Synthesis and characterization of Mg–Zn ferrite based flexible microwave composites and its application as SNG metamaterial

In this article, we propose SNG (single negative) metamaterial fabricated on Mg–Zn ferrite-based flexible microwave composites. Firstly, the flexible composites are synthesized by the sol-gel method having four different molecular compositions of MgxZn(1−x)Fe2O4, which are denoted as Mg20, Mg40, Mg60, and Mg80. The structural, morphological, and microwave properties of the synthesized flexible composites are analyzed using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), and conventional dielectric assessment kit (DAK) to justify their possible application as dielectric substrate at microwave frequency regime. Thus the average grain size is found from 20 to 24 nm, and the dielectric constants are 6.01, 5.10, 4.19, and 3.28, as well as loss tangents, are 0.002, 0.004, 0.006, and 0.008 for the prepared Mg–Zn ferrites, i.e., Mg20, Mg40, Mg60, and Mg80 respectively. Besides, the prepared low-cost Mg–Zn ferrite composites exhibit high flexibility and lightweight, which makes them a potential candidate as a metamaterial substrate. Furthermore, a single negative (SNG) metamaterial unit cell is fabricated on the prepared, flexible microwave composites, and their essential electromagnetic behaviors are observed. Very good effective medium ratios (EMR) vales are obtained from 14.65 to 18.47, which ensure the compactness of the fabricated prototypes with a physical dimension of 8 × 6.5 mm2. Also, the proposed materials have shown better performances comparing with conventional FR4 and RO4533 materials, and they have covered S-, C-, X-, Ku-, and K-band of microwave frequency region. Thus, the prepared, flexible SNG metamaterials on MgxZn(1−x)Fe2O4 composites are suitable for microwave and flexible technologies.


Experimentals
, 20% of Mg nitrate and 80% of Zn nitrate is used. Similarly, (2) for Mg 40 , 40% of Mg nitrate and 60% of Zn nitrate is taken as a molar ratio. In this manner, (3) for the Mg 60 60% of Mg nitrate and 40% of Zn nitrate, and (4) for Mg 80 80% of Mg nitrate and 20% of Zn nitrate is weighted as a molar ratio. The mixers are slowly dissolved in distilled water with magnetic stirring, and also the citric acid (C 6 H 8 O 7 •H 2 O) is added as a chelating agent, and hence, a quite transparent gelatinous solution is found having light red color. Later, the appeared solutions are stirring continuously for about 5 hours at 90 °C. Thus, a reddish gel is formed, which is further dried at 150 °C by transferring the gel into a furnace. The nanoparticle was then achieved by grinding the obtained precursor and, finally, calcined at 750 °C for one hour to complete the chemical process. Figure 2 shows the main preparation steps of the flexible substrate-based metamaterials from their gel states to fabricated metamaterial unit cells. Firstly, the flexible substrate is prepared by adding the synthesized MgZnFe 2 O 4 nanoparticle into the PVA glue at a ratio of 1 g powder to 10 mL of PVA glue and later dried at 80 °C. Finally, the metamaterial unit cell is fabricated upon these Mg 20, Mg 40, Mg 60, and Mg 80 flexible substrates by copper sputtering.  Figure 3a represents the geometrical view of the proposed unit cell. During CST simulation frequency-domain solver is used. The electromagnetic wave propagates from the positive z-axis to the negative z-axis, which is energized by two waveguide ports as the simulation setup is shown in Fig. 3b. The x and y boundaries are set as perfect electric (PEC) and perfect magnetic (PMC) walls. The necessary design parameters, along with their dimensions, are shown in Table 1. The substrate length is chosen 8 mm, and width b is chosen 6.5 mm, where the thickness of the substrate is 0.5 mm for all cases. The unit cell is fabricated on flexible substrates using copper sputtering with a thickness of 0.035 mm. Here, L1 and W1 are the length and width of the outer split ring resonator with a dimension of 5.25 mm and 7.5 mm, respectively. On the other hand, L2 and W2 are the length

Results and discussion
Structure. The Siemens D500 X-ray diffractometer having a Cu Kα anode (40 kV, 20 mA) with a 2θ angle range of 20° to 80° is used to confirm the phase formation of MgZnFe 2 O 4, and the recorded XRD plot for the synthesized samples are shown in Fig. 4.   where λ is the wavelength of the X-ray radiation (1.54060 nm), and β is the full width at half maxima values in radians, and θ is the diffraction angle corresponding to the most intense reflection plane (311). The crystallite sizes of MgZnFe 2 O 4 nano spinel that evaluated from the above equation (5) Table 2, and the compositional variation of lattice constant and average crystallite size concerning Mg 2+ substitution is shown in Fig. 5a. From Fig. 5a, it is seen that due to the increasing percentages of Mg 2+ content x, the values of the lattice constant (a) and crystalline size are decreasing. This scenario has happened by following Vegard's law 52 . Also, these decreasing characteristics can be validated based on theistinction of the ionic length of magnesium and zinc ions.
