Alterations in the magnetic and electrodynamic properties of hard-soft Sr 0.5 Ba 0.5 Eu 0.01 Fe 12 O 19 / Ni x Cu y Zn w Fe 2 O 4 nanocomposites

same contents of Ni and Cu within the soft magnetic phase. The calculated squareness ratios (SQR ¼ M r /M s ) were less than 0.5, indicating incomplete exchange coupling. The coercive ﬁeld (H c ) of the produced NCs reached a maximum value of approximately 2485 Oe at 300 K and 2331 Oe at 10 K with a decrease in the M s values to 56.9 emu/g at 300 K and 78.5 emu/g at 10 K for the H/S Sr 0.5 Ba 0.5 Eu 0.01 Fe 11.99 O 19 /Ni 0.8 Cu 0.1 Zn 0.1 Fe 2 O 4 NC, which contained lower fractions of Zn and Cu and the highest fraction of Ni. The reﬂection/transmission-based waveguide approach was employed to investigate the electrodynamic properties of the H/S NC samples within a frequency band of 7 e 18 GHz. The reﬂection and transmission coefﬁcients (S 11 /S 21 ) were measured using a vector network analyzer (VNA) for the sample placed inside a waveguide. The frequency dispersions of the magnetic permeability and electric permittivity were calculated. © 2021 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).


