Excellent Microwave Absorption Properties in the C Band for the Nitrided Y₂Fe₁₂Co₄Si/Paraffin Composites

Abstract

The nitriding process was employed to optimize the low-frequency microwave absorption properties of Y₂Fe₁₂Co₄Si/paraffin composites. The effects of nitriding temperature on the phase composition, static magnetic properties, electromagnetic parameters, and microwave absorption performance were systematically investigated. As the nitriding temperature increases, lattice expansion results in a significant increase in saturation magnetization and a higher ratio of in-plane to out-of-plane anisotropy fields. This, in turn, boosts the electromagnetic parameters of the composite material. With a further rise in temperature, an increased content of α-Fe is produced and the ratio of the in-plane to out-of-plane anisotropy field diminishes, leading to a decline in electromagnetic parameters. At 500 °C, these factors reach an optimum level, maximizing the composite’s electromagnetic parameters. The composite exhibited a minimum reflection loss (RL_min) of −55.9 dB at 5.58 GHz with a thickness of 2.46 mm. Moreover, at a thickness of 2.21 mm, the composite achieved a maximum effective absorption bandwidth (EAB_max) of 2.95 GHz (5.05–8 GHz). Compared with other low-frequency-absorbing materials, the composite exhibited stronger absorption and a wider absorption bandwidth at a lower thickness in the C band.

1. Introduction

With the development of modern science and technology, electromagnetic wave pollution has become increasingly serious, significantly impacting the environment and the equipment used. Therefore, developing efficient microwave absorbing-materials (MAMs) to mitigate electromagnetic interference is essential. These materials also enhance the stealth performance of military equipment [1,2,3]. Typical MAMs include ferrites [4], magnetic metal materials [5,6,7], and carbon-based materials [8], each with unique absorption characteristics across different frequency bands [9]. Research into C bands (4–8 GHz) faces challenges such as limited broadband absorption, poor compatibility between matching and absorption layers, and increased costs from complex processing [10,11,12].

According to the quarter-wavelength theory, addressing the shortcomings of MAMs at low frequencies (below 8 GHz), such as large matching thickness and limited effective absorption bandwidth (EAB), requires higher electromagnetic parameters. However, excessive dielectric constants hinder impedance matching. Magnetic materials, with higher magnetic permeability than carbon-based materials, have received significant attention for low-frequency microwave absorption [13]. Current research on magnetic materials in the C band focuses on optimizing composition, powder morphology, and surface treatment in isotropic systems. Ma et al. [14] prepared hollow hexagonal NiFe₂O₄/graphite nanosheets, achieving a minimum reflection loss (RL_min) of −57.4 dB at 6.08 GHz with a thickness of 2.9 mm, though their EAB in the C band was only 1.2 GHz, a value which was insufficient for practical use. Liu et al. [15] prepared FeSiAl-xGd (x = 1, 2, 3, 4) composites via vacuum sintering, optimizing Gd content to achieve an RL_min of −54.6 dB at 3.5 mm and an EAB_max of 4.41 GHz (3.98–8.39 GHz), fully covering the C band. Xiang et al. [16] prepared Co@C composite nanofibers by embedding Co particles into carbon nanofibers. The EAB was 3.84 GHz (4.16–8.00 GHz) at a thickness of 6.63 mm. However, the initial permeability and natural resonance frequency of isotropic magnetic materials are constrained by the Snoek limit, limiting their permeability in the low-frequency band and hindering their ability to achieve the requirements of “wide bandwidth” and “thin thickness” [17,18,19,20,21].

Rare earth-3d transition metal compounds have large magnetic moments from transition metal sublattices and magnetocrystalline anisotropy from rare earth sublattices. Easy-face anisotropic R₂M₁₇-based alloys exhibit intrinsic bi-anisotropy, theoretically breaking through the Snoek limit. The Snoek limit formula is as follows (1) [20,21,22,23,24]:

$$({\mu }_i-1)f_r={\displaystyle \frac{\gamma }{2\pi }}{M}_s\sqrt{{\displaystyle \frac{H_{a1}}{H_{a2}}}}$$

(1)

where $f_r$ is the natural resonance frequency, ${\mu }_i$ is the initial magnetic permeability, $\gamma$ is the gyromagnetic ratio, and $M_s$, $H_{a1}$ and $H_{a2}$ are the saturation magnetization, out-of-plane and in-plane magnetocrystalline anisotropy field, respectively. Easy-face anisotropic materials can exceed the Snoek limit by enhancing saturation magnetization and optimizing the ratio of in-plane to out-of-plane magnetocrystalline anisotropy fields, achieving higher magnetic permeability [25].

