Separation of α-Fe phase from Nd2Fe17N3， dramatically broadening microwave absorption bandwidth at low frequency

Separation of α-Fe phase from Nd2Fe17N3， dramatically broadening microwave absorption bandwidth at low frequency | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Separation of α-Fe phase from Nd2Fe17N3， dramatically broadening microwave absorption bandwidth at low frequency Peng Wang, Haiqing Hang, Shibo Wang, Hecheng Zhao, Keli Zhang, and 4 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8571285/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 9 You are reading this latest preprint version Abstract The low-frequency microwave absorption (MA) problem urgently needs to be solved, although very difficult, to address the electromagnetic interference (EMI) issues caused by modern communication systems and devices. In this paper, magnetic nano-composites of Nd 2 Fe 17 N 3 /α-Fe with controllable phase proportion, oxidized surface, high permeability, large magnetic loss, and low permittivity at microwave frequency band, were prepared only by controlling the purity of N 2 used for doping. As a result, strong MA below − 10 dB is achieved at the low frequency C band and S band, when the absorber thickness is 2.8 mm and 5.0 mm respectively, obviously better than the low frequency microwave absorbers reported and thus showing great application prospects. More importantly, it is demonstrated that the design concept proposed by us is feasible, that is combination of easy-plane anisotropic materials like the Nd 2 Fe 17 N 3 and axis anisotropic materials like the α-Fe, capable of helping broaden the MA bandwidth, significantly. Additionally, a kind of particular magnetic loss mechanism on moments from one side to the other side of heterogeneous interfaces of two-phase magnetic nano-composites is proposed and discussed. In short, this paper has presented a novel method of experiment and some unique ideas of design, practical to solve the low-frequency MA problem. Microwave absorption Composite Magnetic Permeability Permittivity Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 1 Introduction Electromagnetic (EM) pollution problem is becoming increasingly serious [ 1 – 3 ] , which not only affects stability of sensitive electronic equipment but also endangers human health. Microwave absorption (MA) materials can convert EM energy into thermal energy through EM loss mechanisms, thus available for solving the EM pollution problem. Over the past decade, various MA materials, including dielectric loss materials, magnetic loss materials, and dielectric/magnetic composites [ 4 – 6 ] , were widely studied and reported, and have exhibited excellent MA performance at high frequency X and K u bands. However, there are huge challenges on the MA studies at the low frequency C, S, and L bands, including large absorber thickness and narrow MA frequency band. Researches show that the absorber thickness can be significantly decreased and MA frequency band can be significantly broadened at low frequency band, when magnetic materials are employed [ 7 – 12 ] , because of their high permeability and large magnetic loss at low frequency band. Moreover, the higher the microwave permeability, the thinner the absorber thickness; the greater the microwave magnetic loss, the wider the MA frequency band. Therefore, we have no choice but increasing the permeability and magnetic loss at low frequency band, to solve the low frequency MA problem. Researches show that a part of R 2 Me 17 alloys (R = Ce, Pr, Nd, Me = Fe, Co, Ni) possess high permeability and large magnetic loss at a wide frequency range from 0.1 GHz to 18.0 GHz, because of an easy plane anisotropy [ 13 – 17 ] , thus regarded as a type of highly promising MA materials. However, their permeability, magnetic loss, and MA performance at low frequency band need to be further improved, according to related reports. In 2023, Zheng et al. conducted a research on MA of Ce 2 Fe 17 N 3−δ @FePO 4 composite [ 14 ] , which exhibited an excellent MA performance at the high frequency band from 12.65 GHz to 18.00 GHz at a very thin thickness of 0.94 mm, while the low frequency MA performance is deficient with a narrow MA frequency band from 5.0 GHz to 6.24 GHz. Similarly, the MA frequency band is marrow at low frequency range for Ce 2 Fe 17 N 3−δ /graphite composite [ 18 ] , Ce 2 Fe 17 N 3−δ /silicone composite [ 19 ] , Ce 2 Fe 17 N 3−δ /MWCNTs composite [ 20 ] , Ce 2 Fe 17 N 3−δ @SiO 2 composite [ 21 ] , Ce 2 Fe 17 N 3−δ [ 22 ] , Pr 2 Fe 17 N 3−δ [ 23 ] , Dy x Pr 2−x Fe 17 [ 24 ] , Pr 2 Co 17 [ 25 ] , Nd 2 Co 17 [ 26 ] , Nd 10.2 Fe 89.8−x C x [ 27 ] , La x Ho 2−x Fe 17 [ 28 ] , and RE 2 Co 17 (RE = Y, Ce, Nd, Ho, Er) [ 29 ] due to not very high permeability and not very large magnetic loss at low frequency band. Although narrow, the easy plane anisotropy R 2 Me 17 alloys are worthy of study as a type of promising low frequency MA materials, on account of a relatively high permeability and a relatively large magnetic loss at low frequency band compared with other magnetic materials. Such, we believe that the low frequency MA problem can be solved significantly, with the help of the easy plane anisotropy R 2 Me 17 alloys, as long as their permeability and loss at low frequency band are further increased. Generally, it is difficult to increase the permeability and magnetic loss of a material at microwave frequency band, while composites of various magnetic materials are always effective, demonstrated by lots of researches. For the R 2 Fe 17 N 3−δ alloys, we note that there is always a small quantity of α-Fe separated from the R 2 Fe 17 N 3−δ , but reason is unknown and content is not controlled [ 18 – 23 ] . Now that the α-Fe is unavoidable in the process of preparing the R 2 Fe 17 N 3−δ , whether the permeability and magnetic loss at low frequency range can be increased by controlling the content of α-Fe? Based on this thought, in this paper, we prepared the Nd 2 Fe 17 N 3 /α-Fe composite with a high content of α-Fe by using a not very pure N 2 (99.999%) in the process of N-doping. As a result, the permeability, magnetic loss, and MA performance are significantly improved at low frequency range, which are interesting and may be useful for adjusting and controlling the permeability, magnetic loss, and MA performance of other rare earth-transition metal alloys. 2 Experimental section 2.1 Preparation of samples Both metal Nd and Fe bulks were purchased from ZhongNuo Advanced Material (Beijing) Technology Co., Ltd, with a purity of 99.9% and 99.999%, respectively. Before experiments, the Nd bulks need to be manually polished to remove the surface oxide layer. Then, the Nd and Fe bulks were weighed and mixed in a molar ratio of 2:17. In consideration of volatilization of metal Nd in the process of electric arc melting, the metal Nd was weighed in an excess of 8% original weight. The weighed Nd and Fe bulks were further transferred to an arc production furnace produced by Zhengzhou CY Scientific Instrument Co., Ltd. Subsequently, the cavity of arc production furnace was pumped by a mechanical pump for 3 h, and washed for five times using high-purity argon gas of 99.999%. Nd 2 Fe 17 alloy ingot was prepared by melting the Nd and Fe bulks using electric arc in an argon atmosphere under a pressure of 0.02 MPa. To ensure a uniformity of sample, the alloy ingot was melted and cooled for four times, repeatedly. Further, the alloy ingot was sealed in a silica tube with a vacuum degree of ~ 6.3×10 − 2 Pa, annealed at 1300 K for 156 h, and quickly cooled in cold water of ~ 300 K, in order to obtain a high temperature phase of Nd 2 Fe 17 with uniform composition. Flake-shaped Nd 2 Fe 17 /Fe composite with a small amount of Fe was prepared by planetary ball milling method. Ball milling conditions, including ratio of ball milling beads to powder samples, size of ball milling beads, ball milling media, revolving speed of ball milling machine, and time of ball milling, have an important influence on content of Fe in composite. Optimal conditions of ball milling include mass ratio of balls to materials of 20:1, ball milling beads of ZrO of 5.0 mm, ball milling media of absolute ethyl alcohol filled with the ball milling jars, revolving speed of 300 r/min, and ball milling time of 12 h. Flake-shaped Nd 2 Fe 17 N 3 /Fe composite with a large amount of Fe was prepared by N-doping at high temperature. Doping conditions including: purity of N 2 , temperature, and time have an obvious effect on content of Fe in composite. Optimal conditions of N-doping include: N 2 with a purity of 99.999% and 99.9999%, temperature of 780 K, and time of 1.5 h. The preparation processes above-mentioned are also summarized in Fig. 1 (a). Microwave absorbers of Nd 2 Fe 17 N 3 /Fe/epoxy were prepared by mechanically stirring the Nd 2 Fe 17 N 3 /Fe powders and epoxy at a mass ratio of 7:3, then rotating the Nd 2 Fe 17 N 3 /Fe/epoxy composites in a strong magnetic field of approximately 20000 Oe created by two opposite N52-type permanent magnets, until the epoxy was solidified. All samples prepared in this study, respectively, are named as S1, S2, S3, S4, S5 and S6 described in Table 1 . Table 1 Description for the samples prepared by us in this study. Sample Description S1 Nd 2 Fe 17 particles before ball milling S2 Nd 2 Fe 17 micro-flakes after ball milling S3 99.9999% N 2 doping Nd 2 Fe 17 N 3 /Fe micro-flakes S4 99.999% N 2 doping Nd 2 Fe 17 N 3 /Fe micro-flakes S5 99.9999% N 2 doping Nd 2 Fe 17 N 3 /Fe micro-flakes/epoxy composite after magnetic orientation S6 99.999% N 2 doping Nd 2 Fe 17 N 3 /Fe micro-flakes/epoxy composite after magnetic orientation 2.2 Characterization of sample The phase composition of powder samples and crystal structure of each phase were confirmed by measuring the x-ray diffraction (XRD) pattern using a x-ray diffraction instrument (Rigaku TTR-III) with a Cu-K α radiation source (λ = 0.154056 nm), a voltage of 40 kV, an electric current of 200 mA, a step length of 0.01°, and a scanning speed of 5°/min. Fourier transform infrared spectrometer (FTIR, EQUINOX55, Buker, Germany) and Raman spectrometer (HORIBA JY LabRAM HR Evolution) with 532 nm laser source, were used to find minor or amorphous phases, to make up for the deficiency of XRD measurements. Transmission electron microscope (TEM, FEI Tecnai G2 F30) was used to acquire some crystallographic information from microscopic areas of powder particles, which is difficult if finished through the XRD measurements, and can be used to evidence the measuring results of XRD. Meanwhile, the TEM was used to analyze the elemental composition and distribution of powder particles through the mapping and EDS measurements. Before TEM measurements, the powder sample was dispersed into absolute ethyl alcohol only by shaking the dispersed system, without ultrasound, otherwise the crystallinity of sample will be destructed. Then, the dispersion liquid was transferred to a carbon support membrane using a disposable plastic straw, and dried at room temperature of ~ 16 ℃ as soon as possible, to prevent the sample from oxidizing. During TEM measurements, the acceleration voltage was set as 200 kV and the focal distance was adjusted to a state under focus. The morphology and micro-structure of powder samples were observed by a scanning electron microscope (SEM, ZEISS SUPRA55 VR), while the SEM was used to analyze the elemental composition and distribution of powder particles through the mapping and EDS measurements. The surface chemical composition of powder particles was studied by x-ray photoelectron spectra (XPS) measurements using an x-ray photoemission spectrometer (Thermo Scientific Escalab 250Xi) with an Al-K α source, a voltage of 15 kV, and an electric current of 10 mA. The static magnetic properties of powder samples were studied by measuring hysteresis loops at room temperature in magnetic field range of \(\:\pm\:\) 20000 Oe to \(\:\mp\:\) 20000 Oe, using a SQUID-VSM system (MPMS XL, Quantum Design, America). Complex permeability and permittivity of microwave absorbers in the frequency band of 1–18 GHz were measured by a vector network analyzer (VNA, Agilent E8363B) using coaxial method. 