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The influence of indium on the properties of Cu0.5Zn0.5Fe2O4 nano ferrites synthesized by the solgel auto-combustion technique was investigated. X-ray diffraction (XRD) analysis demonstrated that pure and substituted ferrites possessed a cubic spinel structure. The lattice parameter increases with the incorporation of In3+ for x ≤ 0.16 and decreased thereafter. Crystallite size was found to decrease linearly as the concentration of indium increased. X-ray density, strain, and dislocation density increase as indium content increases. Hopping lengths as well as radii of A and B sites show increasing behavior up to x = 0.16 and decreased thereafter. The spectral bands indicated the formation of the spinel structure. The band positions are changed as In3+ contents increased. The value of dielectric parameters is raised with the inclusion of indium ions. The substitution of indium in Cu-Zn ferrites decreases the magnetic parameters. The increase in dielectric parameters and a decrease in magnetization proposed that synthesized magnetic oxides may have potential in the fabrication of switching and high-frequency devices. 1. Introduction The researchers and scientists have great attention towards spinel ferrites due to utilization in many fields i.e circulators, transformers core, data storage devices, switching, gas sensing, radio frequency circuits, antennas, microwave, drug delivery, and high-frequency appliances. [1-4]. Spinel ferrites have interesting and excellent structural, electrical, dielectric, and magnetic properties. The characteristics of spinel ferrites depend upon many factors i.e structure, size, density, synthesis method, and substitution of cations [5-9]. The cations distribution among tetrahedral and octahedral sites strongly affected the properties of synthesized ferrites. The interactions between the cations are mainly affected by the magnetic properties of spinel ferrites. When A-B superexchange interaction is stronger than the other two it indicated collinear spin structure while A-A or B-B superexchange interaction is comparable with A-B exchange interaction then it leads to non-collinear spin structure [10, 11]. Copper zinc ferrites are crystallized as spinel ferrites and exhibited ferrimagnetic and semiconducting behavior. The antiparallel spins of the magnetic moment of atoms among interstitial sites give the ferrimagnetic characteristics. Substituting various kinds of an atom having a different radius, magnetization, and atomic weight in the spin-lattice leads to the control of structural properties [12-15]. Copper ferrites possessed an inverse spinel structure and showed ferrimagnetic at room temperature. Zinc ferrites owing normal spinel structure and exhibited 1 ferrimagnetic below Neel's temperature. ZnFe2O4 nano ferrites become paramagnetic at room temperature. Cu2+ ions preferentially occupied at octahedral sites while Zn2+ preferentially occupied on tetrahedral sites. Fe3+ cations are disturbed among tetrahedral and octahedral sites [10, 16, 17]. Variation in the magnetic properties of Cu-Zn nano ferrites is attributed to the distribution of iron ions on two sites with tetrahedral and octahedral coordination of oxygen ions. If divalent cations go to tetrahedral voids only while trivalent cations occupied octahedral voids then the structure was said to be normal spinel. Special attention is required to understand the structural and magnetic properties of copper-zinc ferrites. Novel synthesis techniques are applied to attain definite structural and magnetic properties [1820]. Various methods are used to obtain spinel ferrites but some most common methods are listed as hydrothermal, co-precipitation, solid-state reaction, sol-gel, and micro-emulsion route (references). Sol-gel auto combustion route offers various advantages over other methods including definite particle size, lab friendly, cost-effective, and processed at low temperature with the best homogeneity. Single-phase Cu-Zn nano ferrites can easily be obtained by the auto combustion method [21]. The deviation in magnetic properties is due to variation in structural parameters. The change in structural parameters of copper-zinc ferrites is observed by the substitution of indium ions with different concentrations. The present work aims to explore the influence of indium on the structural, spectral, dielectric as well as magnetic properties of Cu0.5Zn0.5InxFe2-xO4 ferrites. These ferrites are synthesized by a novel synthesis route i.e sol-gel auto-combustion. Here we reported our results on structural, spectral bands as well as bond length, magnetic and dielectric properties which are characterized by XRD, FTIR, VSM, and dielectric. 