XRD was used to analyze the phase of the samples. Figure 1 shows the XRD spectra of CCF, NiFe₂O₄/CCF-1, NiFe₂O₄/CCF-2, NiFe₂O₄/CCF-3, and NiFe₂O₄/CCF-4. For CCF, wide peaks at approximately 10°–30° and 40–50° indicate amorphous carbon [34]. For NiFe₂O₄/CCF-1, NiFe₂O₄/CCF-2, NiFe₂O₄/CCF-3, and NiFe₂O₄/CCF-4, apart from the diffraction peaks of amorphous carbon, all diffraction peaks correspond to the NiFe₂O₄ spinel phase (JCPDS NO.10-0325). The sharp diffraction peaks indicate the good crystallinity of NiFe₂O₄. From NiFe₂O₄/CCF-1 to NiFe₂O₄/CCF-4, the diffraction peaks of NiFe₂O₄ gradually increase, and the amorphous carbon peaks gradually weaken, indicating the increase in the loading amount of NiFe₂O₄. These results show that the coexistence of NiFe₂O₄ and amorphous carbon can be achieved by this method. In addition, the XRD patterns have no impurity peak, indicating the high purity of NiFe₂O₄ nanoparticles.

The morphology of the synthetic samples was studied by SEM, as shown in Figure 2. Figure 2a,b shows the SEM images of CCF, and it is clear that the CCF is twisted and fibrous with a smooth fiber surface. Figure 2c–i shows the SEM images of NiFe₂O₄/CCF-1, NiFe₂O₄/CCF-2, NiFe₂O₄/CCF-3, and NiFe₂O₄/CCF-4. It is obvious that the NiFe₂O₄ nanoparticles are successfully loaded on CCF. Moreover, the NiFe₂O₄ nanoparticles have octahedral morphology, and the particle size is about 100 nm. From NiFe₂O₄/CCF-1 to NiFe₂O₄/CCF-4, the number of the NiFe₂O₄ nanoparticles on CCF shows an upward trend, indicating the increase in the loading amount of NiFe₂O₄ nanoparticles, which is consistent with the XRD results. The TEM images of the NiFe₂O₄ nanoparticles were shown in Figure S1. It is clear that the NiFe₂O₄ nanoparticles have clear edges and corners, and the size of the vast majority of nanoparticles is in the range of 80–120 nm. Figure 3 shows the EDS element diagram and elemental mapping images of NiFe₂O₄/CCF-2. It displays that the NiFe₂O₄/CCF-2 is mainly composed of C, O, Fe, and Ni elements, and the O, Fe, and Ni elements are displayed along the NiFe₂O₄ nanoparticles.

To quantitatively analyze the loading amount of NiFe₂O₄ nanoparticles on the NiFe₂O₄/carbonized cotton fibers, we studied the TG curves of NiFe₂O₄/carbonized cotton fibers in the air atmosphere, as shown in Figure 4. The TG curves show the mass percentage of the residue in the process of temperature change. Since the carbonized cotton fiber can be burned completely in air atmosphere, and the residual product is only NiFe₂O₄, the loading amount of NiFe₂O₄ nanoparticles can be estimated according to the TG curves. It is estimated that the loading amount of NiFe₂O₄ nanoparticles in NiFe₂O₄/CCF-1, NiFe₂O₄/CCF-2, NiFe₂O₄/CCF-3, and NiFe₂O₄/CCF-4 is 7.6 wt% (weight percentage), 21.0 wt%, 42.3 wt%, and 55.3 wt%, respectively. In addition, the NiFe₂O₄/carbonized cotton fibers have small weight loss below 450 °C, displaying good thermal stability.

To study the magnetic properties of NiFe₂O₄/CCF-1, NiFe₂O₄/CCF-2, NiFe₂O₄/CCF-3, and NiFe₂O₄/CCF-4, we measured the hysteresis loops at room temperature within the magnetic field of −20 kOe–20 kOe, as shown in Figure 5. It can be seen that these four samples all show ferromagnetic behavior. The saturation magnetization (M_S) of NiFe₂O₄/CCF-1, NiFe₂O₄/CCF-2, NiFe₂O₄/CCF-3, and NiFe₂O₄/CCF-4 is 4.7 emu/g, 10.5 emu/g, 22.3 emu/g, and 30.3 emu/g, respectively. It is obvious that the M_S increases with the increase in the loading amount of NiFe₂O₄ nanoparticles, and this result is consistent with the XRD, SEM, and TG results.