Studies have demonstrated that the replacement of rare earth atoms with transition metals, or the incorporation of interstitial atoms, induces lattice distortion and promotes the formation of new phases. These changes exert a profound influence on the saturation magnetization and magnetocrystalline anisotropy fields, ultimately enabling the modulation of microwave absorption characteristics [20,26,27,28,29,30,31,32]. Despite extensive investigations into nitrogen-doped 2:17 intermetallic compounds, aimed at optimizing their microwave absorption capabilities, the effects of nitriding temperatures on both their static magnetic and microwave absorption properties have not been comprehensively elucidated [33,34,35,36,37].

In previous work, we reported an easy-plane-type Y₂Fe₁₂Co₄Si/paraffin composite material with superior microwave absorption properties in the X-Ku band (8–18 GHz). The saturation magnetization $M_s$ of the sample was 147 emu/g, and the real part of the magnetic permeability µ′ (0.5 GHz) was 2.69 [38]. In this study, Y₂Fe₁₂Co₄Si nitridation product/paraffin composites were fabricated, and the influence of nitriding temperatures on their electromagnetic parameters and microwave absorption properties was systematically investigated. The intrinsic relationship among phase composition, static magnetic properties, electromagnetic parameters, and microwave absorption properties was determined to reveal the mechanisms governing the performance of the materials.

We find that raising the nitriding temperature increases lattice expansion and the formation of secondary phases. When the content of secondary phases is low, it enhances saturation magnetization and electromagnetic parameters, shifting the natural resonance frequency to lower frequencies and achieving excellent low-frequency absorption performance. However, excessive secondary phase content reduces the in-plane to out-of-plane anisotropy field and weakens polarization, leading to a decline in the composite’s magnetic permeability and dielectric constant. This study presents an effective strategy for enhancing the microwave absorption properties of 2:17-type materials via nitriding treatment, providing a theoretical basis for optimizing material design. Additionally, it stimulates further research into other nitriding parameters, aiming to unlock the full potential of nitriding in the development of high-performance microwave-absorbing materials.

2. Materials and Methods

2.1. Sample Preparation

The raw materials (purity > 99.9%) were arc-melted five times in an argon atmosphere to produce Y₂Fe₁₂Co₄Si alloy ingots, with 1 wt.% excess yttrium (Y) added to compensate for melting losses. The ingots were manually crushed, sieved, and planetarily ball-milled in anhydrous ethanol (parameters in reference [38]). The ball-milled alloy powder was placed in a tubular furnace with a nitriding atmosphere of 95% N₂ and 5% H₂. To prevent oxidation, the tube was purged with N₂ for 10 min before nitriding. The furnace was heated to 400 °C, 500 °C, and 600 °C at a rate of 10 K/min, respectively, and maintained at each temperature for 1 h. During cooling, a high flow of N₂ was used for purging. The nitride powder was uniformly mixed with paraffin wax at a weight ratio of 3:2 to fabricate a composite material. Subsequently, the composite was oriented within a magnetic field using a coaxial ring mold. Figure 1 is the schematic diagram of composite preparation.

2.2. Properties and Characterization

The magnetic powder’s microstructure was characterized by scanning electron microscopy (SEM, Phenom Star, Thermo Fisher, Waltham, MA, USA) at 10 kV. The phase composition and crystal structure of the alloy powder and composite were analyzed by X-ray diffraction (XRD, X’Pert Pro MPD, Panalytical, NLD) with Co-Kα radiation. Hysteresis loops of the composite and powder were measured using a vibrating-sample magnetometer (VSM, Model 3105, East Changing Technologies, Beijing, China) in a magnetic field range from −1.8 T to 1.8 T. Electromagnetic parameters of the composite were measured by the coaxial method with a vector network analyzer (VNA, PNA N5224B, Keysight, Santa Rosa, FL, USA) at 0.5–18 GHz.