3 Results and discussion 3.1 Phase analyses Phase composition of powder samples is first studied by XRD measurements, as shown in Fig. 1 (b). Before ball milling, the sample S1 is mainly composed of Nd 2 Fe 17 , mixed with a little α-Fe. After ball milling, the XRD peaks of α-Fe in the S2 sample are significantly enhanced, which are ascribed to an oxidation of zero-valent Nd in the process of ball milling, leading to α-Fe separated out. After doping of 99.9999% N 2 , the XRD peaks of α-Fe in the S3 sample are further enhanced, due to further oxidation of O 2 and water vapor residual in the 99.9999% N 2 for the zero-valent Nd, leading to α-Fe further separated out. It is worth mentioning that more α-Fe is separated out in the S4 sample after doping of 99.999% N 2 , on account of more O 2 and water vapor residual in the 99.999% N 2 leading to more serious oxidation. Therefore, the α-Fe separated out has different contents in the S3 and S4 samples, just because the N 2 with different purity was used in the process of N-doping, which is significant for tailoring the microwave permeability, magnetic loss, and MA properties, as we will see later. Further, phase composition of powder samples is carefully studied by FTIR and Raman measurements, as shown in Fig. 1 (d) (e), where oxides and hydroxides of Fe and Nd are detected [ 30 – 39 ] , showing that the S3 and S4 samples were indeed oxidized by the O 2 and water vapor residual in the N 2 in the process of N-doping, as we have described above. Moreover, the contents of oxides and hydroxides in the S4 sample are higher than that in the S3 sample, according to the relative peak intensity of the FTIR and Raman spectrums, attributed to more O 2 and water vapor residual in the 99.999% N 2 than the 99.9999% N 2 and more serious oxidation. The oxides or hydroxides of Fe and Nd with high electrical resistivity contribute to a suppressed interfacial polarization, low microwave permittivity, improved impedance matching degree, and better MA performance, as we will see later. Magnetic orientation is always necessary for increasing the microwave permeability, improving the impedance matching degree, and MA performances of magnetic particles, according to lots of reports [ 40 – 43 ] . The XRD of the S5 and S6 samples with magnetic orientation are shown in Fig. 1 (c), where it is observed that the (006) peaks of Nd 2 Fe 17 N 3 phase are dramatically enhanced, compared with the (303) peaks of Nd 2 Fe 17 N 3 phase, which are consistent with related reports, showing that the S5 and S6 samples have been oriented successfully by an applied magnetic field. Meanwhile, it is observed that the (111) peak of α-Fe in the S6 sample is stronger than that of the S5 sample, showing that the content of α-Fe is more in the S6 sample than that in the S5 sample. Further, the phase composition, distribution, and crystallinity of powder samples are studied by measurements of elemental mappings, high-resolution crystal lattice, and selected area electron diffraction using TEM, as shown in Fig. 2 . For the S4 sample, it is observed that the Nd, Fe and N elements are distributed uniformly in the elemental mappings (Fig. 2 (a) (b) (c)), without clear phase boundary, indicating that the α-Fe and Nd 2 Fe 17 N 3 phases may be very fine and interweaving with each other at the nano-scale, which are different from common composites or hybrids. Ultra-fine and interweaving α-Fe and Nd 2 Fe 17 N 3 phases, contribute to a relatively high electrical resistivity, weak electron displacement polarization, and low microwave permittivity due to many phase interfaces. On the other hand, magnetic loss can be increased because of anisotropy on the phase interfaces. From the high-resolution and electron diffraction images (Fig. 2 (e) (f) (g) (h)), it is observed that both the Nd 2 Fe 17 N 3 and α-Fe phases are nano-crystalline with good crystallinity, and coexisting within a nano-scale of ~ 20 nm, showing that the S4 sample, indeed, is one nano-composite, as we analyzed for the elemental mappings, while is one nano-composite of nano-crystalline rather than nano-particle. There are two advantages for this kind of nano-composite. One is that the phases are closely connected and will not be separated from each other, and the other is having good soft magnetic properties due to nano-crystalline. The O elements are distributed uniformly in the elemental mappings (Fig. 2 (d)), showing that the Nd 2 Fe 17 N 3 /α-Fe particles have been uniformly oxidized on the surface, which contributes to a suppressed interfacial polarization and eddy-current loss, resulting in low permittivity and high permeability in the microwave frequency band. Compared with the S4 sample, there are two obvious differences for the S3 sample. One is that the O elements are less (Fig. 2 (l)), and the other is that only the Nd 2 Fe 17 N 3 crystalline phase is detected (Fig. 2 (m) (n) (o) (p)), since the S3 sample was prepared in the N 2 with higher purity, not being heavily oxidized, and without lots of α-Fe separated out, which is consistent with the measuring results of XRD. 3.2 Surface composition The surface composition of S3 and S4 powder samples was studied by XPS measurements after etched using Ar ion beams. Relevant results are shown in Fig. 3 . For the S4 sample, with the increase of etching depth ( ED ), the relative intensity of XPS peaks of zero-valent Fe 0 to Fe 3+ ion gradually increases (Fig. 3 (a)), showing that there is a layer of iron oxide on the surface of Nd 2 Fe 17 N 3 /Fe particles. As the Ar ion etching, gradually, the iron oxide was removed and the zero-valent Fe 0 was exposed. The XPS peaks of Nd 3+ ion are hardly affected by the ED (Fig. 3 (b)), showing that the zero-valent Nd 0 was deeply oxidized from the surface to the inside of Nd 2 Fe 17 N 3 /Fe particles. Such, although the Ar ion etching was adopted, the neodymium oxide was not removed. Compared to the S4 sample, two obvious differences are that the zero-valent Fe 0 was completely exposed after Ar ion etching of only ~ 20 nm (Fig. 3 (c)), and the zero-valent Nd 0 was gradually exposed with the Ar ion etching (Fig. 3 (d)), showing that the surface oxidation of S3 sample is not very serious, relative to the S4 sample, which is consistent with the measuring results of TEM. As we discussed above, the metal oxides on the surface of Nd 2 Fe 17 N 3 /Fe particles contribute to suppressed interface polarization and eddy-current loss, resulting in a low permittivity, a high permeability, a good impedance matching degree in microwave frequency band, and an excellent MA performance. 3.3 Static magnetic properties Static magnetic properties of materials, including saturation magnetization ( M s ) and coercive force ( H c ) are closely associated with microwave permeability and loss. From Fig. 4 (a), it is observed that magnetization ( M ) of S1 sample at \(\:\pm\:\) 20000 Oe and 300 K, equal to 109.2 emu/g, is lower than the M s value reported elsewhere [ 44 ] due to unsaturation magnetization. The M of S2 sample at \(\:\pm\:\) 20000 Oe and 300 K is 115.0 emu/g (Fig. 4 (a)), higher than that of S1 sample due to separation of α-Fe in the S2 sample in the process of ball milling. The M of S3 and S4 samples at \(\:\pm\:\) 20000 Oe and 300 K is 155.1 emu/g and 168.4 emu/g (Fig. 4 (b)), respectively, higher than that of S2 sample due to further separation of α-Fe and formation of Nd 2 Fe 17 N 3 in the process of N 2 doping. The M of S4 sample at \(\:\pm\:\) 20000 Oe and 300 K is higher than that of S3 sample due to more α-Fe separated in the S4 sample in the process of N 2 doping. The M of S5 and S6 samples at \(\:\pm\:\) 20000 Oe and 300 K is 197 emu/g and 212 emu/g (Fig. 4 (c) (d)), respectively, when magnetized along the in-plane direction, greater than the M of S3 and S4 samples at \(\:\pm\:\) 20000 Oe and 300 K due to saturation magnetization. By contrast, when magnetized along the out-of-plane direction, the M of S5 and S6 samples at \(\:\pm\:\) 20000 Oe and 300 K is low due to unsaturation magnetization, equal to 142 emu/g and 158 emu/g, respectively. The difference between the in-plane and out-of-plane magnetization, shows that there is a strong easy plane magnetic anisotropy in the S5 and S6 samples oriented, namely the in-plane magnetization is easier relative to the out-of-plane magnetization. The H c of S1 sample is approximately 29 Oe (inset of Fig. 4 (a)), showing that Nd 2 Fe 17 is not a typical soft magnetic material. The H c of S2 sample reaches 75.9 Oe (inset of Fig. 4 (a)), larger than that of S1 sample because of crystal lattice distortion and internal stress introduced in the process of ball milling. The H c of S3 and S4 samples is 55.8 Oe and 9.5 Oe (inset of Fig. 4 (b)), respectively, smaller compared with the S2 sample, since the internal stresses were greatly eliminated in the process of N 2 doping at high temperature. The H c of S4 sample is obviously smaller than that of S3 sample, attributed to more α-Fe separated in the S4 sample in the process of N 2 doping, which increases the thickness of soft magnetic layer, decreases the gradient of domain wall energy, reduces the pinning field and H c , according to relevant report [ 45 ] . The in-plane and out-of-plane H c of S5 and S6 samples is 56.1 Oe, 107.4 Oe, 0.65 Oe, and 25.7 Oe (inset of Fig. 4 (c) (d)), respectively. It is obvious that the in-plane H c is smaller than the out-of-plane H c for the S5 and S6 samples, further showing a strong easy plane magnetic anisotropy in the S5 and S6 samples oriented. In summary, the more the α-Fe separated from Nd 2 Fe 17 N 3 , the higher the M s and the lower the H c . Additionally, the in-plane H c is obviously smaller than the out-of-plane H c . These factors make the S6 sample with high content of α-Fe possessing an excellent in-plane soft magnetic property, which contribute to a high microwave permeability and good MA performance. 3.4 MA performances MA performances of the S5 and S6 absorbers backed with a metal plate and at normal incidence, are evaluated by calculating the reflection losses (RLs) at different frequencies and thicknesses using the formulas reported elsewhere [ 46 ] , based on the EM parameters of the S5 and S6 absorbers. It is found that both the S5 and S6 absorbers possess very excellent low frequency MA performances ranging from 2 GHz to 8 GHz involving the S band and C band (Fig. 5 (a) (e)). In particular, the S6 absorber with more α-Fe separated out is even better than the S5 absorber with less α-Fe separated out, served as the low frequency microwave absorber, whose minimum reflection loss (RL min ) is lower and maximum effective absorption bandwidth (EAB max ) of RL≤-10 dB is wider (Fig. 5 (c) (d) (g) (h)), up to -59.46 dB at 4.24 GHz at a thickness of 3.60 mm and 4.0 GHz ranging from 4.0 GHz to 8.0 GHz at a thickness of 2.8 mm, respectively. The superior MA performances of S6 absorber are first attributed to its better impedance matching degree, as shown in Fig. 5 (i) (j), where the relative input impedances |Z in /Z 0 | of S6 absorber are closer to 1.0, in comparison with the S5 absorber, on the whole. Such, more EM waves after incidence can enter into the S6 absorber and then be absorbed, instead of being reflected. From Fig. 5 (k), it can be clearly seen that the reflectivity of incident EM waves on the surface of the S6 absorber is lower than that of the S5 absorber at the frequency band of 2–8 GHz. Further, lower reflectivity of the S6 absorber should be ascribed to its lower microwave permittivity (Fig. 7 (a)) and higher microwave permeability (Fig. 7 (d)), resulting in larger |µ r /ε r | values (Fig. 5 (l)) and lower reflectivity. On the other hand, destructive interference between the EM waves reflected by the air/absorber interface and absorber/metal plate interface, plays an important role in the excellent low frequency MA performances of the S5 and S6 absorbers, as shown in Fig. 5 (b) (f), where the data points determined by MA peak frequency and absorber thickness are in good agreement with the t-f curves determined by the quarter wave-length formula ( \(\:t=c/\left(4f\sqrt{{\epsilon\:}_{r}{\mu\:}_{r}}\right)\) ), showing that there is an in-negligible destructive interference. It's worth noting that it is not very perfectly coincident for the S6 absorber on the t-f curve, showing that both the impedance matching and destructive interference are together responsible for the excellent low frequency MA performances of the S6 absorber. To further evaluate the MA performance of the S6 absorber and explore possibility of application, MA bandwidths of magnetic absorbers reported recently and famous for excellent low frequency MA performance, are summarized in Fig. 6 . It is observed that the S6 absorber is the most prominent one among the magnetic absorbers reported, if the MA bandwidth and absorber thickness need to be considered simultaneously, no matter which frequency band of C band (Fig. 6 (a)) and S band (Fig. 6 (b)). As everyone knows, modern electronic devices are working at all kinds of different frequency bands, and have caused serious EM pollution. In this case, employing the microwave absorbers with wider MA frequency band, various EM pollution signals can be widely eliminated. In this respect, the S6 absorber with wide MA bandwidth possesses an incomparable advantage, in comparison with most of the microwave absorbers in Fig. 6 . Meanwhile, modern electronic devices are developing towards the direction of miniaturization. To adapt this kind of new development tendency, microwave absorbers used for anti-EM interference of electronic devices should be thin as much as possible. In this regard, the advantage of S6 absorber with thin absorber thickness is very obvious, compared with the microwave absorbers in Fig. 6 . Thus, the S6 absorber is available for significantly solving the low frequency EM pollution and interference issues of modern electronic devices. Additionally, wide MA bandwidth and thin absorber thickness are strongly desired in military field, to improve the EM stealth performances of military weapons and equipment. On the score, the S6 absorber with wide MA bandwidth and thin absorber thickness have exhibited great potential, as an excellent low frequency EM stealth material. In short, the S6 absorber (Nd 2 Fe 17 N 3 /α-Fe/epoxy composite) with lots of α-Fe separated out have great application prospects in both the civil and military fields, and is worthy of further study by optimizing the elemental composition and ratio of phases. 