Influence of B-site Gd+3 substitution on various properties of Co-ferrite nanoparticles

The effect of rare earth (Gd+3) substitution on various properties of nanocrystalline cobalt ferrite material with composition CoFe2-xGdxO (x = 0, 0.02, 0.04, 0.06, 0,08, 0.1) prepared using combustion method was investigated. The alterations produced in the structural parameters of the spinel lattice of cobalt ferrite were investigated using X-ray diffraction (XRD). The structural parameter such as lattice constant, mass density, strain, and crystallite size showed irregular variation triggered due to rare earth inclusion. The chemical composition analysis was done using energy-dispersive X-ray spectroscopy (EDS). Morphological investigations were done using a scanning electron microscope (SEM). The dependence of A.C. susceptibility and other crucial magnetic properties on rare earth content in the ferrite matrix was also investigated. The temperature dependence of electrical properties such as DC resistivity, dielectric constant, and dielectric loss was also investigated to study the alterations caused due to incorporation of rare earth ions. The rare-earth-doped cobalt ferrite nanoparticles prepared and analyzed in this research work were seen to possess magnetic characteristics which are essential for various biomedical applications.


Introduction
The remarkable advancement in the research based on magnetic nanoparticles such as Co ferrite, Ni ferrite, Mn Zn ferrite, and numerous combinations of transition and inner transition metal oxides in the last two decades is mainly due to their promising application in ferrofluid technology, biomedical sensors, advanced magnetic storage technology, electromagnetic interference suppression technology, targeted drug delivery systems and hyperthermia [1][2][3]. In recent times cobalt ferrite (CoFe 2 O 4 ) in its nano dimensional form has been subjected to extensive investigations mainly because of properties structural, electrical, and magnetic properties that are highly tuneable and has been done using various factors such as different dopant ions, radiation exposure, pressure, and heat treatment, etc. [4][5][6][7][8]. Out of three classes of ferrites namely normal, inverse and mixed spinel, Cobalt ferrite belongs to inverse spinel class of ferrite family. The Co +2 ions occupy octahedral site while the Fe +3 ions are equally distributed over the tetrahedral site and the octahedral site [9,10].
The effects of substitution of rare earth elements such as La +3 , Ce +3 , Nd +3 , Gd +3 , Sm +3 , Er +3 , Ho +3 , Dy +3 , etc. on various combinations of ferrites have been extensively studied. According to the reports, even a minute substitution affects structural, electrical, and magnetic properties significantly. These property alterations have been attributed to larger ionic radii, magnetic ordering in rare-earth metal ions. Specifically, doping of Gd + 3 in spinel systems is known to produces remarkable alterations in electrical and predominantly magnetic properties of the material [11][12][13][14][15][16][17][18].
However, it is difficult to substitute a larger quantity of rare earth dopant ions due to the large size and tendency to occupy the octahedral site replacing the smaller Fe +3 ions which may lead to the formation of a secondary phase [19][20][21][22][23][24][25][26]. Numerous methods are being employed by researchers all over the globe to synthesize ferrite nanoparticles. Methods such as the co-precipitation method, hydrothermal synthesis, sol-gel method are very frequently used methods. Researchers have also employed methods like Molten Salt method for obtaining monophasic compounds [24][25][26][27][28]. The selection of a suitable method for material synthesis is done based upon various parameters such as yield percentage, purity of phase, energy consumption, the time required for the synthesis, and repeatability [28][29][30]. Compared to these material preparation methods, the combustion method appears to be the most ensuring method because of its utmost simplicity, high productivity, cost-effectiveness, and approximate control over the factors such as size, morphology, composition, and agglomeration degree by varying the experimental conditions such as temperature, time, reactants, and stirring rate [27][28][29][30][31].
In this study, we intend to report the effect of increasing rare-earth (Gd +3 ) substitution on structural, morphological, elemental, and electrical properties of and inverse spinel cobalt ferrites with composition CoFe 2-x Gd x O 4 (x restricted to 5% to reduce the chances of secondary phase formation) nanoparticles prepared using combustion method.