Introduction
Spinel ferrites play important roles owing to their ease of synthesis, chemical stability, and high electrical resistivity.They are employed in numerous areas, such as data storage, sensors, medicines, telecommunications, photo-catalysts, microwave, magneto-optical, cancer treatment, and photoactive materials [1e4].In recent years, hard/soft (H/S) nanocomposites (NCs) have been utilized extensively owing to their exceptional microwave absorption, electromagnetic (EM), and magnetization characteristics [5,6].There are numerous composites based on hard/soft ferrites, such as Ni 0.5 Zn 0.5 Fe 2 O 4 /BaFe 12 O 19 [7], NiFe 2 O 4 /SrCo 0.2 Fe 11.8 O 19 [8], Ni 0.6 Zn 0.4 Fe 2 O 4 /SrFe 12 O 19 [9], SrFe 12 O 19 /FeCo [10], BaFe 12 O 19 / CoFe 2 O 4 [11], Mn 0.6 Zn 0.4 Fe 2 O 4 /Sr 0.85 Ba 0.15 Fe 12 O 19 [12], Ni 0.5- Zn 0.5 Fe 2 O 4 /SrFe 12 O 19 [13], SrFe 12 O 19 /Ni 0.5 Zn 0.5 Fe 2 O 4 [14], BaFe 12 O 19 /CoFe 2 O 4 [15], Li 0.3 Co 0.5 Zn 0.2 Fe 2 O 4 /SrFe 12 O 19 [16], and (Mn 0.8 Mg 0.2 Fe 2 O 4 ) (1-x) /(BaZn 0.15 Al 0.15 Fe 11.7 O 19 ) x [17].It has been determined that a high coercive field (H c ) and high saturation magnetization (M s ) can be realized via exchange coupling of the hard and soft phases [18,19].Many studies have reported the behavior of exchange coupling in nanopowders, metal alloys, and thin films such as SmeCo/Fe [20], SmCo xe Co [21], Pr 2 Fe 12 B [22], and FeNi/FePt [23].Recently, nanocomposite magnetic materials have attracted attention as a result of their exceptional magnetic properties.Some significant factors have been identified, such as the grain size, magnetic interaction, composition, and microstructure of soft and hard ferrites [18].It has been noted that the appropriate calcination process and mass ratio can improve the magnetic properties of H/S ferrites [24,25].It is difficult to realize high magnetic saturation and high coercivity simultaneously because most magnetic materials exhibit a high value of coercivity and a low magnetization saturation [26].Consequently, it is necessary to synthesize composite materials to achieve the exchange coupling of hard and soft magnetic structures [27,28].In addition, spinel hard and soft ferrite composites have excellent electrical resistivity and high corrosion resistance [14].Soft spinel ferrites play an important role in microwave applications because of their less anisotropic properties [29].It has been reported that hard hexagonal ferrites (such as BaFe 12 O 19 ) exhibit excellent anisotropy and a high resonant frequency that is adequate up to the W-band [30].Consequently, the chemistry of highly anisotropic hard ferrites and slightly anisotropic soft ferrites can be used for diverse microwave applications [31,32], such as the construction of radar-absorbing materials (RAM) used for military and defense applications [5].Nevertheless, a wideband frequency range, thin absorption layer, and high absorption peak cannot be achieved with a single material of an exemplary radar [6,33].The exchange between hard and soft ferrite phases constructs standard nanocomposites that may be useful and appropriate [34].The magnetic and dielectric nanocomposites exhibit high permeability (m r ) and large permittivity (ε r ) attributable to the exchange-coupling effect.However, certain nanocomposites operating at frequencies in the range of 12e18 GHz are used in military and sensitive applications [35,36].Functional composites based on transition metal oxides (Fe 3 O 4 , BaFe 12-x D x O 19 ) with a polymer matrix [37,38] have been widely investigated.
With the synthesis and development of new nanocomposite materials and their varied use in new applications, it has become crucial to employ highly accurate techniques for the characterization of their EM properties [39e41].Furthermore, this characterization must be viable for a broad spectrum of materials.Nondestructive testing (NDT), which serves as a tool to characterize nanocomposite materials without causing any damage to the sample or permanently altering its structure, can be adopted through EM techniques to measure material properties smoothly and accurately.Typical reflection/transmission (R/T) or reflection-only NDT methods include free-space reflection and open-ended probe techniques [39e44].These are preferred as they do not disrupt the material integrity, and they are utilized in several areas such as the biomedical, construction, and mechanics fields.However, these methods are inaccurate and complex in mathematical modeling compared with guided R/T techniques based on microwave waveguides.
As such, it is important to characterize the EM properties of H/S NCs accurately to remove sources of error in this study.Various characterization techniques have been reported in the scientific literature [44e46].These can be broadly categorized into two groups: resonance techniques and R/T methods.Resonance techniques can deliver highly accurate EM characterization, although the sample under study and the designed resonator need to be fabricated with a great degree of precision [45].Additionally, these methods are generally limited to a narrow range of materials and the measurement of properties such as permeability, permittivity, and the sample size.The challenges associated with resonance techniques include their cost and impracticality when used in research laboratories where material characterization is not the primary focus, but rather serves as the first step in the design of a device.
On the other hand, R/T methods based on EM devices such as coaxial lines, waveguides, and free-space propagation can be used to characterize composite materials.R/T characterization is a wideband method, unlike other methods that work on single or multiple frequencies.The working frequency band for performing the characterization is dependent on the type of R/ T device used for measurement.Waveguides and coaxial cables employ a unimodal propagation band for material characterization.Coaxial cables are practical as they can provide a wider bandwidth and, in theory, support low-frequency measurements owing to the absence of a cut-off band.The problem with this technique is that the material should be prepared in such a shape that it can conform to the concentric corona of the cable, which is problematic from a manufacturing perspective.On the other hand, waveguide characterization requires simpler sample preparation, which makes it preferable to the use of coaxial cables, despite its lower allowable bandwidth.
The R/T-based waveguide approach is used to study the electrodynamic properties of H/S S1eS5 NCs in this study.First, the coefficients of the reflected and transmitted energy (S 11 eS 21 ) are measured using a vector network analyzer (VNA) for a sample placed in a waveguide with a cut-off frequency of 7 GHz.Consequently, the frequency dispersions of the permittivity and magnetic permeability are calculated based on the measured S 11 eS 21 .
Therefore, we have selected a nano-composition consisting of Sr 0.5 Ba 0.5 Eu 0.01 Fe 12 O 19 as a hard ferrite and Ni x Cu y Zn w Fe 2 O 4 as a soft ferrite.These materials were selected based on the improvement in the structural and magnetic properties of Eudoped M-type hexaferrite and NieZneCu spinel ferrite according to various research reports.C. Liu et al. studied the impact of Eu substitution in Sr-hexaferrites on their structure and magnetic features [46].They found that there was a considerable improvement in the saturation magnetization (M s ) and intrinsic coercivity (H c ) with substitution Moreover, A. Zafar et al. investigated the electrical and magnetic characteristics of Eu-doped barium hexaferrites.The samples exhibited a decreased conductivity and increased saturation magnetization and coercivity with Eu doping [47].B. Thangjam et al. and D. Venkatesh et al. explored the magnetic properties of Cusubstituted NieZn spinel ferrites.They observed superior magnetic characteristics when the Cu concentration was varied [48,49].As a result, we produced H/S Sr 0.5 Ba 0.5 Eu 0.01 Fe 12 O 19 / Ni x Cu y Zn w Fe 2 O 4 (S1eS5) NCs and explored the effects of varying the Ni, Cu, and Zn ratios on the structural parameters and magnetic and microwave (MW) properties.To the best of our knowledge, there have been no similar studies on this composition, and thus the results presented here can contribute significantly to the research field of H/S ferrite NCs.