3.5 EM parameters Complex permittivity ( ε r = ε'+iε'' ) and complex permeability ( µ r = µ'+iµ'' ) are two key parameters determining the impedance matching degree, EM loss capacity, and MA performances. The ε r and µ r of the S5 and S6 absorbers in 2–8 GHz are shown in Fig. 7 (a) and Fig. 7 (d), respectively. From Fig. 7 (a), it is observed that the real part ε' and imaginary part ε'' of ε r of S6 absorber are lower than that of the S5 absorber, which are easy to be understood. Briefly, surface oxidation of the S6 (S4) sample is much more serious than the S5 (S3) sample according to the TEM and XPS measurements, leading to formation of a layer of thinner oxidation film on the surface of the S6 (S4) sample, which suppresses the migration of free electrons, interfacial polarization, and relaxation loss (Fig. 7 (b) (c)), and decreases the ε' and ε'' . Low ε' and ε'' of the S6 absorber contribute to a better impedance matching degree and MA performance. From Fig. 7 (d), it is observed that the real part µ' of µ r of S6 absorber is higher than that of the S5 absorber in 2–4 GHz, and the imaginary part µ'' of µ r of S6 absorber is higher than that of the S5 absorber in 2–8 GHz, which are attributed to the α-Fe separated out, with higher content of α-Fe in the S6 (S4) sample relative to the S5 (S3) sample. To support this view, the µ r -f spectrums of the S5 and S6 absorbers are fitted using the common formulas reported elsewhere [ 85 ] , as shown in Fig. 7 (e) (f). According to the fitted results, the α-Fe separated out has an important contribution to the µ' and µ'' at the low frequency band of 2–8 GHz, while the Nd 2 Fe 17 N 3 phase is responsible for the µ' and µ'' at the entire frequency band of 2–8 GHz. Thus, the more the α-Fe separated out, the higher the µ' and µ'' at the low frequency band of 2–8 GHz. This is the reason why the S6 absorber has a higher µ' and µ'' than the S5 absorber at the low frequency band of 2–4 GHz. On the other hand, the α-Fe has a higher conductivity than the Nd 2 Fe 17 N 3 , due to the metal Nd with lower electrical conductivity than the metal Fe, and because of the N atom, which means that there is a greater eddy current loss in the S6 (S4) sample with more α-Fe separated out, compared with the S5 (S3) sample with less α-Fe separated out. As a result, the µ' of the S6 absorber is lower than that of the S5 absorber and the µ'' of the S6 absorber is higher than that of the S5 absorber when the frequency increases from 4 GHz to 8 GHz, due to eddy-current loss (Fig. 7 (h)). Overall, high µ' and µ'' of the S6 absorber contribute to a better impedance matching degree and MA performance. 3.6 EM losses EM loss capacities are directly determined by the complex permittivity and complex permeability, and further determine the attenuating capacities of materials for the incident EM waves. The EM loss capacities are usually estimated by the dielectric and magnetic loss angle tangents ( \(\:{tan\delta\:}_{e}={\epsilon\:}^{{\prime\:}{\prime\:}}/{\epsilon\:}^{{\prime\:}}\) and \(\:{tan\delta\:}_{m}={\mu\:}^{{\prime\:}{\prime\:}}/{\mu\:}^{{\prime\:}}\) ). The \(\:{tan\delta\:}_{e}\) and \(\:{tan\delta\:}_{m}\) values of the S5 and S6 absorbers are shown in Fig. 7 (g), where it is observed that the \(\:{tan\delta\:}_{e}\) values are much smaller than the \(\:{tan\delta\:}_{m}\) values, indicating that the S5 and S6 absorbers are typical magnetic loss microwave absorbers, rather than dielectric loss absorbers. The \(\:{tan\delta\:}_{e}\) values approximately 0.1 for the S5 absorber and 0.03 for the S6 absorber should be from weak relaxation loss of electrons on the interfaces between phases and on the surfaces of particles, weaker for the S6 absorber because of more serious surface oxidation that greatly suppressed the displacement and relaxation loss of electrons on the surfaces of particles. Thus, the \(\:{tan\delta\:}_{e}\) values of the S6 absorber are lower than that of the S5 absorber. The \(\:{tan\delta\:}_{m}\) values of both the S5 and S6 absorbers are high, while the \(\:{tan\delta\:}_{m}\) values of the S6 absorber are higher than that of the S5 absorber, which are attributed to high natural resonance, large eddy-current loss, and fascinating relaxation loss in the rotating process of magnetic moments (Fig. 7 (g) (h) (i)). Specifically, when the frequency of incident EM waves approaches the natural resonance frequency of the α-Fe and Nd 2 Fe 17 N 3 , magnetic moments in the α-Fe and Nd 2 Fe 17 N 3 will be in precession around their easy magnetization axes, rather than quickly towards their easy magnetization axes. In the precession process of magnetic moments, lots of energy of incident EM waves will be consumed, to help the magnetic moments overcoming the resistance of magneto-crystalline anisotropy field in α-Fe and Nd 2 Fe 17 N 3 (Fig. 7 (i)). Secondly, the free electrons in the α-Fe and Nd 2 Fe 17 N 3 will be in a circling motion namely the eddy current under the action of Lorentz force, when the Nd 2 Fe 17 N 3 /α-Fe particles are in microwave magnetic field. In the process of circling motion, lots of energy of incident EM waves will be dissipated, to help the free electrons overcoming the electrical resistance of α-Fe and Nd 2 Fe 17 N 3 (Fig. 7 (i)). Thirdly, the α-Fe and Nd 2 Fe 17 N 3 phases are interweaving with each other in the Nd 2 Fe 17 N 3 /α-Fe particles, forming lots of phase interfaces, where there is a strong interfacial magnetic anisotropy. To overcome the interfacial magnetic anisotropy, lots of energy of incident EM waves will be lost by magnetic relaxation loss (Fig. 7 (i)), in the flipping process of magnetic moments from the α-Fe (Nd 2 Fe 17 N 3 ) phase to Nd 2 Fe 17 N 3 (α-Fe) phase, under the action of microwave magnetic field. It is the three loss mechanisms described above that make the S5 and S6 absorbers with huge magnetic loss. In particular, the natural resonance loss in α-Fe is higher at the low frequency band of 2–8 GHz, the eddy-current loss is larger in α-Fe at the high frequency band of 2–8 GHz, and the magnetic relaxation loss is higher at the entire frequency band of 2–8 GHz for the S6 absorber with more α-Fe separated out and more phase interfaces formed, resulting in larger magnetic loss in the S6 absorber. 4 Conclusion The purity of N 2 used for N-doping has an obvious effect on the phase composition, microstructure, magnetic, dielectric, EM parameters, and MA performances of the rare earth-transition metal alloy R 2 Me 17 such as the Nd 2 Fe 17 . The lower the purity of N 2 , the more the α-Fe separated out, and the smaller the phase size, contributing to stronger static magnetic properties, larger electrical resistivity, higher microwave permeability, higher microwave magnetic loss, lower microwave permittivity, better impedance matching degree, and more superior MA performances at the low frequency S band and C band than magnetic microwave absorbers reported. Through controlling the purity of N 2 , the Nd 2 Fe 17 N 3 /α-Fe/epoxy composite absorber prepared at present, has exhibited an excellent low frequency MA performance, with a wide MA frequency band ranging from 2 GHz to 4 GHz or from 4 GHz to 8 GHz, when the absorber thickness is 5.0 mm and 2.8 mm, respectively, significant for solving the low frequency EM interference problem. Declarations Competing interests: the authors declare no competing interests. Acknowledgements We thank the staff members of the Physical Property Measurement System ( https://cstr.cn/31125.02.SHMFF.PPMS ) at the Steady High Magnetic Field Facility, CAS ( https://cstr.cn/31125.02.SHMFF ), for providing technical support and assistance in data collection and analysis. We thank the Changsha Meiqi Instrument Equipment Co., Ltd, China (https://mitrcn.cnpowder.com.cn/), for providing a free service of planetary ball milling. We also thank Man Wei and Lanlan Shi from Shiyanjia Lab (www.shiyanjia.com) for the measurements and analysis of TEM and XPS in arrears with payments. Author contributions Peng Wang: Conceptualization, Methodology, Software, Validation, Formal analysis, Investigation, Resources, Data Curation, Writing - Original Draft, Writing - Review & Editing, Visualization, Supervision, Project administration, Funding acquisition; Haiqing Hang, Shibo Wang, Xingcheng Li, WenXiang Jin, Hecheng Zhao, and Keli Zhang: Validation; Yuyan Han: Software, Formal analysis; Jiaheng Wang: Resources. Funding This work is supported by Anhui Provincial Natural Science Foundation (No. K120162089) and Anhui Provincial Natural Science Foundation (No. 2308085ME131). Data availability No datasets were generated or analysed during the current study. References A. Ahlbom, M. Feychting, Electromagnetic radiation: Environmental pollution and health, British Medical Bulletin 68(1) (2003) 157–165. G. Redlarski, B. Lewczuk, A. Żak, A. Koncicki, M. Krawczuk, J. Piechocki, K. Jakubiuk, P. Tojza, J. Jaworski, D. Ambroziak, Ł. Skarbek, D. 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Hatakeyama, Permeability spectra of yttrium iron garnet and its granular composite materials under dc magnetic field, Journal of Applied Physics 110 (2011) 053909. Additional Declarations No competing interests reported. Supplementary Files Supplementarymaterial.docx Abstract.png Cite Share Download PDF Status: Under Review Version 1 posted Editorial decision: Revision requested 31 Mar, 2026 Reviews received at journal 31 Mar, 2026 Reviews received at journal 04 Mar, 2026 Reviewers agreed at journal 04 Mar, 2026 Reviewers agreed at journal 03 Mar, 2026 Reviewers invited by journal 02 Mar, 2026 Editor assigned by journal 02 Mar, 2026 Submission checks completed at journal 11 Jan, 2026 First submitted to journal 10 Jan, 2026 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. 