Tri-layer Co@Co_xFe_1−x@Fe@Fe₃O₄ thorny core–shell composite particles and their electromagnetic absorption properties

Abstract

By the combination of chemical liquid-phase reduction, in situ self-oxidation and thermal treatment, tri-layer Co@Co_xFe_1−x@Fe@Fe₃O₄ thorny core–shell composite magnetic particles were synthesized, and their microstructure, static magnetic properties, and electromagnetic wave absorption performance were investigated. Results show that the tri-layer Co@Co_xF_1−x@Fe@Fe₃O₄ thorny particles largely retained the morphology of thorny Co@Fe@Fe₃O₄ particles, but some particles are aggregated, resulting in an increase in the particle size. The tri-layer Co@Co_xF_1−x@Fe@Fe₃O₄ thorny particles exhibit a typical core–shell structure with an internal core and 200–300 nm thick external coating. The center of the particle contains Co, while the outer layer comprises Fe and O. With heat treatment, a discernible transition phase of CoFe and Co₇Fe₃ was formed at the Co/Fe interface. With the increase in self-oxidation temperature, the specific saturation magnetization of the Co@Co_xF_1−x@Fe@Fe₃O₄ particles exhibits a slight increase before a downward trend, while the coercivity decreases slightly and then increases. At an oxidation temperature of 70 °C, the tri-layer Co@Co_xFe_1−x@Fe@Fe₃O₄ thorny particle samples exhibit optimal absorption performance, with a minimum reflection loss of −24.42 dB at a coating thickness of 1.6 mm and a maximum effective absorption bandwidth of 4.80 GHz.

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1 Introduction

Electromagnetic wave absorption materials are a class of functional materials capable of effectively absorbing and attenuating the energy of incident electromagnetic waves. The primary mechanism underlying the absorption of electromagnetic waves in these materials involves the conversion of electromagnetic energy into thermal energy or other forms of energy dissipation through dielectric loss, magnetic loss, and impedance matching characteristics.^1–4

Dielectric loss arises from the polarization relaxation of electric dipoles within the material under the influence of an alternating electromagnetic field. This polarization process exhibits a phase difference relative to the external electromagnetic field, resulting in the conversion of electromagnetic energy into thermal energy, which is subsequently dissipated. Magnetic loss occurs through irreversible magnetic hysteresis, magnetic domain movement, and phenomena such as magnetic resonance within the material under an alternating magnetic field, leading to the conversion of magnetic energy into thermal energy or other forms of energy, thereby facilitating effective absorption of electromagnetic wave energy.^5–10

Magnetic wave-absorbing materials constitute an important component of the electromagnetic wave absorption material system and primarily consist of magnetic metals or their alloys, ferrites, and other components with magnetic loss characteristics. Compared with dielectric wave-absorbing materials or other types of absorbers, magnetic wave-absorbing materials offer significant advantages, including high magnetic permeability, broad bandwidth absorption, low reflection loss, and excellent impedance matching properties.^11–14 These materials can generate significant hysteresis loss, eddy current loss, and natural resonance loss under the influence of high-frequency electromagnetic fields, effectively dissipating electromagnetic wave energy. Simultaneously, their synergistic control over complex magnetic permeability and complex permittivity facilitates better impedance matching with free space, reducing the reflection of electromagnetic waves at the material's surface and enhancing the absorption efficiency.^15–18

Cobalt, as an important magnetic material, possesses a high Curie temperature, excellent magnetic crystalline anisotropy, and good chemical stability, making it widely applicable in fields such as magnetic storage, sensors, and permanent magnets.^19–21 As an absorber material, cobalt exhibits favorable magnetic permeability, permittivity, and impedance matching characteristics, while also demonstrating improved thermal stability at elevated temperatures, thereby optimizing its wave absorption performance. These properties highlight cobalt's significant application prospects and immense potential in electromagnetic wave absorption, stealth technology, and electromagnetic shielding.^22–25