3. Results

3.1. Phase Composition and Morphology

Figure 2 presents the particle size distribution of Y₂Fe₁₂Co₄Si alloy powder after ball milling. The ball-milling process reduces the average particle size, which is beneficial for the subsequent magnetic field orientation process [39]. Figure 3 shows the XRD patterns of Y₂Fe₁₂Co₄Si alloy after nitriding at different temperatures for 1 h. As shown in Figure 3a, two changes in phase composition are observed after nitriding. Firstly, as the nitriding temperature increases, the intensity of the main diffraction peak corresponding to α-Fe increases significantly, which implies that the content of α-Fe within the alloy rises in tandem with the temperature. Secondly, with a higher nitriding temperature, the nitride phase (Fe, Co)N gradually becomes produced. The emergence of these two new phases leads to a decrease in the content of the main phase (2:17 phase), potentially influencing the high-frequency magnetic properties of the material. As depicted in Figure 3b, after nitriding, the diffraction peak corresponding to the (302) crystal plane of the majority phase undergoes a shift towards a lower angle. This phenomenon suggests that nitrogen atoms have intercalated into the lattice, inducing lattice expansion and consequently resulting in the displacement observed in the diffraction peak [40]. With the elevation of the nitriding temperature, the offset phenomenon becomes increasingly pronounced. This can be attributed to the positive correlation between the diffusion rate of nitrogen atoms and temperature. As the temperature rises, a greater number of nitrogen atoms diffuse into the crystal lattice.

Figure 4 presents the microstructural characteristics and elemental distribution of Y₂Fe₁₂Co₄Si alloy powders under varying nitriding temperatures. The ball-milled powder particles exhibit an irregular morphology, which remains largely unchanged after nitriding. Moreover, no significant agglomeration was observed in the samples. According to the EDS analysis results, the nitrogen content within the powders rises as the nitriding temperature increases, reaching 5.14 at.%, 7.03 at.%, and 7.78 at.%, respectively (

Table 1. Chemical composition of Y₂Fe₁₂Co₄Si alloy powder after nitriding at different temperatures.

Sample	Chemical Composition of Y2Fe12Co4Si Alloy (at.%)
	Y	Fe	Co	Si	N
Original	8.19	66.40	22.06	3.35	—
400 °C	9.74	61.52	19.70	3.90	5.14
500 °C	8.43	62.31	19.00	3.23	7.03
600 °C	8.50	61.21	19.31	3.20	7.78

). Evidently, the nitrogen content exhibits a positive correlation with temperature, which is in accordance with the XRD analysis outcomes.

3.2. Static Magnetic Properties

Following the nitriding of Y₂Fe₁₂Co₄Si alloy powders at varying temperatures, an increase in saturation magnetization was observed (Figure 5). This enhancement arose from the fact that nitrogen atoms occupied the lattice interstitial positions, expanding the Fe-Fe atomic spacing. As a result, the detrimental exchange interaction is mitigated, leading to an augmentation of the average magnetic moment per Fe atom [41]. As the nitriding temperature increases, the alloy exhibits the gradual formation of α-Fe and (Fe,Co)N nitride phases. At room temperature, α-Fe shows ferromagnetic behavior with a high saturation magnetization value, while (Fe,Co)N nitrides exhibit antiferromagnetic or paramagnetic properties with relatively low saturation magnetization [42]. Therefore, an increased content of α-Fe leads to a rise in the overall saturation magnetization of the material. The coercivity of the nitrided alloy surpasses that of the original powder, a notable enhancement attributed to two mechanisms. Firstly, the incorporation of nitrogen atoms gives rise to lattice distortion, inducing internal stress within the material. Secondly, the formation of a “core–shell structure”, with surface nitrides encapsulating the internal 2:17 phase, plays a crucial role. The phase interface functions analogously to a grain boundary, effectively pinning magnetic moments and thereby augmenting coercivity.

By comparing the normalized magnetization curves of the Y₂Fe₁₂Co₄Si/paraffin composites measured in the easy-magnetization and difficult-magnetization directions (Figure 6a,b), the influence of nitriding temperature on the in-plane and out-of-plane magnetocrystalline anisotropy field values was explored further. The relevant magnetic properties are listed in

Table 2. Magnetic properties of Y₂Fe₁₂Co₄Si/paraffin composites after nitriding at different temperatures.

Sample	Ha1 (kOe)	Ha2 (kOe)	Ha1/Ha2	Ms (emu/g)	$2{\mathit{M}}_{\mathit{s}}\sqrt{{\mathit{H}}_{\mathit{a}1}}/3\sqrt{{\mathit{H}}_{\mathit{a}2}}$	${\mathsf{\mu }}_0^{\prime }$
Original	11.21	3.88	2.89	147	166.57	2.53
400 °C	9.69	4.08	2.38	156	160.27	2.61
500 °C	9.19	3.63	2.53	158	167.59	3.07
600 °C	8.29	3.43	2.42	159	164.79	2.65