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Wang","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA1klEQVRIiWNgGAWjYDADfgYeEMVMghbJBpK1GBwgVovB8bOHXxdU3LHbfPzsMQmGCuvEBvazB/BrOZOXZj3jzLPkbUCGBMOZ9MQGnrwEvFrMDuSYGfO2HU4GMSQY2w4nNkjwGODXcv4NUMu/w8nG/W+AWv4Ro+VGjvFj3obDdgYSIFsaiNBif+ONGfOMY4cTJG68MbZIOJZu3MaTg1+LZH+O8eeCmsP2/P05hjc+1FjL9rOfwa8FCNikgURiA4iZAOISUg8EzJ9BDiRC4SgYBaNgFIxUAAAX7EaFT6BURQAAAABJRU5ErkJggg==","orcid":"","institution":"Anhui University","correspondingAuthor":true,"prefix":"","firstName":"Peng","middleName":"","lastName":"Wang","suffix":""},{"id":600735709,"identity":"c2281950-635a-492b-9270-82d5829111e3","order_by":1,"name":"Haiqing Hang","email":"","orcid":"","institution":"Anhui University","correspondingAuthor":false,"prefix":"","firstName":"Haiqing","middleName":"","lastName":"Hang","suffix":""},{"id":600735711,"identity":"ec74380c-90de-4379-bee3-eb7965a77973","order_by":2,"name":"Shibo Wang","email":"","orcid":"","institution":"Anhui University","correspondingAuthor":false,"prefix":"","firstName":"Shibo","middleName":"","lastName":"Wang","suffix":""},{"id":600735716,"identity":"c54c730a-965b-492a-b66a-888e779ae1fb","order_by":3,"name":"Hecheng Zhao","email":"","orcid":"","institution":"Anhui University","correspondingAuthor":false,"prefix":"","firstName":"Hecheng","middleName":"","lastName":"Zhao","suffix":""},{"id":600735717,"identity":"491bacca-4032-49bb-a6a0-b772970b22e5","order_by":4,"name":"Keli Zhang","email":"","orcid":"","institution":"Anhui University","correspondingAuthor":false,"prefix":"","firstName":"Keli","middleName":"","lastName":"Zhang","suffix":""},{"id":600735718,"identity":"ee106f2a-65a5-4d40-8c5b-cc746c0a7392","order_by":5,"name":"Xingcheng Li","email":"","orcid":"","institution":"Anhui University","correspondingAuthor":false,"prefix":"","firstName":"Xingcheng","middleName":"","lastName":"Li","suffix":""},{"id":600735719,"identity":"1f950c50-7884-473f-8936-ab555529be4e","order_by":6,"name":"WenXiang Jin","email":"","orcid":"","institution":"Anhui University","correspondingAuthor":false,"prefix":"","firstName":"WenXiang","middleName":"","lastName":"Jin","suffix":""},{"id":600735723,"identity":"3e6bbeaf-3b47-4143-8e0f-cccf4c6a8078","order_by":7,"name":"Yuyan Han","email":"","orcid":"","institution":"Chinese Academy of Sciences","correspondingAuthor":false,"prefix":"","firstName":"Yuyan","middleName":"","lastName":"Han","suffix":""},{"id":600735733,"identity":"ab9cc5bf-aa30-4e41-8d4a-b20754b8ef25","order_by":8,"name":"Jiaheng Wang","email":"","orcid":"","institution":"Hefei University of Technology","correspondingAuthor":false,"prefix":"","firstName":"Jiaheng","middleName":"","lastName":"Wang","suffix":""}],"badges":[],"createdAt":"2026-01-11 03:53:23","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-8571285/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-8571285/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":104022890,"identity":"86e7b419-3c7e-439e-992b-ef65878fba68","added_by":"auto","created_at":"2026-03-05 19:09:15","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":4680914,"visible":true,"origin":"","legend":"\u003cp\u003ePreparation processes of samples (a), XRD of samples before ball milling, after ball milling, and after N-doping (b), XRD of sample after magnetic orientation (c), FTIR spectroscopy of sample after N-doping (d), and Raman spectrum of sample after N-doping (e).\u003c/p\u003e","description":"","filename":"Fig.1.png","url":"https://assets-eu.researchsquare.com/files/rs-8571285/v1/687f2b297d4f428b522ee59a.png"},{"id":104022717,"identity":"b8fd3217-046d-42d0-a038-5d598c0230fe","added_by":"auto","created_at":"2026-03-05 19:08:38","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":31379530,"visible":true,"origin":"","legend":"\u003cp\u003eElemental mappings of the S4 sample (a) (b) (c) (d), high-resolution crystal lattices of the S4 sample (e) (f) (g), selected area electron diffraction of the S4 sample (h), elemental mappings of the S3 sample (i) (j) (k) (l), high-resolution crystal lattices of the S3 sample (m) (n) (o), and selected area electron diffraction of the S3 sample (p).\u003c/p\u003e","description":"","filename":"Fig.2.png","url":"https://assets-eu.researchsquare.com/files/rs-8571285/v1/b2f776074b9740d256a046f8.png"},{"id":104022895,"identity":"bf68c689-1919-4e2e-ac86-68b3fd5d7292","added_by":"auto","created_at":"2026-03-05 19:09:17","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":5427069,"visible":true,"origin":"","legend":"\u003cp\u003eXPS in the Fe2p and Nd3d regions of S4 sample (a) (b), and S3 sample (c) (d).\u003c/p\u003e","description":"","filename":"Fig.3.png","url":"https://assets-eu.researchsquare.com/files/rs-8571285/v1/e0a2848f206628d696f3762b.png"},{"id":104023088,"identity":"48a9aa12-03b3-4473-b8ce-893a840e031a","added_by":"auto","created_at":"2026-03-05 19:09:54","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":1217654,"visible":true,"origin":"","legend":"\u003cp\u003eHysteresis loops at 300 K of S1, S2 samples (a), S3, S4 samples (b), S5 and S6 samples (c) (d).\u003c/p\u003e","description":"","filename":"Fig.4.png","url":"https://assets-eu.researchsquare.com/files/rs-8571285/v1/47d2adef77a8ee875ca4ccec.png"},{"id":104022928,"identity":"f9f1460e-4626-468f-9634-8aa1fb8ed042","added_by":"auto","created_at":"2026-03-05 19:09:28","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":2712802,"visible":true,"origin":"","legend":"\u003cp\u003eOne-dimensional RL-f curves (a) (e), two-dimensional RL maps (c) (g), three-dimensional RL maps (d) (h), input impedance (i) (j), reflectivity (k), |μ\u003csub\u003er\u003c/sub\u003e/ε\u003csub\u003er\u003c/sub\u003e| values (l), and t-f curves (b) (f).\u003c/p\u003e","description":"","filename":"Fig.5.png","url":"https://assets-eu.researchsquare.com/files/rs-8571285/v1/3dd60aaff7122ae58ca93e65.png"},{"id":104022910,"identity":"040c190b-399f-4225-b285-b8240c4f0fbe","added_by":"auto","created_at":"2026-03-05 19:09:24","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":17714870,"visible":true,"origin":"","legend":"\u003cp\u003eMA frequency band of RL≤-10 dB, bandwidth of RL≤-10 dB, and thickness of magnetic microwave absorbers reported recently and working at the low frequency C waveband (a) and S waveband (b), which were obtained from references \u003csup\u003e[4, 7, 28, 47-84]\u003c/sup\u003e previously reported.\u003c/p\u003e","description":"","filename":"Fig.6.png","url":"https://assets-eu.researchsquare.com/files/rs-8571285/v1/962003e983c11cfb2532215c.png"},{"id":104022778,"identity":"076be496-e38e-4fbe-b6e4-1988b9d3b0ef","added_by":"auto","created_at":"2026-03-05 19:08:54","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":4113604,"visible":true,"origin":"","legend":"\u003cp\u003eMeasured frequency spectrums of complex permittivity of S5 and S6 absorbers (a), \u003cem\u003eε''-ε'\u003c/em\u003e curves of S5 and S6 absorbers (b) (c), measured and fitted frequency spectrums of complex permeability of S5 and S6 absorbers (d) (e) (f), magnetic and dielectric loss angle tangent values of S5 and S6 absorbers (g), calculated \u003cem\u003e(f)\u003c/em\u003e\u003csup\u003e\u003cem\u003e-1\u003c/em\u003e\u003c/sup\u003e\u003cem\u003e(μ')\u003c/em\u003e\u003csup\u003e\u003cem\u003e-2\u003c/em\u003e\u003c/sup\u003e\u003cem\u003e(μ'')-f\u003c/em\u003e curves of S5 and S6 absorbers (h), and magnetic loss mechanism diagram of S5 and S6 absorbers (i).\u003c/p\u003e","description":"","filename":"Fig.7.png","url":"https://assets-eu.researchsquare.com/files/rs-8571285/v1/dc6cdd6c0402b3fa5c8b77c1.png"},{"id":104402703,"identity":"9277fc72-ba9c-4a34-8235-f68db5006f85","added_by":"auto","created_at":"2026-03-11 12:16:09","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":77045941,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8571285/v1/0233714e-166d-403c-a500-d4688f2f1e8d.pdf"},{"id":104022922,"identity":"ced7daad-4588-4f43-80e7-47a9bcd7535e","added_by":"auto","created_at":"2026-03-05 19:09:27","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":14744188,"visible":true,"origin":"","legend":"","description":"","filename":"Supplementarymaterial.docx","url":"https://assets-eu.researchsquare.com/files/rs-8571285/v1/d35372d4fac53bc655ad6a51.docx"},{"id":104022802,"identity":"d4a19b7b-3e69-4553-88fb-231f798563b7","added_by":"auto","created_at":"2026-03-05 19:09:00","extension":"png","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":28043567,"visible":true,"origin":"","legend":"","description":"","filename":"Abstract.png","url":"https://assets-eu.researchsquare.com/files/rs-8571285/v1/4571017c5fd3b0b7ef023b61.png"}],"financialInterests":"No competing interests reported.","formattedTitle":"Separation of α-Fe phase from Nd2Fe17N3， dramatically broadening microwave absorption bandwidth at low frequency","fulltext":[{"header":"1 Introduction","content":"\u003cp\u003eElectromagnetic (EM) pollution problem is becoming increasingly serious\u003csup\u003e[\u003cspan additionalcitationids=\"CR2\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]\u003c/sup\u003e, which not only affects stability of sensitive electronic equipment but also endangers human health. Microwave absorption (MA) materials can convert EM energy into thermal energy through EM loss mechanisms, thus available for solving the EM pollution problem. Over the past decade, various MA materials, including dielectric loss materials, magnetic loss materials, and dielectric/magnetic composites\u003csup\u003e[\u003cspan additionalcitationids=\"CR5\" citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]\u003c/sup\u003e, were widely studied and reported, and have exhibited excellent MA performance at high frequency X and K\u003csub\u003eu\u003c/sub\u003e bands. However, there are huge challenges on the MA studies at the low frequency C, S, and L bands, including large absorber thickness and narrow MA frequency band. Researches show that the absorber thickness can be significantly decreased and MA frequency band can be significantly broadened at low frequency band, when magnetic materials are employed\u003csup\u003e[\u003cspan additionalcitationids=\"CR8 CR9 CR10 CR11\" citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]\u003c/sup\u003e, because of their high permeability and large magnetic loss at low frequency band. Moreover, the higher the microwave permeability, the thinner the absorber thickness; the greater the microwave magnetic loss, the wider the MA frequency band. Therefore, we have no choice but increasing the permeability and magnetic loss at low frequency band, to solve the low frequency MA problem.\u003c/p\u003e \u003cp\u003eResearches show that a part of R\u003csub\u003e2\u003c/sub\u003eMe\u003csub\u003e17\u003c/sub\u003e alloys (R\u0026thinsp;=\u0026thinsp;Ce, Pr, Nd, Me\u0026thinsp;=\u0026thinsp;Fe, Co, Ni) possess high permeability and large magnetic loss at a wide frequency range from 0.1 GHz to 18.0 GHz, because of an easy plane anisotropy\u003csup\u003e[\u003cspan additionalcitationids=\"CR14 CR15 CR16\" citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]\u003c/sup\u003e, thus regarded as a type of highly promising MA materials. However, their permeability, magnetic loss, and MA performance at low frequency band need to be further improved, according to related reports. In 2023, Zheng et al. conducted a research on MA of Ce\u003csub\u003e2\u003c/sub\u003eFe\u003csub\u003e17\u003c/sub\u003eN\u003csub\u003e3\u0026minus;δ\u003c/sub\u003e@FePO\u003csub\u003e4\u003c/sub\u003e composite\u003csup\u003e[\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]\u003c/sup\u003e, which exhibited an excellent MA performance at the high frequency band from 12.65 GHz to 18.00 GHz at a very thin thickness of 0.94 mm, while the low frequency MA performance is deficient with a narrow MA frequency band from 5.0 GHz to 6.24 GHz. Similarly, the MA frequency band is marrow at low frequency range for Ce\u003csub\u003e2\u003c/sub\u003eFe\u003csub\u003e17\u003c/sub\u003eN\u003csub\u003e3\u0026minus;δ\u003c/sub\u003e/graphite composite\u003csup\u003e[\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]\u003c/sup\u003e, Ce\u003csub\u003e2\u003c/sub\u003eFe\u003csub\u003e17\u003c/sub\u003eN\u003csub\u003e3\u0026minus;δ\u003c/sub\u003e/silicone composite\u003csup\u003e[\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]\u003c/sup\u003e, Ce\u003csub\u003e2\u003c/sub\u003eFe\u003csub\u003e17\u003c/sub\u003eN\u003csub\u003e3\u0026minus;δ\u003c/sub\u003e/MWCNTs composite\u003csup\u003e[\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]\u003c/sup\u003e, Ce\u003csub\u003e2\u003c/sub\u003eFe\u003csub\u003e17\u003c/sub\u003eN\u003csub\u003e3\u0026minus;δ\u003c/sub\u003e@SiO\u003csub\u003e2\u003c/sub\u003e composite\u003csup\u003e[\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]\u003c/sup\u003e, Ce\u003csub\u003e2\u003c/sub\u003eFe\u003csub\u003e17\u003c/sub\u003eN\u003csub\u003e3\u0026minus;δ\u003c/sub\u003e\u003csup\u003e[\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]\u003c/sup\u003e, Pr\u003csub\u003e2\u003c/sub\u003eFe\u003csub\u003e17\u003c/sub\u003eN\u003csub\u003e3\u0026minus;δ\u003c/sub\u003e\u003csup\u003e[\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]\u003c/sup\u003e, Dy\u003csub\u003ex\u003c/sub\u003ePr\u003csub\u003e2\u0026minus;x\u003c/sub\u003eFe\u003csub\u003e17\u003c/sub\u003e\u003csup\u003e[\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]\u003c/sup\u003e, Pr\u003csub\u003e2\u003c/sub\u003eCo\u003csub\u003e17\u003c/sub\u003e\u003csup\u003e[\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]\u003c/sup\u003e, Nd\u003csub\u003e2\u003c/sub\u003eCo\u003csub\u003e17\u003c/sub\u003e\u003csup\u003e[\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]\u003c/sup\u003e, Nd\u003csub\u003e10.2\u003c/sub\u003eFe\u003csub\u003e89.8\u0026minus;x\u003c/sub\u003eC\u003csub\u003ex\u003c/sub\u003e\u003csup\u003e[\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]\u003c/sup\u003e, La\u003csub\u003ex\u003c/sub\u003eHo\u003csub\u003e2\u0026minus;x\u003c/sub\u003eFe\u003csub\u003e17\u003c/sub\u003e\u003csup\u003e[\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]\u003c/sup\u003e, and RE\u003csub\u003e2\u003c/sub\u003eCo\u003csub\u003e17\u003c/sub\u003e (RE\u0026thinsp;=\u0026thinsp;Y, Ce, Nd, Ho, Er)\u003csup\u003e[\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]\u003c/sup\u003e due to not very high permeability and not very large magnetic loss at low frequency band. Although narrow, the easy plane anisotropy R\u003csub\u003e2\u003c/sub\u003eMe\u003csub\u003e17\u003c/sub\u003e alloys are worthy of study as a type of promising low frequency MA materials, on account of a relatively high permeability and a relatively large magnetic loss at low frequency band compared with other magnetic materials. Such, we believe that the low frequency MA problem can be solved significantly, with the help of the easy plane anisotropy R\u003csub\u003e2\u003c/sub\u003eMe\u003csub\u003e17\u003c/sub\u003e alloys, as long as their permeability and loss at low frequency band are further increased.