Extensive research indicates that core–shell structured particles exhibit superior performance in the domain of wave-absorbing materials. This exceptional capability primarily stems from their unique structural design, which effectively regulates the matching characteristics of electromagnetic parameters and enhances the loss capacity of electromagnetic waves through interface polarization and multiple scattering mechanisms.^26–28 Compared to single-layer-coated core–shell structures, multilayer-coated core–shell structures offer advantages such as more complex interface designs, greater flexibility in tuning electromagnetic parameters, and optimized impedance matching characteristics. By incorporating various loss mechanisms, such as conductive loss, relaxation loss, and resonance loss, these multilayer structures can further enhance the wave absorption performance of the materials.^29–32

Sun et al.³³ successfully synthesized novel CuNC composite CMOF particles by embedding copper nanoclusters (CuNC) into cobalt/zinc (Co/Zn) CMOF. When the matching thickness was 2.30 mm, the RL_min reached −45.1 dB, and when the thickness was 3.10 mm, the EAB reached 8.80 GHz. Liu et al.³⁴ successfully synthesized a Co@CoO@microwave-irradiated porous carbon (MPC) composite material with a bilayer core–shell structure, with RL_min reaching −30 dB and EAB reaching 5.7 GHz. Sun et al.³⁵ modified magnetic permalloy particles by coating 2-methylimidazole cobalt salt (ZF-67) and 2-methylimidazole zinc salt (ZF-8). Composite particles of PM@ZIF67, PM@ZIF8, PM@ZIF67@ZIF8 and PM@ZIF8@ZIF67, with different core–shell structures, were successfully synthesized. Their minimum reflection loss and effective absorption bandwidth reached −47.3 dB and 5.3 GHz at thicknesses of 3.8 mm and 5.5 mm, respectively.

Moreover, studies have demonstrated that particle structures with unconventional geometries (e.g., thorny or flowered shapes) significantly enhance the electromagnetic wave absorption performance. This improvement is primarily attributed to their unique morphological features, which effectively manipulate the propagation pathways of electromagnetic waves and mechanisms of energy dissipation.^36–38 Irregular structures can increase the specific surface area and the number of interfaces within the material, which, in turn, enhances the multiple reflections and scatterings of electromagnetic waves within the material. This leads to extended propagation paths of electromagnetic waves and improved energy loss efficiency. Additionally, structures with thorny or flowered formations can introduce more tip effects and localized electric field concentrations, thereby enhancing the local polarization effect and dielectric loss. Furthermore, the geometric asymmetry of these irregular structures can disrupt the electromagnetic isotropy of the material, optimizing magnetic anisotropy and domain structures to enhance magnetic loss capacity. Therefore, irregular particle structures demonstrate significant application potential in the design of wave-absorbing materials.^39–41

However, research on heterogeneous cobalt-based multilayer core–shell structured wave-absorbing materials remains relatively limited, particularly regarding the specific case of three-layer coated heterogeneous cobalt-based core–shell materials, for which significant reports are yet to be seen.^42–45 This paucity of research largely arises from the complexities associated with the fabrication processes of multilayer core–shell structures, including the selection of materials for each layer, interface compatibility, and structural stability. Due to the absence of systematic experimental data and theoretical analyses, the scientific inquiries into the electromagnetic wave absorption mechanisms, interface effects, and multi-interface synergistic interactions of three-layer coated heterogeneous cobalt-based core–shell structures have yet to be thoroughly explored. This research gap not only limits a deeper understanding of the relationship between the microstructure and macroscopic properties of cobalt-based composite wave-absorbing materials but also hinders their further development and application in the field of wave-absorbing materials.

In light of this, the present study focuses on multilayer-coated heterogeneous cobalt-based wave-absorbing materials. Through chemical liquid-phase deposition, high-temperature heat treatment, and in situ self-oxidation, we prepared tri-layer coated Co@Co_xFe_1−x@Fe@Fe₃O₄ composite particles and compared the effects of different self-oxidation temperatures on the electromagnetic wave absorption performance of Co@Co_xFe_1−x@Fe@Fe₃O₄ composite particles. This study aims to provide valuable insights for the systematic investigation of tri-layer coated heterogeneous cobalt-based core–shell structured wave-absorbing materials, as well as contribute to the theoretical development and practical applications of cobalt-based wave-absorbing materials.

