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Spinel ferrites have a significant role in high-tech applications. In the present work nano-crystalline ferrites having general formula Co0.5Cd0.5BixFe2−xO4 with (x = 0.0, 0.05, 0.1, 0.15, 0.2, and 0.25) are synthesized via micro-emulsion route. Powder x-ray diffraction (XRD) studies discover the FCC spinel structure. Crystalline size is calculated in a range of 11 nm–15 nm. Lattice parameter calculations are reduced due to its substitution which leads to the exchange of large ionic radius of Fe3+ for small ionic radius of Bi3+. The x-ray density is analyzed to increase with doping. Fourier transform infrared spectroscopy (FTIR) is performed to analyze absorption band spectra. The two absorption bands are observed in a range of 400 cm−1–600 cm−1, and they are the characteristic feature of spinel structure. Thermo-gravimetric analysis (TGA) reveals the total weight loss of nearly 1.98%. Dielectric analysis is carried out by impedance analyzer in a frequency span from 1 MHz to 3 GHz by using the Maxwell Wagner model. Dielectric studies reveal the decrease of dielectric parameters. The alternating current (AC) conductivity exhibits a plane behavior in a low frequency range and it increases with the applied frequency increasing. This is attributed to the grain effects in a high frequency range or may be due to the reduction of porosity. Real and imaginary part of impedance show the decreasing trend which corresponds to the grain boundary action. The imaginary modulus shows the occurrence of peak that helps to understand the interfacial polarization. Cole-Cole graph shows a single semicircle which confirms that the conduction mechanism is due to the grain boundaries at low frequency. Dielectric studies reveal the applicability of these ferrites in high frequency equipment, microwave applications, high storage media, and semiconductor devices.
Ferrites are the materials containing iron oxide as its main component, and they are ferromagnetic oxides possessing high permeability and resistivity.[1] They have vast area of applications in all the fields because they are suitable for the applications in high frequency, resistivity and high corrosion resistance.[2] Their commercial use appeared after 1950 in electronic devices like computer circuitry, microwave devices, television, telephones, radio, magnetic memories and transformers and so on. The ferrites possess a large number of bulk applications but the phenomenon of magnetism lies in nano-scale domain.[3] Spinel ferrites have great significance because of their electrical and dielectric characteristics, and these characteristics depend on the chemical composition and the method of preparation. Recently, with the development in information technology, high performance storage devices have received great attention.[4] Multiferroic materials have significance due to their ferroelectric, ferromagnetic and ferroelastic properties. The application of external electric field, magnetic field and stress can cause the change in spontaneous magnetization, polarization and deformation, respectively. Mutual coupling of these phenomena has revealed their applications in sensing, storage and spintronic devices {Wang, 2019 #135}.[5–7] The ferrites possess characteristics of behaving like inhomogeneous dielectric material consisting of highly conducting grains detached by air gap or insulating layers called grain boundaries.[8] Dielectric properties of ferrite like dielectric constant, tangent loss, AC conductivity and impedance can be controlled by annealing and composition.[9,10] Nowadays cobalt ferrites gained much attention due to their applications in high electromagnetic, industrial and biomedical applications. They have good electromagnetic properties and hardness just because of their high chemical stability.[11–13] The characteristics of these ferrites greatly depend on synthesis methodology and the annealing temperature. It should be noted that size of such ferrites strictly relies on annealing temperature.[14–21] In a current refined era, a limited number of researches have been done on cobalt–cadmium spinel ferrites, and researchers have paid magnificent attention to the DC electrical conductivity which exhibits an increasing trend with the rise of temperature.[22] Researchers have also reported the increase in conductance characteristics of cobalt–cadmium ferrites as well as catalytic decomposition of 2-propanol. The cobalt-cadmium ferrites with variational composition, magnetic coupling leads to a strengthening effect on A–B interaction.[14] Research groups have also reported an increase in dielectric properties of material as well as AC conductivity.[23]
In the present study, the cobalt-cadmium ferrite having bismuth as a substituent are prepared via micro-emulsion route. The prepared ferrite material is characterized by thermo-gravimetric analysis (TGA), x-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and dielectric properties analysis.