\u003c/p\u003e \u003cp\u003eGenerally, it is difficult to increase the permeability and magnetic loss of a material at microwave frequency band, while composites of various magnetic materials are always effective, demonstrated by lots of researches. For the R\u003csub\u003e2\u003c/sub\u003eFe\u003csub\u003e17\u003c/sub\u003eN\u003csub\u003e3\u0026minus;δ\u003c/sub\u003e alloys, we note that there is always a small quantity of α-Fe separated from the R\u003csub\u003e2\u003c/sub\u003eFe\u003csub\u003e17\u003c/sub\u003eN\u003csub\u003e3\u0026minus;δ\u003c/sub\u003e, but reason is unknown and content is not controlled\u003csup\u003e[\u003cspan additionalcitationids=\"CR19 CR20 CR21 CR22\" citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]\u003c/sup\u003e. Now that the α-Fe is unavoidable in the process of preparing the R\u003csub\u003e2\u003c/sub\u003eFe\u003csub\u003e17\u003c/sub\u003eN\u003csub\u003e3\u0026minus;δ\u003c/sub\u003e, whether the permeability and magnetic loss at low frequency range can be increased by controlling the content of α-Fe? Based on this thought, in this paper, we prepared the Nd\u003csub\u003e2\u003c/sub\u003eFe\u003csub\u003e17\u003c/sub\u003eN\u003csub\u003e3\u003c/sub\u003e/α-Fe composite with a high content of α-Fe by using a not very pure N\u003csub\u003e2\u003c/sub\u003e (99.999%) in the process of N-doping. As a result, the permeability, magnetic loss, and MA performance are significantly improved at low frequency range, which are interesting and may be useful for adjusting and controlling the permeability, magnetic loss, and MA performance of other rare earth-transition metal alloys.\u003c/p\u003e"},{"header":"2 Experimental section","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Preparation of samples\u003c/h2\u003e \u003cp\u003eBoth metal Nd and Fe bulks were purchased from ZhongNuo Advanced Material (Beijing) Technology Co., Ltd, with a purity of 99.9% and 99.999%, respectively. Before experiments, the Nd bulks need to be manually polished to remove the surface oxide layer. Then, the Nd and Fe bulks were weighed and mixed in a molar ratio of 2:17. In consideration of volatilization of metal Nd in the process of electric arc melting, the metal Nd was weighed in an excess of 8% original weight. The weighed Nd and Fe bulks were further transferred to an arc production furnace produced by Zhengzhou CY Scientific Instrument Co., Ltd. Subsequently, the cavity of arc production furnace was pumped by a mechanical pump for 3 h, and washed for five times using high-purity argon gas of 99.999%. Nd\u003csub\u003e2\u003c/sub\u003eFe\u003csub\u003e17\u003c/sub\u003e alloy ingot was prepared by melting the Nd and Fe bulks using electric arc in an argon atmosphere under a pressure of 0.02 MPa. To ensure a uniformity of sample, the alloy ingot was melted and cooled for four times, repeatedly. Further, the alloy ingot was sealed in a silica tube with a vacuum degree of ~\u0026thinsp;6.3\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e Pa, annealed at 1300 K for 156 h, and quickly cooled in cold water of ~\u0026thinsp;300 K, in order to obtain a high temperature phase of Nd\u003csub\u003e2\u003c/sub\u003eFe\u003csub\u003e17\u003c/sub\u003e with uniform composition. Flake-shaped Nd\u003csub\u003e2\u003c/sub\u003eFe\u003csub\u003e17\u003c/sub\u003e/Fe composite with a small amount of Fe was prepared by planetary ball milling method. Ball milling conditions, including ratio of ball milling beads to powder samples, size of ball milling beads, ball milling media, revolving speed of ball milling machine, and time of ball milling, have an important influence on content of Fe in composite. Optimal conditions of ball milling include mass ratio of balls to materials of 20:1, ball milling beads of ZrO of 5.0 mm, ball milling media of absolute ethyl alcohol filled with the ball milling jars, revolving speed of 300 r/min, and ball milling time of 12 h. Flake-shaped Nd\u003csub\u003e2\u003c/sub\u003eFe\u003csub\u003e17\u003c/sub\u003eN\u003csub\u003e3\u003c/sub\u003e/Fe composite with a large amount of Fe was prepared by N-doping at high temperature. Doping conditions including: purity of N\u003csub\u003e2\u003c/sub\u003e, temperature, and time have an obvious effect on content of Fe in composite. Optimal conditions of N-doping include: N\u003csub\u003e2\u003c/sub\u003e with a purity of 99.999% and 99.9999%, temperature of 780 K, and time of 1.5 h. The preparation processes above-mentioned are also summarized in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e(a). Microwave absorbers of Nd\u003csub\u003e2\u003c/sub\u003eFe\u003csub\u003e17\u003c/sub\u003eN\u003csub\u003e3\u003c/sub\u003e/Fe/epoxy were prepared by mechanically stirring the Nd\u003csub\u003e2\u003c/sub\u003eFe\u003csub\u003e17\u003c/sub\u003eN\u003csub\u003e3\u003c/sub\u003e/Fe powders and epoxy at a mass ratio of 7:3, then rotating the Nd\u003csub\u003e2\u003c/sub\u003eFe\u003csub\u003e17\u003c/sub\u003eN\u003csub\u003e3\u003c/sub\u003e /Fe/epoxy composites in a strong magnetic field of approximately 20000 Oe created by two opposite N52-type permanent magnets, until the epoxy was solidified. All samples prepared in this study, respectively, are named as S1, S2, S3, S4, S5 and S6 described in Table \u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eDescription for the samples prepared by us in this study.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"2\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSample\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eDescription\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eS1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eNd\u003csub\u003e2\u003c/sub\u003eFe\u003csub\u003e17\u003c/sub\u003e particles before ball milling\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eS2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eNd\u003csub\u003e2\u003c/sub\u003eFe\u003csub\u003e17\u003c/sub\u003e micro-flakes after ball milling\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eS3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e99.9999% N\u003csub\u003e2\u003c/sub\u003e doping Nd\u003csub\u003e2\u003c/sub\u003eFe\u003csub\u003e17\u003c/sub\u003eN\u003csub\u003e3\u003c/sub\u003e/Fe micro-flakes\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eS4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e99.999% N\u003csub\u003e2\u003c/sub\u003e doping Nd\u003csub\u003e2\u003c/sub\u003eFe\u003csub\u003e17\u003c/sub\u003eN\u003csub\u003e3\u003c/sub\u003e/Fe micro-flakes\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eS5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e99.9999% N\u003csub\u003e2\u003c/sub\u003e doping Nd\u003csub\u003e2\u003c/sub\u003eFe\u003csub\u003e17\u003c/sub\u003eN\u003csub\u003e3\u003c/sub\u003e/Fe micro-flakes/epoxy composite after magnetic orientation\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eS6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e99.999% N\u003csub\u003e2\u003c/sub\u003e doping Nd\u003csub\u003e2\u003c/sub\u003eFe\u003csub\u003e17\u003c/sub\u003eN\u003csub\u003e3\u003c/sub\u003e/Fe micro-flakes/epoxy composite after magnetic orientation\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Characterization of sample\u003c/h2\u003e \u003cp\u003eThe phase composition of powder samples and crystal structure of each phase were confirmed by measuring the x-ray diffraction (XRD) pattern using a x-ray diffraction instrument (Rigaku TTR-III) with a Cu-K\u003csub\u003eα\u003c/sub\u003e radiation source (λ\u0026thinsp;=\u0026thinsp;0.154056 nm), a voltage of 40 kV, an electric current of 200 mA, a step length of 0.01\u0026deg;, and a scanning speed of 5\u0026deg;/min. Fourier transform infrared spectrometer (FTIR, EQUINOX55, Buker, Germany) and Raman spectrometer (HORIBA JY LabRAM HR Evolution) with 532 nm laser source, were used to find minor or amorphous phases, to make up for the deficiency of XRD measurements. Transmission electron microscope (TEM, FEI Tecnai G2 F30) was used to acquire some crystallographic information from microscopic areas of powder particles, which is difficult if finished through the XRD measurements, and can be used to evidence the measuring results of XRD. Meanwhile, the TEM was used to analyze the elemental composition and distribution of powder particles through the mapping and EDS measurements. Before TEM measurements, the powder sample was dispersed into absolute ethyl alcohol only by shaking the dispersed system, without ultrasound, otherwise the crystallinity of sample will be destructed. Then, the dispersion liquid was transferred to a carbon support membrane using a disposable plastic straw, and dried at room temperature of ~\u0026thinsp;16 ℃ as soon as possible, to prevent the sample from oxidizing. During TEM measurements, the acceleration voltage was set as 200 kV and the focal distance was adjusted to a state under focus. The morphology and micro-structure of powder samples were observed by a scanning electron microscope (SEM, ZEISS SUPRA55 VR), while the SEM was used to analyze the elemental composition and distribution of powder particles through the mapping and EDS measurements. The surface chemical composition of powder particles was studied by x-ray photoelectron spectra (XPS) measurements using an x-ray photoemission spectrometer (Thermo Scientific Escalab 250Xi) with an Al-K\u003csub\u003eα\u003c/sub\u003e source, a voltage of 15 kV, and an electric current of 10 mA. The static magnetic properties of powder samples were studied by measuring hysteresis loops at room temperature in magnetic field range of \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\pm\\:\\)\u003c/span\u003e\u003c/span\u003e20000 Oe to \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\mp\\:\\)\u003c/span\u003e\u003c/span\u003e20000 Oe, using a SQUID-VSM system (MPMS XL, Quantum Design, America). Complex permeability and permittivity of microwave absorbers in the frequency band of 1\u0026ndash;18 GHz were measured by a vector network analyzer (VNA, Agilent E8363B) using coaxial method.