Experimental
For the H/S Sr 0.5 Ba 0.  transmission electron microscope (TEM).The magnetic properties of the products were investigated using a Quantum Design SQUID-PPMS vibrating sample magnetometer (PPMS DynaCool, Quantum Design, San Diego, CA, USA).The MW properties in the range of 7e18 GHz were calculated for the five investigated samples using an Agilent VNA (Model E5071C) employing the transmission-line approach for the measurement.The samples were placed inside a rectangular waveguide to interfere with the propagating waves.The measured coefficients of the reflected (S 11 ) and transmitted (S 21 ) energies were transformed to permittivity and permeability values (real and imaginary parts) using the Nic-olsoneRosseWeir algorithm [38] based on the measured Sparameters at different frequencies.

3.
Results and discussion

Microstructure
The XRD patterns of the S1eS5 H/S NCs are illustrated in Fig. 1.The XRD patterns of several NCs presented peaks associated with spinel ferrite and SreBa hexaferrite, which confirms the occurrence of hard and soft phases together with no configuration of any secondary phases.The cell parameters of the S1eS5 H/S NCs and the proportions of both hard and soft ferrites were estimated using Match!v. 3 Full Proof software and are listed in Table 1.The cell parameters of both phases fluctuated slightly with changes in the ratios of Ni, Cu, and Zn in the soft side because of the redistribution of ions in the spinel structure [46].Furthermore, the crystallite size (D XRD ) of the S1eS5 H/S NCs was obtained using the Scherrer equation.The crystallite size decreased with the variation in the amounts of Ni, Cu, and Zn.Moreover, the crystal size of the hard phase was nearly 57% greater than that of the soft phase [47].The variation in crystallite size is due to the boundary migration that generally occurs through extended-range diffusion among hard and soft phase systems [48].

Morphological investigation
The FE-SEM analysis of Sr 0.5 Ba 0.5 Eu 0.01 Fe 12 O 19 and the S1eS5 H/S NCs is shown in Fig. 2. The images show an  NCs was performed via EDX and elemental mapping (Fig. 3).
The results demonstrate the presence of Sr, Ba, Eu, Ni, Zn, Cu, Fe, and O, which confirms the creation of the H/S phase without any impurities.Sr 0.8 Ca 0.2 Fe 12Àx Eu x O 19 (x ¼ 0.00e0.25).It was shown that the H c value increased with increasing Eu 3þ ion content, from approximately 2412 Oe for the non-substituted sample (x ¼ 0.00) to 4045 Oe for x ¼ 0.25.The authors attributed this enhancement to the improvement in shape anisotropy resulting from the decrease in grain size with the increase in Eu 3þ content; thus, the inclusion of Eu plays the role of an inhibitor for the growth of grains.In a recent study, we investigated the physical properties of EueNd co-substituted SrBa hexaferrite [62].An improvement in the H c values was observed at both 300 and 10 K under the substitution effect.The maximum H c values were approximately 2.78 and 2.41 kOe at 300 and 10 K, respectively.Fig. 5 illustrates the MÀH data obtained at low and room temperatures for the magnetic S1eS5 H/S NCs.Magnetization measurements were conducted under an applied magnetic field of 70 kOe at both 300 and 10 K.The MÀH curves are smooth for the various prepared S1eS5 H/S NCs.It has been established that a hysteresis loop with a smooth shape suggests a 'collaborative' magnetic switching of the two interacting magnetic grains across a good exchange coupling between the two phases.For the various produced NCs, the reversal in magnetization is not smooth, and the MÀH curves do not exhibit a single-phase character in the second quadrant, signifying that the exchange interaction is incomplete.Based on previous reports, the exchange interaction may be significant for the magnetic characteristics of the NC when the size of the soft magnetic phase does not exceed twice the domain wall width of the hard phase [67e70].Taking into account that the M-type hexaferrite has a domain wall width of approximately 10 nm [71], the soft magnetic phase may exhibit a mean size of approximately 20 nm.Furthermore, Y. Sun et al. [72] confirmed these calculations and reported that the average dimension of hard magnetic grains may be greater than 40 nm.In comparison, soft magnetic grains may be smaller than approximately 20 nm to establish an excellent exchange-coupling effect.Nevertheless, here, the size of the grains for NiCuZn ferrite nanoparticles exceeds 30 nm (Table 1), which is inconsistent with the assumptions of the theoretical calculations.Hence, it will be challenging for the exchange interaction to occur between the two magnetic grains in these NCs, and consequently, the dipolar interactions will be dominant [27,73].This results in the observation of a kink in the second quadrant of the MÀH curve (Fig. 5) for NCs exhibiting weak exchange coupling.