From the above analysis, NiFe₂O₄/carbonized cotton fibers with different loading amounts of NiFe₂O₄ nanoparticles were successfully prepared. Generally speaking, electromagnetic parameters are the most direct characterization parameters of the interaction between absorbing materials and electromagnetic waves, so the regulation of electromagnetic parameters is the most direct and effective means of designing absorbing materials. The most important electromagnetic parameters are complex permittivity (ε_r = ε′ − jε″) and complex permeability (μ_r = μ′ − jμ″). ε′ and ε″, respectively, represent the capacity and loss of electric field energy, while μ′ and μ″, respectively, represent the storage and loss of magnetic energy [35,36]. Figure 6 shows the complex permittivity and complex permeability of CCF, NiFe₂O₄/CCF-1, NiFe₂O₄/CCF-2, NiFe₂O₄/CCF-3, and NiFe₂O₄/CCF-4 with the filling ratio of 25 wt% in paraffin matrix. As can be seen from Figure 6a,b, the ε′ all show a downward trend in the frequency range of 2–18 GHz. This is a typical frequency dispersion phenomenon, which widely exists in carbon materials and is conducive to microwave absorption [37,38]. In addition, the ε′ decreases with the increase in NiFe₂O₄ content owing to the poor conductivity of NiFeO₄ nanoparticles. This is because the introduction of the NiFeO₄ nanoparticles into carbonized cotton fibers hinders the electron migration, resulting in fewer conductive network connections and reduced conductivity of the composite. For NiFe₂O₄/CCF-4, the ε″ is stable at about 2.1 in the frequency range of 2–18 GHz, while the ε” slowly decreases from 7.6, 7.4, 4.4, and 3.5 to 5.1, 4.9, 3.4, and 2.4, respectively, for CCF, NiFe₂O₄/CCF-1, NiFe₂O₄/CCF-2, and NiFe₂O₄/CCF-3. According to the Debye theory, the relationship between ε′ and ε″ can be further deduced as the following formula: [39,40]. where ε_s and ε_∞ are static dielectric constant and dielectric constant at infinite frequency, respectively. Therefore, the Debye relaxation process can be represented by the Cole–Cole semicircle, and a semicircle represents a relaxation process. As shown in Figure S2, the CCF, NiFe₂O₄/CCF-1, NiFe₂O₄/CCF-2, and NiFe₂O₄/CCF-3 display two Cole–Cole semicircles, and the NiFe₂O₄/CCF-4 displays three Cole–Cole semicircles. The distinct Cole–Cole semicircles indicate the occurrence of the polarization relaxation process, and NiFe₂O₄/CCF-4 shows a stronger polarization relaxation behavior. As can be seen from Figure 6c,d, the μ′ and μ″ are very low for CCF, showing a weak storage and loss of magnetic energy in the range of 2–18 GHz. Moreover, the μ″ shows three vibration peaks at about 7 GHz, 12 GHz, and 17 GHz, and increases with the increase in NiFe₂O₄ content due to the magnetic properties of NiFe₂O₄ nanoparticles. Generally speaking, the magnetic loss is usually related to natural resonance, exchange resonance, and eddy current loss [41]. Eddy current loss can be determined by C₀ (C₀ = μ″μ′⁻²f⁻¹ = 2πμ₀σd²/3) [42,43]. If the main reason for the magnetic loss is eddy current loss, the value of C₀ is constant. The C₀ displays a similar vibration trend to that of μ″ in the frequency range 2–18 GHz (Figure S3), confirming that the natural resonance and exchange resonance are the main magnetic loss mechanism.