\u003c/p\u003e \u003c/div\u003e"},{"header":"3 Results and discussion","content":"\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e3.1 Phase analyses\u003c/h2\u003e \u003cp\u003ePhase composition of powder samples is first studied by XRD measurements, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e(b). Before ball milling, the sample S1 is mainly composed of Nd\u003csub\u003e2\u003c/sub\u003eFe\u003csub\u003e17\u003c/sub\u003e, mixed with a little α-Fe. After ball milling, the XRD peaks of α-Fe in the S2 sample are significantly enhanced, which are ascribed to an oxidation of zero-valent Nd in the process of ball milling, leading to α-Fe separated out. After doping of 99.9999% N\u003csub\u003e2\u003c/sub\u003e, the XRD peaks of α-Fe in the S3 sample are further enhanced, due to further oxidation of O\u003csub\u003e2\u003c/sub\u003e and water vapor residual in the 99.9999% N\u003csub\u003e2\u003c/sub\u003e for the zero-valent Nd, leading to α-Fe further separated out. It is worth mentioning that more α-Fe is separated out in the S4 sample after doping of 99.999% N\u003csub\u003e2\u003c/sub\u003e, on account of more O\u003csub\u003e2\u003c/sub\u003e and water vapor residual in the 99.999% N\u003csub\u003e2\u003c/sub\u003e leading to more serious oxidation. Therefore, the α-Fe separated out has different contents in the S3 and S4 samples, just because the N\u003csub\u003e2\u003c/sub\u003e with different purity was used in the process of N-doping, which is significant for tailoring the microwave permeability, magnetic loss, and MA properties, as we will see later. Further, phase composition of powder samples is carefully studied by FTIR and Raman measurements, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e(d) (e), where oxides and hydroxides of Fe and Nd are detected\u003csup\u003e[\u003cspan additionalcitationids=\"CR31 CR32 CR33 CR34 CR35 CR36 CR37 CR38\" citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e]\u003c/sup\u003e, showing that the S3 and S4 samples were indeed oxidized by the O\u003csub\u003e2\u003c/sub\u003e and water vapor residual in the N\u003csub\u003e2\u003c/sub\u003e in the process of N-doping, as we have described above. Moreover, the contents of oxides and hydroxides in the S4 sample are higher than that in the S3 sample, according to the relative peak intensity of the FTIR and Raman spectrums, attributed to more O\u003csub\u003e2\u003c/sub\u003e and water vapor residual in the 99.999% N\u003csub\u003e2\u003c/sub\u003e than the 99.9999% N\u003csub\u003e2\u003c/sub\u003e and more serious oxidation. The oxides or hydroxides of Fe and Nd with high electrical resistivity contribute to a suppressed interfacial polarization, low microwave permittivity, improved impedance matching degree, and better MA performance, as we will see later. Magnetic orientation is always necessary for increasing the microwave permeability, improving the impedance matching degree, and MA performances of magnetic particles, according to lots of reports\u003csup\u003e[\u003cspan additionalcitationids=\"CR41 CR42\" citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e]\u003c/sup\u003e. The XRD of the S5 and S6 samples with magnetic orientation are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e(c), where it is observed that the (006) peaks of Nd\u003csub\u003e2\u003c/sub\u003eFe\u003csub\u003e17\u003c/sub\u003eN\u003csub\u003e3\u003c/sub\u003e phase are dramatically enhanced, compared with the (303) peaks of Nd\u003csub\u003e2\u003c/sub\u003eFe\u003csub\u003e17\u003c/sub\u003eN\u003csub\u003e3\u003c/sub\u003e phase, which are consistent with related reports, showing that the S5 and S6 samples have been oriented successfully by an applied magnetic field. Meanwhile, it is observed that the (111) peak of α-Fe in the S6 sample is stronger than that of the S5 sample, showing that the content of α-Fe is more in the S6 sample than that in the S5 sample.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFurther, the phase composition, distribution, and crystallinity of powder samples are studied by measurements of elemental mappings, high-resolution crystal lattice, and selected area electron diffraction using TEM, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. For the S4 sample, it is observed that the Nd, Fe and N elements are distributed uniformly in the elemental mappings (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(a) (b) (c)), without clear phase boundary, indicating that the α-Fe and Nd\u003csub\u003e2\u003c/sub\u003eFe\u003csub\u003e17\u003c/sub\u003eN\u003csub\u003e3\u003c/sub\u003e phases may be very fine and interweaving with each other at the nano-scale, which are different from common composites or hybrids. Ultra-fine and interweaving α-Fe and Nd\u003csub\u003e2\u003c/sub\u003eFe\u003csub\u003e17\u003c/sub\u003eN\u003csub\u003e3\u003c/sub\u003e phases, contribute to a relatively high electrical resistivity, weak electron displacement polarization, and low microwave permittivity due to many phase interfaces. On the other hand, magnetic loss can be increased because of anisotropy on the phase interfaces. From the high-resolution and electron diffraction images (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(e) (f) (g) (h)), it is observed that both the Nd\u003csub\u003e2\u003c/sub\u003eFe\u003csub\u003e17\u003c/sub\u003eN\u003csub\u003e3\u003c/sub\u003e and α-Fe phases are nano-crystalline with good crystallinity, and coexisting within a nano-scale of ~\u0026thinsp;20 nm, showing that the S4 sample, indeed, is one nano-composite, as we analyzed for the elemental mappings, while is one nano-composite of nano-crystalline rather than nano-particle. There are two advantages for this kind of nano-composite. One is that the phases are closely connected and will not be separated from each other, and the other is having good soft magnetic properties due to nano-crystalline. The O elements are distributed uniformly in the elemental mappings (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(d)), showing that the Nd\u003csub\u003e2\u003c/sub\u003eFe\u003csub\u003e17\u003c/sub\u003eN\u003csub\u003e3\u003c/sub\u003e/α-Fe particles have been uniformly oxidized on the surface, which contributes to a suppressed interfacial polarization and eddy-current loss, resulting in low permittivity and high permeability in the microwave frequency band. Compared with the S4 sample, there are two obvious differences for the S3 sample. One is that the O elements are less (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(l)), and the other is that only the Nd\u003csub\u003e2\u003c/sub\u003eFe\u003csub\u003e17\u003c/sub\u003eN\u003csub\u003e3\u003c/sub\u003e crystalline phase is detected (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(m) (n) (o) (p)), since the S3 sample was prepared in the N\u003csub\u003e2\u003c/sub\u003e with higher purity, not being heavily oxidized, and without lots of α-Fe separated out, which is consistent with the measuring results of XRD.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003e3.2 Surface composition\u003c/h2\u003e \u003cp\u003eThe surface composition of S3 and S4 powder samples was studied by XPS measurements after etched using Ar ion beams. Relevant results are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. For the S4 sample, with the increase of etching depth (\u003cem\u003eED\u003c/em\u003e), the relative intensity of XPS peaks of zero-valent Fe\u003csup\u003e0\u003c/sup\u003e to Fe\u003csup\u003e3+\u003c/sup\u003e ion gradually increases (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(a)), showing that there is a layer of iron oxide on the surface of Nd\u003csub\u003e2\u003c/sub\u003eFe\u003csub\u003e17\u003c/sub\u003eN\u003csub\u003e3\u003c/sub\u003e/Fe particles. As the Ar ion etching, gradually, the iron oxide was removed and the zero-valent Fe\u003csup\u003e0\u003c/sup\u003e was exposed. The XPS peaks of Nd\u003csup\u003e3+\u003c/sup\u003e ion are hardly affected by the \u003cem\u003eED\u003c/em\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(b)), showing that the zero-valent Nd\u003csup\u003e0\u003c/sup\u003e was deeply oxidized from the surface to the inside of Nd\u003csub\u003e2\u003c/sub\u003eFe\u003csub\u003e17\u003c/sub\u003eN\u003csub\u003e3\u003c/sub\u003e/Fe particles. Such, although the Ar ion etching was adopted, the neodymium oxide was not removed. Compared to the S4 sample, two obvious differences are that the zero-valent Fe\u003csup\u003e0\u003c/sup\u003e was completely exposed after Ar ion etching of only\u0026thinsp;~\u0026thinsp;20 nm (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(c)), and the zero-valent Nd\u003csup\u003e0\u003c/sup\u003e was gradually exposed with the Ar ion etching (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(d)), showing that the surface oxidation of S3 sample is not very serious, relative to the S4 sample, which is consistent with the measuring results of TEM. As we discussed above, the metal oxides on the surface of Nd\u003csub\u003e2\u003c/sub\u003eFe\u003csub\u003e17\u003c/sub\u003eN\u003csub\u003e3\u003c/sub\u003e/Fe particles contribute to suppressed interface polarization and eddy-current loss, resulting in a low permittivity, a high permeability, a good impedance matching degree in microwave frequency band, and an excellent MA performance.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003e3.3 Static magnetic properties\u003c/h2\u003e \u003cp\u003eStatic magnetic properties of materials, including saturation magnetization (\u003cem\u003eM\u003c/em\u003e\u003csub\u003e\u003cem\u003es\u003c/em\u003e\u003c/sub\u003e) and coercive force (\u003cem\u003eH\u003c/em\u003e\u003csub\u003e\u003cem\u003ec\u003c/em\u003e\u003c/sub\u003e) are closely associated with microwave permeability and loss. From Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(a), it is observed that magnetization (\u003cem\u003eM\u003c/em\u003e) of S1 sample at \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\pm\\:\\)\u003c/span\u003e\u003c/span\u003e20000 Oe and 300 K, equal to 109.2 emu/g, is lower than the \u003cem\u003eM\u003c/em\u003e\u003csub\u003e\u003cem\u003es\u003c/em\u003e\u003c/sub\u003e value reported elsewhere\u003csup\u003e[\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e]\u003c/sup\u003e due to unsaturation magnetization. The \u003cem\u003eM\u003c/em\u003e of S2 sample at \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\pm\\:\\)\u003c/span\u003e\u003c/span\u003e20000 Oe and 300 K is 115.0 emu/g (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(a)), higher than that of S1 sample due to separation of α-Fe in the S2 sample in the process of ball milling. The \u003cem\u003eM\u003c/em\u003e of S3 and S4 samples at \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\pm\\:\\)\u003c/span\u003e\u003c/span\u003e20000 Oe and 300 K is 155.1 emu/g and 168.4 emu/g (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(b)), respectively, higher than that of S2 sample due to further separation of α-Fe and formation of Nd\u003csub\u003e2\u003c/sub\u003eFe\u003csub\u003e17\u003c/sub\u003eN\u003csub\u003e3\u003c/sub\u003e in the process of N\u003csub\u003e2\u003c/sub\u003e doping. The \u003cem\u003eM\u003c/em\u003e of S4 sample at \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\pm\\:\\)\u003c/span\u003e\u003c/span\u003e20000 Oe and 300 K is higher than that of S3 sample due to more α-Fe separated in the S4 sample in the process of N\u003csub\u003e2\u003c/sub\u003e doping. The \u003cem\u003eM\u003c/em\u003e of S5 and S6 samples at \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\pm\\:\\)\u003c/span\u003e\u003c/span\u003e20000 Oe and 300 K is 197 emu/g and 212 emu/g (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(c) (d)), respectively, when magnetized along the in-plane direction, greater than the \u003cem\u003eM\u003c/em\u003e of S3 and S4 samples at \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\pm\\:\\)\u003c/span\u003e\u003c/span\u003e20000 Oe and 300 K due to saturation magnetization. By contrast, when magnetized along the out-of-plane direction, the \u003cem\u003eM\u003c/em\u003e of S5 and S6 samples at \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\pm\\:\\)\u003c/span\u003e\u003c/span\u003e20000 Oe and 300 K is low due to unsaturation magnetization, equal to 142 emu/g and 158 emu/g, respectively. The difference between the in-plane and out-of-plane magnetization, shows that there is a strong easy plane magnetic anisotropy in the S5 and S6 samples oriented, namely the in-plane magnetization is easier relative to the out-of-plane magnetization. The \u003cem\u003eH\u003c/em\u003e\u003csub\u003e\u003cem\u003ec\u003c/em\u003e\u003c/sub\u003e of S1 sample is approximately 29 Oe (inset of Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(a)), showing that Nd\u003csub\u003e2\u003c/sub\u003eFe\u003csub\u003e17\u003c/sub\u003e is not a typical soft magnetic material. The \u003cem\u003eH\u003c/em\u003e\u003csub\u003e\u003cem\u003ec\u003c/em\u003e\u003c/sub\u003e of S2 sample reaches 75.9 Oe (inset of Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(a)), larger than that of S1 sample because of crystal lattice distortion and internal stress introduced in the process of ball milling. The \u003cem\u003eH\u003c/em\u003e\u003csub\u003e\u003cem\u003ec\u003c/em\u003e\u003c/sub\u003e of S3 and S4 samples is 55.8 Oe and 9.5 Oe (inset of Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(b)), respectively, smaller compared with the S2 sample, since the internal stresses were greatly eliminated in the process of N\u003csub\u003e2\u003c/sub\u003e doping at high temperature. The \u003cem\u003eH\u003c/em\u003e\u003csub\u003e\u003cem\u003ec\u003c/em\u003e\u003c/sub\u003e of S4 sample is obviously smaller than that of S3 sample, attributed to more α-Fe separated in the S4 sample in the process of N\u003csub\u003e2\u003c/sub\u003e doping, which increases the thickness of soft magnetic layer, decreases the gradient of domain wall energy, reduces the pinning field and \u003cem\u003eH\u003c/em\u003e\u003csub\u003e\u003cem\u003ec\u003c/em\u003e\u003c/sub\u003e, according to relevant report\u003csup\u003e[\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e]\u003c/sup\u003e. The in-plane and out-of-plane \u003cem\u003eH\u003c/em\u003e\u003csub\u003e\u003cem\u003ec\u003c/em\u003e\u003c/sub\u003e of S5 and S6 samples is 56.1 Oe, 107.4 Oe, 0.65 Oe, and 25.7 Oe (inset of Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(c) (d)), respectively. It is obvious that the in-plane \u003cem\u003eH\u003c/em\u003e\u003csub\u003e\u003cem\u003ec\u003c/em\u003e\u003c/sub\u003e is smaller than the out-of-plane \u003cem\u003eH\u003c/em\u003e\u003csub\u003e\u003cem\u003ec\u003c/em\u003e\u003c/sub\u003e for the S5 and S6 samples, further showing a strong easy plane magnetic anisotropy in the S5 and S6 samples oriented. In summary, the more the α-Fe separated from Nd\u003csub\u003e2\u003c/sub\u003eFe\u003csub\u003e17\u003c/sub\u003eN\u003csub\u003e3\u003c/sub\u003e, the higher the \u003cem\u003eM\u003c/em\u003e\u003csub\u003e\u003cem\u003es\u003c/em\u003e\u003c/sub\u003e and the lower the \u003cem\u003eH\u003c/em\u003e\u003csub\u003e\u003cem\u003ec\u003c/em\u003e\u003c/sub\u003e. Additionally, the in-plane \u003cem\u003eH\u003c/em\u003e\u003csub\u003e\u003cem\u003ec\u003c/em\u003e\u003c/sub\u003e is obviously smaller than the out-of-plane \u003cem\u003eH\u003c/em\u003e\u003csub\u003e\u003cem\u003ec\u003c/em\u003e\u003c/sub\u003e. These factors make the S6 sample with high content of α-Fe possessing an excellent in-plane soft magnetic property, which contribute to a high microwave permeability and good MA performance.