Preparation of Chain-like CoBiNi Alloy as Soft Magnetic Materials for High Permeability and Low Loss

Abstract

5G communication commercialization is accelerating in many countries. At present, a large number of communication materials are deployed to transmit millimeter waves for 5G base stations. However, it brings huge energy consumption due to the shortcomings of the current materials. Therefore, a novel soft magnetic material with high magnetic permeability and low dielectric constant is urgently needed to reduce the energy loss of 5G base stations. In this work, a series of CoBiNi alloys were prepared using the hydrothermal reduction method, with bismuth (Bi) as the dopant. The results indicate that Bi can regulate the magnetic permeability of soft magnetic materials; the permeability of the Co20Bi5Ni75 alloy fluctuates stably around 1.50 within the frequency range of 14.00–18.00 GHz. The saturation magnetization exhibits an upward trend with increasing Bi doping, with the Co20Bi5Ni75 sample reaching a saturation magnetization of 73.11 emu/g. The coercivity and residual magnetization characteristics confirm that Co20Bi5Ni75 is a typical soft magnetic material. The microwave return loss (RL) of the Co20Bi5Ni75 alloy was consistently higher than −6.89 dB across the 1.00–18.00 GHz frequency range when the sample thickness was 5 mm. The increased magnetic permeability of the Co20Bi5Ni75 alloy is attributed to the ability of Bi³⁺ to suppress carrier migration, thereby increasing the resistivity of the crystal structure and consequently improving the material’s magnetic permeability. These findings provide new insights into the preparation of high-permeability soft magnetic materials.

1. Introduction

In today’s Internet of Things era, the development of 5G communication drives the explosion of mobile data traffic, cloud services and hyper-realistic media enabled by big-data analysis [1,2,3]. Compared with 4G communication, the 5G millimeter-wave communication operates at a higher frequency band and has weaker diffraction capability, resulting in significantly increased base station energy consumption. As a result, conventional electromagnetic materials can no longer balance signal transmission efficiency with low energy consumption requirements [4,5,6,7]. Therefore, developing novel soft magnetic materials with high permeability and low magnetic loss is the core key to achieving efficient and low-power operation of 5G base stations. In order to realize low-power signal transmission in 5G-based stations, the soft magnetic materials simultaneously need to meet four basic properties of low dielectric constant, low dielectric loss, high magnetic permeability and low magnetic loss as far as possible. Among these, permeability and magnetic loss are the two core indicators that determine the energy efficiency of 5G communication transmission.

The magnetic permeability of soft magnetic materials may be achieved by optimizing their intrinsic material parameters, thereby overcoming Snoek’s limit. In recent years, a significant amount of research has explored various feasible methods for achieving this breakthrough through the modulation of relevant parameters [8,9,10,11,12]. Ceramic ferrites as magnetic cores have been widely used in high-power and high-frequency inductors due to their low cost and relatively low magnetic losses around 100 KHz [13,14]. However, conventional ferrite soft magnetic materials are constrained by the Snoek limit, resulting in low permeability at high frequencies, making them unsuitable for high-frequency application scenarios in 5G.

In contrast, CoNi-based metallic alloys offer advantages such as high saturation magnetization, tunable permeability, and easily controllable microstructure, making them highly promising candidate materials for high-frequency soft magnetic applications [15,16]. Kurlyandskaya et al. found that the CoNi alloy has a wide range of physical and chemical properties in the fields of microwave absorption and magnetic resonance imaging agents. The applications of CoNi alloys depend on two factors: composition and morphology, which together determine their intrinsic magnetic moment, magnetic permeability and magnetic properties [17,18,19,20,21]. Liu et al. [9] synthesized CoNi alloys of different shapes by using surfactants as structure-directing, the results indicated that surfactants play a crucial role in enhancing the magnetic permeability of cobalt–nickel alloys.