Magnetization
The values of M s , H c , and remanence (M r ) are presented in Fig. 6 for different NCs.Maximum M s values of 63.1 and 93.9 emu/g were obtained at 300 and 10 K, respectively, for sample S2, which contained the highest Zn content and the same Ni and Cu contents in the soft magnetic phase part.For the various produced NCs, M r was practically the same.Generally, M s is a fundamental characteristic of a product.Hence, it is strongly affected by the different synthesis methods, morphologies, shapes, dimensions, and structural properties of the ferrite (nano)-particles, as well as the distribution of constituent phases within the NCs [74e76].The calculated values of the squareness ratio (SQR ¼ M r /M s ) are shown in Fig. 7.An SQR value of greater than 0.5 is considered a convincing criterion for the existence of the exchange coupling effect, while an SQR value of less than 0.5 is an indication on incomplete exchange between the two magnetic phases [77].In the  present study, the obtained SQR values were less than 0.5 for all of the synthesized NCs, confirming that the exchange coupling was incomplete.
The H c values for different NCs were located amid those of the individual hard and soft magnetic phases.The coercive field of the produced NCs reached a maximum value of approximately 2485 Oe at 300 K and 2331 Oe at 10 K with a drop in M s values to 56.9 emu/g at 300 K and 78.5 emu/g at 10 K for sample S5.This sample contained lower fractions of Zn and Cu, and the highest fraction of Ni.Without the presence of a soft phase, the magnetic moments of the hard grains are coupled, leading to deviations in the magnetic moments from the easy axis.Generally, when the soft phase is inserted into the hard grains, the effect of dipolar interactions is weakened owing to the appearance of hardesoft interactions in addition to the direct hardehard coupling interaction.For the lower soft phase fraction, the hard grains exert a powerful exchange force on the magnetic moments of the soft grains, and hence the coercivity will be increased.However, above a certain level, when the soft fraction increases, the exchange coupling will be reduced, while the dipolar interactions (softesoft interactions) will become significant [27,73].Thus, the coercive field will decrease.Furthermore, some soft grains may switch the magnetic moments of the neighboring hard grains once the demagnetizing field is reversed.The extra demagnetizing field can also reduce the coercive field in the whole NC [67].Additionally, numerous factors, including the interface condition, morphology, and grain size, affect the coercivity.Quantification of these diverse parameters is challenging.Therefore, it is difficult to monitor the coercive force in NCs.
The magnetization curves were differentiated to assess the tendency of irreversible susceptibility with respect to the reversing field.Fig. 8 presents the plots of dM/dH versus H at 300 and 10 K for different NCs.Such an analysis can provide information about the interphase exchange coupling.Typically, two separate peaks are anticipated for non-coupled H/S mixtures, whereas a single peak is observed for wellexchanged composites [78,79].Additionally, the width of the dM/dH peaks is an indicator of the effectiveness of the H/S interphase exchange coupling.The wider the peaks, the worse the exchange coupling between the hard and soft phases is.The manifestation of a maximum in the vicinity of zero fields suggests that a large fraction of soft crystallites are uncoupled by the exchange effect.This broad peak shows a large range of crystallite sizes that exhibit various degrees of exchange coupling.Overall, the different NCs presented two peaks at the two measured temperatures.This indicates that the reversal in magnetization is not accomplished in one step in these NCs.