The volume (V) of the unit cell of the synthesized samples can be computed with the help of lattice constant (a) by the following formula: As the unit cell volume values are directly proportional to the lattice parameter values, thus it shows the same trend as the values of the lattice constant. Hence, the X-ray density factor calculation is necessary to characterize materials. Therefore, the values of the X-ray density (d X ) can be determined from the following equation: where Z represents the cubic lattice coordination number, M represents the molecular weight of individual concentrations, V is the volume of the unit cell, and N A is the Avogadro's number (i.e., N A = 6.022 × 10 23 ). Figure 5b represents the X-ray density (d x ) and the bulk density (d B ) variation with various compositions. It is investigated that the values of the X-ray density (d x ) and the bulk density (d B ) are decreases with an increase of Mg 2+ ions. It may happen due to the molecular weight loss of the synthesized samples. Finally, from the values of the X-ray density (d x ) and the bulk density (d B ), the percentage of porosity (P) is calculated by the equation below: The calculated values of the X-ray density (d x ) and the bulk density (d B ), and the percentage of porosity (P) are also summarized in Table 2.

Morphology. The FESEM (Field Emission Scanning Electron Microscopy) technique is used to investigate
the morphology of the synthesized nanoparticle. Figures 6 and 7 represents the FESEM images and particle size histograms of Mg 20 , Mg 40 , Mg 60 , and Mg 80. A high-energy ball mill EMAX (Retsch, Germany) was utilized for grinding the Mg x Zn (1−x) Fe 2 O 4 species to nanocrystals which allows faster grinding with a maximum revolutionary speed of 2000 rpm and produce unique size particles. Zirconium oxide grinding balls having 0.5 mm size were used to grind the materials for about 3 hours. The mean grain size of the synthesized nanoparticle is   It is also observed that with increases in Mg content, the grain size and porosity are decreased, which also significantly affects on dielectric properties of the materials and leads to having tunable properties.
Optical and photoluminescence analysis. The optical energy bandgap (E g ) of the prepared samples is determined using the UV-Vis spectrophotometer. The wavelength of the applied light is chosen from 300 nm to 800 nm. For evaluating the optical energy bandgap (E g ) values, the most popular "Tauc plot" [(αhν) 2 v/s (E g )] was drawn from the UV-Vis absorbance spectral data. The "Tauc plot" was drawn using the following relation, and the plots are displayed in Fig. 8a.
