https://doi.org/10.52131/jmps.2021.0202.0016 54 Journal of Materials and Physical Sciences Volume 2, Number 2, 2021, Pages 54 - 68 Journal Homepage: https://journals.internationalrasd.org/index.php/jmps Role of Tb-Cr Substitution on the Structural and Magnetic Properties of Cobalt Ferrites Abdul Hameed1, Muhammad Waqas Khaliq2* 1 Institute of Physics, The Islamia University of Bahawalpur, Bahawalpur-63100, Pakistan 2 ALBA Synchrotron Light Source, Carrer de la Llum, 2, 26, 08290 Cerdanyola del Vallès, Barcelona, Spain ARTICLE INFO ABSTRACT Article History: Received: July 18, 2021 Revised: September 16, 2021 Accepted: December 29, 2021 Available Online: December 31, 2021 The impact of Tb-Cr substitution in cobalt-based ferrites having composition Co1-xTbxFe2-yCryO4 (x=0.0-0.1, y=0.0-0.5) prepared by solid state reaction method on structural, magnetic and dielectric parameters was explored. Structural analysis was carried out by X-ray diffraction technique. A variation in lattice parameter (a) was observed as a function of doping. The lattice constant ‘a’ increases from 8.37 Å to 8.39 Å, whereas crystallite size (D) decreases from 44 nm to 28 nm with Tb-Cr substitution. Unit cell volume was found in the range 586.8 Å3 - 590.6 Å3. X-ray density, bulk density as well as porosity were also affected with the inclusion of Tb-Cr ions. Saturation magnetization and remanence were observed to decrease from 62 emu/g to 51 emu/g and 13.38 emu/g to 8.29 emu/g, respectively with the addition of Tb-Cr contents. The incorporation of Tb-Cr cations caused the decrease in dielectric parameters like dielectric constant as well as dielectric loss. Keywords: Spinel Ferrites X-Ray Diffraction Magnetization Coercive Force Dielectric Properties © 2021 The Authors, Published by iRASD. This is an Open Access article under the Creative Common Attribution Non-Commercial 4.0 *Corresponding Author’s Email: mkhaliq@cells.es 1. Introduction Ferrites, magnetic materials, are relatively inexpensive and thermally stable than other magnetic materials. These materials have extensive industrial applications like high- frequency circuits, transformer cores, high-quality filters, and operating devices (Shirsath, Jadhav, Toksha, Patange, & Jadhav, 2011). The field of spinel ferrite is an important class of ferrite family due to its potential applications in different fields such as electronics, modern technology, and electrical and microwave industries. Multilayer chip indicators are unique, essential elements of modern electronics products used in videos cameras, notebook computers, cellular phones, and silver electrodes (Bhandare, Jamadar, Pathan, Chougule, & Shaikh, 2011). The researchers and engineers have investigated that Nano- ferrites show tremendous properties compared to their bulk counterparts. In the nano regime, grain boundaries control physical properties instead of grains (Gadkari, Shinde, & Vasambekar, 2013). Recently, cobalt ferrites have been extensively focused on their use in electrical and microwave industries, such as reflection coils, antennas, inductors, transformers cores, and nano-metric scales as ultrafine powders (Patange et al., 2011). Though, the uses of ferrites depend on their soft magnetic and hard magnetic nature. Ferrites exhibiting a high coercive field (Hc) are an essential parameter as these materials having significant coercivity can be used in many advanced applications. Cobalt ferrite is considered a hard magnetic material because of its large coercivity and reasonable saturation magnetization, a prerequisite for high-density magnetic recording and data storage devices. Their high coercivity value results from the high anisotropy of the Co2+ ions because of its significant spin-orbit coupling (Mallapur et al., 2009). Electrical and magnetic characteristics of ferrites https://journals.internationalrasd.org/index.php/jmps mailto:mkhaliq@cells.es Abdul Hameed, Muhammad Waqas Khaliq 55 can be