Generally speaking, a RL value lower than −10 dB means more than 90% microwave absorption. Only absorbent with RL lower than −10 dB can be used in practice, so the effective bandwidth represents the width of the frequency range when RL = −10 dB. According to the formulas (1) and (2) above, the RL is closely related to the thickness of the absorbent and the frequency of the electromagnetic wave. The minimum RL and effective bandwidth are obtained from the RL curve, so the minimum RL and effective bandwidth of microwave absorption are closely related to the thickness of the absorbent. Therefore, we simulated the RL curves of CCF, NiFe₂O₄/CCF-1, NiFe₂O₄/CCF-2, NiFe₂O₄/CCF-3, and NiFe₂O₄/CCF-4 at different thicknesses, as shown in Figure 7. As can be seen from Figure 7a, for CCF, the effective bandwidth is 4.5 GHz (13.5–18 GHz) at a thickness of 1.5 mm. For NiFe₂O₄/CCF-1, when the thickness is 2 mm, the effective bandwidth is also 4.5 GHz (10.5–15 GHz). For NiFe₂O₄/CCF-2, when the thickness is 2 mm, the effective bandwidth can reach 5.8 GHz (11.7–17.5 GHz). For NiFe₂O₄/CCF-3, when the thickness is 2.5 mm, the effective bandwidth reaches 6.5 GHz (11.3–17.8 GHz) with a minimum RL of −33.2 dB; when the thickness rises to 3 mm, the minimum RL is −45.3 dB. For NiFe₂O₄/CCF-4, the minimum RL is −18.8 dB with an effective bandwidth of 5.6 GHz (11.9–17.5 GHz). In addition, as shown in Figure 7a–e, it can be seen that the RL peak moves to the low-frequency region with increasing thickness. Figure 7f shows the RL curves at 2.5 mm for these five samples. Obviously, the RL peak shifts to the high-frequency region by decreasing the complex permittivity under the same thickness by comparing these five samples, and this can be explained by the geometric effect [44,45]. When the thickness (t_m) of the absorbent follows the 1/4 wavelength model: t_m = nλ₀/4(|μ_r||ε_r|)^1/2 = nc/4f_m(|μ_r||ε_r|)^1/2(n =1,3,5, …), the input impedance is the same as the air wave impedance, and the RL (dB) reaches a minimum value. Simulation and calculation of the absorber thickness (t_m) versus peak frequency (f_m) for these five samples are shown in Figure S4, and it is obvious that the calculated results agree well with the simulated values. For the 1/4 wavelength model, the f_m is inversely proportional to the t_m and complex permittivity. For the same sample with the same μ_r and ε_r, the f_m is inversely proportional to the t_m, so the RL curve moves to the low frequency with increasing the thickness. For different sample in the same thickness, the RL curve moves to the high frequency with decreasing the electromagnetic parameters. Thus, from CCF to NiFe₂O₄/CCF-4, it is understandable that the optimal RL moves to the high thickness with decreasing the complex permittivity for these five samples. As an essential standard for evaluating microwave absorption performance, the minimum RL value and effective bandwidth are also compared with other reported carbon-based absorbents, as shown in Table 1 [46,47,48,49,50,51,52,53]. Obviously, the NiFe₂O₄/carbonized cotton fibers show wider effective bandwidth and stronger RL with a low filling rate. What is more, compared with the microwave absorbents such as ferrite, magnetic metal, magnetic metal oxide, and alloy, the NiFe₂O₄/CCF has a lower density; compared with the preparation of carbon fiber, the CCF has a very simple preparation method [54]. In a word, the use of cotton fibers, simple preparation methods, and excellent microwave absorption performance give NiFe₂O₄/CCF broad application prospects as an economical, lightweight, and broadband microwave absorbent.

Impedance matching and electromagnetic loss are two key factors affecting microwave absorption performance. The impedance matching can be expressed by the impedance matching coefficient (Z = Z_in/Z₀ = (μ_r/ε_r)^1/2) [55,56]. The closer the Z is to 1, the better the impedance matching is. Recent studies have found that the ε_r value is much higher than the μ_r value for carbon materials [57,58]. Therefore, the decrease in the ε_r can increase the impedance matching coefficient and improve the impedance matching of the absorbent. Moreover, the electromagnetic loss can be determined by the dielectric loss factor (tanδ_e = ε″/ε′) and magnetic loss factor (tanδ_m = μ″/μ′) [59,60]. Figure 8 shows the impedance matching coefficient, tanδ_e, and tanδ_m of CCF, NiFe₂O₄/CCF-1, NiFe₂O₄/CCF-2, NiFe₂O₄/CCF-3, and NiFe₂O₄/CCF-4. As seen in Figure 8a, the Z follows the order of CCF < NiFe₂O₄/CCF-1 < NiFe₂O₄/CCF-2 < NiFe₂O₄/CCF-3 < NiFe₂O₄/CCF-4, which is opposite to the complex permittivity. So the existence of NiFe₂O₄ with relatively lower conductivity improves the percolation threshold in the NiFe₂O₄/biomass-based carbon fibers system and hence maintains a good impedance matching [61]. As seen in Figure 8b,c, it is obvious that the tanδ_e is much larger than the tanδ_m, showing that the dielectric loss plays a major role. What is more, the tanδ_e shows an upward trend in the frequency of 2–18 GHz, which is highly beneficial to high-frequency microwave absorption. The tanδ_m shows a similar tendency to that of μ″, showing that the magnetic loss is mainly influenced by the imaginary part of the complex permeability. For CCF and NiFe₂O₄/CCF-1, although the tanδ_e is higher than 0.4 in the frequency of 2–18 GHz, the impedance matching coefficient is much lower than that of NiFe₂O₄/CCF-3, so the microwave absorption performance is slightly worse. For NiFe₂O₄/CCF-4, although the impedance matching is the best, the electromagnetic loss is relatively small, which is not conducive to microwave absorption. For NiFe₂O₄/CCF-3, the tanδ_e is only lower than NiFe₂O₄/CCF-1, while the tanδ_m is only lower than NiFe₂O₄/CCF-4, so the NiFe₂O₄/CCF-3 has relatively high electromagnetic loss. In addition, the low value of −45.3 dB at 11 GHz may be attributed to the high tanδ_e and tanδ_m value in the range of 10–14 GHz. Therefore, the excellent microwave absorption performance of NiFe₂O₄/CCF-3 can be attributed to the appropriate loading amount of NiFe₂O₄ nanoparticles with moderate electromagnetic parameters, and the balance of impedance matching and electromagnetic loss.