Moreover, saturation magnetization (Ms) is an internal parameter of the material, which is determined by the number of unpaired electrons outside the core of the soft magnetic materials. Meanwhile, the Ms values of multiphase materials are usually higher than those of monophase materials [22,23]. For example, Fe-Co, Fe-Ni, and Fe-Si-Al composites have higher Ms values than the monophase materials such as Fe, Co, Ni, Si, and Al. Importantly, the doping of critical metals may significantly enhance the magnetic permeability of alloy materials. Current research indicates that dopants such as Al, Cr, V, Bi and rare-earth ions are employed in CoNi-based ferrite systems. In particular, Bi³⁺ has a noticeably larger ionic radius (1.17 Å at a coordination number of 6), which is far greater than that of Fe³⁺ and other transition metal ions [24]. Where Bi³⁺ primarily substitutes for Fe³⁺ at the octahedral B sites in the spinel lattice, this size mismatch introduces local lattice distortion and alters the cation distribution [25]. Furthermore, contrary to the expectation that the dilution of non-magnetic Bi³⁺ would simply reduce the magnetic susceptibility, an appropriate amount of Bi³⁺ substitution can improve magnetic permeability through microstructural optimization. Sertkol et al. (2022) found that Bi³⁺-substituted Co_0.5Ni_0.5Bi_xFe_2−xO₄ nanofibres exhibited enhanced coercivity and rectangularity at x = 0.04, indicating an improved domain structure [24]. Similarly, Alqarni et al. (2022) reported that in a Co-Ni spinel ferrite synthesized via hydrothermal synthesis, Bi³⁺ doping significantly modified the magnetic domain configuration; the coercivity of the samples was tunable within the range of 101 to 1038 Oe depending on the Bi content, and Bi0.04 doping led to the formation of a single-domain structure with uniaxial symmetry, a configuration that enhances magnetic permeability [26].

In addition, Mohammad et al. prepared thin films of luteium-iron garnet substituted by bismuth element (Bi) with low coercivity. It has a wide range of potential applications in next-generation integrated optics, magneto-photonics and magnetic field sensors [27,28]. In addition, Bi atoms could inhibit the movement of charge carriers and increase the resistivity of the crystal structure, thereby improving the magnetic permeability. By introducing bismuth trioxide, Attia Aslam et al. prepared nanoscale powder materials of CoFe₂O₄ and Sr₂NiMnFe₁₂O₂₂ [29]. In this context, Bi₂O₃ acted as a scattering center for carriers, hindering carrier hopping between octahedral sites and suppressing the electronic transition mechanism between Fe²⁺ and Fe³⁺ ions. This enhances the resistivity of the composite material and significantly improves its magnetic permeability properties [30,31].

In summary, appropriate Bi³⁺ doping induces lattice distortion, which promotes the magnetic domain transition from a multi-domain state to a single-domain configuration and facilitates cation redistribution between the A-site and B-site. This optimization effectively tailors the magnetic anisotropy and domain wall dynamics, thereby significantly improving the initial permeability of the prepared nanoferrites [32,33].

Therefore, Bi atoms and their corresponding crystal structure played an important role in overcoming Snoek’s limit of soft magnetic materials. In this work, a series of CoBiNi alloys with different Bi contents was prepared by the hydrothermal reduction method. These results indicated that the magnetic permeability of CoBiNi alloys could be adjusted by adjusting the doping content. The magnetodielectric properties of the series of cobalt–bismuth–nickel alloys were tested, including the real and imaginary parts of the dielectric constant, as well as the real and imaginary parts of the magnetic permeability; simultaneously, the microwave reflection loss (RL) and effective absorption bandwidth of the alloys were verified. This research provides new insights for the development of high-permeability, low-loss soft magnetic materials.

2. Experimental

2.1. Materials

Nickel acetate tetrahydrate (C₄H₆NiO₄·4H₂O, AR, 98.0%), cobalt acetate tetrahydrate (C₄H₆CoO₄·4H₂O, AR, 99.5%), ethylene glycol (C₂H₆O₂, EG, 99%), sodium hydroxide (NaOH, AR, 96%), absolute ethanol (C₂H₅OH, ≥99.7%) and diethylenetriamine (C₄H₁₃N₃, DETA, AR) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Bismuth nitrate pentahydrate (Bi(NO₃)₃·5H₂O, AR, 99.5%) was supplied by Shanghai Qiangshun Chemical Co., Ltd. (Shanghai, China).