Bismuth substituted cobalt–cadmium ferrites were manufactured by micro-emulsion technique. The general composition formula used was Co0.5Cd0.5BixFe2−xO4 (x = 0.0, 0.05, 0.1, 0.15, 0.2, and 0.25). The materials used were cobalt (II) nitrate hexa-hydrate (Co (No3)2.6H2O), cadmium nitrate tetra-hydrate (Cd (No3)2.4H2O), bismuth (III) nitrate penta-hydrate (Bi (No
Thermo-gravimetric analysis (TGA) as well as differential thermal analysis (DTA) of prepared sample Co0.5Cd0.5Fe2O4 is accomplished in a temperature range of 0°C–1010°C. The analysis of percentage weight loss against temperature was made with the help of TGA curve, which exhibits that the decomposition process is completed step by step.
A weight loss of about 1.98% is observed in the whole temperature range. Rapid changes in the curve occur at 55°C and 420°C. Below 100°C there is a weight loss of approximately 0.38%, which is due to the evaporation of water.[24,25] At 358°C the weight loss of about 1.38% occurs which is due to the decomposition of matter, evaporation of water and oxidation of organic matter.[26,27] Secondly, most of the weight loss is nearly 1.36% occurring in the temperature range of 100°C–420°C. Then in the last step, a 0.17% weight loss is found in the temperature range of 420°C–974°C. From TGA graph it follows that the phase formation starts at 900°C after which there is very small weight loss, beyond 1010°C there exists no weight loss, which confirms the creation of stable phase. The DTA curve shows that the decomposition is carried out step by step due to some exothermic peaks. The first sharp peak is observed at 94°C, the second oneoccurs at 234°C and the third one appears at 405°C, indicating the burning of additional constituents.[24,28] Figure
Bismuth doped cobalt–cadmium ferrites having general composition formula Co0.5Cd0.5BixFe2−xO4 (x = 0.0, 0.05, 0.1, 0.15, 0.2, and 0.25) are synthesized via micro-emulsion route. To study the crystalline structure in detail and identify the crystalline phase formation, powder XRD is carried out on PANalytical XPert Pro.[29] It is very basic and important characterization technique to determine certain parameters of crystal structure like, crystalline size, lattice constant, bulk density, x-Ray density, stacking fault, lattice strain, micro-strain and dislocation density, and so on. The crystalline phase of the material is confirmed because the obtained XRD pattern consists of very sharp peaks. At
The crystalline size is calculated for the hkl value of (311) from the Debye Sherrer’s formula given by
The average lattice constant is calculated to be in a range of 8.46 Å–8.5 Å. The variations in the “a” is described on the basis of Bi3+ and Fe3+ ion radius. Lattice parameter is found to decrease with the substitution of Bi ion content.
This is due to the replacement of large radius of Fe3+ ion (0.76 Å) by smaller radius of Bi3+ ion (0.74 Å) at the octahedral site.[34–36] Lattice constant first decreases rapidly with the substitution and then increases, in the end it again decreases, which s may be due to the segregation of Bi3+ ions on grain boundaries.[1] The x-ray density of synthesized ferrite is found from the following formula:
![]() | Table 1.
Structural parameters of Co0.5Cd0.5BixFe2−xO4 (x = 0.0, 0.05, 0.1, 0.15, 0.2, and 0.25). . |
Fourier transform infrared spectrum (FTIR) is recorded the on iS50 FT-IR for further detailed analysis of crystal structure. The spinel structures of the prepared ferrites having compositional formula Co0.5Cd0.5BixFe
From the frequency band data the force constants are also calculated, which are Kt and Ko for tetrahedral and octahedral sites by the following formulas:
These tetrahedral and octahedral radii are seen to decrease inhomogenously, which proves the decrease in bond length with the concentration of Bi3+ increasing.[24] The values of these parameters are given in Table
![]() | Table 2.
Different parameters of FTIR analysis. . |
The dielectric properties of prepared ferrite samples having general formula of composition Co0.5Cd0.5BixFe2−xO4 (x =0.0, 0.05, 0.1, 0.15, 0.2, and 0.25) are calculated with the help of impedance analyzer in a span of frequency from 1 MHz to 3 GHz at room temperature (as shown in Table
![]() | Table 3.
Dielectric parameters for Co0.5Cd0.5BixFe2−xO4 (x=0.0, 0.05, 0.1, 0.15, 0.2 and 0.25). . |
Dielectric constant, dielectric loss, and tangent loss of Co0.5Cd0.5BixFe2−xO4 (x = 0.0, 0.05, 0.1, 0.15, 0.2, and 0.25) are calculated with respect to the frequency of applied field.