. When the magnetization intensity of the material under a specific field strength attains 85% of the saturation magnetization intensity, then the field strength can be regarded as the anisotropy field of the material in this direction [24]. Notably, the out-of-plane anisotropy field value of the nitrided Y₂Fe₁₂Co₄Si alloy is significantly lower than that of the original powder. The decrease in the overall out-of-plane magnetocrystalline anisotropy field can be attributed to two factors [43]. Firstly, the incorporation of nitrogen atoms into the lattice interstices induces lattice expansion. Secondly, the precipitation of α-Fe, which has a low magnetocrystalline anisotropy field, contributes to this reduction. Conversely, the value of the in-plane magnetocrystalline anisotropy field exhibits a trend of increasing initially and then decreasing as the temperature rises.

After undergoing nitriding, the magnetic permeability of Y₂Fe₁₂Co₄Si/paraffin composites at 0.5 GHz was moderately enhanced. Nevertheless, the calculated $2M_s\sqrt{H_{ha}}/3\sqrt{H_{ea}}$ exhibited insignificant changes relative to the original powder, indicating that the nitriding process had a negligible impact on the Snoek limit of the alloy. Consequently, the natural resonance frequency of the alloy shifted to a lower frequency (Figure 6b). As shown in Equation (1), the enhancement in permeability stems from the shift in the natural resonance frequency toward lower ranges.

3.3. Electromagnetic Parameters

Nitriding modifies the crystal structure and electron cloud distribution of the alloy powder, thereby inducing alterations in the dielectric constant and permeability of the Y₂Fe₁₂Co₄Si alloy. As depicted in Figure 7a, the real part of the permeability of the composite at 8 GHz exhibits an increase after nitriding. With the increment in nitriding temperature, the permeability initially ascends and subsequently descends. Below 500 °C, nitrogen atoms occupy interstitial sites, thereby enhancing the saturation magnetization and reducing the in-plane magnetocrystalline anisotropy field. These modifications collectively contribute to a significant improvement in permeability. However, when the temperature increases, the 2:17 phase content decreases, while the content of α-Fe with low magnetocrystalline anisotropy and nitrides increases. As a result, the ratio of the in-plane to out-of-plane magnetocrystalline anisotropy fields declines, leading to a gradual decrease in the increment of permeability. Notably, above 8 GHz, the permeability of the nitrided alloy is lower than that of the original alloy due to the shift in the natural resonance frequency to lower values after nitriding. Moreover, as illustrated in Figure 7b, the imaginary part of the permeability experiences a substantial increase, which significantly enhances the magnetic loss and electromagnetic wave attenuation capabilities of the composites.

The doping of nitrogen atoms caused lattice distortion, leading to an increase in lattice vibration frequency. This limited the amplitude of atomic displacement, thereby reducing the polarization ability of the material [44]. However, at 400 °C, a small amount of α-Fe phase (with a high dielectric constant) was generated inside the composite material. An electric dipole moment formed at the interface, which increased the material’s dielectric constant. This formation of α-Fe counteracted the above effects, leading to negligible changes in the dielectric constant [45].

With the increase in nitriding temperature, the doping amount of nitrogen atoms and the content of precipitated phases continued to increase. At 500 °C, α-Fe became the main precipitate phase, and the interface polarization effect was strengthened, resulting in a significant increase in dielectric constant. When the temperature reached 600 °C, considerable (Fe,Co)N phase content was widely formed on the surface of α-Fe precipitates, hindering the formation of conductive networks and the accumulation of free space charges. This reduction in space charge polarization, as depicted in Figure 7c,d, resulted in the dielectric constant exhibiting a trend of first increasing and then decreasing.

These findings indicate that nitriding at 500 °C resulted in the highest values of permeability and dielectric constant, suggesting that the composite achieved optimal microwave absorption performance at low frequencies under this condition.

3.4. Microwave Absorption Properties

For an absorber to dissipate electromagnetic energy effectively, achieving good impedance matching is essential. When this occurs, electromagnetic waves can penetrate the absorber without significant surface reflection, facilitating energy dissipation. The impedance matching performance of the absorber is quantitatively evaluated using the Delta (|Δ|) function. This function is calculated by integrating relevant Equations (2)~(4) [46], enabling a precise assessment of the impedance matching degree.

$$\left|∆\right|=\left|sin{h}^2\left(Kfd\right)-M\right|$$

(2)

$$K={\displaystyle \frac{4\pi \sqrt{{\mu }^{\prime }{\epsilon }^{\prime }}\times sin\left(\frac{{\delta }_{\epsilon }+{\delta }_{\mu }}{2}\right)}{c\times cos{\delta }_{\epsilon }\times cos{\delta }_{\mu }}}$$