2.2. The Preparation Process of Co₂₀BiXNi_80−x

Co₂₀Bi_XNi_80−x (X = 1, 2, 5, 10) alloys were prepared in a solvothermal reduction system as follows: C₄H₆NiO₄·4H₂O, C₄H₆CoO₄·4H₂O and Bi(NO₃)₃·5H₂O were used as precursors and dissolved in a 40 mL C₂H₆O₂ solution; The NaOH concentration was 0.75 mol·L⁻¹. The precursor concentration was 0.75 mol·L⁻¹. The Co₂₀Bi_xNi_80−x (X = 1, 2, 5, 10) mixture was dissolved by ultrasound at room temperature, and 1 ml of diethylenetriamine (DETA) was added, then transferred into a Teflon-lined stainless steel autoclave (100 mL capacity). The autoclave was sealed and maintained at 200 °C for 6 h, then cooled to room temperature. The blackish-gray solid product at the bottom of the reactor was collected and washed twice with anhydrous ethanol, followed by a single wash with deionized water. Finally, the samples were dried in a vacuum oven at 60 °C, achieving a dry sample yield of over 85%.

2.3. Characterization

The morphologies and sizes of the Co₂₀Bi_XNi_80−x (X = 1, 2, 5, 10) alloys were characterized by a field-emission scanning electron microscope (SEM, Regulus8100, Hitachi, Tokyo, Japan). The distribution of elements of the Co₂₀Bi_XNi_80−x (X = 1, 2, 5, 10) alloys was characterized by energy dispersive spectroscopy (EDS, Regulus8100, Hitachi, Tokyo, Japan). The structure of the Co₂₀Bi_XNi_80−x (X = 1, 2, 5, 10) alloys was examined by X-ray diffraction (XRD) with a Cu K radiation source at room temperature (D/max-2200/PC, Rigaku, Tokyo, Japan). The density of the samples was measured by the Archimedes drainage method. The valence state of the element was measured by X-ray photoelectron spectroscopy (Nexsa, Thermo Fisher, Waltham, MA, USA). The magnetic properties of the Co₂₀Bi_XNi_80−x (X = 1, 2, 5, 10) alloys were determined with a magnetometer (MPMS-VSM, Quantum Design Company, San Diego, CA, USA). Dielectric constant (ε′), dielectric constant (ε″), magnetic permeability (μ′) and magnetic loss (μ″) values of the samples were measured by a Vector network analyzer (E5080B, Keysight, Santa Rosa, CA, USA). The samples were prepared by homogeneously mixing paraffin wax with composites (mass:ratio = 7:3) and then pressed into toroidal-shaped samples (ø_out = 7.0 mm, ø_in = 3.0 mm).

3. Results and Discussion

The synthesis process of a series of CoBiNi alloys with different Bi contents was illustrated in Figure 1a. As the content increases, both the microstructure and grain size of the cobalt-bismuth-nickel alloy will change. EDS mappings illustrate the existence of Co, Ni and Bi elements in Figure S1. Simultaneously, the difference in Co, Ni and Bi contents needs further analysis during the preparation of the Co₂₀Bi₅Ni₇₅ sample. The EDS spectrum is used to test the contents of these elements in the Co₂₀Bi₅Ni₇₅ sample, as listed in Table S1. Co contents in three parts of the Co₂₀Bi₅Ni₇₅ sample are 7.76%, 15.06% and 5.93%, respectively. Ni contents for those are 87.06%, 83.00% and 93.73%, respectively. Meanwhile, Bi is 5.17%, 1.93% and 0.33%, respectively. Co, Ni and Bi contents at different ports vary greatly, which is consistent with the previous results in Figure S1. SEM images indicate that the Co₂₀Ni₈₀ alloy presents a chain-like structure (Figure 1b). The length of a single Co₂₀Ni₈₀ crystal is about 80 μm. In the presence of DETA surfactant, the Co₂₀Ni₈₀ microspheres self-assemble into a chain-like structure with multiple branches. The preparation conditions of Co₂₀Bi₁Ni₇₉, Co₂₀Bi₂Ni₇₈, Co₂₀Bi₅Ni₇₅, Co₂₀Bi₁₀Ni₇₀ and Co₂₀Bi₈₀ are the same as those of the Co₂₀Ni₈₀ alloy. A series of CoBiNi alloys all exhibit the chain-like shape. Among them, the diameters of the Co₂₀Bi₁Ni₇₉ and Co₂₀Bi₂Ni₇₈ alloys are not obviously different from that of Co₂₀Ni₈₀, whose diameter is below 1μm, especially in the microspheres’ edges. It may be caused by the addition of Bi atoms. With the increase in Bi content, the Co₂₀Bi₅Ni₇₅ alloy is assembled into a chain-like structure with a diameter of about 2 μm microspheres in Figure 1e. Its length is about 20 μm. As shown in Figure 1f, the Co₂₀Bi₁₀Ni₇₀ alloy is also assembled into a chain-like structure with a diameter of about 1.5 μm and loses the microsphere shape. Meanwhile, there are also some large particles with a rough surface in the Co₂₀Bi₁₀Ni₇₀ alloy. As the Bi content increases, the particle size decreases significantly, indicating that Bi atoms inhibit the growth of CoNi grains [26]. The microstructure of Co₂₀Bi₈₀ alloy is spherical (Figure 1g) with a smooth surface and non-uniform particle size. Meanwhile, the minimum diameter of the observed particles is approximately 500 nm.

