Substitution Effect on the Structural, Magnetic, and ... - IEEE Xplore

IEEE TRANSACTIONS ON MAGNETICS, VOL. 54, NO. 1, JANUARY 2018

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Substitution Effect on the Structural, Magnetic, and Electrical Properties of Co1−xZnxFe2 O4 Nanocrystalline Ferrites (x = 0–1) Prepared via Gelatin Auto-Combustion Method M. A. Gabal1 , N. H. Al-Zahrani1 , Y. M. Al Angari1 , and A. Saaed2 1 Chemistry 2 Physics

Department, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia Department, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia

Co1−x Zn x Fe2 O4 ferrites (x = 0–1) were successfully synthesized, for the first time, via sol-gel auto-combustion route using gelatin fuel. The auto-combustion was characterized using DTA-TG-DSC up to ferrite formation, and an appropriate gelation mechanism was suggested. The structural, morphological, magnetic, and electrical properties were investigated through X-ray diffraction (XRD), Fourier transform infrared (FT-IR), transmission electron microscopy (TEM), vibrating sample magnetometer (VSM), ac-conductivity, and dielectric constant measurements. XRD indicated the need for further calcination at 350 °C to obtain well crystalline ferrites. The slight changes in the lattice values up to x = 0.8 suggested the substitution of Zn2+ ions for the Co2+ ions located in the octahedral sites. The large change at x = 1 suggested the conversion into the normal spinel structure. Based on the structural data, an appropriate distribution for cations was suggested. This distribution was reinforced using FT-IR and magnetic measurements. TEM showed dense agglomeration at low substitutions released by increasing Zn-content. VSM exhibited hard magnetic properties with an obvious transition from ferromagnetic to paramagnetic by increasing zinc. The maximum saturation magnetization (56.7 emu/g) was obtained for Co0.8 Zn0.2 Fe2 O4 . The behavior of magnetization with Zn-substitution was explained in the view of the cationic stoichiometry. The coercivity decreases by increasing zinc, which was attributed to the anisotropic nature of zinc. AC-conductivity versus temperature revealed a semiconducting behavior with an obvious change from ferro- to paramagnetic by rising temperature. The conduction mechanism as well as the type of the charge carriers was discussed in the view of calculated activation energies and the frequency dependence of conductivity. The measured dielectric constants gave results that agreed well with the conductivity data. Index Terms— Co–Zn ferrites, cation distribution, conductivity, dielectric, vibrating sample magnetometer (VSM).

I. I NTRODUCTION ERROSPINELS with molecular formula MFe2 O4 (M = Fe2+ , Co2+ , Ni2+ , Zn2+ , . . . etc.), represented an important type of technologically advanced materials with remarkable electromagnetic characteristics which made them suitable for a lot of applications, such as lithium batteries, recording media, transducers, actuators, sensors, printing, electromagnetic interference shielding, biotechnological applications, and drugs delivery [1]–[4]. Among them, CoFe2 O4 and ZnFe2 O4 have been deeply studied, since they exhibited the typically inverse and normal spinel ferrites, respectively. CoFe2 O4 exhibited ferromagnetic characteristics in which all Co2+ ions occupied octahedral sites, whereas Fe3+ ions were equally distributed between tetrahedral and octahedral sites. On the other hand, ZnFe2 O4 revealed anti-ferromagnetic behavior, since the Zn2+ ions preferably occupied the tetrahedral sites, and all the Fe3+ ions occupied the octahedral sites [5]–[7]. CoFe2 O4 nanoparticles were proposed as a promising candidate in many biomedical applications, such as magnetic drug delivery, hyperthermia, magnetic resonance imaging, and