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003e3.4 MA performances\u003c/h2\u003e \u003cp\u003eMA performances of the S5 and S6 absorbers backed with a metal plate and at normal incidence, are evaluated by calculating the reflection losses (RLs) at different frequencies and thicknesses using the formulas reported elsewhere\u003csup\u003e[\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e]\u003c/sup\u003e, based on the EM parameters of the S5 and S6 absorbers. It is found that both the S5 and S6 absorbers possess very excellent low frequency MA performances ranging from 2 GHz to 8 GHz involving the S band and C band (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(a) (e)). In particular, the S6 absorber with more α-Fe separated out is even better than the S5 absorber with less α-Fe separated out, served as the low frequency microwave absorber, whose minimum reflection loss (RL\u003csub\u003emin\u003c/sub\u003e) is lower and maximum effective absorption bandwidth (EAB\u003csub\u003emax\u003c/sub\u003e) of RL\u0026le;-10 dB is wider (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(c) (d) (g) (h)), up to -59.46 dB at 4.24 GHz at a thickness of 3.60 mm and 4.0 GHz ranging from 4.0 GHz to 8.0 GHz at a thickness of 2.8 mm, respectively. The superior MA performances of S6 absorber are first attributed to its better impedance matching degree, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(i) (j), where the relative input impedances |Z\u003csub\u003ein\u003c/sub\u003e/Z\u003csub\u003e0\u003c/sub\u003e| of S6 absorber are closer to 1.0, in comparison with the S5 absorber, on the whole. Such, more EM waves after incidence can enter into the S6 absorber and then be absorbed, instead of being reflected. From Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(k), it can be clearly seen that the reflectivity of incident EM waves on the surface of the S6 absorber is lower than that of the S5 absorber at the frequency band of 2\u0026ndash;8 GHz. Further, lower reflectivity of the S6 absorber should be ascribed to its lower microwave permittivity (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e(a)) and higher microwave permeability (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e(d)), resulting in larger |\u0026micro;\u003csub\u003er\u003c/sub\u003e/ε\u003csub\u003er\u003c/sub\u003e| values (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(l)) and lower reflectivity. On the other hand, destructive interference between the EM waves reflected by the air/absorber interface and absorber/metal plate interface, plays an important role in the excellent low frequency MA performances of the S5 and S6 absorbers, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(b) (f), where the data points determined by MA peak frequency and absorber thickness are in good agreement with the \u003cem\u003et-f\u003c/em\u003e curves determined by the quarter wave-length formula (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:t=c/\\left(4f\\sqrt{{\\epsilon\\:}_{r}{\\mu\\:}_{r}}\\right)\\)\u003c/span\u003e\u003c/span\u003e), showing that there is an in-negligible destructive interference. It's worth noting that it is not very perfectly coincident for the S6 absorber on the \u003cem\u003et-f\u003c/em\u003e curve, showing that both the impedance matching and destructive interference are together responsible for the excellent low frequency MA performances of the S6 absorber.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTo further evaluate the MA performance of the S6 absorber and explore possibility of application, MA bandwidths of magnetic absorbers reported recently and famous for excellent low frequency MA performance, are summarized in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e. It is observed that the S6 absorber is the most prominent one among the magnetic absorbers reported, if the MA bandwidth and absorber thickness need to be considered simultaneously, no matter which frequency band of C band (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e(a)) and S band (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e(b)). As everyone knows, modern electronic devices are working at all kinds of different frequency bands, and have caused serious EM pollution. In this case, employing the microwave absorbers with wider MA frequency band, various EM pollution signals can be widely eliminated. In this respect, the S6 absorber with wide MA bandwidth possesses an incomparable advantage, in comparison with most of the microwave absorbers in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e. Meanwhile, modern electronic devices are developing towards the direction of miniaturization. To adapt this kind of new development tendency, microwave absorbers used for anti-EM interference of electronic devices should be thin as much as possible. In this regard, the advantage of S6 absorber with thin absorber thickness is very obvious, compared with the microwave absorbers in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e. Thus, the S6 absorber is available for significantly solving the low frequency EM pollution and interference issues of modern electronic devices. Additionally, wide MA bandwidth and thin absorber thickness are strongly desired in military field, to improve the EM stealth performances of military weapons and equipment. On the score, the S6 absorber with wide MA bandwidth and thin absorber thickness have exhibited great potential, as an excellent low frequency EM stealth material. In short, the S6 absorber (Nd\u003csub\u003e2\u003c/sub\u003eFe\u003csub\u003e17\u003c/sub\u003eN\u003csub\u003e3\u003c/sub\u003e/α-Fe/epoxy composite) with lots of α-Fe separated out have great application prospects in both the civil and military fields, and is worthy of further study by optimizing the elemental composition and ratio of phases.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003e3.5 EM parameters\u003c/h2\u003e \u003cp\u003eComplex permittivity (\u003cem\u003eε\u003c/em\u003e\u003csub\u003e\u003cem\u003er\u003c/em\u003e\u003c/sub\u003e\u0026thinsp;\u003cem\u003e=\u0026thinsp;ε'+iε''\u003c/em\u003e) and complex permeability (\u003cem\u003e\u0026micro;\u003c/em\u003e\u003csub\u003e\u003cem\u003er\u003c/em\u003e\u003c/sub\u003e\u0026thinsp;\u003cem\u003e=\u0026thinsp;\u0026micro;'+i\u0026micro;''\u003c/em\u003e) are two key parameters determining the impedance matching degree, EM loss capacity, and MA performances. The \u003cem\u003eε\u003c/em\u003e\u003csub\u003e\u003cem\u003er\u003c/em\u003e\u003c/sub\u003e and \u003cem\u003e\u0026micro;\u003c/em\u003e\u003csub\u003e\u003cem\u003er\u003c/em\u003e\u003c/sub\u003e of the S5 and S6 absorbers in 2\u0026ndash;8 GHz are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e(a) and Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e(d), respectively. From Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e(a), it is observed that the real part \u003cem\u003eε'\u003c/em\u003e and imaginary part \u003cem\u003eε''\u003c/em\u003e of \u003cem\u003eε\u003c/em\u003e\u003csub\u003e\u003cem\u003er\u003c/em\u003e\u003c/sub\u003e of S6 absorber are lower than that of the S5 absorber, which are easy to be understood. Briefly, surface oxidation of the S6 (S4) sample is much more serious than the S5 (S3) sample according to the TEM and XPS measurements, leading to formation of a layer of thinner oxidation film on the surface of the S6 (S4) sample, which suppresses the migration of free electrons, interfacial polarization, and relaxation loss (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e(b) (c)), and decreases the \u003cem\u003eε'\u003c/em\u003e and \u003cem\u003eε''\u003c/em\u003e. Low \u003cem\u003eε'\u003c/em\u003e and \u003cem\u003eε''\u003c/em\u003e of the S6 absorber contribute to a better impedance matching degree and MA performance. From Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e(d), it is observed that the real part \u003cem\u003e\u0026micro;'\u003c/em\u003e of \u003cem\u003e\u0026micro;\u003c/em\u003e\u003csub\u003e\u003cem\u003er\u003c/em\u003e\u003c/sub\u003e of S6 absorber is higher than that of the S5 absorber in 2\u0026ndash;4 GHz, and the imaginary part \u003cem\u003e\u0026micro;''\u003c/em\u003e of \u003cem\u003e\u0026micro;\u003c/em\u003e\u003csub\u003e\u003cem\u003er\u003c/em\u003e\u003c/sub\u003e of S6 absorber is higher than that of the S5 absorber in 2\u0026ndash;8 GHz, which are attributed to the α-Fe separated out, with higher content of α-Fe in the S6 (S4) sample relative to the S5 (S3) sample. To support this view, the \u003cem\u003e\u0026micro;\u003c/em\u003e\u003csub\u003e\u003cem\u003er\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e-f\u003c/em\u003e spectrums of the S5 and S6 absorbers are fitted using the common formulas reported elsewhere\u003csup\u003e[\u003cspan citationid=\"CR85\" class=\"CitationRef\"\u003e85\u003c/span\u003e]\u003c/sup\u003e, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e(e) (f). According to the fitted results, the α-Fe separated out has an important contribution to the \u003cem\u003e\u0026micro;'\u003c/em\u003e and \u003cem\u003e\u0026micro;''\u003c/em\u003e at the low frequency band of 2\u0026ndash;8 GHz, while the Nd\u003csub\u003e2\u003c/sub\u003eFe\u003csub\u003e17\u003c/sub\u003eN\u003csub\u003e3\u003c/sub\u003e phase is responsible for the \u003cem\u003e\u0026micro;'\u003c/em\u003e and \u003cem\u003e\u0026micro;''\u003c/em\u003e at the entire frequency band of 2\u0026ndash;8 GHz. Thus, the more the α-Fe separated out, the higher the \u003cem\u003e\u0026micro;'\u003c/em\u003e and \u003cem\u003e\u0026micro;''\u003c/em\u003e at the low frequency band of 2\u0026ndash;8 GHz. This is the reason why the S6 absorber has a higher \u003cem\u003e\u0026micro;'\u003c/em\u003e and \u003cem\u003e\u0026micro;''\u003c/em\u003e than the S5 absorber at the low frequency band of 2\u0026ndash;4 GHz. On the other hand, the α-Fe has a higher conductivity than the Nd\u003csub\u003e2\u003c/sub\u003eFe\u003csub\u003e17\u003c/sub\u003eN\u003csub\u003e3\u003c/sub\u003e, due to the metal Nd with lower electrical conductivity than the metal Fe, and because of the N atom, which means that there is a greater eddy current loss in the S6 (S4) sample with more α-Fe separated out, compared with the S5 (S3) sample with less α-Fe separated out. As a result, the \u003cem\u003e\u0026micro;'\u003c/em\u003e of the S6 absorber is lower than that of the S5 absorber and the \u003cem\u003e\u0026micro;''\u003c/em\u003e of the S6 absorber is higher than that of the S5 absorber when the frequency increases from 4 GHz to 8 GHz, due to eddy-current loss (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e(h)). Overall, high \u003cem\u003e\u0026micro;'\u003c/em\u003e and \u003cem\u003e\u0026micro;''\u003c/em\u003e of the S6 absorber contribute to a better impedance matching degree and MA performance.