According to SEM and EDS elemental mapping results, Bi exhibits a non-uniform distribution in the CoBiNi alloys with local elemental enrichment. Such an inhomogeneous distribution induces local composition fluctuation and heterogeneous interfacial structure, which regulates magnetic domain movement and magnetic exchange coupling, and consequently affects the magnetic permeability and coercivity. To quantitatively reveal the inhibitory effect of Bi on CoNi grain growth, the average crystallite size of all samples was calculated from XRD patterns via the Scherrer equation. With the increase in Bi content, the grain sizes of Co₂₀Ni₈₀, Co₂₀Bi₁Ni₇₉, Co₂₀Bi₂Ni₇₈, Co₂₀Bi₅Ni₇₅ and Co₂₀Bi₁₀Ni₇₀ are 171 Å, 146 Å, 132 Å, 130 Å and 128 Å, respectively. The results showed that the crystallite size gradually decreases with increasing Bi content, confirming that Bi doping can effectively restrain the grain growth of CoNi alloy. The variation trend is consistent with the particle size evolution observed from SEM images, which further verifies the grain refinement effect of Bi doping. Furthermore, the lattice parameters were calculated according to XRD peak shifts. Due to the larger atomic radius of Bi compared with Co and Ni, Bi incorporation induces obvious lattice distortion and lattice strain, which modulates the electronic structure and serves as an important intrinsic factor for optimizing magnetic properties.

The crystal structures of Co₂₀Ni₈₀, Co₂₀Bi₁Ni₇₉, Co₂₀Bi₂Ni₇₈, Co₂₀Bi₅Ni₇₅, Co₂₀Bi₁₀Ni₇₀ and Co₂₀Bi₈₀ alloys are characterized by XRD. As shown in Figure 2a, the diffraction peaks appear at 44.5°, 51.8° and 76.7°, corresponding to the (111), (200) and (220) crystal planes, respectively. It indicates the face-centered cubic (fcc) structure of CoNi alloys. In addition, the diffraction peaks of Co₂₀Bi₈₀ alloy at 27.1° and 48.7° correspond to the (012) and (202) crystal planes, respectively. Similarly, the diffraction peaks of a series of CoBiNi alloys at 27.1° and 48.7° also correspond to (012) and (202) crystal planes. Meanwhile, the characteristic peaks of Co₂₀Ni₈₀ alloy are located at 44.5°, 51.8° and 76.7° for (111), (200) and (220) crystal planes. Moreover, the diffraction peak intensities of the CoBiNi alloy series increase with the rise in Bi content. As shown in Figure 2b, the bulk density of CoBiNi alloys also increases with increasing Bi content, and the relative density of alloy samples follows the same trend, verifying that Bi atoms can improve the compactness of the crystal structure [34]. The EDS spectrum indicates that the Co₂₀Bi₅Ni₇₅ contains several elements (Figure S1). By detecting the distribution of these elements, the elemental distribution characteristics of the CoBiNi alloy series can be clarified. EDS mapping of Co, Ni and Bi elements is used as tracers to characterize the composition of Co₂₀Ni₈₀, Co₂₀Bi₅Ni₇₅. Co₂₀Bi₁₀Ni₇₀ samples. As shown in Figure 2c, only Co and Ni elements are found in the Co₂₀Ni₈₀ sample, with no Bi element observed. Comparatively, as shown in Figure 2d,e, Co, Ni and Bi elements are all distributed in the EDS mapping of Co₂₀Bi₅Ni₇₅ and Co₂₀Bi₁₀Ni₇₀ samples. Combined with XPS analysis, the binding energies of the Bi 4f and 4f_5/2 orbitals were found to be 158.27 eV and 163.58 eV, respectively, indicating that Bi has successfully replaced the magnetic host cations, incorporated into the lattice, and formed a lattice solid solution.

