Structural, magnetic, and gigahertz-range electromagnetic wave absorption properties of bulk Ni–Zn ferrite

Nickel–zinc ferrite (Ni0.5Zn0.5Fe2O4) powders were prepared by the conventional solid-state route and sintered at 1100 and 1300 °C for utilization as a tile electromagnetic wave absorber. Structural, magnetic, and microwave absorption properties were investigated by characterization techniques of X-ray diffraction, thermogravimetric analysis, Raman spectroscopy, electron microscopy, vibrating sample magnetometry, and vector network analyzer. The samples sintered at 1300 °C showed high magnetic saturation of 87 emu/g and low coercivity of 4 Oe. Electromagnetic investigations exhibit high reflection losses up to − 48.1 dB at certain high and low gigahertz frequencies, as clearly depicted in the 3D contour plot. The optimized condition between reflection loss, thickness, and bandwidth revealed a reflection loss of about − 36.1 dB at the matching thickness of 3.7 mm for the X-band. Furthermore, the effective working bandwidth at − 10 dB was up to ~ 7.1 GHz for the minimum thickness of 4.3 mm, which thoroughly covered the C-band. The microwave absorption performance of the well-sintered Ni–Zn ferrite was attributed to the incorporation of dielectric and magnetic loss mechanisms in which the magnetic part prevails.

Wireless communication transmitting information from one point to another, without using any connection like wires, cables, or any physical medium, is widely used in mobile phones, GPS receivers, remote controls, and Wi-Fi, owing to its inexpensive, mobility, easiness, and reliability 1,2 . However, there are few drawbacks such as interference, security, and probable health issues for wireless communications [3][4][5] . The interference of electromagnetic (EM) waves from various devices can be led to high amounts of noise in connections by undesired constructive/destructive ways [6][7][8][9] . Among various methods, the shielding of electronic devices by microwave absorbers is an attractive and practical method to be safe against electromagnetic interferences and brings good electromagnetic compatibility (EMC) for the operating devices 10,11 . An ideal absorber material for electromagnetic interference (EMI) shielding should have high dielectric and magnetic losses, chemical stability, low-cost, and lightweight 12,13 . Dielectric 14 , magnetic 15 , and carbon-based 16,17 materials could be utilized as EM wave absorbers in order to attenuate the distinct frequency range and reducing the interference caused by airborne connections 18 . In addition to the attenuation ability, which is related to dielectric and magnetic losses, another challenging issue is that an absorber material should fulfill the impedance matching condition z = µ ε ∼ 1 to receive incident EM waves with the lowest reflectivity. In most cases, absorber materials possess low permeability and high permittivity, while magnetic spinel ferrites have the competency of good impedance matching according to their higher permeability over other absorbers. Spinel ferrites (MFe 2 O 4 , M = Co, Ni, Mn, Zn, Fe, Cu) as soft magnetic materials can be used for diverse applications such as home and industrial electronic devices 19,20 , biomedicals [21][22][23] , catalysts 24,25 , and further in EMI shielding as absorber materials because of their relatively high dielectric/magnetic loss, excellent chemical stability, and low costs 26,27 . Nickel-Zinc ferrite shows better EM wave absorption performances compared to other spinel ferrites due to higher resistivity, high saturation magnetization, and high permeability [28][29][30] . The spinel Ni-Zn ferrite has a cubic crystal structure with the space group of Fd3m in which the cations are distributed between tetrahedral (A) and octahedral [B] sites 12 . The occupation of (A) sites and

Experimental procedure
The Ni-Zn ferrite powders were synthesized by the conventional solid-state method. Analytical grade of Fe 2 O 3 , ZnO, and NiO powders in stoichiometric amounts (1:0.5:0.5) was mixed by using a planetary ball mill (Sanat Ceram, Iran) in a wet ethanol medium. The milling rotational speed was set at 200 rpm for 7 h with the ball to powder ratio (BPR) of 20. The obtained mixture was dried in the oven at 70 °C and then calcined at 900 °C in the air atmosphere. The calcined powders were ground again to break the large agglomerates. The as-calcined NiZn ferrite powders were uniaxially pressed into both disk-shaped (d = 10 mm) and toroidal ones (d in = 3 mm and d out = 7 mm) under the pressure of 250 MPa. The compacted bodies were sintered at 1100 and 1300 °C for 2 h in the air atmosphere. The schematic of the synthesis procedure is shown in Fig. 1.
