Synthesis, Characterization, and Electromagnetic Wave Absorbing Properties of M 12+ –M 24+ Substituted M-Type Sr-Hexaferrites

: Mn–Ti, Zn–Ti, Zn–Zr substituted M-type Sr-hexaferrites (SrM), SrFe 12 − 2x M 1 x M 2 x O 19 (0 ≤ x ≤ 2.0, M 1 = Mn or Zn; M 2 = Ti or Zr) were synthesized, and their solubility, crystalline structure, and high-frequency properties were studied. Zn–Zr substitution caused a relatively large lattice parameter change and resulted in lower solubility ( x ≤ 1.0) in the M-type phase compared with Mn–Ti and Zn–Ti substitutions. However, the ferromagnetic resonance frequency ( f FMR ) effectively decreased with increasing x in SrFe 12 − 2x Zn x Zr x O 19 (Zn–Zr:SrM) (0 ≤ x ≤ 1.0) and the electromagnetic wave (EM) absorption frequency also varied according to the shift in f FMR in the 7–18 GHz range. This is attributed to a gradual decrease in the magnetocrystalline anisotropy of Zn–Zr:SrM (0 ≤ x ≤ 1.0) with an increase in x . Zn–Zr:SrM ( x = 0.9)–epoxy(10 wt%) composites exhibited a high EM absorption in the X-band (8–12 GHz) with the lowest reﬂection loss of < − 45 dB. The sample with x = 0.8 showed a broad Ku band (12–18 GHz) absorption performance satisfying RL < − 19 dB at 11 ≤ f ≤ 18 GHz.

When EM waves are irradiated into a material, the interactions of absorption, reflection, and transmission are classified based on the complex permittivity ε = ε − jε and permeability, µ = µ − jµ of the material. The imaginary parts of the permittivity (ε ) and permeability (µ ) are closely related to EM absorbing performances through the dielectric or magnetic loss mechanism, respectively. In addition, good impendence matching characteristics that are functions of these ε , ε , µ , and µ material parameters are also important for the high EM absorbing performances without high EM reflection [16]. Recently, improved EM absorbing or shielding properties at GHz range have been reported by designing the nanocomposites where EM active materials are employed in template materials [17][18][19]. In the research, high EM absorbing performances with a relatively broad absorption bandwidth could be achieved through controlling the composite structures and its complex permittivity and permeability properties. The EM wave absorption of insulating hexaferrites is mostly dependent on the magnetic loss mechanism, which is closely related to the imaginary part of the permeability (µ ). The µ of hexaferrites increases at the ferromagnetic resonance (FMR) frequency (f FMR ), which is related to the magnetic anisotropy field (H ani ) by the equation below [20], where γ is the gyromagnetic ratio and µ 0 is the permeability of vacuum.
SrM has a high f FMR of~50 GHz, which is too high for hexaferrites to function as EM absorbers in the commercial radar frequency range, such as the X-band (8)(9)(10)(11)(12) and Ku-band (12)(13)(14)(15)(16)(17)(18). It has been reported that Co-Ti co-substitution effectively decreases the f FMR from 50 to a few gigahertz for a substitution x range of 0 ≤ x ≤ 1.5 for SrFe 12−2x Co x Ti x O 19 [5]. In our previous research [7], the EM absorption properties of Co-Ti-substituted SrM were investigated. The f FMR of Co-Ti substituted M-type Sr-hexaferrites (1.1 ≤ x ≤ 1.3) changed gradually from 6 to 15 GHz, which included the X-band (8)(9)(10)(11)(12) of interest in terms of radar applications. It was demonstrated that the EM absorption area can vary according to the gradual shift of the f FMR . As cobalt is an expensive element, it is important to find other substitution elements that are more price competitive. The EM absorption properties of Ti-Mn [8][9][10] and Mn-Zr [11] substituted M-type hexaferrites have been reported. However, systematic studies with varying substitution amounts and correlations among composition, magnetic properties, and EM absorption properties are rare. In particular, to the best of our knowledge, the EM absorption properties of Zn-Zr substituted M-type hexaferrites have not yet been reported. In this study, Zn-Ti, Mn-Ti, and Zn-Zr co-substituted SrM with the chemical formula SrFe 12−2x M 1x M 2x O 19 (0 ≤ x ≤ 2.0, M 1 = Mn or Zn; M 2 = Ti or Zr) were synthesized, and their crystalline structures, microstructures, high-frequency permeability, permittivity, and EM absorption properties were systematically studied.