tailored by doping the transition metals (Ni, Co, Zn, Cd, etc.) as well as the rare earth elements (Gd, Dy, Tb, etc.) synthesized via various techniques (Ahmed, Afify, El Zawawia, & Azab, 2012; Batoo, Kumar, Lee, & Alimuddin, 2009; Zhao et al., 2006). The distribution of cations at octahedral (B) and tetrahedral (A) sites plays a chief role in controlling the structural, electric, dielectric, and magnetic behavior (G. Kumar et al., 2013). The rare earth cations have unpaired 4f electrons with ionic radii larger than Fe cation when substituted in spinel ferrite lattice, ensuring structural, magnetic, and electrical properties (Asif Iqbal et al., 2014). S. G Kakade et al. synthesized the Er3+ substituted cobalt ferrites and studied that structural, electrical, magnetic, and dielectric parameters were changed and exhibited significant changes with doping the rare earth metals. The optimized electric resistivity shows that the prepared samples are applicable for decreasing eddy current losses at large frequency regions and can be suitable for multilayer chip inductor (MLCI) devices (Kakade, Kambale, Kolekar, & Ramana, 2016). H-z Duan et al. investigated the influence of rare earth cation on absorbing and magnetic properties of Ce doped Ni-Co hollow spheres nanoparticles synthesized via hydrothermal method (Duan, Zhou, Cheng, Chen, & Li, 2017). In recent work, we explored the influence of Tb-Cr doping for Fe-Co in cobalt-based ferrites by the standard ceramic method. The cobalt-based ferrites have been explored for their outstanding uses in magnetic recording media and EM attenuation. The central theme of this work is to elucidate the changes in structural parameters, absorption bands, and magnetic as well dielectric properties with doping of transition (Cr) and rare-earth element (Tb). The optimized results of this research are discussed here in detail. 2. Experimental Methods Cobalt ferrite and Tb-Cr substituted cobalt ferrites were fabricated by the standard ceramic method. Analytical reagents were used to prepare Tb and Cr doped cobalt spinel ferrites with composition Co1-xTbxFe2-yCryO4 (x = 0.0-0.1, y=0.0-0.5) using starting materials Fe2O3 (Sigma-Aldrich 99.9 %), CoO (Sigma-Aldrich 99.9 %), Tb4O7 (Sigma-Aldrich 99.9 %) and Cr2O3 (Sigma-Aldrich 99.9 %). These materials were combined in a stoichiometric ratio to prepare the required compositions. The dried residues were then ground for 4 hours using a pestle and mortar to obtain the fine powders. The ground powders were then converted into pellets at a pressure of (~35 KN) for 5 minutes by a Paul-Otto Weber hydraulic press. After that, the samples were firstly sintered at 650 °C for 24 hours and finally sintered at 825 °C for eight hours, followed by air quenching. Differential thermal analysis and thermogravimetric analysis of un-sintered powder were carried out to investigate the sintering temperatures by the thermal analyzer instrument model; Mettler Toledo GC 200), having a heating rate of 10 ºC/min. X-ray diffraction statistics data were collected using Bruker D8-ADVANCED X-ray diffractometer with Cu-Kα radiation (λ=1.54178 Å). Magnetization measurements under an applied magnetic field of ±1T were carried out at room temperature using a quantum design vibrating sample magnetometer (VSM: Model 6000). Dielectric parameters were examined for a frequency range 1MHz to 3GHz using the two probe method. 3. Results and Discussion 3.1. TGA/DTA Analysis Figure 1 shows the thermogravimetric (TGA), differential gravimetric (DTA), and differential scanning calorimetric (DSC) studies for un-annealed Co0.96Tb0.04Fe1.8Cr0.2O4 ferrite sample. TGA curve depicts that the entire weight loss of material was 19.4 % which was split into four portions. The first loss was 6.24 % at 82 °C, followed by a second loss of 3.16 % at 158.7 °C because of water evaporation. In the third part, 6.9 % weight loss was observed at 262 °C because of the decomposition of another phase before the development of spinel ferrite (Mahmood et al., 2013). In the last part, weight loss was 2.1% from 262 to 950 °C because of the transition of metal oxides into ferrite materials. This temperature is the annealing temperature [14]. Further, no additional weight loss after 950°C was seen, which proves the complete Journal of Materials and Physical Sciences 2(2), 2021 56 breakdown of complexes and shows the completion of phase formation.