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Manuscript received March 21, 2017; revised July 25, 2017; accepted August 20, 2017. Date of publication October 23, 2017; date of current version December 20, 2017. Corresponding author: M. A. Gabal (e-mail: [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMAG.2017.2752726

biosensors [8]–[10]. Their magnetic properties are greatly affected by the size, shape, and cationic substitution. For instance, the substitution by a diamagnetic ion such as Zn2+ could have resulted in interesting changes in different properties due to the preferential occupancy of Zn2+ ions in the tetrahedral sites, and in some cases by the octahedral sites, thus by tailoring the entire electromagnetic properties. Therefore, Co–Zn mixed ferrites have attracted considerable attentions due to their inclusion of diverse properties for both ZnFe2 O4 and CoFe2 O4 [11]–[15]. Various properties of such materials are mainly dependent on their shape, size, cationic distribution, and structure, which are strongly affected by the synthetic processes [16]. The selection of suitable processing conditions will affect their morphology, crystal size, and cation distribution, and hence, resulting in modified structural and electromagnetic properties. In the literature, an enormous number of preparation techniques were utilized for the synthesis of various ferrites including sol-gel, complexometric, ball milling, co-precipitation, thermal decomposition, polymeric assisted, hydrothermal, reverse micelles, auto-combustion, and the micro-emulsion methods [14]–[24]. Among these methods, the auto-combustion synthesis one [11], [13], [15], based on reacting metal nitrates (acting as oxidizing agents) with organic fuels (acting as reducing agents), has attracted significant attention due to its cheap precursors, short time, and low-temperature synthesis. Many fuels

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have been utilized in these auto-combustion syntheses such as urea [25], starch [13], sucrose [15], glycine [26], chitosan [27], egg-white [28], and fuel mixtures [29]. In this paper, we are focusing on the synthesis of Zn-substituted cobalt ferrites nanocrystals via simple, economic, and environmentally friendly auto-combustion method employing gelatin as a fuel. The structure as well as the morphology of the prepared ferrites was characterized using X-ray diffraction (XRD), Fourier transform infrared (FT-IR), and transmission electron microscopy (TEM) measurements. The magnetic and electrical behaviors were studied using vibrating sample magnetometer (VSM), ac-conductivity, and dielectric properties measurements to shed light on the effect of diamagnetic substitutions on the electromagnetic properties of the entirely studied ferrites. An appropriate distribution was suggested according to the obtained results and discussion. The sol-gel auto-combustion reaction followed by the ferrites formation was also estimated and discussed. II. E XPERIMENTAL P ROCEDURE A. Materials and Ferrites Preparation All the reagents used are of analytical grade and used as received. The reagents, obtained from BDH, include cobalt nitrate hexahydrate; Co(NO3 ).2 6H2 O, iron nitrate nonahydrate; Fe(NO3 ).3 9H2 O, and zinc nitrate hexahydrate; Zn(NO3).2 6H2 O. Gelatin powder was supported by Fluke. The procedure for the preparation of cobalt ferrite and its substituted zinc system, Co1−x Znx Fe2 O4 (where x = 0, 0.2, 0.4, 0.6, 0.8, and 1), using sol-gel Gelatin method was followed as previously reported in [30]. Stoichiometric amounts of the entire nitrates were dissolved in 100 ml water. In another beaker, 10 g of gelatin powder was dissolved in 100 ml cold water under stirring until a clear solution was obtained. The gelatin solution was then added under heating and constant stirring to the nitrate solution. The mixture was then heated to obtain a homogeneous solution. On evaporation at 90 °C, a viscous gel was obtained which started to auto-combust, by rising heat, with the formation of dry powder (as-prepared gel precursor). The auto-combustion reaction was accompanied by the evolution of enormous amount of gases with producing a foamy powder. The resulting powder was grounded and calcined at 350 °C in an electrical oven for 1 h. After cooling to room temperature in a desiccator, the obtained powders were collected and given the name as-prepared powders. B. Techniques In order to investigate different properties of the prepared ferrite sample, variety of experimental techniques were used. Each experimental technique provides special information about structure, morphology, thermal, particle size, magnetic, or electrical properties. The decomposition of the dry gel precursors was followed up to the ferrite formation using Perkin–Elmer thermal analyzer. DTA-DSC-TG measurements were carried out in air at the heating rate of 5 °C min−1 .

Fig. 1. (a) Gelatin structure. (b) Chelation mechanism between gelatin and metal ion.