Results and Discussion
Figure 1a-c shows the XRD patterns of the Zn-Ti, Mn-Ti, and Zn-Zr substituted SrM (0 ≤ x ≤ 2.0) powders after second calcination at 1250 • C. The diffraction peaks of the samples were indexed based on the international center for diffraction data (PDF search number; SrM: 00-033-1340) and the hexagonal magnetoplumbite structure with the space group P6 3 /mmc (ICDD 0801198). As shown in Figure 1a, Zn-Ti substituted SrM (Zn-Ti:SrM) exhibits a single M-type hexaferrite. It is believed that Zn-Ti is fully soluble in the SrM phase in the substitution range of x ≤ 2.0. For the case of Mn-Ti:SrM shown in Figure 1b, a single M-type phase can be identified for x ≤ 1.5 and a small number of second phase peaks of Fe 2 TiO 5 can be observed. Unidentified secondary phase peaks are denoted by an asterisk (*). Unlike Zn-Ti and Mn-Ti:SrM, Zn-Zr:SrM exhibits large second phase peaks of ZnFe 2 O 4 , SrZrO 3 , and ZrO 2 for x ≥ 1.5 in Figure 1c. For x = 2.0, ZrFe 2 O 4 exhibits a primary peak. In order to reveal the solubility limit of Zn-Zr (x) in SrM, additional XRD analysis was carried out on samples with x between 0.6 and 1.1 with an interval of 0.1. A clear second phase peak of the ZrFe 2 O 4 phase starts appearing at x = 1.0 and its intensity grows larger with increasing x. In the inset of Figure 1c, the (220) peaks of the samples around 2θ = 65 • are presented. Going from x = 0 to x = 0.5, the peak shift to the left is large, and the peak position slightly moves to the left between x = 0.5 and x = 1.0, but beyond that the peak position does not change. For all samples shown in Figure 1a-c, the lattice parameters, a and c, are calculated from the values of d hkl corresponding to the (2011) and (220) peaks according to the following equation: where d hkl is the interplanar spacing, and h, k, and l are the Miller indices. The values of a, c, and the cell volume of the sample are listed in Table 1, and their % changes are plotted in Figure 2a-c.     For Zn-Ti and Mn-Ti substitution (0 ≤ x ≤ 2.0), shown in Figure 2a,b, a, c, and the cell volume increase gradually with increasing x. Meanwhile, for the Zn-Zr substitution shown in Figure 2c, the increase in lattice parameters with x is much greater, but at x = 1.0 and above the cell parameter values are constant. Based on the lattice changes with substitution amount x, the solubility limit for Zn-Ti and Mn-Ti is estimated to be above x = 2.0, and for Zn-Zr, it is estimated to be x = ~1.0.
The high-frequency permeability characteristics of the three groups of samples, SrFe12−2xZnxTixO19, SrFe12−2xMnxTixO19, and SrFe12−2xZnxZrxO19, were evaluated. In the case of hexaferrites, utilization as an EM absorber in the GHz range requires a peak increase in μ″ in the corresponding frequency band, which can be caused by FMR phenomena. Figure  3a-f show the μ′ and μ″ spectra of these samples. As can be seen in the μ′ spectra, the  Figure 2c, the increase in lattice parameters with x is much greater, but at x = 1.0 and above the cell parameter values are constant. Based on the lattice changes with substitution amount x, the solubility limit for Zn-Ti and Mn-Ti is estimated to be above x = 2.0, and for Zn-Zr, it is estimated to be x =~1.0.