2. Experimental procedure The indium incorporated Cu-Zn nanocrystalline ferrites were synthesized by the sol-gel autocombustion route. The analytical grade copper nitrate, zinc nitrate, Iron nitrate, indium nitrate, and citric acid were utilized without any purification in the stoichiometric ratio of metals nitrate to citric acid was 1:1. All the reagents were weighed correctly and dissolved in the deionized water. After that, all solutions were heated at 45 °C with continuous stirring. The pH value was maintained with the addition of the ammonia solution ~ 7. Moreover, all the solution mixture 2 was continuously stirred as well as heating the temperature maintain up to 80-90 °C. The water molecules were evaporated and the formation of the sol was developed. The gel was obtained by stirring the solutions for 3 hours as well as heated at 80-90 °C. The gel was self-ignited and burned entirely by the auto combustion method to make fluffy powder. Finally, the prepared powder was sintered at 500 °C for 4.30 hours to remove the organic matter and spinel phase formation. To form a pellet 5% polyvinyl alcohol (PVA) was added to calcinated powder as a binder and pressed to obtain a pellet of diameter 7mm. The pellet was heated at 160 °C for 2 hours. The prepared pellet was used for the dielectric study. D8 Advance Bruker, CuK α was used to recognize the crystal structure of sintered powder. FTIR spectra were recorded by Nickolet TM 5700 spectrophotometer. The dielectric property of synthesized ferrites was carried out by the Keithley LCR meter model-197. M-H loops were obtained using a vibrating sample magnetometer (VSM) Lake-shore model-7300. 3. Results and discussion 3.1 Thermal analysis The phase transformation of the as-received sample of copper-zinc ferrite was studied by thermal analysis. Thermal analysis of pure ferrite depicted in Fig. 1. The weight loss in TGA and DTA curves occurred through four successive steps. There are four peaks observed in the TGA and DTA curves due to the disintegration of residual components. First weight loss of ~ 2% was observed due to water evaporation. Second weight loss ~ 3% and third weight loss ~ 2% were observed due to disintegration of residual components. After that very small weight loss was observed this indicated the spinel phase formation. The first exothermic that appeared below 100 °C was attributed due to moister or water trapped in the pores of prepared ferrite. The exothermic peaks occurred at 115 °C and 148 °C was due to the decomposition of metal nitrates. The peak that appeared at 278 °C exhibited crystallization of the spinel phase. The overall weight loss reduced in the thermal analysis was 8%. The spinel phase formation occurred after 400 °C which can be seen from TGA curves. Three exothermic peaks that appeared in the DSC curve are associated with DTA peaks. DSC analysis provides information about reactions i.e exothermic or endothermic. DSC curve revealed that the decomposition process that occurred in the present study is strongly exothermic. The sintering temperature for indium substituted copper ferrites 3 was estimated from the TGA curve. The sintering of synthesized nano ferrites was carried out at 500 °C for 4.3 hours [22, 23]. 3.2 Structural analysis X-ray diffraction patterns of Cu0.5Zn0.5InxFe2-xO4 nanocrystalline ferrites were represented in Fig.2. XRD pattern of the pure sample exhibited only spinel phase and no additional phase appeared during the sintering process. All the peaks were well indexed and matched with the ICDD card no. 01-089-7409. In the present study, ferrites were crystallized in a spinel structure having cubic symmetry with the space group Fd3m [24]. The various parameters such as crystallites size, the lattice constant, and cell volume are elucidated from XRD data and listed in Table 1. Lattice parameter increases with the substitution of In3+ up to x = 0.16 and decreases for further x. The variation occurred in the value of the lattice parameter ascribed via the difference in ionic radii. The increase in the lattice parameter due to the incorporation of higher ionic radius In3+ ions than Fe3+ ions and decrease due to the presence of an additional phase [25]. The crystallite size (D) determined from Scherrer’s relation and decreased monotonically with the increase of indium contents presented in Fig.3. The value of ‘D’ remains in the range of 9.3714.76 nm. The decrease in ‘D’ was attributed to an increase in strain. When higher ionic radii In3+ was substituted in place of Fe3+ ions then lattice strain was produced because larger ions enter the spinel lattice of copper-zinc ferrites produced internal stress [26]. It was observed that the most intense peak (311) was shifted towards a lower angle which may be due to an increase in cell volume and lattice constant. When indium concentration increases In3+ ions isolate from the crystal structure of Cu0.5Zn0.5Fe2O4 and formed the InFeO3 phase. The extra phase that appeared in XRD patterns was InFeO3 and matched with the ICDD no. 01-085-2306. The impurity phase that appeared in XRD was indicated by (*). Initially, cell volume was increased and decreased for higher x. X-ray density was found to increase as indium content increases. The increased in X-ray density was due to the incorporation of denser ions in the place of lighter ions. Indium ions possessed high molecular weight and density that's why an increase in theoretical density was expected. The bulk density has nonlinear behavior. Overall bulk density was increased with the increase of dopant concentration. The value of bulk density was smaller than X-ray density due to the occurrence of pores which depends on sintering conditions. 