The different phases were characterized by XRD. A Bruker axs D8 diffractometer, with Cu-Kα radiation (λ = 0.15418 nm), was used. The size and morphology were characterized through TEM measurements by a JEOL-2010 using a voltage of 100 kV. FT-IR was analyzed by a Perkin–Elmer in the range between 600 and 200 cm−1 . Magnetic properties were measured at room temperature under applied magnetic field up to 10 kOe using a VSM (VSM-9600-1 LDJ, USA). The electrical measurements (ac-conductivity and dielectric) were measured versus temperature (up to 750 K) at different frequencies (100 Hz–4 MHz) on a Hioki LCR-3531 bridge utilizing the two-probe method. 1 cm diameter and about 1 mm thickness pellets of the compressed samples were painted with silver. III. R ESULTS AND D ISCUSSION A. Gelation and Ferrites Formation Reaction Gelatin is a poly-dispersed mixture of relatively low molecular weight polypeptides. It can be easily obtained through acid or alkaline hydrolysis of collagen. Its general formula can be presented, as shown in Fig. 1(a) [31]. In alkaline medium, it is negatively charged; therefore, its proton-accepting capability is more pronounced than in aqueous solutions [32]. During the gelation process, the gelatin peptides aggregated and physically cross-linked with other forming gels. The presence of metal ions could be accelerating such aggregation. The negatively charged carboxylic acid groups (COO− ) attract positively charged metal ions through electrostatic interactions forming ionic bonds [Fig. 1(b)] [33]. The metal complex gelatin matrix is converted into dry gel by successive evaporation till dryness, after which a violence auto-combustion reaction, accompanied by the evolution of enormous amount of gases, was initiated resulting in the formation of fluffy powder. This fluffy mass was collected, kept in a dessicator, and named as-prepared precursor.

GABAL et al.: SUBSTITUTION EFFECT ON STRUCTURAL, MAGNETIC, AND ELECTRICAL PROPERTIES

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Fig. 2. DTA-TG-DSC curves in the air of precursor with x = 0. Heating rate = 5 °C min−1 .

This auto-combustion reaction was followed through studying the decomposition behavior of the dry gelatin precursor. Fig. 2 shows DTA-TG-DSC curves characterizing the decomposition process of gelatin precursor. The decomposition proceeds through four TG steps up to 385 °C. The first step, owing to its endothermicity, can be attributed to the precursor dehydration process. Three exothermic DTA peaks at 163 °C, 255 °C, and 360 °C are characterized by the following successive TG steps. The exothermicity of these steps can be assigned to auto-combustion occurred between gelatin moiety and nitrates. The above DTA-TG-DSC measurements ensure the formation of the entire ferrites just after the complete decomposition of their entire gel precursors. Thus, if the energy generated during the auto-combustion was enough to the complete decomposition and ferrite formation, no further calcination for the obtained as-prepared precursors will be needed and in case this energy is low, further calcination should be carried out to obtain crystalline ferrites. This situation can be easily solved through using XRD measurements. B. Structural Characterization 1) XRD Measurements: Room temperature XRD patterns of as-prepared Co1−x Znx Fe2 O4 system are presented in Fig. 3(a). The patterns showed some of the characteristic diffraction peaks of the cubic spinel ferrites with relatively broadness and low intensity. This result indicated that the energy generated during auto-combustion reaction is insufficient for the formation of crystalline ferrites and further calcination should be needed. Fig. 3(b) shows XRD spectra for samples calcined at 350 °C for 1 h. The obtained XRD peaks are sharper and intense than

Fig. 3. (a) XRD patterns of the as-prepared Co1−x Znx Fe2 O4 system. (b) XRD patterns of the calcined Co1−x Znx Fe2 O4 system.

those obtained in Fig. 3(a), indicating the effect of calcination on the entire ferrites crystallinity. The experimental lattice parameters (aExp) were calculated using the inter-planar distances (d-values), Miller indices (hkl), and relation from [28]. The presented values in Table I are observed to be varied between 8.4052 and 8.4464 Å. These two obtained limits are in consistent with those reported for CoFe2 O4 and ZnFe2 O4 , respectively [4], [16], [17]. Generally, according to [4], [14], and [34], the lattice constant was observed to increase linearly obeying Vegard’s law on the substitution of cobalt by zinc in the Co1−x Znx Fe2 O4 system. In this process, Zn2+ ions begin to replace Fe3+ ions presented in the tetrahedral sites. Thus, Fe3+ ions preferably migrate to octahedral sites displacing Co2+ ions, and thereby, changing the system into a normal structure. In the present case, the situation is completely different, since the Zn-substitution causes a very slight change in the lattice parameter values up to x = 0.8 before inversion into normal spinel at x = 1.

