{4}\) content. It can be seen from Figure 6a that with an increase of frequency, the real part of the complex permeability gradually decreases, because the speed of the dielectric magnetization in the nano-Fe\({}_{3}\)O\({}_{4}\)/PLA composite absorber cannot keep up with the speed of the magnetic field change, forming a hysteresis phenomenon [16]. When the content of nano-Fe\({}_{3}\)O\({}_{4}\) in the nano-Fe\({}_{3}\)O\({}_{4}\)/PLA composite absorber is 30 wt%, the real part of the complex permeability is the highest, followed by when the content is 20 wt%, and the lowest is when the content is 10 wt%. The increase of nano-Fe\({}_{3}\)O\({}_{4}\) brings about improvement in magnetic storage.