4 Dislocation density, tetrahedral and octahedral radii, and hopping length were calculated and written in Table 2. Dislocation density has a direct relation with strain and an inverse relation with crystallite size. The decrease in D and increase in ε raised the value of density. The hopping length of tetrahedral sites (LA) was the distance between magnetic ions at tetrahedral sites and LB was the distance between ions at octahedral sites. The hopping length improved as the percentage of indium increase up to a certain level and reduced for higher x. An increase in hopping length indicated that charge carriers required more energy for the hopping between one cation side to another. Sometimes hopping was also called the distance between centers of adjacent ions. The tetrahedral and octahedral radii are estimated from XRD data. The behavior of these radii is similar to the lattice constant [27, 28]. 3.3 Spectral analysis The structural and chemical changes that take place during combustion were observed from spectroscopic analysis. The combustion reaction phenomenon understands with the help of infrared analysis. Figure 4 shows the infrared spectra of indium substituted copper zinc ferrites. The band υ1 (539-560 cm-1) and υ2 (454-464 cm-1) correspond to cubic spinel structure [29-31]. The band υ1 occurred due to stretching vibration of metal-oxygen (Mtetra-O) at tetrahedral sites while octahedral band υ2 in the FTIR spectra attributed to vibration of Mocta-O at octahedral sites. FTIR analysis also confirmed the spinel structure of synthesized ferrites [32]. The highfrequency band tilted towards low frequency with the substitution of In3+ ions. Both bands related inversely with each other. The shifting of bands depends upon cations mass, lattice parameter, and bonding of cations with oxygen. The shifting of bands towards lower frequency may ascribe the decrease in metal stretching vibrations energies. As a result, covalent bonding between metal-oxygen decreased. The value of absorption band υ1 was high as compared to υ2 due to variation in bond length [33]. The bond length exhibited an increasing trend for x ≤ 0.16 as well as decreasing behavior for higher x. The peak appeared about 1638 cm-1 attributed to O-H stretching vibrations [34]. The variation in the lattice parameter was related to the expansion and contraction of the MetalOxygen bond length of crystallographic sites [35]. When high atomic mass In3+ was incorporated in copper-zinc ferrites transfer low atomic mass ferroic ions from B sites to A sites. That's why the vibrational frequency of the B site increase and A-site decrease [36]. The behavior of the 5 force constant was explained based on vibrational frequencies. Table 3 revealed the force constant of the tetrahedral site followed by the vibrational frequencies of metals oxygen at tetrahedral sites and having the same behavior. The force constant of octahedral sites varied as the vibrational frequency of metals oxygen at octahedral sites [37]. 3.4 VSM analysis The magnetic response of synthesized ferrites was estimated from M-H loops recorded at room temperature by VSM. The synthesized nano ferrites exhibited narrow hysteresis loops with a small value of coercivity which was evidence for the soft nature of prepared samples. It was clear from Fig.5 that pure and substituted Cu0.5Zn0.5InxFe2-xO4 ferries revealed ferromagnetic nature. All the magnetic parameters were determined from magnetic data and presented in Table 4. Variations in magnetization as a function of the applied field are studied and depicted in Fig. 5. The value of Ms is reduced with the inclusion of indium [38]. The declined in Ms value with an increase of In3+ concentration was attributed to the nonmagnetic ions entered the lattice at B sites and reduces the number of Fe3+ ions on octahedral sites. Another important factor that affects saturation magnetization was crystallite size. The decrement in the value of Ms may be ascribed by reducing in crystallite size. Present results were similar to the previously reported results [23, 39]. The decreased in the value of Ms can be described by Neel’s two sublattice model given by: M = MB - M A (4.1) Where MB and MA were represented magnetic moments of octahedral and tetrahedral sites. The magnetic moment is originated due to the spin orientation of two sublattices which are aligned in an antiparallel direction. The exchange interaction (A-B) was predominant than the other two types. If the nonmagnetic In3+ ions preferred octahedral sites and replaced magnetic ions Fe3+ from B-site. The magnetic moment of B sites reduced even magnetic moment of A-site remains the same which may cause a decrease in saturation magnetization. This decrease in saturation magnetization may be due to the appearance of secondary phases. It was seen from Table 4 that Bohr’s magneton was found to decrease due to an increase of In3+ content. It is another reason for the decrease in Ms value [40]. The small value of MR and Hc indicated the presence of singledomain particles [41]. 