Thermogravimetric analysis was operated with a simultaneous thermal analyzer (BAHR 504) from room temperature up to 1500 °C with the same condition used for calcination and sintering to show the thermal stability of the samples. The structural properties were characterized using an Ultima IV X-ray diffractometer (Rigaku, Japan) with Cu Kα (λ = 1.5406 A) radiation. Crystal structure analysis of the XRD patterns was done using Highscore plus software, and the lattice parameter and cation distribution of the samples were calculated by the Rietveld refinement method. The nickel-zinc ferrite belongs to the spinel Fd3m space group; therefore, oxygen positions (x = y = z) were taken as free parameters, while other atomic fractional positions were held as fixed. Moreover, lattice constants, isothermal parameters, occupancies, scale factors, and shape parameters were also taken as free parameters. Refinement steps were continued until the goodness of fit ( σ 2 ) reaches to the values close to 1.5, showing the convergence of the refined profile with the observed patterns and confirming the quality of fit 53 . Cation distribution was obtained by taking into account the presence of both inverse and normal spinel structures. The nickel ferrite was modeled as complete inverse, while the zinc ferrite was considered as complete normal spinel. Furthermore, Raman spectroscopy was done using an XploRA PLUS (HORIBA, Japan) with a 785 nm laser line operated at room temperature. The microstructural characteristics were observed by a Mira 3 field-emission scanning electron microscope (TESCAN). An energy-dispersive detector with an accelerating voltage of 15 kV was used to depict the elemental composition. A vibrating sample magnetometer (Meghnatis Daghigh Kavir Co., Iran) was used to measure the magnetic properties at room temperature. The applied field www.nature.com/scientificreports/ was swept from − 10 to10 kOe. Electromagnetic properties, including permittivity and permeability spectra, were obtained using an 8722 ES network analyzer (Agilent/HP) in the wide frequency range of 1-18 GHz. Figure 2 represents the DTA-TG plot of as-prepared powders. The initial mass reduction (about 5%) starts from ambient temperature up to 600 °C due to the elimination of physically absorbed water in most (3.5% up to 250 °C) and dehydroxylation and decomposition of residual organic compounds. Three critical temperatures in TG curve at 900, 1100, and 1300 °C were chosen for further thermal treatments of calcination and sintering where the main mass reduction occurs, spinel phase formation initiates, and sintering phase completes, respectively. Moreover, fluctuations of DTA curve at about 930, 1120, and 1280 °C were attributed to the mentioned critical temperatures, where the broad endothermic peak at 1200-1400 °C can also be assigned to the grain growth step of the sintering. Small fluctuations and negligible mass changes in the thermally analyzed sample illustrate the stability of the compound at higher temperatures 54 . Figure 3 shows the XRD patterns of the initial mixture, as-calcined Ni-Zn ferrite powder, and sintered samples. During the calcination process, the finely crushed NiO, ZnO, and Fe 2 O 3 powders disappear by reacting the    Figure 4 shows the Raman spectra of the sintered samples at 1100 and 1300 °C. Small and broad peaks at about 210, 300, 460, 640, and 680 cm −1 are related to the five active Raman modes of T 2g (1), E g , T 2g (2), T 2g (3), and A 1g 58 . Low Raman shifts revealed the presence of octahedral B-sties (BO 6 ), while the observed peaks at about 640-700 cm −1 are attributed to oxygen motion in the tetrahedral A-sites (AO 4 ) having A 1g mode Raman character 59,60 . This broad containing shoulder peak was also been assigned to the order-disorder effect of ions over A and B sites 61 . The related active Raman shifts presented in Table 2 confirms the formation of cubic spinel ferrite, Fd3m, in the sintered samples. Also, all Raman peaks blue shifted slightly toward higher wavenumber due to the increase in sintering temperature (to 1300 °C), where the crystal defects and strain reduced and consequent lower lattice parameter values were obtained 30 .