Material preparation
The nanoparticles of Gd +3 doped cobalt ferrite powders with chemical formula CoFe 2-x Gd x O 4 (x = 0.0, 0.02, 0.04, 0.05, 0.06, 0.08 and, 0.1) were prepared using combustion method [29]. The metal salts such as Cobaltous (II) nitrates (Co (NO 3 ) 2 6H 2 O), Ferric (III) nitrate nonahydrate (Fe(NO 3 ) 3 9H 2 O), and Gadolinium acetate were dissolved in double distilled water along with nitrilotriacetic acid (N(CH 2 CO 2 H) 3 ) as complexing agents and urea (CO(NH 2 ) 2 ) as fuel. The clear solution obtained from the mixture of the above reagents heated at an elevated temperature of 90 °C was heated further till the ignition temperature is reached. The dry residue obtained as a result of the combustion process was finely crushed and ground for two hours to obtained ferrite nano-powders [20,24,26,27]. The calculated amounts of salts utilized in synthesis of CoFe 2-x Gd x O 4 are tabulated in Table 1

Material characterization
The nanocr ystalline samples with composition CoFe 2-x Gd x O 4 (x = 0.0, 0.02, 0.04, 0.06, 0.08, 0.1) prepared using combustion synthesis were analyzed using several experimental investigations. The X-ray diffraction (XRD) patterns were obtained on the Rigaku X-Ray diffractometer (Cu Kα, λ = 1.5418 Å) in 2θ scanning range of 20° to 80° with a step size of 0.02°. Rietveld refinement on the XRD patterns was carried out using Full Prof software. The Fourier transforms infrared spectra of Gd +3 doped cobalt ferrite nanoparticles were recorded on Shimadzu FTIR 8900 setup using KBr pellets (diameter 1.5 mm, radius 5 mm) containing 2 mg of the sample. The samples were exposed to gold sputtering for 15 s before taking SEM images in order to obtain a good quality SEM image. The Scanning Electron Microscope (SEM) Carl Zeiss EVO18 was used to obtain SEM images of ferrite nanoparticles. The Hitachi transmission electron microscope was employed to obtain images of ferrite material. The variation of normalized AC susceptibility with the temperature of rare earth doped cobalt ferrite samples was studied using the high temperature AC susceptibility set up supplied by ADEC Embedded Technology & Solutions Pvt Ltd. Magnetic hysteresis data was recorded on Quantum Design Versa Lab's 3 T vibrating sample magnetometer (VSM)The samples were pressed into pellets of diameter 5 mm and thickness 2.5 mm with an approximate weight of 0.75 g and were used to investigate electrical properties. The dielectric constant 'ε' variation with frequency of applied field (10 Hz to 3 MHz) for different concentrations of Gd +3 was investigated with Wayne Kerr precision component analyzer 6440B. The variation of DC resistivity 'ρ' with temperature over a range of 30 °C to 500 °C for rare-earth-doped ferrite nano-powders was studied using a two-probe setup.

X-ray diffraction
The Rietveld refinement of XRD patterns of nanocrystalline CoFe 2-x Gd x O 4 powders recorded at room temperature indicated the existence of inverse spinel phase belonging to the Fd3m space group (Fig. 1). The absence of any extra peak in the diffraction pattern indicated that the rareearth ions were perfectly dissolved into the inverse spinel structure without any traces of hematite formation [20,24,[29][30][31][32]. The substitution of Gd +3 (ionic radii 1.02 Å) at the octahedral site replacing a Fe +3 without any secondary phase formation can be considered as a prominent feature of the combustion method. The refinement parameters obtained from the refinement of XRD patterns of CoFe 2-x Gd x O 4 nanoparticles are presented in Table 2.
The samples with concentrations x = 0.04, 0.05, and 0.1showed a drastic increase in lattice parameters and unit cell volume indicating successful substitution of Gd +3 at the octahedral site. While the other concentrations (x = 0.06 and 0.08) showed a marginal increase in structural parameters indicating occupancy of Gd +3 ions at the interstitial sites. The X-ray peak intensities were seen to decrease with increasing Gd concentration. This reduction could be attributed to the amorphous nature introduced by the larger rare earth ions as these ions are responsible for the increment in the strain as depicted in Fig. 2b [33][34][35][36][37][38][39][40].

Fourier transforms infrared spectroscopy (FTIR)
The FTIR technique is used to obtain crucial information about the ferrite structure and the changes that are caused by the different dopants in the ferrite structure. The technique was used to confirm the formation of desired structure at the end of combustion process. The FTIR spectra recorded at room temperature for CoFe 2-x Gd x O 4 nano-crystalline powders are shown in Fig. 3. A measured quantity (2 mg) was mixed with 100 mg of KBr and the mixture was pressed into pallets with a thickness of 1 mm and 10 mm in diameter. The FTIR spectra for all the samples showed two major absorption bands in the range 400-650 cm −

Scanning electron microscopy
The morphological study of Gd +3 doped cobalt ferrite powders was done using the scanning electron microscope (SEM) instrument. The scanning electron micrographs of CoFe 1.98 Gd 0.02 O 4 and CoFe 1.96 Gd 0.04 O 4 are shown in Fig. 4. The SEM micrographs showed a highly agglomerated structure indicating strong magnetic behavior which is a characteristic of transition elements Fe and, Co, and also of inner transition element Gadolinium present in the ferrite samples under investigation [43][44][45][46].