(Farhadi & Rashidi, 2010). Figure 1: TGA, DTA, DSC curves for as-prepared Co0.96Tb0.04Fe1.8Cr0.2O4 sample DTA analysis describes the decomposition rate of material. The amount of the reacting substances can be measured through the area of the peak. Endothermic peaks describe phase transition, dehydration, and reduction, while exothermic peaks describe the crystallization and oxidation processes. The differential scanning calorimetry (DSC) curve of the sample is also shown in Figure 1. In the DTA curve, peaks appeared at 65.31 °C, and 215.5 °C designate the breakdown of anions, whereas a sharp peak at 356 °C depicts the breakdown of metallic complexes. The peak at 228.95 °C describes the exothermic behavior due to the heat evolved during the crystallization of ferrite material. 3.2. Structural Analysis Figure 2 shows the XRD spectra of terbium and chromium co-doped cobalt ferrites with composition Co01-xTbxFe2-yCryO4 (x=0-0.1, y=0.1-0.5).The XRD spectra of entire samples reveal the XRD peaks, which are indexed as (111), (220), (311), (222), (400), (422), (511), and (440) reflections (Muhammad Azhar Khan et al., 2015; Mukhtar et al., 2015). These XRD peaks are the characteristics planes of single-phase cubic spinel structure. The addition of Tb and Cr in the cobalt ferrite exhibits the presence of the TbFeO3 residual phase (*). This peak is observed at an angle of 33. 47° and is found as (322) plane of TbFeO3 orthorhombic phase confirmed through ICDD # 88-0144. The Tb-Cr co-doped cobalt soft ferrites reveal the secondary phase peak, and its intensity gradually increases with the incorporation of dopants. It is also observed that the incorporation of larger radii rare earth cations increases the lattice constant due to the formation of the secondary phase (M. Azhar Khan et al., 2009). The crystallite size of all prepared samples was calculated by Scherrer’s formula given as (Iqbal, Ahmad, Meydan, & Nlebedim, 2012). D=   cos 9.0 (1) D shows crystallite size, β shows full width at half maximum (FWHM), λ is X-ray's wavelength, and θ shows diffraction angle. The structural parameters such as lattice constant, cell volume, X-ray density, bulk density, and crystallite size were evaluated from the recorded data of XRD. All these parameters are listed in table 1. Crystallite size exhibits Abdul Hameed, Muhammad Waqas Khaliq 57 a non-linear behavior with the increasing contents of Tb and Cr in cobalt ferrite. The decrease in crystallite size has also been reported in the literature by including rare earth cations (Rezlescu, Rezlescu, Pasnicu, & Craus, 1994). The lattice constant increases from 8.372 Å to 8.39 Å with the increase of Tb3+ and Cr3+ concentration in the spinel lattice of cobalt ferrites. Figure 2: XRD patterns of Co1-xTbxFe2-yCryO4 (x = 0.0- 0.10 and y = 0.0- 0.5) ferrites The cell volume was also increased from 586 (Å)3 to 590 (Å)3. The slight increase in the lattice constant is attributed to the larger ionic radius of Tb3+ (0.93 Å) than Co2+ (0.74 Å) and smaller ionic radius of Cr3+ (0.63 Å) as compared to Fe3+ (0.67 Å) (B. Cheng, 2006; John Berchmans, Kalai Selvan, Selva Kumar, & Augustin, 2004). The replacement of smaller cations (Co) with the larger cations (Tb) justified the increase in lattice constant. The bulk densities of all samples were observed to be smaller than X-ray densities, which revealed the presence of pores in these soft ferrites obtained in the preparation or sintering of the ferrite samples. The value of X-ray density