2 Preparation and characterization of tri-layer Co@Co_xFe_1−x@Fe@Fe₃O₄ thorny particles

2.1 Preparation of tri-layer Co@Co_xFe_1−x@Fe@Fe₃O₄ thorny particles

The preparation of tri-layer Co@Co_xFe_1−x@Fe@Fe₃O₄ thorny particles started with the synthesis of thorny Co particles. Cobalt(II) sulfate heptahydrate (CoSO₄·7H₂O) was dissolved to prepare a CoSO₄ solution, and potassium sodium tartrate (C₄H₄O₆KNa·4H₂O) was dissolved to form a C₄H₄O₆KNa solution. Then, the two solutions were mixed fully. The temperature of the reaction mixture was controlled at 50 °C with an ultrasonic generator. Under mechanical stirring and ultrasonic vibration, concentrated hydrazine hydrate (N₂H₄·H₂O) solution was added to the reaction mixture, facilitating a rapid reaction that yielded thorny Co particles. The reaction equations are as follows:

Co²⁺ + n(C₄O₆H₄)²⁻ ↔ [CO(C₄O₆H₄)]²⁻²ⁿ_n (1)

[CO(C₄O₆H₄)]²⁻²ⁿ_n + 3N₂H₄ → [CO(N₂H₄)₃]²⁺ + n(C₄O₆H₄)²⁻ (2)

[CO(N₂H₄)₃]²⁺ + N₂H₄ + OH⁻ → Co↓ + N₂↑ + H₂↑ + NH₃ + O (3)

Building upon this, a specific mass of ferrous sulfate heptahydrate (FeSO₄·7H₂O), potassium sodium tartrate (C₄H₄O₆KNa·4H₂O), and trisodium citrate (C₆H₅Na₃O₇·2H₂O) were dissolved in 500 mL of deionized water to form the coating solution. Sodium borohydride (NaBH₄) was dissolved to prepare the reducing solution. At a reaction temperature of 50 °C, the reducing solution was added to the coating solution, followed by washing with deionized water to obtain Co@Fe particles. The chemical reactions are represented as follows:

Fe²⁺ + n(C₄O₆H₄)²⁻ ↔ [Fe(C₄O₆H₄)²⁻²ⁿ_n] (4)

[Fe(C₄O₆H₄)²⁻²ⁿ_n] + 2BH₄⁻ + 6H₂O → Fe↓ + 2B(OH)₃ + 7H₂↑ + n(C₄O₆H₄)²⁻ (5)

Based on this, sodium nitrite (NaNO₂) was used as the oxidizing agent, and an appropriate amount of sodium hydroxide (NaOH) was added to adjust the pH. The Co@Fe particles undergo self-oxidation in the water bath environment at temperatures of 50 °C, 60 °C, 70 °C, and 80 °C, resulting in partial oxidation to Fe₃O₄ and yielding dual-layer thorny Co@Fe@Fe₃O₄ particles. The chemical reaction equations for this process are:

3Fe + NaNO₂ + 5NaOH → 3Na₂FeO₂ + NH₃↑ + H₂O (6)

6Na₂FeO₂ + NaNO₂ + 5H₂O → 3Na₂FeO₂ + NH₃↑ + 7NaOH (7)

Na₂FeO₂ + Na₂FeO₄ + 2H₂O → Fe₃O₄ + 4NaOH (8)

For the dual-layer Co@Fe@Fe₃O₄ thorny particles, heat treatment was performed at 500 °C for 1 hour, and tri-layer Co@Co_xFe_1−x@Fe@Fe₃O₄ thorny particles were finally produced. The heat-treated particles are designated as 50 °C-R, 60 °C-R, 70 °C-R, and 80 °C-R, respectively.

2.2 Characterization of tri-layer Co@Co_xFe_1−x@Fe@Fe₃O₄ thorny particles

The Co@Co_xFe_1−x@Fe@Fe₃O₄ composite particles were observed with scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The crystallographic structure of the samples was determined by X-ray diffraction (XRD). The static magnetic properties of the prepared samples were measured with a vibrating sample magnetometer (VSM), and the electromagnetic wave absorption properties were studied with a vector network analyzer.

3 Results and discussion

3.1 Morphology and phase structure of tri-layer Co@Co_xFe_1−x@Fe@Fe₃O₄ thorny particles

Fig. 1 shows SEM images of thorny Co and thorny Co@Fe particles. The thorny Co particles demonstrate a nearly equiaxed morphology, featuring a surface adorned with numerous sharp, thorny protrusions, and possess a particle diameter ranging approximately from 3 to 5 µm. The thorny Co@Fe particles, synthesized from the thorny Co particles, retain the geometric configuration of the initial thorny Co particles. In contrast to the original sword-shaped Co particles, the surface protrusions of the thorny Co@Fe particles remain pronounced, albeit with their tips being somewhat blunted.