Microwave properties
The MW properties of the nanocomposite samples were investigated at frequencies of 7e18 GHz using a VNA with the waveguide R/T method, as shown in Fig. 9.The permittivity (ε) and permeability (m) values of the samples were determined using the NicolsoneRosseWeir method based on the scattering S-parameters measured over the wide frequency range of interest [80e82].To determine the electromagnetic parameters of the investigated materials, we used a section of the transmission line with a waveguide (air filling), as shown in Fig. 9.
Figs. 10e13 depict the dependences of the real and imaginary parts of ε and m as functions of the frequency of the investigated composites.Because the electrodynamic parameters were measured at frequencies of 7e18 GHz, the main contributor to the formation of the dielectric constant value is the polarization losses (dipole polarization).The dipole polarization that determines the value of the real part of the permittivity is a result of the dipole orientations in the AC electric field.Because the studied material samples are a mixture of several components, the main contributors to the dielectric constant value are nickel, copper, and zinc.
The absorption of the electromagnetic field energy is manifested at frequencies coinciding with the natural frequencies of the precession of the magnetic moments of the electronic system of the ferromagnetic sample in the internal effective magnetic field.This is the excitation in the entire volume of the sample of oscillations of the uniform precession of the magnetization vector, which is caused by a magnetic microwave field.In general, this is ferromagnetic resonance.The main characteristics of ferromagnetic resonance, i.e., the resonance frequencies, relaxation, shape and width of the absorption lines, and nonlinear effects, are determined by the collective multielectron nature of ferromagnetism.In this Fig. 9 e Setup for measurement of the microwave properties of samples: (a) detail of the waveguide structure when the three pieces are moved away from each other, (b) waveguide connected to the vector network analyzer using low-loss RF cables, and (c) disassembled waveguide structure.
case, the presence of a domain structure in the ferromagnet complicates the process, leading to the possibility of the appearance of several resonance peaks, and the resonant absorption of MW energy causes local heating.The ferromagnetic resonance frequency of a flat sample in a parallel external field is calculated using the Kittel equation: where M is the magnetization of the material.The magnetization depends on the p m vector of the magnetic moment for the entire set of atoms in a given volume of the material, V.
Accordingly, by increasing the number of ions in the material, the vector of the magnetic moment increases (with a constant volume), and the magnetization and frequency of absorption of the energy of the electromagnetic field increase.
The dipole moment can be calculated using following equation: where e is the charge of the partial dipole, and r is the distance between negative and positive charges.Fig. 10 shows that an increase in the Ni content affects the permittivity of the mixture.For example, sample S5 has a nickel content of 0.8, which is the highest among all samples, while the copper and zinc contents are minimal at 0.1.Hence, S5 exhibits the highest dielectric constant.On the other hand, increasing the copper and zinc contents will lead to a rapid decrease in permittivity because the dipole moments for copper and zinc are opposite to the main dipole moment of nickel.
Owing to the differences in the shapes and polarizabilities of the f-, p-, and s-electronic orbitals, the center of the dipole moment is shifted towards nickel by r, which results in generation of the so-called homopolar moment of the dipole.An increase in nickel content accompanied by a decrease in the copper and zinc contents in the composite maximizes the real   part of the permittivity (ε 0 ) over the entire frequency range (see Fig. 10).
The dielectric losses of materials are determined by the imaginary part of the permittivity (ε'').At lower frequencies, when dipole polarization exists, these losses are substantial.With increasing frequency, the orientations of the dipoles do not have time to change in the AC field.This results in transformation of the dipole polarization to a polarization due to the ion-relaxation processes.This type of polarization (i.e., ion relaxation) can be described by the ion shifts in the crystal lattice.
The permeability (m) of the composites exhibits distinct peculiarities over the considered frequency range in a similar manner to the permittivity, as shown in Figs. 12 and 13.All of the studied samples behave like ferromagnets (where m > 1).Larmor's theorem can explain this change in permeability with frequency.Samples of a substance in a quiescent state exhibit a magnetic moment with a small value that does not drastically alter the whole magnitude of the permeability.On the other hand, when a time-varying magnetic field excites the substance, precession of the magnetic moment of electrons will begin according to Larmor's theorem.
Because nickel, copper, and zinc have practically the same number of electrons (they are located in series sequentially), an increase in the frequency provokes an increase in the precession of the electrons' magnetic moment.This results in an increase in the real part of the permeability (m 0 ).Because of the insignificant value of the magnetic moment of zinc, an increase in the Zn content marginally alters the global value of m'.The number of electrons for the Ni ion is smaller than that for Cu.It is observed that an increase in the Cu concentration affects the total precession of the magnetic moments of the electrons insignificantly and increases the real part (m 0 ).
The increase in m'' with increasing x, y, and z contents can be clarified as follows: an increase in the frequency induces an increase in the precession of the magnetic moments of electrons and a growth in the energy expended to preserve the precessions, i.e., losses.
Herein, the reflection coefficient of a semi-infinite surface is calculated.Let the EM waves fall normally at the interface between two media: air and the material under study, whose parameters were measured as ε 2 and m 2 .In this case, the reflection coefficient is expressed as To analyze the obtained values of the reflection coefficient, Fig. 14 shows the dependence of the reflection coefficient magnitude on the frequency, which can be expressed in decibels (dB) as where |R| is the reflectance magnitude, representing the ratio of the reflected voltage amplitude relative to the amplitude of the incident voltage in dB.
Fig. 14 shows the frequency dispersion of the reflection losses (R).It is evident that the value of the coefficient of the reflection is lower than À13 dB in the range of 7e14 GHz, which indicates that the produced nanocomposite materials can absorb adequate energy through losses in polarization.This behavior of small reflectivity suggests exciting applications in radar-based systems such as stealth aircraft with low observability in military applications.