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TABLE I S TRUCTURAL D ATA OF THE Co1−x Znx Fe2 O 4 S YSTEM

Based on the Shannon tables [35], the ionic radii of the entire studied ions are as follows: rCo2+ (tet.) = 0.58 Å, rCo2+ (oct.) = 0.745 Å, rZn2+ (tet.) = 0.60 Å, rZn2+ (oct.) = 0.740 Å, rFe3+ (tet.) = 0.490, and rFe3+ (oct.) = 0.645 Å. The very slight obtained changes in the lattice values by the addition of zinc suggested the successive substitution of Zn2+ ions for the Co2+ ions located in the octahedral sites with the same ionic radius. The change in the lattice value from 8.4052 to 8.4147 Å at x = 0.2 can be owed to the incorporation of Zn2+ ions in the tetrahedral sites. The large change in the lattice parameter at x = 1 suggested the conversion into the normal spinel structure in which all Zn2+ ions prefer tetrahedral occupation. In accordance with the obtained lattice values (aExp) discussed above, an appropriate distribution was suggested and presented in Table I. The theoretical parameters (aTh ) were calculated based on this suggested distribution using the ionic radii of the tetrahedral (r A ) and octahedral (r B ) sites, and equation from [28]. The reported value (Table I) agreed well with those experimentally (aExp) obtained indicating the validity of entire distribution in discussing the entire system. This suggested cation distribution will be also reinforced in the following discussions via infrared and electromagnetic properties. X-ray data enabled us also to study the effect of Zn-substitution on different structural parameters, such as crystallite size (L), X-ray density (Dx ), distance between magnetic ions (jumping length) in tetrahedral sites (L A ) and octahedral sites (L B ), oxygen positional parameter (u), and inversion factor (γ ), besides the intercationic distances including bond length of tetrahedral site (dAX ), bond length of octahedral site (dBX ), tetrahedral edge (dAXE ), shared octahedral edge (dBXE ), and unshared octahedral edge (dBXU ). These parameters were calculated using equations from [28] and were presented in Table I. It can be observed from Table I that the slight change in the crystallite sizes and jumping lengths with increasing Zn-content agreed well with the obtained value of the lattice parameters. The constant value of oxygen positional parameter

Fig. 4.

FT-IR spectra of the Co1−x Znx Fe2 O4 system.

up to x = 0.8 indicated constant tetrahedral composition and suggested preferential occupancy of Zn2+ ions in the octahedral sites. In addition, the slight changes in the intercationic distances up to x = 0.8, which are related to the radius of entire ions, could be interpreted according to the constant lattice parameter (a) and oxygen positional parameter (u) obtained at different Zn-substitution (Table I). The successive increase in X-ray density (Dx ) by increasing zinc up to x = 0.8 can be mainly related to the molecular weight of the entire ferrites and their lattice parameters (aExp). The substitution of Co2+ ions of lower atomic weight (58.933 amu) with higher weight Zn2+ ions (65.409 amu) is accompanied by a slight change in the lattice parameter and this will result in increasing density. The obvious decrease in the density at x = 1 can be attributed to the abrupt increase in the lattice parameter value. 2) Fourier Transforms Infrared Spectra: FT-IR spectral analysis of the entire calcined ferrites, carried out at room temperature in the range of 800–300 cm−1 , is presented in Fig. 4. This measurement was carried out to ensure the ferrites phase formation, detect the chemical bonds present in the spinel structure, confirm the XRD structural characterization, and reinforce the suggested cation distribution of the system.


Fig. 5.

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TEM images of the Co1−x Znx Fe2 O4 system.