Fig. 1 SEM images of (a) Co particles and (b) Co@Fe particles.

Fig. 2 presents the SEM images of thorny Co@Fe@Fe₃O₄ particles with in situ self-oxidation at 50 °C, 60 °C, 70 °C, and 80 °C water-bath temperatures. It can be seen that during the self-oxidation process, the thorny Co@Fe@Fe₃O₄ particles are further passivated at the tip, and the surface becomes less smooth at the same time, which is mainly due to part of the Fe₃O₄ that formed in the in situ self-oxidation process entering the solution.

Fig. 2 SEM images of thorny Co@Fe@Fe₃O₄ particles with different in situ self-oxidation temperature (a) 50 °C; (b) 60 °C; (c) 70 °C; and (d) 80 °C.

Fig. 3 presents SEM images of the tri-layer Co@Co_xF_1−x@Fe@Fe₃O₄ thorny particles. It can be observed that these particles have largely retained the morphology of the Co@Fe@Fe₃O₄ particles. However, during the heat-treatment process in the preparation of Co@Co_xF_1−x@Fe@Fe₃O₄ from Co@Fe@Fe₃O₄ particles, some particles aggregated, resulting in an increase in the particle size.

Fig. 3 SEM images showing the morphology of tri-layer Co@Co_xF_1−x@Fe@Fe₃O₄ thorny particles prepared with different in situ self-oxidation temperatures: (a and b) 50 °C; (c and d) 60 °C; (e and f) 70 °C; and (g and h) 80 °C.

To further investigate the internal material characteristics of tri-layer Co@Co_xF_1−x@Fe@Fe₃O₄ thorny particles, Fig. 4 presents SEM images and cross-sectional EDS analysis results of the tri-layer Co@Co_xF_1−x@Fe@Fe₃O₄ thorny particles obtained with a focused ion beam (FIB). In Fig. 4(b), it can be seen that the Co@Co_xF_1−x@Fe@Fe₃O₄ particles exhibit a typical core–shell structure with an internal core and 200–300 nm thick external coating. The EDS line scan results obtained from the FIB cross-section (Fig. 4(c)) indicate that the center of the particle contains Co, while the outer layer is composed of Fe and O. Fig. 4(d)–(f) show the elemental distribution maps of Co, Fe, and O within the particle, demonstrating that Fe and O are mainly located in the outer layer.

Fig. 4 Thorny Co@Co_xF_1−x@Fe@Fe₃O₄ particles prepared under oxidation at 70 °C: (a) SEM image; (b) cross-sectional SEM image; (c) elemental distribution line scan; and (d)–(f) element analysis maps.

3.2 Phase structure of tri-layer Co@Co_xF_1−x@Fe@Fe₃O₄ thorny particles

Fig. 5 presents the XRD patterns of Co@Co_xF_1−x@Fe@Fe₃O₄ thorny particles. Compared to standard cards for FCC-Co (PDF#15-0806), HCP-Co (PDF#05-0727), Fe₃O₄ (PDF#88-0866), CoFe (PDF#44-1433), Co₇Fe₃ (PDF#50-0795), and Fe (PDF#06-0696), the XRD patterns of the particles that underwent heat treatment at oxidation temperatures of 70 °C and 80 °C exhibit characteristic diffraction peaks at 2θ = 30.1°, 35.5°, 57.1°, and 62.5°, corresponding to the (220), (311), (511), and (440) crystal planes of Fe₃O₄, respectively. For the particles that were heat-treated with oxidation temperatures of 50 °C and 60 °C, the characteristic diffraction peaks previously corresponding to the (220), (511), and (400) planes of Fe₃O₄ disappeared, leaving only the (311) plane observable. The diffraction peak at 2θ = 45.1° corresponds to the (110) planes of CoFe, α-Fe, and Co₇Fe₃, while the peak at 2θ = 65.7° corresponds to the (200) planes of CoFe and Co₇Fe₃. The peak at 2θ = 83.1° corresponds to the (211) planes of CoFe and Co₇Fe₃. The XRD results indicate that the heat treatment of the Co@Fe@Fe₃O₄ thorny particles produced the CoFe and Co₇Fe₃ compounds, which exhibit higher saturation magnetization. As CoFe and Co₇Fe₃ form at the Co/Fe interface, it is suggested that one of the two original interfaces in the particles has become indistinct due to the heat treatment, which is likely the Co/Fe interface.