The hysteresis loops of a series of CoBiNi alloys measured at 300 K show typical ferromagnetic properties (Figure 3). The saturation magnetizations (Ms) of Co₂₀Ni₈₀, Co₂₀Bi₁Ni₇₉, Co₂₀Bi₂Ni₇₈, Co₂₀Bi₅Ni₇₅, Co₂₀Bi₁₀Ni₇₀ and Co₂₀Bi₈₀ alloys are 50.81 emu g⁻¹, 68.05 emu g⁻¹, 70.78 emu g⁻¹, 73.11 emu g⁻¹, 56.91 emu g⁻¹ and 0.05 emu g⁻¹, respectively. The coercive force (Hc) values of the six samples are 100.59 Oe, 153.80 Oe, 80.98 Oe, 66.28 Oe, 87.36 Oe and 14.47 Oe, while the remanent magnetization (Mr) of the six samples are 4.44 emu g⁻¹, 7.14 emu g⁻¹, 4.80 emu g⁻¹, 2.82 emu g⁻¹, 2.81 emu g⁻¹ and 0.002 emu g⁻¹ in sequence. Compared with other alloys, the Ms value of the chain-like Co₂₀Bi₅Ni₇₅ alloy is the highest due to the existence of multiphases. In addition, the microsphere size of Co₂₀Bi₅Ni₇₅ alloy is smaller than that of Co₂₀Ni₈₀. The interface area of microspheres in Co₂₀Bi₅Ni₇₅ alloy increases correspondingly, which strengthens the interfacial exchange coupling effect between these microspheres, thus effectively enhancing the Ms values [35]. Among all the samples, Co₂₀Bi₁Ni₇₉ alloy has the largest coercivity, followed by Co₂₀Ni₈₀ alloy, due to its large shape anisotropy and large particle size [20]. However, Co₂₀Bi₅Ni₇₅ alloy has the lowest coercivity of 66.28 Oe, which is attributed to its small particle size [9]. The residual magnetization of all five main alloys is relatively low, indicating that these materials are typical soft magnetic materials.

Figure 4 presents the four magnetic and dielectric properties of Co₂₀Ni₈₀, Co₂₀Bi₁Ni₇₉, Co₂₀Bi₂Ni₇₈, Co₂₀Bi₅Ni₇₅ and Co₂₀Bi₁₀Ni₇₀ alloys. Figure 4a shows the real permittivity ε′ of five alloy materials. It is obvious that the ε′ of Co₂₀Ni₈₀ decreases at 1.00–18.00 GHz, which is mainly caused by the motion hysteresis of charged ions under an alternating electric field, resulting in the magnetic moment relaxation phenomenon. The ε′ of materials reflects their polarization capability [8,36], and a higher ε′ corresponds to a higher polarization degree. After the doping of Bi atoms, the ε′ values of Co₂₀Bi₁Ni₇₉, Co₂₀Bi₅Ni₇₅, Co₂₀Bi₂Ni₇₈ and Co₂₀Bi₁₀Ni₇₀ are significantly lower than those of Co₂₀Ni₈₀ at 1.00–18.00 GHz. The ε′ value of Co₂₀Bi₅Ni₇₅ also decreases with increasing frequency, and remains stable at approximately 10.26 in the range of 9.56–18.00 GHz. This phenomenon is attributed to the incorporation of Bi atoms, which increases the material’s resistivity and reduces the electron hopping probability in the crystal structure. As shown in Figure 4b, the imaginary permittivity (ε″) of Co₂₀Ni₈₀ reaches 15.98 at 4.53 GHz, and exhibits obvious fluctuations across the entire tested frequency band of 1.00–18.00 GHz. After the addition of the Bi element, the ε″ value of Co₂₀Bi₁Ni₇₉ and Co₂₀Bi₂Ni₇₈ remains stable. In the range of 1.00–18.00 GHz, the ε″ values of Co₂₀Bi₅Ni₇₅ alloy fluctuate slightly from 23.04 to 0.56, while obvious sharp fluctuations appear at 1.00–2.00 GHz and 8.00–10.00 GHz, respectively. The ε″ peak value of Co₂₀Bi₁₀Ni₇₀ alloy is approximately 5.80 at 11.63 GHz.