Generally, two main absorption bands, characteristic for the spinel structure, appeared. The band (ν1 ), observed in the range of 542–592 cm−1 , can be related to the stretchingvibrations of the metal cation-oxygen bond in the tetrahedral sites, while the low band (ν2 ) appeared in the range of 390–415 cm−1 can be assigned to the stretching-vibrations of the metal cation-oxygen bond in the octahedral sites [36]. The band positions were estimated and reported in Table I. From Table I, it can be noticed that, by increasing Zn2+ ions content, stretching frequencies: ν1 exhibited slight decrease, whereas stretching frequencies: ν2 showed a slight increase. The obtained values at different concentrations agree well with the estimated radii at tetrahedral (r A ) and octahedral (r B ) sites using XRD analysis. The increase in the ionic radii

is accompanied by a decrease in the frequency value and vice versa. This behavior of the frequency changes reinforced the suggested distribution. The observed broadness of the stretching bands by increasing Zn-content can be assigned to the perturbation in Fe–O bonds due to Zn-substitution [37]. The weak band appeared at about 330 cm−1 (ν3 ) in the samples with x ≥ 0.8 can be attributed to the divalent stretching frequency arised from Zn2+ ions situated in the tetrahedral sites [38]. 3) Morphology Investigation: The morphology of the studied system was investigated using TEM analysis. TEM images of the substituted ferrites are shown in Fig. 5. Direct observation of the images revealed that the obtained nanoparticles are cubic in shape with nearly homogeneous size distribution.

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TABLE II E LECTROMAGNETIC PARAMETERS OF THE Co1−x Znx Fe2 O 4 S YSTEM

Fig. 6. Room temperature hysteresis loops for the Co1−x Znx Fe2 O4 system.

The observed sizes (L TEM ) agreed well with those estimated via XRD measurements (L XRD ). The images showed also dense inter particle agglomeration at low substitutions and by increasing Zn-content, this agglomeration was observed to release. This agglomeration phenomenon could be due to the small-size single domain particles, which are permanently magnetized owing to experience a magnetostatic interaction proportional to their volume. The interfacial surface tension in this case cannot be also neglected. C. Magnetic Characterization Fig. 6 shows room temperature magnetic field versus magnetization for the Co–Zn ferrite system. The corresponding magnetic parameters, namely, maximum magnetization (Mmax ), remanent magnetization (Mr ), coercivity (Hc ), magnetic moment (η B ), and squareness value (Mr /Ms ) are summarized in Table II. A close view to the hysteresis curves evidenced that the saturation cannot be reached in all samples even at maximum utilized magnetic field strength. This behavior can be attributed to the small sizes of the entire particles and the dependence

of the field required to saturate the magnetization on this size [39]. In addition, while all the samples showed the S-shape hysteresis characteristic for the ferromagnetic materials, the sample with Zn-content x = 1, i.e., ZnFe2 O4 sample exhibited very small value for the maximum magnetization (3.7 emu/g), indicating its paramagnetic type behavior. Singh et al. [12] previously reported a similar behavior for zinc ferrite prepared via reverse micelle technique. Careful analysis of the reported data (Table II) evaluated that, although zero-magnetic moment Zn2+ ions are introduced in the cobalt ferrite spinel structure, the saturation magnetization was increased from 40.5 at x = 0–56.7 emu/g at x = 0.2 followed by a subsequent significant decreases with increasing Zn-content. A similar trend was also reported by other investigators for the Co1−x Znx Fe2 O4 system prepared via other methods [4], [11], [16], [40]. The measured saturation magnetization for pure cobalt ferrite (40.5 emu/g) is obviously lower than that of bulk cobalt ferrite (80 emu /g) [41] as well as the measured values via other different preparation routes [4], [11]–[16], [40]. This can be discussed based on the smaller crystallite sizes obtained via the entire studied method. On the other hand, the measured coercivity value (1048 Oe), which is relatively higher than that of the bulk cobalt ferrite (750–980 Oe) [42], indicated hard magnetic properties and suggested the ability of being used as permanent magnets. Generally, the change in magnetization with Zn-substitution can be explained according to Neel’s two sub-lattice model [43]. In this model, the moment per formula unit (η B (x)) can be theoretically calculated by subtracting the magnetic moments of tetrahedral sub-lattice (M A ) from that of octahedral one (M B ), i.e., using equation: η B (x) = M B (x) − M A (x). Thus, the obtained variation in magnetization can be treated in the view of suggested distribution and the preferable occupancy in the different sites. According to the above theory, the obvious increment of magnetization at x = 0.2 can be due to the preferable occupation of Zn2+ ions by the tetrahedral sites to replace Fe3+ ions. The migrated Fe3+ ions to the octahedral sites (B-sites),