Fig. 5 XRD patterns of tri-layer Co@Co_xF_1−x@Fe@Fe₃O₄ thorny particles.

3.3 Magnetic properties of tri-layer Co@Co_xFe_1−x@Fe@Fe₃O₄ thorny particles

Fig. 6 illustrates the hysteresis loops of the tri-layer Co@Co_xF_1−x@Fe@Fe₃O₄ thorny particles. Table 1 summarizes the saturated magnetization (M_s), coercive force (H_c), and remnant magnetization (M_r) values obtained from the hysteresis loops of theCo@Co_xF_1−x@Fe@Fe₃O₄ particles.

Fig. 6 Hysteresis loops of tri-layer Co@Co_xF_1−x@Fe@Fe₃O₄ thorny particles.

Table 1 Static magnetic properties of tri-layer Co@Co_xF_1−x@Fe@Fe₃O₄ thorny particles

Sample	M s (emu g^−1)	H c (Oe)	M r (emu g^−1)
50 °C-R	166.4	74.0	7.6
60 °C-R	171.2	67.7	7.6
70 °C-R	173.2	87.4	7.2
80 °C-R	154.6	96.0	9.5

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As shown in Table 1, with the increase in self-oxidation temperature, the specific saturation magnetization of the particles exhibits a slight increase before showing a downward trend, while the coercivity decreases slightly and then increases. This behavior can be attributed to the static magnetic properties of Co@Co_xF_1−x@Fe@Fe₃O₄, which are primarily influenced by the four components: Co, Co_xF_1−x, Fe, and Fe₃O₄. Through in situ self-oxidation, Fe₃O₄ with lower saturation magnetization and higher coercivity is introduced into the Co@Fe particles. As the oxidation temperature rises, the content of Fe₃O₄ gradually increases, leading to a decrease in specific saturation magnetization and an increase in coercivity. Additionally, during the high-temperature heat-treatment process, the formation of CoFe and Co₇Fe₃ phases at the Co/Fe interface also influences the specific saturation magnetization and coercivity, so the observed decrease in specific saturation magnetization and increase in coercivity exhibit fluctuations.

3.4 Electromagnetic wave absorption properties of tri-layer Co@Co_xF_1−x@Fe@Fe₃O₄ thorny particles

3.4.1 Complex dielectric constant and complex permeability of tri-layer Co@Co_xFe_1−x@Fe@Fe₃O₄ thorny particles. Fig. 7 illustrates the variations of the real part of the complex dielectric constant (ε′) and the imaginary part of the complex dielectric constant (ε″) for the Co@Co_xFe_1−x@Fe@Fe₃O₄ thorny particle samples within the frequency range of 2–18 GHz.

Fig. 7 (a) Real part of the dielectric constant ε′ and (b) imaginary part of the dielectric constant ε″ for Co@Co_xF_1−x@Fe@Fe₃O₄ thorny samples.

From the figure, it can be observed that the real part of the complex dielectric constant (ε′) for the Co@Co_xFe_1−x@Fe@Fe₃O₄ thorny samples displays a gradually decreasing trend with increasing frequency of electromagnetic waves. Specifically, under oxidation temperatures of 50 °C, 60 °C, 70 °C, and 80 °C, the ranges of the ε′ values for the samples are 10.04–9.42, 10.12–9.85, 11.14–9.80, and 9.85–8.78, respectively. As the oxidation temperature increases, the maximum ε′ value of the Co@Co_xFe_1−x@Fe@Fe₃O₄ thorny samples first increases and then decreases. Within the frequency range of 2–14 GHz, the sample treated at an oxidation temperature of 70 °C exhibits the highest ε′ value, indicating superior charge storage capability. In the range of 14–18 GHz, the ε′ value of the samples of 60 °C is the highest, suggesting enhanced charge storage ability in this frequency range.