(3)

$$M={\displaystyle \frac{4{\mu }^{\prime }cos{\delta }_{\epsilon }\times {\epsilon }^{\prime }cos{\delta }_{\mu }}{({\mu }^{\prime }cos{\delta }_{\epsilon }-{\epsilon }^{\prime }cos{\delta }_{\mu }{)}^2+{\left[tan\left(\frac{{\delta }_{\epsilon }+{\delta }_{\mu }}{2}\right)\right]}^2\times ({\mu }^{\prime }cos{\delta }_{\epsilon }+{\epsilon }^{\prime }cos{\delta }_{\mu }{)}^2}}$$

(4)

where ε′ and μ′ are the real parts of the dielectric constant and magnetic permeability, respectively; ε″ and μ″ are the imaginary parts of the dielectric constant and magnetic permeability, respectively; d is the matching thickness of the absorber; $tan{\delta }_{\epsilon }={\displaystyle \frac{{\epsilon }^{″}}{{\epsilon }^{\prime }}}$ is the dielectric loss factor; and $tan{\delta }_{\mu }={\displaystyle \frac{{\mu }^{″}}{{\mu }^{\prime }}}$ is the magnetic loss factor. Within a certain thickness range, smaller |Δ| values indicate better impedance matching. At a given absorber thickness, impedance matching is deemed satisfactory when the value of |Δ|≤ 0.4.

Moreover, when the absorber thickness (d_m) and the frequency (f) corresponding to the RL_min satisfy the quarter-wavelength condition, interference cancellation takes place, thereby further enhancing the impedance matching. The larger the overlap area between the matching thickness curve and Delta |Δ|≤ 0.4, the better the impedance matching performance. This functional relationship is quantitatively described by Equation (5) [47]:

$$d={\displaystyle \frac{n\lambda }{4}}={\displaystyle \frac{nc}{4f\sqrt{\left|{{\mu }_r\epsilon }_r\right|}}}(n=1,3,5,\dots )$$

(5)

After nitriding, the electromagnetic parameters of the Y₂Fe₁₂Co₄Si alloy were enhanced. Consequently, the matching thickness curve shifted towards lower frequencies and smaller thicknesses, which expanded the overlapping area between the impedance matching curve and the region where |Δ|≤ 0.4, as depicted in Figure 8. With the increase in nitriding temperature, the impedance matching characteristics of the alloy undergo changes. Between 400 °C and 500 °C, the frequency corresponding to the optimal impedance matching shifts to lower values, thereby enhancing the impedance matching performance at low frequencies. This enhancement can be attributed to the increment in electromagnetic parameters as the nitriding temperature rises, which in turn causes the matching thickness curve to shift towards lower frequencies and thicknesses. Conversely, when the nitriding temperature increases from 500 °C to 600 °C, a decrease in the electromagnetic parameters occurs, leading to a shift in the optimal impedance matching frequency to higher values.

The microwave absorption performance of composites is commonly evaluated using reflection loss (RL). Specifically, an RL value lower than −10 dB corresponds to 90% absorption of incident electromagnetic waves, whereas an RL value below −20 dB denotes 99% absorption. The effective absorption bandwidth (EAB), which represents the frequency span where RL < −10 dB, is a crucial parameter reflecting the overall absorption performance. The RL values can be calculated using Equations (6) and (7) [17]:

$$Z_{in}=Z_0\sqrt{{\displaystyle \frac{{\mu }_r}{{\epsilon }_r}}}\mathit{tanh}\left(j{\displaystyle \frac{2\pi fd}{c}}\sqrt{{\mu }_r{\epsilon }_r}\right)$$

(6)

$$RL\left(dB\right)=20\mathit{lg}\left|{\displaystyle \frac{Z_{in}-Z_0}{Z_{in}+Z_0}}\right|$$

(7)

Manipulating the nitriding temperature serves as an effective strategy for engineering the low- and mid-frequency microwave absorption behavior of the composites, as depicted in Figure 9. The Y₂Fe₁₂Co₄Si/paraffin composites exhibit excellent microwave absorption properties in the X-Ku band. However, their absorption performance at low frequencies remains suboptimal and warrants further enhancement. Nitriding treatment significantly improves the electromagnetic parameters of the alloy powder and refines impedance matching, enabling electromagnetic waves to penetrate the absorber without significant reflection. This process leads to a notable improvement in the reflection loss (RL) characteristics of the composites. For composites with identical thickness, the frequency corresponding to the RL_min of the nitrided Y₂Fe₁₂Co₄Si/paraffin composites shifts towards lower values. This shift indicates robust absorption performance across both low- and mid/high-frequency bands.