To further clarify the magnetic loss mechanism, the eddy current loss coefficient C₀ = μ″(μ′)⁻²f⁻¹ was calculated, and the results are presented in Figure 5. Theoretically, if eddy current loss dominates the magnetic loss, C₀ should remain constant over the measured frequency range. However, all samples in this work show obvious frequency-dependent C₀ values, demonstrating that eddy current loss is not the dominant magnetic loss mechanism within 1–14 GHz. Only Co₂₀Ni₈₀ exhibits evident C₀ fluctuation above 14 GHz, indicating a minor contribution of eddy current loss at high frequencies. With the increase in Bi content, the C₀ value decreases gradually and becomes more stable. Combined with SEM, EDS, XRD and XPS results, the evolution of electromagnetic properties is closely related to microstructure characteristics: Bi doping refines grain size, inhibits grain growth, and maintains the chain-like self-assembled morphology; the non-uniform distribution of Bi induces local composition fluctuation and increases electrical resistivity, which effectively suppresses long-range electron migration and eddy current effect. Therefore, magnetic loss is mainly dominated by natural resonance and exchange resonance, which is intrinsically determined by the chain-like structure, grain size effect and inhomogeneous Bi distribution of CoBiNi alloys.

Figure 6c,d separately present the μ′ and μ″ curves of Co₂₀Ni₈₀, Co₂₀Bi₁Ni₇₉, Co₂₀Bi₂Ni₇₈, Co₂₀Bi₅Ni₇₅ and Co₂₀Bi₁₀Ni₇₀ alloys. The μ′ value of Co₂₀Ni₈₀ remains about 1.17 within the frequency range of 1.00–18.00 GHz. When the Bi doping content is increased to 1% and 2%, the μ′ values of Co₂₀Bi₁Ni₇₉ and Co₂₀Bi₂Ni₇₈ are still maintained at around 1.17 throughout the tested frequency band of 1.00–18.00 GHz, respectively. As the Bi content increases to 5%, the μ′ values of Co₂₀Bi₅Ni₇₅ fluctuate around 1.50 in the range of 14.00–18.00 GHz, which may be attributed to the combined effect of eddy current loss, anomalous loss and enhanced polarization ability caused by the incorporation of Bi atoms [36]. The μ″ values of Co₂₀Ni₈₀, Co₂₀Bi₁Ni₇₉ and Co₂₀Bi₂Ni₇₈ basically fluctuate within the scope of 0–0.30 at 1.00–18.00 GHz. During the same range, the μ′ values show a continuous decreasing trend, which is caused by eddy current losses and abnormal losses at high frequency. The μ″ value of the Co₂₀Bi₅Ni₇₅ alloy fluctuates around 0.50 at the frequency range of 1.00–12.00 GHz. Similarly, the μ″ value increases in the range of 12.00–18.00 GHz and finally reaches 1.21 at 18.00 GHz.

According to the above electromagnetic parameters, the RL values of the five alloys were calculated, as shown in Figure 6a. The Co₂₀Ni₈₀ sample exhibits no microwave absorption capacity at a thickness of 1 mm. The RL value of the Co₂₀Bi₅Ni₇₅ sample exceeds −6.89 dB at 2.34 GHz under a thickness of 5 mm. Therefore, Bi atoms can effectively improve the signal transmission efficiency for 5G communication. To further illustrate the effect of Bi atoms on microwave absorption performance, the correlation between absorption performance and frequency for alloy samples with different thicknesses was systematically calculated. Figure 6b–f show the 3D RL curves of CoNi, Co₂₀Bi₁Ni₇₉, Co₂₀Bi₂Ni₇₈, Co₂₀Bi₅Ni₇₅ and Co₂₀Bi₁₀Ni₇₀ samples with different thicknesses. Co₂₀Ni₈₀ has a maximum RL value of 36.98 dB at 1.5 mm thickness due to its high real permittivity. As shown in Figure 6c, the RL values of Co₂₀Bi₁Ni₇₉ alloy are −14.82 dB at 11.97 GHz with a thickness of 2.15 mm. The RL values of Co₂₀Bi₂Ni₇₈ alloy are −26.47 dB at 8.90 GHz with a thickness of 2.15 mm (Figure 6d). Significantly, with the increase in Bi content, the Co₂₀Bi₅Ni₇₅ alloy delivers an optimal RL value of −23.75 dB at a thickness of 1.00 mm, and its corresponding effective absorption bandwidth reaches 5 GHz (Figure 6f). Similarly, the RL value of Co₂₀Bi₁₀Ni₇₀ alloy is −36.88 dB at 10.86 GHz under a thickness of 2.60 mm. Combined with the magnetic properties and microwave absorption performance, moderate doping of Bi atoms is beneficial to improve the microwave absorption properties of the soft magnetic materials.