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Fig. 7. Relation between lnσ and reciprocal of absolute temperature at different substitutions as a function of applied frequency for the Co1−x Znx Fe2 O4 system.

and hence, strengthen the B-B interaction and increasing the magnetic interaction. The subsequent decrease in the magnetization by Zn-substitution (x ≥ 0.4) can be, thus, owed to the occupation of the diamagnetic zinc ions in B-sites, substituting the Co2+ ions. This process will result in weakening the B-B interaction and thus diluting the entire magnetic moment. In ZnFe2 O4 (x = 1), the very small negligible saturation magnetization obtained (3.7 emu/g) suggested the formation of a normal spinel with all Zn2+ ions occupy tetrahedral sites, and all Fe3+ ions are in the octahedral sites with anti-parallel moments alignment. This behavior suggested that Zn-doping in CoFe2 O4 interestingly brought a transition from ferro- to paramagnetic. The paramagnetic characteristics in the spinel ferrite structure can be attributed to the small crystallite sizes resulting in the single domain structure [12]. Generally, the obvious agreement between the behaviors of Neel’s magnetic moments (η B (x)), calculated at different Zn-contents, with the experimentally calculated one (η B ) (Table II) reinforced the suggested cation distribution and

discussed well the observed changes in the lattice parameter values estimated through the XRD measurements. The obtained paramagnetic characteristics for ZnFe2 O4 sample illustrates the agglomeration release, at higher Zn-content, observed through the TEM measurements. The weakening of the magnetic properties by Zn-substitution decreases the magnetic interaction between particles and released agglomeration. The smaller squareness ratios (Mr /Mmax ) less than 0.5 reported in Table II, suggested that the uni-axial particles with cubic magnetocrystalline anisotropy are randomly oriented [11]. The obvious sharp decrease in this ratio at x = 1 (i.e., ZnFe2 O4) indicated the very weak exchange coupling between magnetic ions in agreement with the supposed cation distribution and paramagnetic characteristics of this sample. The sharp decrease in coercivity by the addition of zinc (Table II) exhibited the clear change from hard to soft ferrite. This behavior of coercivity change is understandable since the

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Fig. 8.


Relation between lnσ and applied frequency as a function of temperature for the Co1−x Znx Fe2 O4 system.

Zn substantially increases inside the grain with respect to the increase in the unit cell volume [44]. In addition, the lower anisotropic nature of Zn+2 ions (−1 × 104 erg/cm3) compared to that of Co2+ ions, according to Stoner Wolfforth model for nanoparticles [45], cannot be neglected. D. Electrical Measurements Fig. 7 illustrates the changes in the electrical conductivity (σ ) as a function of the frequency in the temperature range up to 500 °C. For all the samples, the main behavior observed is the decrease in the conductivity values up to about 120 °C, followed by linear increases at higher ranges. This obvious decrease in conductivity can be related to the water adsorbed on ferrites surfaces during preparation and pellets compression. Water is known as an electron donor and by rising temperature, the successive desorption of water molecules can led to this decrease in conductivity [46]. At higher temperatures, the conductivity versus 1000/T relations for all the samples showed semiconducting characteristics. The conductivity linearly increases with temperature. The conduction activation energies can be calculated from the

slopes using the Arrhenius equation [15]. The obvious changes in the slopes by increasing temperature indicate changes in the investigated ferrites magnetic properties (from ferromagnetic to paramagnetic) accompanied by a change in the conduction mechanism and charge carrier types [15], [47], [48]. The calculated values of the activation energies measured at ferromagnetic (E f ) and paramagnetic regions (E p ) as well as the observed Curie transition temperatures (TC ) are summarized in Table II. The observed decrease in Curie temperatures, TC values by increasing Zn-content can be owed to decrease in the ferromagnetic region on the expense of the paramagnetic regions by increasing concentration of zero-magnetic moment zinc ions. This observation agreed well with the previously obtained magnetic data. The conductivity in cobalt ferrite can be explained via simultaneous electron exchange occurred in n-type (Fe3+ /Fe2+ ) and p-type (Co2+ /Co3+ ), mainly situated adjacent in the octahedral B-sites [49]. This is because of the sufficiently reduced distances between the different valences states of the same elements inside octahedral B-sites facilitate electronic transitions than A-A or A-B transitions [15].