In the same frequency range (2–18 GHz), the ε″ values for the Co@Co_xFe_1−x@Fe@Fe₃O₄ thorny samples at oxidation temperatures of 50 °C, 60 °C, 70 °C, and 80 °C are in the ranges of 0.22–0.66, 0.41–1.46, 0.72–1.34, and 0.56–0.67, respectively. With increasing oxidation temperature, the ε″ values also exhibit a trend of first increasing and then decreasing. Within the range of 2–15.44 GHz, the samples treated at 70 °C show the highest ε″ values, featuring two pronounced dielectric resonances at 8.8 GHz and 14.16 GHz. In the frequency range of 15.44–18 GHz, the samples with 60 °C exhibit higher ε″ values.

Fig. 8 illustrates the behaviors of the real part of complex permeability (μ′) and the imaginary part of complex permeability (μ″) for the Co@Co_xFe_1−x@Fe@Fe₃O₄ thorny samples in the frequency range of 2–18 GHz.

Fig. 8 (a) Real part of permeability μ′ and (b) imaginary part of permeability μ″ for Co@Co_xFe_1−x@Fe@Fe₃O₄ thorny samples.

The results indicate that with the increase of electromagnetic wave frequency, the real part of complex permeability (μ′) presented a decreasing trend, and in the frequency range of 2–10 GHz, at oxidation temperatures of 50 °C, 60 °C, 70 °C, and 80 °C, the μ′ values for the samples are basically the same, with the differences being relatively small. In the range of 10–18 GHZ, the differences of samples with different in situ self-oxidation temperatures gradually increased, and the real part of complex permeability (μ′) of the in situ self-oxidation samples at 60 °C was the lowest. The real part of complex permeability (μ′) of the in situ self-oxidation samples at 80 °C was relatively high, and the whole real part of complex permeability (μ′) of the samples ranged from 1.54 to 0.99.

In the frequency range of 2–14 GHz, the imaginary part of complex permeability (μ″) for the Co@Co_xFe_1−x@Fe@Fe₃O₄ thorny samples exhibits a decreasing trend at oxidation temperatures of 50 °C, 60 °C, 70 °C, and 80 °C. In the range of 14–18 GHz, the μ″ values for the samples treated at 50 °C and 80 °C remain stable, whereas the μ″ value for the samples treated at 70 °C shows a slight increase, and a substantial decrease is noted for the samples treated at 60 °C.

3.4.2 Dielectric loss and magnetic loss of Co@Co_xFe_1−x@Fe@Fe₃O₄ thorny particles. Fig. 9 shows the variations of dielectric loss (tan δ_e) and magnetic loss (tan δ_m) values of the Co@Co_xFe_1−x@Fe@Fe₃O₄ thorny particle samples as a function of electromagnetic wave frequency within the range of 2–18 GHz. The maximum dielectric loss values (tan δ_e) for samples at oxidation temperatures of 50 °C, 60 °C, 70 °C, and 80 °C were found to be 0.069, 0.152, 0.133, and 0.078, respectively. Correspondingly, the maximum magnetic loss values (tan δ_m) were 0.303, 0.295, 0.311, and 0.271. This indicates that both dielectric loss and magnetic loss mechanisms coexist in the Co@Co_xFe_1−x@Fe@Fe₃O₄ thorny particle samples. The magnetic loss (tan δ_m) values of the Co@Co_xFe_1−x@Fe@Fe₃O₄ thorny particle samples in the 2–18 GHz frequency were greater than the dielectric loss (tan δ_e) values, indicating that magnetic loss remains the primary contributor to electromagnetic wave attenuation for the samples.

Fig. 9 (a) Dielectric loss (tan δ_e) and (b) magnetic loss (tan δ_m) of Co@Co_xFe_1−x@Fe@Fe₃O₄ thorny particle samples.

Within the 2–18 GHz frequency range, the tan δ_e value exhibited minimal variation; however, the values across most frequency bands presented increasing trends. In the frequency range of 2–12 GHz, the magnetic loss decreases with the increase in the in situ self-oxidation temperature, and the regularity is no longer obvious in the frequency range of 12–18 GHz.