Energy-dispersive spectra (EDS)
The EDS spectra recorded for CoFe 2 O 4 and CoFe 0.6 Gd 0.04 O 4 are shown in Fig. 5a,b and the elemental analysis data of three samples are presented in Table 3. All the doped samples showed the presence of Co, Fe, Gd, and O. The Au peaks were observed in the spectra due to gold sputtering which was done to improve SEM image quality. The weight percentage of Gd in the material was seen to increase with increasing rare earth substitution levels in the cobalt ferrite matrix [43,44]. The Au peaks occurring in the EDS spectra.

Magnetic properties using vibrating sample magnetometer
The hysteresis loops obtained for Gd +3 doped cobalt ferrite samples are shown in Fig. 6. The variation of saturation magnetization (Ms), retentivity (M R ), coercive field (H C ), and anisotropy constant 'K' with Gd +3 concentration are shown in Fig. 7 (a,b,c). Although the magnetic parameters  M S , M R, and H C showed a random variation with increasing Gd +3 concentrations, the M S and M R were seen to vary inversely to lattice constant and while a direct analogy was observed between H C and (311) peak height depicting the relation between crystalline nature of the ferrite material. The magnetic environment in cobalt ferrite is administered by Co +2 , Fe +3 ions. With the spinel geometry, cobaltous ions occupy the tetrahedral site and ferric ions occupy the octahedral site. Generally, the magnetic moment of the materials is expressed in terms of Eq. 1 [44].
where M is the magnetic moment of the spinel ferrite system, M B is the magnetic moment of the octahedral site and M A is the net magnetic moment of the tetrahedral site. However, with the substitution of a larger rare earth ion Gd +3 at the octahedral site, the system experiences an overall lattice expansion and cationic redistribution which changes the overall magnetic moment of the system. This may reduce the strength of super-exchange interaction between the Co +2 and Fe +3 ions at two cationic sublattice and incomplete ordering of Co +2 and Fe +3 at the tetrahedral sites and the octahedral sites in the spinel structure. The variations in the magnetic moment can be also caused by changes in canting Yafet-Kittel angle (q YK ) due to rare-earth substitution. The magnetic moment of the rareearth-doped system in this situation is expressed in terms of Eq. 2 [45][46][47][48][49].

Magnetic susceptibility
The AC susceptibility plots obtained for as prepared CoFe 2-x Gd x O 4 are shown in Fig. 8. This measurement proves vital information about the Curie temperature and the type of magnetic domains present in the material. Depending upon the grain size, the domains present in the ferrite are categorized into three categories namely Single domain (SD), multi-domain (MD), and superparamagnetic domain (SP). The MD grains are characterized by the presence of multiple walls within the grain. If the grain size is comparable to that of the wall thickness then these grains are termed as single domains. Superparamagnetic behavior is predominantly seen in particles with dimensions < 10 nm. Here, the magnitude of thermal energy is comparable to that of magnetic anisotropy energy.
It can be seen from Fig. 8 that there is a gradual increase in normalized AC susceptibility followed by a steep rise creating a peak at high temperature before a sharp fall in the vicinity of the Curie temperature. This indicates that the as prepared samples contain a large section of single domain and a good number of multi-domain grains as well. The values of curie temperature for all the samples were in the range of 525-575 °C. The variation in the Curie temperature can be attributed to spin reordering triggered by rare earth doping resulting in enhanced magnetic character of nanomaterials [22,50,51]

DC resistivity
The plots of temperature-dependent variation of DC resistivity (ρ) over a range of 300-773 K for rare-earth-doped cobalt ferrite samples are presented in Fig. 9. The rare-earthdoped ferrite powders showed an increase in resistivity with increasing temperature initially till a resistivity maximum of the order 10 7 Ω-cm in the temperature range of 473-523 K. These high resistivity values are the attributes of the removal of moisture present in the interstitial sites that facilitates the conduction mechanism. The high resistivity of the order 10 7 Ω-cm can be attributed to the smaller grain size and the large number of grain boundaries available in the nanomaterials. These grain boundaries are characterized by their insulating  [52][53][54][55]. The rare-earth ions (Gd +3 ) being a larger ion occupies the spacious octahedral site replacing Fe +3 ions which are generally known as a source of charge carriers. Hence this reduction in Fe +3 ions also cause the depletion of charge carriers [56][57][58]. The larger ionic radii of Gd +3 ions also alter the electron hopping mechanism adversely by causing lattice expansion. This increases the separation between Co +2 and Fe +3 ions reducing the electronic exchange [59,60].