As the nitriding temperature increases, the RL_min values of the composites are −48.4 dB (2.03 mm, 9.77 GHz), −55.9 dB (2.46 mm, 5.58 GHz), and −49.9 dB (2.37 mm, 7.24 GHz), respectively. The RL_min first increases and then decreases, which is consistent with the trend of electromagnetic parameters. Additionally, the frequency corresponding to the RL_min first shifts to lower frequencies and then to higher frequencies, aligning with the movement of the matching thickness curve.

To comprehensively evaluate the microwave absorption properties of nitrided Y₂Fe₁₂Co₄Si/paraffin composites at different temperatures in the C band, the EAB_max within the C band and the corresponding sample thickness were compared (Figure 10). For the sample nitrided at 500 °C, effective absorption (RL ≤ −10 dB) was achieved over the frequency range of 5.05–8 GHz with a thickness of 2.21 mm.

Table 3. Comparison of effective absorption bandwidth (EAB) among representative magnetic absorbers and this study in C band.

Composite	Loading	RLmin (dB)	RL < −10 dB		Ref.
			EAB (GHz)	Thickness (mm)
MnO2@NPC/paraffin	50 wt.%	−54.96	2.00 (6.00–8.00)	3.23	[48]
FeSiAl-3Gd	sinter	−54.60	4.00 (4.00–8.00)	3.50	[15]
MoSe2/paraffin	50 wt.%	−51.60	2.40 (4.40–6.80)	4.45	[47]
Flaky porous Fe3O4/paraffin	60 wt.%	−22.50	2.80 (4.80–7.60)	4.20	[49]
MXene@CNTs /paraffin	30 vol.%	−29.90	2.20 (5.80–8.00)	3.80	[51]
FeCoNi@MOF/paraffin	38 wt.%	−27.70	2.30 (5.70–8.00)	4.23	[50]
Ce2Fe17N3-δ/paraffin	30 vol.%	−34.40	2.00 (4.00–7.00)	2.20	[35]
Co2FeGe nanoflakes/paraffin	70 wt.%	−48.60	2.60 (4.00–6.60)	4.50	[52]
Ce1.2MM0.8Co1.7/paraffin	80 wt.%	−31.98	2.40 (5.80–8.00)	1.80	[32]
Y2Fe12Co4Si/paraffin *	60 wt.%	−55.90	2.95 (5.05–8.00)	2.46	This work

Note: * Y₂Fe₁₂Co₄Si alloy treated at 500 °C nitriding temperature.

presents a comparison of the EAB of a selected C-band-absorbing material [15,32,35,47,48,49,50,51,52]. The results demonstrate that the composite’s optimal thickness and EAB surpass those of many previously reported materials in the same frequency range, highlighting the potential of nitrided Y₂Fe₁₂Co₄Si/paraffin composites as low-frequency MAMs.

4. Conclusions

As the nitriding temperature increases, the lattice volume expansion of the alloy intensifies, accompanied by the gradual formation of the α-Fe and the (Fe, Co)N phases. These changes slightly enhance the saturation magnetization and reduce the out-of-plane anisotropy field. Concurrently, the presence of greater α-Fe and (Fe, Co)N phase content reduces the in-plane anisotropy field and weakens polarization. Hence, the ratio of in-plane to out-of-plane anisotropy fields initially increases and then decreases with temperature. This behavior reflects a similar trend in the composite’s electromagnetic parameters and natural resonance frequency.

At 500 °C, the lattice distortion and phase composition significantly enhance the alloy’s saturation magnetization and anisotropy fields, optimizing the electromagnetic parameters and shifting the natural resonance frequency to a lower range. This optimization enables optimal impedance matching and a reflection loss performance in the low-frequency region, achieving a minimum reflection loss (RL_min) of −55.9 dB and a maximum effective absorption bandwidth (EAB_max) of 2.95 GHz. These results underscore the critical role of appropriate nitriding processes in enhancing the low-frequency microwave absorption properties of rare-earth transition metal compounds with easy-plane anisotropy values. However, a high powder filling content (60 wt.%) is not suitable for practical applications. Thus, the systematic optimization of the alloy composition and preparation process is essential to achieve higher magnetic permeability at a lower filling ratio.