3.4.3 Electromagnetic wave loss mechanism of Co@Co_xFe_1−x@Fe@Fe₃O₄ thorny particles. Fig. 10 presents the Cole–Cole curves of the Co@Co_xFe_1−x@Fe@Fe₃O₄ thorny particle samples. Multiple semi-circular arcs were observed in all samples, indicating a pronounced dielectric relaxation polarization phenomenon. The presence of linear portions at the ends of the Cole–Cole arcs in all four sample groups indicates the existence of conduction losses in the Co@Co_xFe_1−x@Fe@Fe₃O₄ thorny particle samples.

Fig. 10 Cole–Cole curves of Co@Co_xFe_1−x@Fe@Fe₃O₄ thorny particle samples at (a) 50 °C, (b) 60 °C, (c) 70 °C, and (d) 80 °C.

For the Co@Co_xFe_1−x@Fe@Fe₃O₄ thorny particles, diffusion between Co and Fe led to the formation of the Co_xFe_1−x phase, resulting in the generation of Co/Co_xFe_1−x and Co_xFe_1−x/Fe interfaces. Simultaneously, the oxidation of the Fe surface resulted in the formation of a Fe₃O₄ layer, establishing a Fe/Fe₃O₄ interface that significantly facilitated interfacial polarization in the particles.

Fig. 11(a) displays the magnetic loss C₀ and the decay constant α of the Co@Co_xFe_1−x@Fe@Fe₃O₄ thorny particle samples. The C₀ values of the samples exhibited a monotonous decreasing trend across the 2–18 GHz frequency range, with no significant fluctuations. Notably, within the 2–6 GHz range, the decline occurred rapidly, suggesting that magnetic loss in this frequency range is primarily dominated by natural resonance. In the 6–18 GHz range, the decrease in C₀ values slowed, indicating the possible contribution of eddy current losses in this frequency band.

Fig. 11 (a) Magnetic loss C₀, and (b) decay constant α of Co@Co_xFe_1−x@Fe@Fe₃O₄ thorny particle samples.

Fig. 11(b) presents the decay constants α of the Co@Co_xFe_1−x@Fe@Fe₃O₄ thorny particle samples. It is evident that with increasing frequency, the α values of the samples gradually increase, and the maximum α values at oxidation temperatures of 50 °C, 60 °C, 70 °C, and 80 °C were recorded as 195.32, 201.71, 218.60, and 188.94, respectively. At an oxidation temperature of 70 °C, the Co@Co_xFe_1−x@Fe@Fe₃O₄ thorny particles exhibited the highest decay constant α, demonstrating that the material had the strongest attenuation capability.

3.4.4 Impedance matching of Co@Co_xFe_1−x@Fe@Fe₃O₄ thorny particles. Fig. 12 illustrates the relationship between the reflection loss and impedance matching (Z values) of the Co@Co_xFe_1−x@Fe@Fe₃O₄ thorny particle samples. As shown in Fig. 12(a)–(d), the minimum reflection losses corresponding to the impedance matching Z values for the Co@Co_xFe_1−x@Fe@Fe₃O₄ thorny particle samples at oxidation temperatures of 50 °C, 60 °C, 70 °C, and 80 °C are 1.26, 1.21, 1.12, and 1.38, respectively. With an increase in oxidation temperature, the impedance matching Z values of the Co@Co_xFe_1−x@Fe@Fe₃O₄ thorny particle samples exhibit a trend of initially decreasing and then increasing. When the oxidation temperature is set at 70 °C, the impedance matching Z value of the Co@Co_xFe_1−x@Fe@Fe₃O₄ thorny particle sample is relatively close to 1, indicating a better impedance matching performance.

Fig. 12 Reflection loss versus impedance matching curve of Co@Co_xFe_1−x@Fe@Fe₃O₄ thorny particle samples under self-oxidation at (a) 50 °C, (b) 60 °C, (c) 70 °C, and (d) 80 °C and (e) Z value curve.

As shown in Fig. 12(e), it can be observed that at oxidation temperatures of 50 °C, 60 °C, and 80 °C, the Z values of the Co@Co_xFe_1−x@Fe@Fe₃O₄ thorny particle samples deviate significantly from the effective Z value range (0.8–1); the Z value at an oxidation temperature of 70 °C was relatively better. The optimization of impedance matching allows more electromagnetic waves to penetrate into the interior of the particles, which will be helpful for enhancing the electromagnetic wave absorption properties.