Figure
Figure
The AC conductivity is one of the important characteristics of dielectric material. The AC conductivity of the prepared ferrite samples Co0.5Cd0.5BixFe2−xO4 (x = 0.0, 0.05, 0.1, 0.15, 0.2, and 0.25) are calculated in a frequency range from 1 MHz to 3 GHz. ssThe formula to be used for the calculation of AC conductivity is
From the figure it can be observed that for most of samples the AC conductivity has an increasing trend in the low frequency range, whereas in the high frequency range it presents the dispersive behavior . Maxwell–Wagner model and Koop’s phenomenological theory both explain that the ferrites materials have high conducting grains which are separated by grain boundaries having resistive nature.[45] At low frequency the behaviors of all samples seem to be planar and the same due to high resistance of grain boundaries. At high frequencies, as a result of the effects of grains and the hopping phenomena in Fe2+ and Fe3+ at octahedral sites, the conductivity is enhanced. The increasing behavior of conductivity may also be due to the reduction of porosity. At low frequencies it is seen that the conductivity has a grain boundary effect whereas in the high frequency range the conducting effects of grains are observed, therefore the dispersion trend can be seen.[1,24,26]
Impedance analysis is a powerful tool to relate dielectric properties of synthesized material with its microstructural composition.[46,47] The real and imaginary impedance part are calculated for each of the ferrite Co0.5Cd0.5BixFe2−xO4 (x = 0.0, 0.05, 0.1, 0.15, 0.2, and 0.25). These impedance parts dependent on the applied field frequency are calculated as depicted in Figs.
Impedance spectrum shows that the increase of applied frequency reduces the real and imaginary impedance parts. With the further increase of frequency the impedance curves of all samples merge with each other and at higher frequency impedance show constant behavior, which is attributed to the release of space charges.[48] These space charges are produced because of the disparity of concentration and also due to the inhomogeneity of applied field which tends to accumulate these charges on the grain boundaries. The decreasing real and imaginary impedance parts show the improvement of conductivity with the increase of field frequency.[47,48]
Real and imaginary modulus of the prepared ferrite having compositional formula Co0.5Cd0.5BixFe2−xO4 (x = 0.0, 0.05, 0.1, 0.15, 0.2, and 0.25) are calculated as a function of applied frequency. The relations used for the calculation of real and imaginary modulus are
Figure
If the grain boundary region occupies a greater volume, then the graph between
![]() | Table 4.
AC conductivity, impedance, and modulus values for Co0.5Cd0.5BixFe2−xO4 (x = 0.0, 0.05, 0.1, 0.15, 0.2, and 0.25) in the frequency range from 1 MHz to 3 GHz. . |
The Cole–Cole diagram is shown in Fig.
Nano-crystalline ferrites with general formula Co0.5Cd0.5BixFe2−xO4 (x = 0.0, 0.05, 0.1, 0.15, 0.2, and 0.25) are effectively synthesized by the micro-emulsion method which has proved to be an easiest method of synthesizing such ferrites. The XRD studies confirm the FCC spinel structure with the appearance of sharp peaks. Crystalline size is found to be in nanosize range from 11 nm to 15 nm and is calculated by Debye–Scherrer formula. Lattice constant shows the decreasing behavior with Bi3+ substitution increasing due to the replacement of large-radius Fe3+ ion by smaller-radius Bi3+ ion. Two frequency stretching bands are observed by FTIR studies, which are corresponding respectively to tetrahedral and octahedral site and are the characteristic bands of spinel structure. The weight loss of the ferrite material is calculated by TGA and DTA analyses, which shows the percentage weight loss of about 1.98% in a specified temperature range due to the evaporation of moisture and impurity content. Dielectric studies are carried out in a frequency range of 1 MHz–3 GHz, which follows the Maxwell Wagner model, and reveal the dielectric parameters decreasing with applied field frequency increasing. The AC conductivity of material is found to increase with frequency increasing due to the grain effect at high frequency. The impedance analysis shows decreasing tendency which in turn results in an increasing conductivity of material. It is attributed to the grain boundary effect. The real and imaginary modulus analysis conduces to the understanding of the bulk and grain boundary effect and the Cole–Cole explanation reveals the formation of single semicircle that confirms that the conduction mechanism is due to the grain boundaries at low frequency. These dielectric properties propose the probable usage of these nano-ferrites in high frequency applications, microwave applications, storage media, and semiconductor devices.
Authors thank the ORIC of Balochistan University of Information Technology Engineering and Management Sciences (BUITEMS) Quetta-PK, for help and financial support to accomplish this research work in the Department of Physics.
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