The high-frequency permeability characteristics of the three groups of samples, 19 , and SrFe 12−2x Zn x Zr x O 19 , were evaluated. In the case of hexaferrites, utilization as an EM absorber in the GHz range requires a peak increase in µ in the corresponding frequency band, which can be caused by FMR phenomena. Figure 3a-f show the µ and µ spectra of these samples. As can be seen in the µ spectra, the samples have µ values between 1.5 and 2.0, and µ commonly decreases to close to 1.0 in the range of f < 1 GHz. This µ spectral transition (f < 1 GHz) is associated with magnetic domain wall motion [21]. In addition, the transition of µ in the frequency range of f < 1 GHz corresponds to the magnetic loss associated with magnetic domain wall motion. Therefore, it is believed that domain wall motion cannot contribute to the permeability at frequencies higher than 1 GHz, and the peak increase of µ > 1 and µ > 0 at f > 1 GHz is mostly caused by electron spin motions, that is, FMR [6,7]. It is known that non-substituted M-type Sr-hexaferrite, SrFe 12 O 19 , has f FMR~5 0 GHz. Thus, no µ peak is observed in the measured frequency range of f ≤ 18 GHz. When Zn-Ti is substituted into SrM, f FMR decreases with increasing x. In Figure 3a,b sharp increases in the µ , µ peaks can be observed at the right edge. We can see that f FMR is approximately 18 GHz for SrFe 12−2x Zn x Ti x O 19 (x = 2.0), and that it is above 18 GHz for samples with x < 2.0. For the case of Mn-Ti substitution (Figure 3c,d), no clear peaks of µ , µ spectra are absorbed at f > 1.0 GHz for all samples. Meanwhile, a clear FMR signal is absorbed by the Zn-Zr substituted samples, as shown in Figure 3e,f. In the vicinity of 10 GHz in the µ spectra, a peak from FMR is shown for x = 1.0, and the height gradually decreases as x reaches 2.0. For the sample with x = 0.5, there is no FMR peak at f >1 GHz because its f FMR is greater than 18 GHz. Although the FMR peak is supposed to shift to a lower frequency upon increasing x to 1.5, and 2.0, its position is almost the same. This is because even for x higher than 1.0, no substitution of Zn-Zr can occur, as mentioned previously in Figure 2c. In addition, the decrease in µ peak height with an increase in x up to 2.0 is due to an increase in the volume fraction of the non-magnetic secondary phase (Figure 1c).   (Figure 4f), all grains show nearly equal contrast, but brighter grains begin to appear at x = 1.5 (Figure 4g), and brighter spots increase at x = 2.0 (Figure 4h). Considering the XRD analysis results (Figure 1c) and average atomic weights of the second phases, the bright spots in Figure 4g,h correspond with SrZrO 3 . Figure 5 shows the magnetization curves for the SrFe 12−2x Zn x Zr x O 19 -epoxy composition (x = 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1) samples. Because the solubility limit of SrFe 12−2x Zn x Zr x O 19 was found to be approximately x = 1.0, sample compositions with smaller intervals of x below x = 1.0 (x = 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1) were also studied. The saturation magnetization (4πM S ) and coercivity (H C ) values of these samples are presented in Table 2. The 4πM-H curves in the low magnetic field range are shown in the inset. The 4πM S of the sintered samples are expected to be twice these values because the volume fraction of the nonmagnetic epoxy binder is~50%. The 4πM S of the x = 0.5 sample is 1.875 kG, and it decreases with increasing x. The reason for this is that non-magnetic ions of Zn 2+ -Zr 4+ are substituted for the magnetic Fe 3+ ions. It has also been reported that a small amount of Zn substitution (x ≤ 0.3) into SrM could increase M S due to a selective substitution of Fe in the down-spin state [22,23]. It is notable that the substitution amount (x ≥ 0.5) in this study is too high to expect such an enhancement of M S in the SrM. H C decreases noticeably for an increase in x from x = 0.5 to x = 0.7 but decreases slightly from x = 0.7 to x = 0.9 and remains roughly constant above x = 0.9. The H C value depends on both the intrinsic and extrinsic characteristics of the magnetic materials. Grain size is a dominating factor influencing H C . It is well known that H C decreases with increasing grain size of hexaferrites [24][25][26]. Considering that the average grain size of the x = 0.5 sample is larger than that of the sample with x = 1.0, the smaller H C for a higher x is due to a strong intrinsic property change caused by the Zn-Zr substitution, which overcomes the disadvantageous extrinsic factor.   