increases from 4.79 to 5.09 g/cm3 with an increasing amount of dopants. It is also obvious from Table 1 that bulk density increases from 3.68 to 3.93 g/cm3 with the increase of dopants content, and it might be because the Tb ion has a higher atomic weight than the Co cation. The porosity varies from 24.6 to 22.8 %. Similar results were reported in the literature for rare earth substituted cobalt spinel ferrites (Dascalu, Popescu, Feder, & Caltun, 2013; Tahar et al., 2008). Table 1 Crystallite size (D), Lattice constant (ɑ), Cell volume (Vcell), X-ray density (ρx), Bulk density (ρb), and Porosity (P %) of Co1-xTbxFe2-yCryO4 (x = 0.0-0.1, y=0.0- 0.5) nano-ferrites Composition D (nm) a (Å) Vcell (Å3) ρx (gm/cm3) ρb (gm/cm3) P (%) x=0.00, y=0.0 44 8.372 586.8 4.79 3.68 23 x=0.02, y=0.1 43 8.374 587.2 4.86 3.72 23.5 x=0.04, y=0.2 39 8.377 587.8 4.97 3.75 24.5 x=0.06, y=0.3 41 8.382 588.9 5.01 3.78 24.6 x=0.08, y=0.4 32 8.386 589.7 5.06 3.84 24.1 x=0.10, y=0.5 28 8.390 590.6 5.09 3.93 22.8 3.3. Magnetic Properties M–H loops of pure and Tb-Cr co-doped cobalt ferrites were obtained up to an applied magnetic field of 10 kOe and are exhibited in figures 3a to 3f. Magnetic parameters, i.e., Journal of Materials and Physical Sciences 2(2), 2021 58 coercivity (Hc), saturation magnetization (Ms), and remanence (Mr), were taken out from these loops and are tabulated in table 2. The Ms can also be calculated using the law of approach to saturation (Grossinger, 1981). All the fitted curves of saturation magnetization are calculated using this law at 300 K are shown in figures 4(a-f). All samples revealed high Ms values while coercivity (Hc) values were relatively low, indicating that all the materials showed strong magnetism (Ozkaya et al., 2009). Coercivity values range from a few hundred oersteds for entire materials, and these elucidate the intrinsic behavior of soft ferrites. It was noticed that co-doping of Tb and Cr in Co soft ferrites substantially lowers the coercivity. This decrement in coercivity was attributed to the decrease of the magneto- crystalline anisotropy of Co ferrite; as the concentration of Co decreases that has a high value of magneto-crystalline anisotropy from other cations present in these materials (Kambale, Song, Won, Lee, & Hur, 2012; Ren & Xu, 2014; Sodaee, Ghasemi, Paimozd, Paesano Jr, & Morisako, 2013). Figure 3a: M-H loop of CoFe2O4 ferrite Figure 3b: M-H loop of Co0.98Tb0.02Fe1.9Cr0.1O4 ferrite Abdul Hameed, Muhammad Waqas Khaliq 59 Figure 3c: M-H loop of Co0.96Tb0.04Fe1.8Cr0.2O4 ferrite Figure 3d: M-H loop of Co0.94Tb0.06Fe1.7Cr0.3O4 ferrite Figure 3e: M-H loop of Co0.92Tb0.08Fe1.6Cr0.4O4 ferrites Journal of Materials and Physical Sciences 2(2), 2021 60 Figure 3f: M-H loop of Co0.90Tb0.10Fe1.5Cr0.5O4 ferrite The coercive force can be co-related with crystallite size. The larger crystallites provide minimum pinning of domain walls due to the smaller volume fraction of the grain boundaries. It was found that crystallite size exhibits non-linear behavior by increasing doping concentration. Similarly, coercivity first decreases from 425 Oe to 364 Oe, and later it increases for higher dopants concentration. The Ms reduces from 62 to 51 emu/g by increasing the dopants concentration. The decrement in the Ms can be associated with exchange interaction among tetrahedral cations (A-sites) and octahedral cations (B-sites). The decrease in the Ms is due to the dilution of magnetization for Tb3+ and Cr3+ in the B-site (Panneer Muthuselvam & Bhowmik, 2010; Wells & Ramana, 2013). When a small amount of these dopants are introduced in these ferrites, these paramagnetic cations tend to arrive in the B-sublattice (Y. Y. Li, 1974) and substitute ferromagnetic Co cations at B-sites. Hence, the magnetization decreases, resulting in the Ms decreases for all Tb-Cr co-doped ferrites (L. Kumar & Kar, 2012). The remanence reduces from 13.38 to 8.12 emu/g by increasing the contents of dopants. The