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Fig. 9. Relation between real part of dielectric constant (ε ) and the absolute temperature as a function of applied frequency for the Co1−x Znx Fe2 O4 system.

The different valence states, i.e., Fe2+ and Co3+ can be formed in the octahedral B-sites as a result of zinc ions loss during sintering process or mainly due to exchanging electrons between Co2+ and Fe3+ ions as can be observed from equation: Co2+ + Fe3+ → Co3+ + Fe2+ , suggested by Nikumbh et al. [46]. The most predominant conduction mechanism in the ferromagnetic region is the electron hopping mechanism; while in the paramagnetic region, the conduction is mainly attributed to a small positive holes (polarons) migrated inside the sample due to large thermal energy [50]. This clarified the higher activation energies obtained for paramagnetic regions than those of ferromagnetic ones (Table II). On the other hand, the obtained behavior of ac-conductivity versus frequency, as a function of temperature (Fig. 8) in which a gradual increase was observed at low temperatures, while almost linear behavior was obtained at higher temperatures assured the changing in the conduction mechanism and consequently the type of charge carriers by increasing temperature [30].

The conductivity values (σ ) measured in the ferromagnetic regions at 444 K and 100 kHz as a function of Zn-content are summarized in Table II. Using Table II, it is observable that the conductivity values are slightly changed with the addition of zinc. This is indicated that during cobalt substitution, the added zinc does not alter the Fe2+ /Fe3+ ratio responsible for the hopping conduction in this ferromagnetic region. The calculated activation energy values E f (Table II) agree well with the obtained change in conductivities. Frequency dependences for real part (ε ) as well as the imaginary part (ε ) of dielectric constant in the frequency range from 100 Hz to 4 MHz and temperature ranging between room temperature and 700 K are shown in Figs. 9 and 10. Generally, the dielectric values are observed to increase with increasing temperature showing an obvious decrease with increasing frequency. In addition, the temperature dependence is not significant at high frequencies compared with that of the lower ones. A similar trend was previously reported by Haque et al. [51] for lanthanum substituted cobalt ferrite. The obvious decrease in dielectric by enhanc-

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Fig. 10. Relation between imaginary part of dielectric constant (ε ) and the absolute temperature as a function of applied frequency for the Co1−x Znx Fe2 O4 system.

ing frequency can be treated as a normal behavior in all spinels [15]. For all the samples, a broad dielectric relaxation peak was observed. The position of this relaxation peak agreed well with the previously measured Curie temperatures through conductivity measurements (Table II). Thus, this change attributed to the transition from ordered (ferromagnetic) to disordered (paramagnetic) states can be assigned to a magnetic relaxation [15]. IV. C ONCLUSION A new sol-gel method using gelatin was successfully used for synthesizing Co1−x Znx Fe2 O4 (where x = 0, 0.2, 0.4, 0.6, 0.8, and 1) nanocrystalline ferrites. A mechanism for the gelation process was suggested and discussed. XRD measurements indicated the formation of nanocrystalline ferrites only after calcining at 350 °C. Based on the obtained lattice parameters, a suitable distribution of the entire system was suggested.

FT-IR spectra were used to explain tetrahedral and octahedral clusters and reinforced the suggested cation distribution. TEM images revealed aggregated cubic nanoparticles having sizes agreed well with those estimated via XRD measurements. VSM indicated that the ferromagnetic characteristics of the samples changed to paramagnetic by the addition of zinc. Maximum magnetization was obtained for the sample with x = 0.2, and the variation in magnetization values was discussed in the view of the cation redistribution. The obtained coercivity value of CoFe2 O4 (1048 Oe) indicated hard magnetic properties and suggested the ability of being used as permanent magnets. The sharp decrease in the coercivity by increasing zinc exhibited a change from hard to soft ferrite. Conductivity measurements as a function of temperature and frequency revealed semiconducting behavior with an obvious change from ferromagnetic to paramagnetic by rising temperature. The conduction mechanism as well as the type of the charge carriers was


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