Figure 5 shows the magnetization curves for the SrFe12−2xZnxZrxO19-epoxy composition (x = 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1) samples. Because the solubility limit of SrFe12−2xZnxZrxO19 was found to be approximately x = 1.0, sample compositions with smaller intervals of x below x = 1.0 (x = 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1) were also studied. The saturation magnetization (4πMS) and coercivity (HC) values of these samples are presented in Table 2. The 4πM-H curves in the low magnetic field range are shown in the inset. The 4πMS of the sintered samples are expected to be twice these values because the volume fraction of the nonmagnetic epoxy binder is ~50%. The 4πMS of the x = 0.5 sample is 1.875 kG, and it decreases with increasing x. The reason for this is that non-magnetic ions of Zn 2+ -Zr 4+ are substituted for the magnetic Fe 3+ ions. It has also been reported that a small amount of Zn substitution (x ≤ 0.3) into SrM could increase MS due to a selective substitution of Fe in the down-spin state [22,23]. It is notable that the substitution amount (x ≥ 0.5) in this study is too high to expect such an enhancement of MS in the SrM. HC decreases noticeably for an increase in x from x = 0.5 to x = 0.7 but decreases slightly from x = 0.7 to x = 0.9 and remains roughly constant above x = 0.9. The HC value depends on both the intrinsic and extrinsic characteristics of the magnetic materials. Grain size is a dominating factor influencing HC. It is well known that HC decreases with increasing grain size of hexaferrites [24][25][26]. Considering that the average grain size of the x = 0.5 sample is larger than that of the sample with x = 1.0, the smaller HC for a higher x is due to a strong intrinsic property change caused by the Zn-Zr substitution, which overcomes the disadvantageous extrinsic factor.    Table 2, the real (ε ) and imaginary (ε ) parts of the permittivity at 1 GHz, the real part of the permeability (µs ) in the steady region after the first transition (~3 GHz), and the frequencies of the maximum µ values (f FMR ) are presented. In the ε and ε spectra shown in Figure 6a,b, as the frequency increases, for all samples, ε decreases and converges upon~7, and ε approaches zero. The real (µ ) and imaginary (µ ) parts of the permeability spectra are shown in Figure 6c,d, the first transitions of µ and µ are associated with magnetic domain wall motion (f < 2 GHz), and the peaks at higher frequencies (f > 2 GHz) are caused by FMR. In the µ spectra (Figure 6d), a gradual µ peak shift to the left is observed upon increasing x from 0.7 to 0.9, and a very small peak shift can be found between x = 0.9 and x = 1.0. The µ peak frequency corresponds to f FMR and is directly proportional to the magnetic anisotropy field (H a ), as expressed in Equation (1). The change in f FMR is related to an intrinsic property change and can be caused by cation substitution. This is consistent with the change in H C shown in Figure 5, as mentioned above. The µ peak positions and H C values of the samples with x = 0.9-1.1 are very similar, because the solubility limit of Zn-Zr in the M-phase is approximately x = 1.0. The gradual increase in µs upon increasing x up to 0.9 can be explained by Snoek's limit law [27], presented below:   is approximately x = 1.0. The gradual increase in μs′ upon increasing x up to 0.9 can be explained by Snoek's limit law [27], presented below: Here, γ is a constant known as the gyromagnetic ratio, and Ms is the saturation magnetization value. A decrease in static real permeability (μs′) causes an increase in fFMR. This implies that changes in the high-frequency permeability property are governed by the intrinsic magnetic parameter of the magnetocrystalline anisotropy, which can be con- Here, γ is a constant known as the gyromagnetic ratio, and M s is the saturation magnetization value. A decrease in static real permeability (µs ) causes an increase in f FMR . This implies that changes in the high-frequency permeability property are governed by the intrinsic magnetic parameter of the magnetocrystalline anisotropy, which can be controlled by Zn-Zr substitution.