Conclusion
A one-pot solegel auto-combustion approach was used to obtain S1eS5 Sr 0.5 Ba 0.5 Eu 0.01 Fe 12 O 19 /Ni x Cu y Zn w Fe 2 O 4 H/S NCs.
The XRD results showed the formation of both hard and soft phases with crystal sizes in the range of 20 and 85 nm, respectively.The surface analysis showed aggregation of a hexagonal plate covered by a cluster of cubic particles, indicating the co-occurrence of hexaferrite and spinel ferrite.Analysis of the magnetic measurements revealed that the reversal in magnetization was not smooth, and the MÀH curves did not present a single-phase character in the second quadrant.Furthermore, the plots of dM/dH versus H presented two separate peaks.Additionally, the calculated SQR values were lower than 0.5.All of these findings indicate that the exchange interaction between the two magnetic phases was incomplete.Maximum M s values were obtained for the sample that contained the highest content of Zn and the same contents of Ni and Cu within the soft magnetic phase (i.e., H/S Sr 0.5 Ba 0.5 Eu 0.01 Fe  calculate the frequency dispersions of ε and m (real and imaginary parts).It was observed that the stoichiometry of the NiCuZn ions in the spinel fraction critically influenced the electrodynamic properties.It was demonstrated that this type of composite effectively absorbed energy (>-13 dB) in the range of 7e14 GHz due to losses in polarization.This research opens a vast scope for applications in radar-absorbing technologies owing to the adequate level of absorption that must be close to the natural absorption of air media.

Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Fig. 2 e
Fig. 2 e SEM micrographs of the S1eS5 H/S NCs.

Fig. 3 eFig. 4 e
Fig. 3 e Elemental mapping and EDX spectra of the S2 and S3 H/S NCs.
j o u r n a l o f m a t e r i a l s r e s e a r c h a n d t e c h n o l o g y 2 0 2 1 ; 1 5 : 1 4 1 6 e1 4 2 9

Fig. 5 e
Fig. 5 e Magnetization versus field curves for S1eS5 H/S NCs performed at (a,b) T ¼ 300 K and (c,d) T ¼ 10 K.
j o u r n a l o f m a t e r i a l s r e s e a r c h a n d t e c h n o l o g y 2 0 2 1 ; 1 5 : 1 4 1 6 e1 4 2 9

Fig. 6 eFig. 7 eFig. 8 e
Fig. 6 e Evolution of the M s , M r , and H c values for S1eS5 H/S NCs.

Fig. 10 eFig. 11 e
Fig. 10 e Values of the real part of the permittivity of S1eS5 H/S NCs over the 7e18 GHz range.

Fig. 12 eFig. 13 e
Fig. 12 e Values of the real part of the permeability of S1eS5 H/S NCs over the 7e18 GHz range.
j o u r n a l o f m a t e r i a l s r e s e a r c h a n d t e c h n o l o g y 2 0 2 1 ; 1 5 : 1 4 1 6 e1 4 2 9

Fig. 14 e
Fig. 14 e Frequency dispersions of the reflection losses of S1eS5 H/S NCs.

Table 1 e
Determined structural parameters for various H/S NCs.
11.99 O 19 /Ni 0.2 Cu 0.2 Zn 0.6 Fe 2 O 4 ).In contrast, H c reached a maximum value for the sample that contained lower fractions of Zn and Cu and the highest fraction of Ni (i.e., H/S Sr 0.5 Ba 0.5 Eu 0.01 Fe 11.99 O 19 /Ni 0.8 Cu 0.1 Zn 0.1 Fe 2-O 4).Investigations of the electrodynamic properties of the SrBaEu-hexaferrite/NiCuZn-spinel composites with fixed SrBaEu compositions and varying NiCuZn compositions were performed at frequencies of 7e18 GHz.The measurements were performed using the waveguide reflection/transmission method.S-parameters (S 11 eS 21 ) were measured and used to