To further clarify the regulatory effect of Bi doping, Figure 7 displays the full XPS survey and high-resolution spectra of the Co₂₀Bi₅Ni₇₅ alloy. Figure 7a shows the three characteristic peaks of the C1s spectrum, which correspond to C-C (284.91 eV), C-O-C (286.03 eV) and O-C=O (288.86 eV), respectively. The high-resolution Co 2p spectrum confirms the coexistence of metallic Co and Co²⁺ ions. The peaks located at 778.75 eV and 793.33 eV are assigned to Co 2p^3/2 and Co 2p^1/2, respectively. These results verify that the main valence state of Co is metallic Co, while the peaks at 782.65 eV and 798.25 eV belong to Co²⁺ ions. According to the Ni 2p spectrum in Figure 7c, the peaks at 853.85 and 871.20 eV are assigned to the characteristic peaks of Ni⁰ for Ni 2p^3/4 and Ni 2p^1/2, respectively. In addition, the peaks at 859.30 and 875.02 eV indicate the presence of Ni²⁺ ions. In Figure 7d, the Bi 4f spectrum contains two peaks located at 158.27 eV for Bi⁰ 4f^7/2 and 163.58 eV for Bi⁰ 4f^5/2. Furthermore, there are two more peaks at 160.45 eV and 165.81 eV, which correspond to Bi³⁺ 4f^7/2 and Bi³⁺ 4f^5/2, respectively. These XPS results further demonstrate that the Co₂₀Bi₅Ni₇₅ sample contains abundant Co²⁺, Ni²⁺ and Bi³⁺ ions besides the corresponding metallic elemental valence states. This result indicates that Bi exists in two coexisting chemical states in the as-prepared alloy; part of Bi is present in the form of metallic clusters, while the rest exists in the oxidized state as Bi³⁺. The Bi³⁺ species may originate from surface oxidation during material preparation or air exposure, and may also segregate at the grain boundaries or surface regions of the alloy. Furthermore, it should be noted that the XPS analysis was only performed on representative samples. For other compositions in the series (e.g., samples with different Bi contents or Co/Ni ratios), the chemical state and relative proportion of Bi may vary with composition and preparation conditions, which constitutes one limitation of this study. The movement of charge carriers among them causes interface polarization and reduces magnetic properties. Based on the available XPS results, we speculate that Bi³⁺ may act as carrier scattering centers to locally increase the resistivity of the material, thereby exerting an indirect influence on magnetic permeability.

4. Conclusions

In this work, a series of CoBiNi alloys were successfully fabricated via a simple structural template combined with a hydrothermal reaction method. The effect and mechanism of Bi doping on regulating the magnetic permeability of the materials were revealed. Adjusting the Bi content can effectively modulate the compactness and magnetic properties of the alloys. Among them, the Co₂₀Bi₅Ni₇₅ sample exhibits the optimal magnetic permeability, reaching 1.5 at 16.56 GHz. Furthermore, microwave reflection loss (RL) and saturation magnetization characterizations demonstrate that Co₂₀Bi₅Ni₇₅ possesses excellent microwave absorption capacity and high saturation magnetization. With a sample thickness of 5 mm, its RL value remains higher than −6.89 dB within the frequency range of 1.00–18.00 GHz. Based on the XPS results, it is preliminarily speculated that the superior magnetic permeability of CoBiNi alloys is attributed to Bi content, which hinders the migration of charge carriers and increases the material resistivity, thereby improving the magnetic permeability. These provide a reference for the design and development of functional materials with tunable magnetic permeability.