According to transmission line theory [28], the reflection loss (RL), which implies the EM wave absorption performance, can be calculated using the following equations: where Z in is the input impedance of the absorber, Z 0 = √ µ 0 ε 0 is the characteristic impedance of free space, c is the speed of light, f is the frequency of the incident EM wave, d is the thickness of the absorber, and ε r and µ r are the complex permittivity (ε r = ε − jε ) and permeability (µ r = µ − jµ ), respectively. Here, the measured µ , µ , ε , and ε spectra can be used to obtain Z in Z o for any thickness d with respect to f. RL calculations were plotted in square f -d maps, as shown in Figure 7a In the RL map shown in Figure 7a-i, the strong EM absorbing area at lower d, m with a rectangular box, starts to be observed at x = 0.7, and it moves gradually w increase in x up to 1.0 (Figure 7c-f). The left edge of the absorption area already app at the right edge of Figure 7b for the sample with x = 0.6. EM absorption in the m area is caused by a magnetic loss mechanism, that is, FMR, which produces a peak μ″ spectra. Thus, the gradual movement of the EM absorbing area with x = 0.7, 0. In the RL map shown in Figure 7a-i, the strong EM absorbing area at lower d, marked with a rectangular box, starts to be observed at x = 0.7, and it moves gradually with an increase in x up to 1.0 (Figure 7c-f). The left edge of the absorption area already appeared at the right edge of Figure 7b for the sample with x = 0.6. EM absorption in the marked area is caused by a magnetic loss mechanism, that is, FMR, which produces a peak in the µ spectra. Thus, the gradual movement of the EM absorbing area with x = 0.7, 0.8, and 0.9 (Figure 6c-e) is attributed to the µ peak shift shown in Figure 6d. It is also observed that the absorption intensity in the RL maps for x from 1.1 to 2.0 (Figure 6g-i) becomes weaker. A decrease in the µ peak height shown in Figure 3f is the reason for the weakening EM absorption. As previously mentioned, the decrease in µ peak height for the x = 1.5 and 2.0 samples is due to an increase in the non-magnetic second phase. Figure 8 shows the RL spectra of the sample (x = 0.7, 0.8, 0.9, 1.0, 1.1) at the optimal thickness, at which the minimum RL (RL min ) point was located. The RL min values and frequency of RL min (f RLmin ) are presented in Table 2. Each sample shows an RL min < −30 dB, and f RLmin shifts to a lower frequency with increasing x. f RLmin shifts to a lower f in large steps as x increases from 0.7 to 0.9, but it moves only slightly at x ≥ 0.9. The tendency for changes in large steps up to x = 0.9, and then for changes in very small steps for x > 0.9, is the same for H C , f FMR , and f RLmin . All these changes are related to the Zn-Zr substitution amount in the M-type phase and its magnetocrystalline anisotropy change. As shown, f RLmin is similar to f FMR for each sample, and FMR is the dominant EM absorbing mechanism in hexaferrites. The x = 0.9 sample demonstrates EM absorption properties optimized for the X-band (8)(9)(10)(11)(12)

Conclusions
From among the three different M1 2+ -M2 4+ substitutions of Mn-Ti, Zn-Ti, Zn-Zr, the Zn-Zr substitution most effectively decreased the intrinsic property of the magnetocrystalline anisotropy of the hexaferrite, although the solubility limit (x = ~1.0) is smaller than that for the other substitutions. The coercivity, natural resonance frequency, and frequency range of the EM absorption via the magnetic loss mechanism decreased in large steps for an increasing substitution x of up to 0.9, and then decreased slightly with increasing x for 1.0 ≤ x ≤ 1.1. All these parameters are closely related to one another, and the changes are due to the magnetic crystalline anisotropy caused by the Zn-Zr substitution. The sample with x = 0.9 demonstrated a high EM absorption in the X-band (8)(9)(10)(11)(12) with the lowest reflection loss of <−45 dB, and the sample with x = 0.8 exhibited a broad Ku band (12-18 GHz) absorption performance satisfying RL < −19 dB at 11 ≤ f ≤ 18 GHz. Herein, we report for the first time the EM absorption properties of Zn-Zr substituted Mtype Sr-hexaferrites and show that they are very promising candidates for X and Ku band EM absorbers.

Conclusions
From among the three different M 1 2+ -M 2 4+ substitutions of Mn-Ti, Zn-Ti, Zn-Zr, the Zn-Zr substitution most effectively decreased the intrinsic property of the magnetocrystalline anisotropy of the hexaferrite, although the solubility limit (x =~1.0) is smaller than that for the other substitutions. The coercivity, natural resonance frequency, and frequency range of the EM absorption via the magnetic loss mechanism decreased in large steps for an increasing substitution x of up to 0.9, and then decreased slightly with increasing x for 1.0 ≤ x ≤ 1.1. All these parameters are closely related to one another, and the changes are due to the magnetic crystalline anisotropy caused by the Zn-Zr substitution. The sample with x = 0.9 demonstrated a high EM absorption in the X-band (8)(9)(10)(11)(12) with the lowest reflection loss of <−45 dB, and the sample with x = 0.8 exhibited a broad Ku band (12-18 GHz) absorption performance satisfying RL < −19 dB at 11 ≤ f ≤ 18 GHz. Herein, we report for the first time the EM absorption properties of Zn-Zr substituted M-type Sr-hexaferrites and show that they are very promising candidates for X and Ku band EM absorbers.