## 3 Results and Discussion
Meanwhile, in the range of 8-18 GHz, the \(\mu\)\" curves of Fe\({}_{3}\)O\({}_{4}\) and Fe\({}_{3}\)O\({}_{4}\)/ZnO composites change to become similar, one \(\mu\)\" value peak of sample B is observed at ~11 GHz, which may be attributable to the dissipation of EM energy. Furthermore, it is noticed that negative values of \(\mu\)\" occur in the high frequency range due to calibration or sensitivity issues of the experimental set-up. The theoretical reflection loss (RL) of the composite absorber at different thicknesses was calculated using the following equations [30; 31; 32]:  
\[RL=20~{}log|(Z_{in}-Z_{0})/(Z_{in}+Z_{0})| \tag{1}\]  
\[Z_{in}=Z_{0}\sqrt{\frac{\mu_{r}}{\varepsilon_{r}}}tanh\left(j\frac{2\pi fd\sqrt{ \mu_{r}\varepsilon_{r}}}{c}\right) \tag{2}\]  
Here, \(Z_{0}\) is the impedance of free space, \(Z_{\rm in}\) is the normalized input impedance of the absorber, \(d\) is the thickness, \(C\) is the velocity of EMW in free space, and \(f\) is the frequency of the incident wave. RL values of \(-10\) dB and \(-20\) dB correspond to 90% and 99% attenuation of the incident EMW energy, and the frequency range where RL is smaller than \(-10\) dB is defined as the effective absorption bandwidth. Figure 6a-e shows the plots of RL versus the frequency of the Fe\({}_{3}\)O\({}_{4}\) NPs and two samples of Fe\({}_{3}\)O\({}_{4}\)/ZnO composites at different thicknesses. As for Fe\({}_{3}\)O\({}_{4}\) NPs, the minimum RL of \(-7.28\) dB is observed at 15.68 GHz with the thickness of 2.5 mm, which indicates that the naked Fe\({}_{3}\)O\({}_{4}\) NPs have a weak EMW absorption property. Furthermore, with the doping of the dielectric component, the EMW absorption property of sample A can be improved slightly. As shown in Figure 6b, the minimum RL is \(-13.91\) dB at 5.52 GHz with the thickness of 4.5 mm. This is because the ZnO particles failed to generate chemical bonds with the Fe\({}_{3}\)O\({}_{4}\) NPs and grew into short rod shapes alone; therefore, the composites were unable to obtain a good impedance match and interfacial polarization. As for the sample B loaded with 30 wt % (Figure 6c), because of the high dispersion in the paraffin matrix, the Fe\({}_{3}\)O\({}_{4}\)/ZnO nanoparticles failed to generate conductive interconnections, so the EMW absorption performance did not show an enhancement in comparison to pure Fe\({}_{3}\)O\({}_{4}\). It is noticed that sample B loaded with 50 wt % shows an enhanced EMW absorption property (Figure 6d). Specifically, the minimum RL value of \(-50.79\) dB can be achieved at 4.38 GHz with the thickness of 4.5 mm. Based on the results of Figure 4d-f, we deduced that the incorporation of the dielectric ZnO into the Fe\({}_{3}\)O\({}_{4}\) NPs  
Figure 5: Frequency dependence on the (**a**) real part and (**b**) imaginary part of relative complex permittivity; (**c**) real part and (**d**) imaginary part of relative complex permeability. may generate a high dielectric constant and loss due to the effective interfaces between the dielectric and magnetic materials, giving them an advantage in terms of matching complex permittivity and permeability. The enhanced EM absorption properties benefit from the uniform core-shell structures, which induce an intensification of interfacial polarization. It is worth noting that the core-shell structured Fe\({}_{3}\)O\({}_{4}\)/ZnO composites are able to achieve an enhanced absorption property in both low and high frequency bands; such dual absorption regions are also competitive in comparison to other materials. It can be observed from Figure 6d that the minimum RL values all shift toward the lower frequency region with increasing thickness, which can be explained by the quarter-wavelength match principle [18]:  
\[t_{m}=\frac{n\lambda}{4}=\frac{nc}{4f_{m}\sqrt{(|\varepsilon\mu r|)}}(n=1,3,5 \ldots) \tag{3}\]  
where \(t_{m}\) is the absorber thickness, \(\mu_{r}\) is the complex permeability at \(f_{m}\), and \(\varepsilon_{r}\) is the complex permittivity at \(f_{m}\).