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003e3.6 EM losses\u003c/h2\u003e \u003cp\u003eEM loss capacities are directly determined by the complex permittivity and complex permeability, and further determine the attenuating capacities of materials for the incident EM waves. The EM loss capacities are usually estimated by the dielectric and magnetic loss angle tangents (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{tan\\delta\\:}_{e}={\\epsilon\\:}^{{\\prime\\:}{\\prime\\:}}/{\\epsilon\\:}^{{\\prime\\:}}\\)\u003c/span\u003e\u003c/span\u003e and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{tan\\delta\\:}_{m}={\\mu\\:}^{{\\prime\\:}{\\prime\\:}}/{\\mu\\:}^{{\\prime\\:}}\\)\u003c/span\u003e\u003c/span\u003e). The \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{tan\\delta\\:}_{e}\\)\u003c/span\u003e\u003c/span\u003e and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{tan\\delta\\:}_{m}\\)\u003c/span\u003e\u003c/span\u003e values of the S5 and S6 absorbers are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e(g), where it is observed that the \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{tan\\delta\\:}_{e}\\)\u003c/span\u003e\u003c/span\u003e values are much smaller than the \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{tan\\delta\\:}_{m}\\)\u003c/span\u003e\u003c/span\u003e values, indicating that the S5 and S6 absorbers are typical magnetic loss microwave absorbers, rather than dielectric loss absorbers. The \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{tan\\delta\\:}_{e}\\)\u003c/span\u003e\u003c/span\u003e values approximately 0.1 for the S5 absorber and 0.03 for the S6 absorber should be from weak relaxation loss of electrons on the interfaces between phases and on the surfaces of particles, weaker for the S6 absorber because of more serious surface oxidation that greatly suppressed the displacement and relaxation loss of electrons on the surfaces of particles. Thus, the \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{tan\\delta\\:}_{e}\\)\u003c/span\u003e\u003c/span\u003e values of the S6 absorber are lower than that of the S5 absorber. The \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{tan\\delta\\:}_{m}\\)\u003c/span\u003e\u003c/span\u003e values of both the S5 and S6 absorbers are high, while the \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{tan\\delta\\:}_{m}\\)\u003c/span\u003e\u003c/span\u003e values of the S6 absorber are higher than that of the S5 absorber, which are attributed to high natural resonance, large eddy-current loss, and fascinating relaxation loss in the rotating process of magnetic moments (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e(g) (h) (i)).\u003c/p\u003e \u003cp\u003eSpecifically, when the frequency of incident EM waves approaches the natural resonance frequency of the α-Fe and Nd\u003csub\u003e2\u003c/sub\u003eFe\u003csub\u003e17\u003c/sub\u003eN\u003csub\u003e3\u003c/sub\u003e, magnetic moments in the α-Fe and Nd\u003csub\u003e2\u003c/sub\u003eFe\u003csub\u003e17\u003c/sub\u003eN\u003csub\u003e3\u003c/sub\u003e will be in precession around their easy magnetization axes, rather than quickly towards their easy magnetization axes. In the precession process of magnetic moments, lots of energy of incident EM waves will be consumed, to help the magnetic moments overcoming the resistance of magneto-crystalline anisotropy field in α-Fe and Nd\u003csub\u003e2\u003c/sub\u003eFe\u003csub\u003e17\u003c/sub\u003eN\u003csub\u003e3\u003c/sub\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e(i)). Secondly, the free electrons in the α-Fe and Nd\u003csub\u003e2\u003c/sub\u003eFe\u003csub\u003e17\u003c/sub\u003eN\u003csub\u003e3\u003c/sub\u003e will be in a circling motion namely the eddy current under the action of Lorentz force, when the Nd\u003csub\u003e2\u003c/sub\u003eFe\u003csub\u003e17\u003c/sub\u003eN\u003csub\u003e3\u003c/sub\u003e/α-Fe particles are in microwave magnetic field. In the process of circling motion, lots of energy of incident EM waves will be dissipated, to help the free electrons overcoming the electrical resistance of α-Fe and Nd\u003csub\u003e2\u003c/sub\u003eFe\u003csub\u003e17\u003c/sub\u003eN\u003csub\u003e3\u003c/sub\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e(i)). Thirdly, the α-Fe and Nd\u003csub\u003e2\u003c/sub\u003eFe\u003csub\u003e17\u003c/sub\u003eN\u003csub\u003e3\u003c/sub\u003e phases are interweaving with each other in the Nd\u003csub\u003e2\u003c/sub\u003eFe\u003csub\u003e17\u003c/sub\u003eN\u003csub\u003e3\u003c/sub\u003e/α-Fe particles, forming lots of phase interfaces, where there is a strong interfacial magnetic anisotropy. To overcome the interfacial magnetic anisotropy, lots of energy of incident EM waves will be lost by magnetic relaxation loss (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e(i)), in the flipping process of magnetic moments from the α-Fe (Nd\u003csub\u003e2\u003c/sub\u003eFe\u003csub\u003e17\u003c/sub\u003eN\u003csub\u003e3\u003c/sub\u003e) phase to Nd\u003csub\u003e2\u003c/sub\u003eFe\u003csub\u003e17\u003c/sub\u003eN\u003csub\u003e3\u003c/sub\u003e (α-Fe) phase, under the action of microwave magnetic field. It is the three loss mechanisms described above that make the S5 and S6 absorbers with huge magnetic loss. In particular, the natural resonance loss in α-Fe is higher at the low frequency band of 2\u0026ndash;8 GHz, the eddy-current loss is larger in α-Fe at the high frequency band of 2\u0026ndash;8 GHz, and the magnetic relaxation loss is higher at the entire frequency band of 2\u0026ndash;8 GHz for the S6 absorber with more α-Fe separated out and more phase interfaces formed, resulting in larger magnetic loss in the S6 absorber.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"4 Conclusion","content":"\u003cp\u003eThe purity of N\u003csub\u003e2\u003c/sub\u003e used for N-doping has an obvious effect on the phase composition, microstructure, magnetic, dielectric, EM parameters, and MA performances of the rare earth-transition metal alloy R\u003csub\u003e2\u003c/sub\u003eMe\u003csub\u003e17\u003c/sub\u003e such as the Nd\u003csub\u003e2\u003c/sub\u003eFe\u003csub\u003e17\u003c/sub\u003e. The lower the purity of N\u003csub\u003e2\u003c/sub\u003e, the more the α-Fe separated out, and the smaller the phase size, contributing to stronger static magnetic properties, larger electrical resistivity, higher microwave permeability, higher microwave magnetic loss, lower microwave permittivity, better impedance matching degree, and more superior MA performances at the low frequency S band and C band than magnetic microwave absorbers reported. Through controlling the purity of N\u003csub\u003e2\u003c/sub\u003e, the Nd\u003csub\u003e2\u003c/sub\u003eFe\u003csub\u003e17\u003c/sub\u003eN\u003csub\u003e3\u003c/sub\u003e/α-Fe/epoxy composite absorber prepared at present, has exhibited an excellent low frequency MA performance, with a wide MA frequency band ranging from 2 GHz to 4 GHz or from 4 GHz to 8 GHz, when the absorber thickness is 5.0 mm and 2.8 mm, respectively, significant for solving the low frequency EM interference problem.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003eCompeting interests: the authors declare no competing interests.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eWe thank the staff members of the Physical Property Measurement System (\u003cem\u003ehttps://cstr.cn/31125.02.SHMFF.PPMS\u003c/em\u003e) at the Steady High Magnetic Field Facility, CAS (\u003cem\u003ehttps://cstr.cn/31125.02.SHMFF\u003c/em\u003e), for providing technical support and assistance in data collection and analysis. We thank the Changsha Meiqi Instrument Equipment Co., Ltd, China (https://mitrcn.cnpowder.com.cn/), for providing a free service of planetary ball milling. We also thank Man Wei and Lanlan Shi from Shiyanjia Lab (www.shiyanjia.com) for the measurements and analysis of TEM and XPS in arrears with payments.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003ePeng Wang: Conceptualization, Methodology, Software, Validation, Formal analysis, Investigation, Resources, Data Curation, Writing - Original Draft, Writing - Review \u0026amp; Editing, Visualization, Supervision, Project administration, Funding acquisition; Haiqing Hang, Shibo Wang, Xingcheng Li, WenXiang Jin, Hecheng Zhao, and Keli Zhang: Validation; Yuyan Han: Software, Formal analysis; Jiaheng Wang: Resources.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis work is supported by Anhui Provincial Natural Science Foundation (No. K120162089) and Anhui Provincial Natural Science Foundation (No. 2308085ME131).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNo datasets were generated or analysed during the current study.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eA. 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Hatakeyama, Permeability spectra of yttrium iron garnet and its granular composite materials under dc magnetic field, Journal of Applied Physics 110 (2011) 053909.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"advanced-composites-and-hybrid-materials","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"achm","sideBox":"Learn more about [Advanced Composites and Hybrid Materials](https://link.springer.com/journal/42114)","snPcode":"42114","submissionUrl":"https://submission.nature.com/new-submission/42114/3","title":"Advanced Composites and Hybrid Materials","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Microwave absorption, Composite, Magnetic, Permeability, Permittivity","lastPublishedDoi":"10.21203/rs.3.rs-8571285/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8571285/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe low-frequency microwave absorption (MA) problem urgently needs to be solved, although very difficult, to address the electromagnetic interference (EMI) issues caused by modern communication systems and devices. In this paper, magnetic nano-composites of Nd\u003csub\u003e2\u003c/sub\u003eFe\u003csub\u003e17\u003c/sub\u003eN\u003csub\u003e3\u003c/sub\u003e/α-Fe with controllable phase proportion, oxidized surface, high permeability, large magnetic loss, and low permittivity at microwave frequency band, were prepared only by controlling the purity of N\u003csub\u003e2\u003c/sub\u003e used for doping. As a result, strong MA below \u0026minus;\u0026thinsp;10 dB is achieved at the low frequency C band and S band, when the absorber thickness is 2.8 mm and 5.0 mm respectively, obviously better than the low frequency microwave absorbers reported and thus showing great application prospects. More importantly, it is demonstrated that the design concept proposed by us is feasible, that is combination of easy-plane anisotropic materials like the Nd\u003csub\u003e2\u003c/sub\u003eFe\u003csub\u003e17\u003c/sub\u003eN\u003csub\u003e3\u003c/sub\u003e and axis anisotropic materials like the α-Fe, capable of helping broaden the MA bandwidth, significantly. Additionally, a kind of particular magnetic loss mechanism on moments from one side to the other side of heterogeneous interfaces of two-phase magnetic nano-composites is proposed and discussed. In short, this paper has presented a novel method of experiment and some unique ideas of design, practical to solve the low-frequency MA problem.\u003c/p\u003e","manuscriptTitle":"Separation of α-Fe phase from Nd2Fe17N3， dramatically broadening microwave absorption bandwidth at low frequency","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-03-05 19:08:09","doi":"10.21203/rs.3.rs-8571285/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2026-03-31T12:12:35+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-03-31T11:41:44+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-03-04T15:35:15+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"61616850330228649450700314419005028530","date":"2026-03-04T12:34:11+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"211395168980709385856937597668995228759","date":"2026-03-03T09:20:17+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2026-03-02T11:49:05+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2026-03-02T11:45:58+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2026-01-12T01:46:50+00:00","index":"","fulltext":""},{"type":"submitted","content":"Advanced Composites and Hybrid Materials","date":"2026-01-11T03:40:22+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"advanced-composites-and-hybrid-materials","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"achm","sideBox":"Learn more about [Advanced Composites and Hybrid Materials](https://link.springer.com/journal/42114)","snPcode":"42114","submissionUrl":"https://submission.nature.com/new-submission/42114/3","title":"Advanced Composites and Hybrid Materials","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"f745af56-ca99-4de3-bd17-42490db36bc1","owner":[],"postedDate":"March 5th, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2026-04-13T12:08:34+00:00","versionOfRecord":[],"versionCreatedAt":"2026-03-05 19:08:09","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8571285","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8571285","identity":"rs-8571285","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}