fitted curves of Ms, acquired from the law of approach to saturation, are calculated at 300 K for all samples and are shown in figures 4a to 4f. Figure 4a: The fitted curve of Ms at 300 K obtained from the law of approach to saturation for CoFe2O4 ferrite Abdul Hameed, Muhammad Waqas Khaliq 61 Figure 4b: The fitted curve of Ms at 300 K was obtained from the law of approach to saturation for Co0.98Tb0.02Fe1.9Cr0.1O4 ferrite Figure 4c: The fitted curve of Ms at 300 K was obtained from the law of approach to saturation for Co0.96Tb0.04Fe1.8Cr0.2O4 ferrite. Figure 4d: The fitted curve of Ms at 300 K was obtained from the law of approach to saturation for Co0.94Tb0.06Fe1.7Cr0.3O4 ferrite. Journal of Materials and Physical Sciences 2(2), 2021 62 Figure 4e: The fitted curve of Ms at 300 K was obtained from the law of approach to saturation for Co0.92Tb0.08Fe1.6Cr0.4O4 ferrite. Figure 4f: The fitted curve of Ms at 300 K was obtained from the law of approach to saturation for Co0.90Tb0.10Fe1.5Cr0.5O4 ferrite Table 2 Saturation magnetization (Ms), Remanence (Mr) and Coercivity (Hc) for Co1- xTbxFe2-yCryO4 (x = 0.0-0.1, y=0.0-0.5) nano-ferrites Composition Ms (emu/g) Mr (emu/g) Hc (Oe) x=0.00, y=0.0 62 13.38 425 x=0.02, y=0.1 61 10.96 397 x=0.04, y=0.2 59 10.11 398 x=0.06, y=0.3 58 8.12 365 x=0.08, y=0.4 56 9.10 399 x=0.10, y=0.5 51 8.29 409 3.4. Dielectric Properties Ferrites are dipolar materials as they have Fe2+ cations in the minority Fe3+ cations in the majority. Dipoles of Fe3+-Fe2+ align in the direction of the electric field. It can be seen from figure 5 that at a lower frequency, the dielectric constant (εꞌ) decreases quickly. Still, it decreases gradually at the higher frequency region and becomes almost independent of frequency. In the literature, other researchers have reported a similar dielectric behavior Abdul Hameed, Muhammad Waqas Khaliq 63 (D.R. Patil, 2009; N. Singh, 2011). In this research work, the movement of electrons between Fe3+ cations and Fe2+ cations and the movement of holes between Ni2+ cations and Ni3+ cations contributes to the local direction of electrons in the electric field, resulting in polarization. Koop’s theory is helpful to interpret this fast decrement of dielectric constant at minimum frequency (Koops, 1951). On the basis of Koop’s phenomenological theory dielectric structure having inhomogeneous medium consisting of two layers and this is also ascribed to the Maxwell- Wagner model (Jacob, Thankachan, Xavier, & Mohammed, 2013). This is evident from figure 5 that the dielectric constant is reduced by incorporating dopants (Tb3+-Cr3+). Figure 5: Dielectric constant (εꞌ) vs frequency for Co1-xTbxFe2-yCryO4 (x = 0.0-0.1, y = 0.0- 0.5) ferrites In ferrites, there are two losses, including magnetic loss and dielectric loss. Figures 6 and 7 indicate the dielectric loss (εꞌꞌ) and tan loss of Tb-Cr doped cobalt ferrite. Figure 7 shows that dielectric tan loss decreases by increasing the number of dopants. The dielectric loss initially decreases rapidly and then increases slowly, and the last composition exhibits strange behavior. The variation in dielectric losses is because of the ion shifting. The loss peak is usually observed when the frequency of the external electric field becomes equal to the hopping frequency of the electron between Fe2+ and Fe3+ ions. Figure 6: Dielectric loss (εꞌꞌ) vs frequency for Co1-xTbxFe2-yCryO4 (x = 0.0-0.1, y = 0.0- 0.5) ferrites Journal of Materials and Physical Sciences 2(2), 2021 64 The resonance phenomenon happens because of the matching of frequencies of external electric field and hopping frequency of transferring electrons. When dopants are incorporated in cobalt ferrite, it occupies the octahedral site, decreasing the number of cobalt and iron ions on that site. Due to this, electrical conduction decreases (Zhijian Penga, 2011). AC conductivity was calculated using the following equation: σac= 2𝜋𝑓 ′ ′′𝑡𝑎𝑛𝛿 (2) Figure 7: Tan loss vs frequency of Co1-xTbxFe2-yCryO4 (x=0.0-0.1,y=0.0-0.5) ferrites Figure 8 shows that AC conductivity rises with the increase of doping contents of Tb3+ - Cr3+. Figure 8 reveals that ac conductivity rise with the rise in frequency. This behavior can be explained using Maxwell Wagner's models and Koop's theory. Based on these models, ferrites are called conducting grains separated by a thin resistive layer called grain boundaries. In the lower frequency region, grain boundaries are more active and contribute to conduction. In higher frequency regions, conductive grains increase conduction and hoping conduction (Asif Iqbal et al., 2014). Peaking behavior is also observed beyond 1.5 GHz, which reveals the resonance phenomenon. The decrement in the AC conductivity with the increased dopants is ascribed to a decrease in the hoping frequency of electrons between Fe2+ and Fe3+. The decrease in AC conductivity may be described by grain size. Since grain size declines for higher dopants concentration and hence grain boundaries become larger, AC conductivity declines. Figure 8: AC conductivity vs frequency for Co1-xTbxFe2-yCryO4 (x=0.0-0.1, y=0.0- 0.5) ferrites Abdul Hameed, Muhammad Waqas Khaliq 65 In ferrites, the conduction mechanism and relaxation behavior can be understood from the interpretation of the dielectric modulus. The real and imaginary parts of the dielectric modulus can be obtained using the following equations: M'=ε'/(ε'2+ε''2) (3) M''=ε''/(ε'2+ε''2) (4) The nature of real (M') and imaginary (M'') parts of electric modulus as a function of applied field frequency is significant to knowing the relaxation mechanism (Asif Iqbal et al., 2014). Figure 9 and figure 10 indicate the real (M') and imaginary (M'') parts of the dielectric modulus as a function of applied frequency. M'' increases with the rise in the biasing oscillating applied electric field and reaches the highest value at some applied frequency called relaxation frequency. An oscillating frequency and applied field frequency become equal. According to these figures (9 &10), when the contents of dopants (Tb3+- Cr3+) increased highest peaks in M'' shifts towards the lower frequency region. This decrease in maxima peak with increased doping concentration for different compositions suggests the contribution of the grain boundary resistance (Pervaiz & Gul, 2014). These graphs show the missing relaxation phenomenon in these doped cobalt ferrites associated with the relaxation process (Pervaiz & Gul, 2014). This may be the reason for low loss, which makes these ferrites bad conducting and low dielectric loss to be used in an instrument that absorbs electromagnetic radiation (Asif Iqbal et al., 2014). Figure 9: Real part of electric modulus vs frequency for Co1-xTbxFe2-yCryO4 (x=0.0- 0.1 and y = 0.0- 0.5) ferrites Figure 10: Imaginary part of electric modulus vs frequency for Co1-xTbxFe2-yCryO4 (x = 0.0-0.1 and y = 0.0- 0.5) ferrites Journal of Materials and Physical Sciences 2(2), 2021 66 4. Conclusion Tb-Cr substituted cobalt soft ferrites having the following compositions Co1-xTbxFe2- yCryO4 (x = 0.0-0.10, y = 0.0-0.5) were synthesized by solid-state reaction method. CoFe2O4 has a single-phase cubic spinel structure, and all other doped compositions exhibited traces of the ortho phase (TbFeO3) and the cubic spinel phase. The structural and physical parameters depended strongly on the amount of dopants concentration. By increasing the dopants' contents following parameters were optimized as lattice constant (a, b, c), cell volume (V), and crystallite size (D). Magnetic parameters like Ms, Mr, and Hc were obtained from M-H loops. Ms was found in 51 emu/g -61 emu/g, and Hc lies in 364 Oe - 425 Oe. The Hc of prepared cobalt ferrites was found to be minimum, indicating that these materials can serve as potential candidates for high-density data storage devices applications. 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