3.4.5 Reflection loss and effective absorption bandwidth of Co@Co_xFe_1−x@Fe@Fe₃O₄ thorny particles. Fig. 13 presents the reflection loss curve and 3D plot of the Co@Co_xFe_1−x@Fe@Fe₃O₄ thorny particle samples, while Table 2 summarizes the electromagnetic wave absorption performance statistics for these samples. The data indicate that at an oxidation temperature of 70 °C, the Co@Co_xFe_1−x@Fe@Fe₃O₄ thorny particle samples exhibit optimal absorption performance, with a minimum reflection loss of −24.42 dB at a coating thickness of 1.6 mm. Within the frequency range of 2–18 GHz, the maximum effective absorption bandwidth of the samples is 4.80 GHz. At the same time, the effective absorption bandwidth and minimum reflection loss of the in situ self-oxidation samples at 50 °C, 60 °C and 80 °C are 4.56 GHz, 4.64 GHz, and 4.16 GHz, and −18.75 dB, −20.20 dB, and −16.05 dB, respectively. Although the effective absorption bandwidth and minimum reflection loss are lower than those of the samples at 70 °C, the coating thickness is thinner than that at 70 °C, the corresponding sample thicknesses are 1.5 mm, 1.4 mm and 1.5 mm, respectively. It is clear that for Co@Co_xFe_1−x@Fe@Fe₃O₄ thorny particles, different processing processes can be selected to prepare electromagnetic wave absorbing materials with wider effective absorption bandwidth and smaller minimum reflection or thinner coating, according to the application requirement.

Fig. 13 Reflection loss curve and 3D plot of Co@Co_xFe_1−x@Fe@Fe₃O₄ thorny particle samples under self-oxidation at (a)–(c) 50 °C, (d)–(f) 60 °C, (g)–(i) 70 °C, and (j)–(l) 80 °C.

Table 2 Electromagnetic wave absorption performance of Co@Co_xFe_1−x@Fe@Fe₃O₄ thorny particles

Sample	RLmin (dB)	d (mm)	EAB (GHz)	Effective absorbing band (GHz)
50 °C-R	−18.75	1.5	4.56	13.28–17.84
60 °C-R	−20.20	1.4	4.64	13.36–18.00
70 °C-R	−24.42	1.6	4.80	12.72–17.52
80 °C-R	−16.05	1.5	4.16	13.84–18.00

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4 Conclusion

By chemical liquid-phase reduction, in situ self-oxidation and heat treatment, core–shell structured Co@Co_xFe_1−x@Fe@Fe₃O₄ thorny composite particles were prepared, and the microstructure, static magnetic properties and electromagnetic wave absorption performance of the particles were investigated.

(1) The tri-layer Co@Co_xF_1−x@Fe@Fe₃O₄ thorny particles largely retain the morphology of Co@Fe@Fe₃O₄ thorny particles, with a typical core–shell structure for the internal core and a 200–300 nm thick external coating. With heat treatment, a discernible transition phase of CoFe and Co₇Fe₃ was formed at the Co/Fe interface.

(2) With the increase in self-oxidation temperature, the specific saturation magnetization of the Co@Co_xF_1−x@Fe@Fe₃O₄ thorny particles exhibits a slight increase before a downward trend, while the coercivity decreases slightly and then increases.

(3) Within the 2–18 GHz frequency range, the tan δ_e value exhibited minimal variation; however, the values across most frequency bands presented increased trends. In the frequency range of 2–12 GHz, the magnetic loss decreases with the increase of in situ self-oxidation temperature, and the regularity is no longer obvious in the frequency range of 12–18 GHz.

(4) At an oxidation temperature of 70 °C, the Co@Co_xFe_1−x@Fe@Fe₃O₄ thorny particle samples exhibit optimal absorption performance, with a minimum reflection loss of −24.42 dB at a coating thickness of 1.6 mm, and the maximum effective absorption bandwidth is 4.80 GHz.

Conflicts of interest

There are no conflicts to declare.

Data availability

Data will be available from the corresponding author upon reasonable request.

Acknowledgements

The authors would like to express gratitude to teacher Lijie Li from School of materials of Beijing institute of technology for her valuable technical assistance in the experiments.

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