During the research, NiFe₂O₄/carbonized hemp fibers were also successfully prepared by changing the cotton fibers to hemp fibers under the same conditions. Figure 9a shows the XRD spectrum of NiFe₂O₄/CHF. In addition to the diffraction peaks of NiFe₂O₄, the wide peaks at 10°–30° and 40–50° are the diffraction peaks of carbonized hemp fibers. Figure 9b shows the TG curves of NiFe₂O₄/CHF in the air atmosphere, revealing that the loading amount of NiFe₂O₄ nanoparticles in NiFe₂O₄/CHF is 8.3 wt%. Figure 10a–d show the SEM images of NiFe₂O₄/CHF at different magnifications. From the SEM images, we can observe that the NiFe₂O₄ nanoparticles are attached to the surface of the carbonized hemp fibers. Figure 10e–i show the element diagram and elemental mapping images of NiFe₂O₄/CHF in Figure 10d. It displays that the NiFe₂O₄/CHF is mainly composed of C, O, Fe, and Ni elements, and the Fe, Ni, and O elements are distributed along the NiFe₂O₄ nanoparticles on the carbonized hemp fibers. In addition, when the hemp fibers were changed to bamboo fibers under the same conditions, NiFe₂O₄ nanoparticles were also loaded on the carbonized bamboo fibers. Moreover, the microwave absorption performance of the NiFe₂O₄/carbonized bamboo fibers was also investigated (See Supporting Information Figure S5–S7). In general, the experimental results indicate that this facile self-assembly technology is universally applicable in designing other types of carbon matrix composites.

The electromagnetic properties of NiFe₂O₄/CHF dispersed in paraffin matrix with 33 wt % are shown in Figure 11. Figure 11a illustrates the complex permittivity of NiFe₂O₄/CHF. The ε′ decreases from 9.4 to 5.5, and the ε″ is in the range of 2.1–2.6 with three vibration peaks. Figure 11b shows the complex permeability of NiFe₂O₄/CHF. Interestingly, the μ″ shows an opposite trend to ε″, showing that there is an obvious electromagnetic synergistic effect, which is conducive to electromagnetic loss. Figure 11c displays the simulated RL curve of NiFe₂O₄/CHF at different thicknesses. The effective bandwidth is 5.85 GHz (12.15–18 GHz) with a minimum RL of −23.1 dB at a thickness of 2.0 mm. Figure 11d shows the impedance matching coefficient of NiFe₂O₄/CHF, which shows an upward trend in the frequency range 2–18 GHz, implying that the well-matched characteristic impedance contributes to high-frequency microwave absorption. Figure 11e illustrates the dielectric loss factor and magnetic loss factor. The curves of dielectric loss tangent and magnetic loss tangent are similar to ε″ and μ″, respectively. This illustrates that the loss tangent is mainly affected by the imaginary part of the complex permittivity and permeability. Moreover, the dielectric and magnetic loss tangent present opposite vibration peaks, indicating a strong electromagnetic coupling effect [62]. In addition, the dielectric loss tangent gradually increases, which is in favor of high-frequency electromagnetic loss. Figure 11f corresponds to the Cole–Cole curve of NiFe₂O₄/CHF, which has three Cole–Cole semicircles, meaning that it has multiple relaxation processes and promoted interfacial polarization. Such polarization processes are conducive to dielectric loss. In a word, the impedance matching, and the synergy between dielectric loss and magnetic loss contribute to the enhanced microwave absorption performance of NiFe₂O₄/CHF.