The optical bandgap energy (E g ) parameters were determined from the tangent drawn at the X-axis of the "Tauc plots, " as shown in Fig. 8a. The values of 'E g ' were found to be in the range of 2.20-2.36 eV. The photoluminescent properties were studied for the prepared samples by the photoluminescence (PL) spectra excited at a wavelength of 400 nm. Fig. 8b demonstrates the PL spectra recorded at 300 K. The distinctive near band-edge emission (NBE) for all the samples was observed at the wavelength range of 524-530 nm. These variations in optical energy bandgap (E g ) and PL spectra is happened due to variations of material concentration as well as grain size, cation distribution, etc.  www.nature.com/scientificreports/ netic material. Figure 9a also shows that the Mg x Zn (1−x) Fe 2 O 4 nanoparticles' maximum magnetization increased with increases in the Mg content. It is noted that the magnetic hysteresis loops of the Mg-Zn ferrite nanoparticles did not reach complete saturation even when the applied magnetic field was 10 kOe. This feature is often found in spinel ferrite nanoparticles. It can be ascribed to the presence of a spin-disordered layer on nanoparticle surfaces, which requires a large magnetic field to saturate together with the concomitant effect of the size of the  Electromagnetic properties of the fabricated metamaterials. There has been a wide interest in the application of metamaterials in recent years. The materials in nature are made of atoms or molecules, while the metamaterial is engineered with artificially ordered repetitive structures. Those newly designed structures laid above the base material, including but not limited to their shape, arrangement, and geometry, dominate the property of the metamaterial. Those repetitive structures, often called periodic unit cells, are usually assembled at a scale below the wavelength to manipulate the electromagnetic property of the wave. The property of a metamaterial depends on the structure of the unit cell, equivalent to the atom or molecule of natural materials. The performance of metamaterial could vary upon the modification of those unit cells. In contrast, metamaterial may be designed to manipulate numerous properties of those applications influenced by the electromagnetic wave. Here, CST microwave studio is used to finalize the structure of the metamaterial unit cell before prototyping. Finally, the electromagnetic properties of the proposed flexible metamaterial were measured and extracted with the PNA vector network analyzer (Agilent N5227A, 10 MHz-67 GHz). The simulated transmission coefficient (S 21 ) for the proposed metamaterial unit cell on all the four flexible substrates, i.e., Mg 20, Mg 40, Mg 60, and Mg 80, are shown in Fig. 11a, and the corresponding measured transmission coefficient are shown in Fig. 11b. The effective permittivity, permeability, and refractive index extracted by the Nicolson-Ross-Wire method 55 are presented in Fig. 11c,d, and e, respectively. Based on the Nicolson-Ross-Wire method, the values of the effective permittivity ( ε r ), permeability ( µ r ), and refractive index ( n r ) can be calculated by the following equations:  Table 5. A very good agreement is found among simulated and measured values with a slight frequency shifting. This frequency shifting may happen due to fabrication error and/or external noise during measurements. All the values of effective permeability (μ r ) were found positive, and the values of effective permittivity (ε r ) were found negative, corresponding to resonance frequencies. Thus the metamaterial can be declared as single negative (SNG) or epsilon negative (ENG) metamaterial. The negative values of the effective permittivity and refractive index are presented in Table 3. The parameters are quite similar in the pattern for all these four metamaterials but at slightly different frequencies due to having slightly different dielectric properties.
To justify the nobility of proposed flexible substrate materials, the transmission of proposed metamaterials was investigated by replacing the substrate materials with conventional FR4 and Rogers RO4533 materials keeping the unit cell structure remain the same. The obtained transmission coefficient regarding the above cases is shown in Fig. 12 Table 4. It is seen that in both cases, the proposed materials offer better performances over commercially available materials, which originates due to having tunable dielectric and  Table 5 in terms of total dimension, substrate material, operating frequency, resonances, metamaterial type and microwave band of applications. Also, a brief comparison of the proposed metamaterial unit cells in terms of (1) physical dimensions, (2) the type of substrate, (3) the number of resonances obtained, (4) the band of applications, (5) the effective medium ratio (EMR), and (6) the type of metamaterials with some existing literature is presented in Table 6. Here, the EMR is an important factor regarding metamaterials design, which governs the compactness of the designed metamaterial. The EMR is calculated by the following equation.
where λ is the electrical wavelength corresponding to the lowest resonance of the transmission parameter (S 21 ) obtained by the metamaterial unit cell and L is the maximum physical length of the metamaterial unit cell. The     18.47. The advantages of the prepared composites are they are cost-effective, highly flexible, lightweight, and applicable in the case of wearable devices, and also have shown better performances comparing with conventional FR4 and Rogers RO4533 substrates. Thus, the overall investigations confirm that the proposed flexible metamaterials are the prominent candidate for the S-, C-, X-, Ku-and K-bands of the microwave frequency range as well as flexible microwave technologies. Applications S-, C-, X-, Ku-, and K-Band S-, C-, X-, Ku-, and K-Band S-, C-, X-, Ku-, and K-Band S-, C-, X-, Ku-, and K-Band