Results and discussion
FESEM micrographs and EDS analysis of the sintered samples at 1100 and 1300 °C are presented in Fig. 5. The samples sintered at 1100 °C have small (~ 0.3 µm) and discrete grains, while the large grains (~ 10 µm) with smooth grain boundaries appear for sintering at 1300 °C. This may affect the magnetic and microwave absorption properties of the samples through the interface effects 62,63 . Also, it can be observed that the Ni-Zn ferrite particles cannot be grown and sintered at the lower temperature. Densification is chiefly related to grain boundary (GB)   www.nature.com/scientificreports/ diffusion, while grain growth is ruled by GB migration. The abnormal grain growth at 1300 °C can be attributed to the small grain size of Ni-Zn ferrite powders, providing a large amount of activation energy for grain boundary migration. Some small pores are also trapped inside the ferrite grains because of the diffusion lag during the sintering process. Although some pores may be trapped, the resultant structure has lower defects and pores and   (Table 1) results in the high saturation magnetization for the sample sintered at 1300 °C. The higher imbalance of Fe 3+ cations for higher sintering temperatures can be attributed to the higher cooling rate 70 and crystal strains. Moreover, the high crystallinity and large grains are also beneficial for the increase of Ms at higher sintering temperatures. Figure 7 shows the permittivity (ε r = ε′ − jε″) and permeability (µ r = µ′ − jµ″) spectra of the sintered sample at 1300 °C. The real parts of spectra are related to the storage of electrical and magnetic energy, while the energy dissipation is shown by imaginary parts. The values of ε′ decrease versus frequency from 6 at 1 GHz to 5.7 at 18 GHz with an average value of about 5.8. The values of ε″ show two characteristic peaks at ~ 8.5 and 14.5 GHz over the 1-18 GHz range. The electrical energy of EM waves is dissipated by the conductivity and polarization mechanisms. The atomic, electronic, and dipolar polarizations are the main mechanisms of dielectric loss 71 . Among them, only dipolar polarizations occur in the microwave range due to the interfacial relaxations in which the accumulation of free charges at grain boundaries and defects of material enhances the dipoles [72][73][74] . The complex permittivity (ε r ) can be effectively modeled by considering the Debye equation for dipolar polarization and σ ωε 0 for conductivity losses as follows 75 : where the first term describes the superposition of different relaxation mechanisms and the second one is related to the contribution of electrical conductivity σ ωε 0 in which σ = σ dc + σ ac . The ω = 2πf is the operating angular frequency, the values of ε r0i and ε r∞i are permittivity at low-frequency and high-frequency, respectively, τ i is the relaxation time, and 0 < α i < 1 is a non-ideality empirical constant in ith relaxation mechanism. Large ferrite grains are partially conductive, whereas grain boundaries have poor conductivity. At high-frequency ranges, large grains www.nature.com/scientificreports/ prevail grain boundaries which were effective at low frequencies. Also, the Maxwell-Wagner double layer model expressed that ε′ is decreased and σ ac is increased as frequency increased 76 . Therefore, the dispersive behavior of ε″ values with the frequency can be attributed to the higher contribution of AC conductivity (σ ac ). Furthermore, the peaks of ε″ spectra are related to the contributed relaxations, such as interfacial polarization between the ferrite grains, as can be confirmed by the Cole-Cole plot (Fig. 8a) 77,78 . Figure 7c and d show the real and imaginary parts of permeability versus frequency. The values of μ′ and μ″ decrease with the increase of frequency because of the magnetization relaxations 79,80 . The magnetic losses in the GHz region are mainly related to the eddy current and ferromagnetic resonance mechanisms 81,82 . The ferromagnetic resonances, including the natural resonance and wall resonance, occur in the MHz range for the soft magnetic NiZn spinel ferrite due to its low anisotropy field (H A ) 83,84 . However, the natural resonance was observed at higher frequencies (GHz range) because of the exchange resonance 29,85,86 . On the other hand, the eddy current effect loss can be the main mechanism in the GHz region for spinel ferrites. The eddy current loss is dominant when the value of C 0 = 2πt 2 σμ 0 = μ″ (μ′) −2 f −1 is independent of frequency 87 . As shown in Fig. 8b, the C 0 values are constant at above 12 GHz, indicating the predominance of the eddy current loss.