Dielectric constant variation with frequency
The plot of frequency dependence of dielectric constant is shown in Fig. 10a and the values of dielectric constant as at 20 Hz for different Gd +3 concentrations are shown in Fig. 10b.
The Dielectric constant 'ε' was seen to decrease with the increasing frequency of the applied electric field and was seen to remain constant beyond a critical value of frequency. The increasing Gd 3+ occupancy at the B site replacing a smaller ion Fe 3+ was also seen to affect the variation of the dielectric constant [61][62][63][64]. The Dielectric behavior of cobalt ferrite nanomaterials material is administered by the electronic exchange between metal ions and primarily dominated by the Fe 3+ /Co 2+ ions. The doping of rare-earth ions  Gd 3+ ions alter the structural parameters such as cell volume, density, lattice constant, etc., and also decreases Fe +3 ion concentrations at the octahedral site. As a result, the electron hopping between Fe 3+ and Co 2+ ions get affected adversely [65][66][67][68]. The dielectric constant profile showed a steep reduction in magnitude to a lower value with an increase in frequency in the lower frequency region attains a constant low value that remains constant over higher frequency values of the applied field. This variation in dielectric properties can be explained using the space charge polarization phenomenon. An ideal dielectric material is comprised of highly conducting grains. These grains are bounded by nonconducting grain boundaries. When the electric field is applied, a potential drop occurs at the grain boundaries as a result of the accumulation of space charge in this region. The influence of the grain boundaries is observed distinctly over the lower frequency range. Beyond a certain value of applied field frequency, the electronic transfer between the metal ions starts lagging behind the rate of variation in the frequency of an applied electric field. This results in uniform lower magnitudes of dielectric constants [66][67][68].

Dielectric loss variation with frequency
The variation of dielectric loss versus frequency for varying Gd 3+ concentrations is presented in Fig. 11a. While the loss values for different Gd +3 concentrations in cobalt ferrite samples at 20 Hz are presented in Fig. 11b. It can be seen that for nanoparticle samples dielectric loss decreases with increasing frequency of applied electric field and attains a low constant value beyond a critical frequency. At lower frequencies, the dielectric curve is seen to exhibit broad peaks in the neighborhood of 100 Hz. The profile depicted in Fig. 1b shows an approximately inverse behavior of dielectric loss to that of dielectric constant at 10 Hz. The variation in the dielectric loss with increasing Gd 3+ concentrations at low frequencies could be due to hindrance in the electronic exchange between Co +2 /Fe +3 and Fe +2 /Fe +3 [67][68][69][70][71][72][73]. At higher frequencies, the dielectric loss decreases drastically which may allow these materials to find their applications in high-frequency devices [74][75][76][77][78][79][80].

Conclusion
The ultrafine powders of Gd +3 doped cobalt ferrite with composition CoFe 2-x Gd x O 4 (x = 0.0, 0.02,0.04,0.05, 0.06, 0.08, 0.1) with cubic spinel structure and crystallite size ranging between 21 and 41 nm were prepared successfully using combustion route. The structural parameters such as lattice constant 'a', X-density 'D X ', and unit cell volume were altered drastically due to Gd +3 substitution at the octahedral site. The particle agglomerates seen in the SEM images indicated a strong magnetic behavior. The EDS spectra showed the presence of Co, Fe, and Gd in desired quantities without any impurities. The A.C. susceptibility measurements indicated the presence of both single domain and multi-domain particles in all the samples. The changes observed in magnetic properties were attributed to cationic redistribution, lattice expansion, and alterations in magnetic ordering due to Gd +3 substitution.
The DC resistivity profile for all the samples showed high room temperature resistivity of the order 10 7 Ω-cm. The dielectric constant values were also seen to be administered by the concentrations of Gd +3 at the octahedral site. The dielectric constant variation was explained using the space charge polarization phenomenon. The variation in the dielectric loss with increasing Gd 3+ concentrations at low frequencies was attributed to hindrance in the electronic exchange between Fe +2 /Fe +3 and Co +2 /Fe +3 . At higher frequencies, all the samples were seen to exhibit low losses making them a potential candidate for high energy applications.