6 The variations in Ms, Hc, and MR are shown in Fig. 6. The coercivity was declined with the substitution of In3+ ions. This decreased was attributed to the effect of magnetocrystalline anisotropy. The indium has zero angular momentum and did not take part in magnetocrystalline anisotropy. When In3+ ions were replaced the iron spin-orbit coupling decreased which reduce the value of coercivity [42]. The anisotropy constant was calculated and depicted in Table 4. Anisotropy constant reduced with the inclusion of In3+ ions. This happened due to indium has a small value of anisotropy constant than iron. As a result, the value of coercivity was decreased. The decreased in the anisotropy constant was due to a decline in domain wall energy [43]. The squareness ratio was estimated from remanence and saturation magnetization. Both the value of MR and Ms decreased which decreased the squareness ratio. The value of the squareness ratio was very small which suggests that indium incorporated copper zinc ferrites were useful for high-frequency applications [44]. 3.5 Dielectric studies The dielectric constant (ε') was used to elucidate the speed of electromagnetic wave travel through the medium. The materials possessed a high value of ε' was used to store charge [45]. The dielectric parameters were recorded in the frequency range of 1 MHz - 3GHz. Fig. 7 showed the frequency-dependent dielectric constant. The ε' has a large value at a low frequency while a small value at a high frequency. This is the general behavior of ferrites materials. Dielectric constant at low frequency decreased rapidly while it decreased slowly at high frequency and become almost frequency independent. The resonance peak was observed above 2.5 GHz. The behavior of the dielectric constant was associated with space charge polarization [46]. The behavior of the dielectric constant was ascribed by the Maxwell Wagoner model. The polarization phenomenon occurred due to hopping between ferrous and ferric ions. When frequency reached up to definite point charge carriers can not follow the variation in the ac applied field. As a result dielectric constant exhibited frequency-independent behavior [47]. The relaxation phenomenon that appeared in the dielectric constant was directly related to the polarization of substances. Dipolar and interfacial polarization contributes to a dielectric constant at a low frequency while electronic polarization was accountable only at high frequency. The space charge polarization was occurred between Fe2+ and Fe3+ due to the hopping phenomenon. According to Koop's theory, two conducting grains layers are separated by a resistive layer 7 known as grain boundaries [48]. The charges are pile up at grain boundaries which mainly contribute to the dielectric constant. The concentration of ferrous ions increased at octahedral sites which may increase space charge polarization consequently dielectric constant increased. Resonance peaks beyond 2.5GHz occurred due to the matching of ions frequency with the externally applied field frequency. Frequency of charge carriers matched with ac applied field then peaking behavior observed which increased power loss. Therefore resonance peak appeared [30, 49]. The dielectric loss of indium incorporated Cu-Zn nano ferrites was depicted in Fig. 8. It was seen that dielectric loss was found to decrease as the externally applied field increased [50]. In dielectric material, a dielectric loss estimated the dissipation of energy. The material possessed high conductivity also has a high dielectric loss and low conductivity as well as low dielectric loss. The resonance peaks observed in the high-frequency region was explained briefly in the above section. The dielectric loss tangent of indium inclusion copper zinc ferrites was displayed in Fig. 9. It was the ratio of dielectric loss to dielectric constant. The variation in tanδ depends upon various factors such as compositions, annealing temperature, and surface morphology. The value of tanδ was reduced with the increase in frequency [51]. Losses were low at high frequency due to charge carriers no longer obey the applied field. The resonance phenomenon was observed due to the coinciding of the hopping frequency with externally applied field frequency. If the content of In3+ was added in a fixed ratio then the conduction mechanism was proportional to the concentration of iron ions which was responsible for a ...
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