The permittivity (ε r ) and permeability (μ r ) parameters can be used for the calculation of normalized impedance of absorber (Z in ) relative to free space (Z 0 ) and reflection loss RL (dB) versus frequency (f) and thickness (t), according to the transmission line theory as follows 88,89 : The dependency of reflection loss on the frequency and thickness is shown as a 3D contour plot in Fig. 9a. The higher magnetic and dielectric loss of EM waves by the absorber leads to a more negative RL because of the lower wave reflection from the absorber. High reflection loss up to − 48.1 dB is attained at the frequency range of 1-18 GHz, as shown in the 3D plot. Also, by data extraction from the plot, well-known Wireless LAN protocols of IEEE 802.11b, y, and j at ~ 2.4, 3.65, and 5 GHz can be fully suppressed by the NiZn ferrite absorber. The energy of EM waves is consumed by both dielectric and magnetic losses, can be compared by loss factor (tanδ) versus frequency (Fig. 10b). It defines as the ratio of imaginary to real components of the complex permittivity and permeability spectra. The magnetic loss is dominant in the range of 1-12 GHz, while the dielectric loss at above 12 GHz. The magnetic loss can be attributed to the resonance mechanisms at the frequency of 1-12 GHz and the eddy current loss at 12-18 GHz.
The total contribution of dielectric and magnetic attenuation ability can be evaluated by the attenuation constant (α), which is defined by the following equation 97 : Figure 11a shows a broad peak in the frequency range of 2-12 GHz in which the increase of α values is related to the high contribution of dielectric and magnetic losses, while the strong reduction of dielectric loss results in  www.nature.com/scientificreports/ the decrease of α values at higher frequencies. The incident EM waves can penetrate the absorber with the minimum reflection when the input impedance is well-matched with free space (Z 0 ). In other words, the modulus of normalized impedance, |Z| = Z in Z 0 , should be approached to 1 98,99 . Figure 11b presents the normalized impedance at different matching thicknesses. It can be found that the values of |Z| are enough close to 1 at higher thicknesses. Therefore, the EM waves can be easily penetrated to the absorber and attenuated by magnetic and dielectric loss mechanisms.

Conclusion
Due to the emergence of wireless communications, the utilization of electromagnetic absorber materials is mandatory to reduce electromagnetic interference and bring electromagnetic compatibility for electronic devices. In this research, Ni-Zn ferrite (Ni 0.5 Zn 0.5 Fe 2 O 4 ) bulk samples were fabricated to use as a tile microwave absorber material in the wide 1-18 GHz frequency range. The sample sintered at 1300 °C had a large grain size (~ 10 µm) with smooth grain boundaries. In addition, higher sintering temperature led to appropriate magnetic properties, including high saturation magnetization of 87 emu/g and low coercivity of 4 Oe. 3D reflection loss plot showed the distribution of loss over the 1-18 GHz frequency range in which high reflection losses up to − 48.1 dB were obtained at certain low and high frequencies. The optimized condition by the preference of working bandwidth was in the range of ~ 3 to 10.1 (7.1) GHz for the sample with the matching thickness of 4.3 mm with RL of − 16.9 dB, while by RL preference, the minimum value of − 36.1 dB was achieved at the thickness of 3.7 mm with the effective absorption bandwidth of 6.7 (4.7-11.4) GHz. The absorption performance of the sintered www.nature.com/scientificreports/ Ni-Zn ferrite was related to easy penetration of EM waves and further attenuation by magnetic and dielectric loss mechanisms. The magnetic loss was attributed to the resonance mechanism, while the interfacial polarization was responsible for the dielectric loss.