Effects of copper substitution on the microstructural, electrical and magnetic properties of Ni0.7Co0.3-xCuxFe2O4 ferrites

Accepted Manuscript Effects of copper substitution on the microstructural, electrical and magnetic properties of Ni0.7Co0.3-xCuxFe2O4 ferrites K. Vijaya Babu, G.V. Santosh Kumar, K. Jalaiah, P.T. Shibeshi PII:

S0022-3697(17)31895-4

DOI:

10.1016/j.jpcs.2018.02.051

Reference:

PCS 8466

To appear in:

Journal of Physics and Chemistry of Solids

Received Date: 7 October 2017 Revised Date:

21 February 2018

Accepted Date: 22 February 2018

Please cite this article as: K.V. Babu, G.V.S. Kumar, K. Jalaiah, P.T. Shibeshi, Effects of copper substitution on the microstructural, electrical and magnetic properties of Ni0.7Co0.3-xCuxFe2O4 ferrites, Journal of Physics and Chemistry of Solids (2018), doi: 10.1016/j.jpcs.2018.02.051. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT Effects of copper substitution on the microstructural, electrical and magnetic properties of Ni0.7Co0.3-xCuxFe2O4 ferrites K. Vijaya Babu1*, G.V. Santosh Kumar1, K. Jalaiah2, P. T. Shibeshi3 1*

Advanced Analytical Laboratory, Andhra University, Visakhapatnam, India 2

3

Department of Physics, Andhra University, Visakhapatnam, India

Department of Physics, College of Natural Science, Arba Minch University, Arba Minch, Ethiopia

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Corresponding Author: [email protected]

Abstract

Spinel ferrites are interesting materials because of their rich physical and magnetic properties. In this study, copper substituted Ni0.7Co0.3-xCuxFe2O4 (x = 0.0, 0.05, 0.1, 0.15 and

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0.2) nanocrystalline ferrites were synthesized using the sol–gel method. The X-ray powder diffraction (XRD) patterns showed that all of the samples had a pure single-phase cubic spinel structure with the Fd-3m space group over the whole composition range. The lattice

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constants, average crystallite size, and grain size were calculated for these compounds based on the XRD patterns and compared with those for nickel ferrite, where most of the values increased with the copper concentration. The DC resistivity increased with the dopant concentration. The magnetic properties were investigated based on the electron spin resonance, which showed that the g-factor decreased in a linear manner as the magnetic field

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and dopant concentration increased.

Keywords: dielectric loss, g-factor, image mapping, spinel ferrite Introduction

Ferrites are magnetic ceramics comprising iron oxide and metal oxides, which have

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potential applications in devices such as permanent magnets, memory storage devices, microwave devices and telecommunication equipment. Ferrites have remarkable electrical

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and magnetic features such as high electrical resistivity, low dielectric loss, high saturation magnetization, high permeability and moderate permittivity [1-3]. Nanocrystalline form ferrites have applications in new fields including magnetically guided drug delivery, magnetic resonance imaging, catalysis, magnetic fluids, and humidity and gas sensors. Depending on the crystal structure, ferrites can be classified according to three types comprising hexagonal ferrite, garnet and spinel ferrite. Spinel ferrites are a regular combination with oxygen with the general formula of AB2O4. The unit cell of spinel ferrites comprises 32 oxygen atoms in a cubic closed packed arrangement distributed in tetrahedral (A) and octahedral sites (B) [4-6]. The chemical and structural properties of spinel nanocrystalline ferrites are highly sensitive to their composition and the synthesis methods 1

ACCEPTED MANUSCRIPT employed, and their corresponding electric and magnetic properties depend on cation substitutions. Among the spinel ferrites, the inverse types are most interesting because of their high magneto-crystalline anisotropy and high saturation magnetization [7-8]. Nickel ferrite (NiFe2O4) is one of the most important materials in the inverse spinel family in terms of its magnetic and electrical properties. Substituted nickel ferrites have also been studied

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widely because of their typical properties, such as high saturation magnetization, low conductivity, low dielectric losses and low cost, which make them suitable candidates for applications as soft magnets and low loss materials at high frequencies [9-10].

In the present study, we prepared and characterized spinel nanocrystalline ferrites with general formula Ni0.7Co0.3-xCuxFe2O4 (x = 0.0, 0.05, 0.1, 0.15 and 0.2) by substituting

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Cu2+ and Co2+ in NiFe2O4. The effects of these substitutions on the structural, electrical, and magnetic properties were investigated. The materials were obtained by the sol–gel method

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using citric acid as a chelating and fuel agent. Synthesis and experimental techniques

The Ni0.7Co0.3-xCuxFe2O4 (x = 0, 0.05, 0.1, 0.15 and 0.2) nanocrystalline ferrites were synthesized using the sol–gel method. Analytical grade chemicals comprising nickel nitrate (Ni(NO3)2.6H2O), cobalt nitrate (Co(NO3)2.6H2O), ferric nitrate (Fe(NO3)3.9H2O), copper

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nitrate Cu(NO3)2.3H2O, and citric acid (C6H8O7) were used in the synthesis process. The molar ratio of metal nitrates relative to citric acid was 1:3. Ammonia solution was added to maintain the pH at 7. The compounds obtained were then dried in an oven for 2 h. The powdered samples were mixed with polyvinyl alcohol as a binder, ground and then pressed at

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a pressure of 5 t for 6 min to obtain a circular disk-shaped pellet. The synthesized powder and pellet were sintered at 1200°C for 5 h, before investigating their structural, morphological,

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magnetic, and electrical properties. The surface layers of the sintered pellet were carefully polished and washed in acetone, and the pellet was then coated with silver paste on opposite faces, which acted as electrodes. The synthesized nanocrystalline Ni0.7Co0.3-xCuxFe2O4 (x = 0.0, 0.05, 0.1, 0.15 and 0.2)

ferrites were characterized using standard techniques comprising X-ray powder diffraction (XRD), scanning electron microscopy (SEM), electron spin resonance (ESR) and with an LCR meter. The XRD patterns were recorded at room temperature in the 2θ range of 10° to 70° using Cu-Kα radiation (λ = 1.5405 Å). The morphology of the particles in the powder was observed based on SEM images obtained with a JEOL JSM-6610L system. Fourier transform infrared (FT-IR) spectra measurements were acquired using a Shimadzu Shimadzu 2

ACCEPTED MANUSCRIPT IR-Prestige21 instrument according to the transmittance method with potassium bromide (KBr) as the IR window in the wave number region of 400 to 1300 cm–1. The magnetic properties were measured using a JEOL-JES-FA100 ESR spectrometer based on the X-band at room temperature. Impedance measurements were obtained with a Wayne–Kerr high frequency LCR meter model 65120 in the frequency range of 20 Hz to 8 MHz at room

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temperature. Initial permeability was determined based on 10 turns of SWG (standard wire gauge) enameled copper wire on toroids and inductance measurements were acquired at various frequencies using a Wayne–Kerr high frequency LCR meter model 65120 in the frequency range of 20 Hz to 8 MHz at room temperature.

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Results and Discussion Structural properties

The XRD patterns obtained for the nanocrystalline Ni0.7Co0.3-xCuxFe2O4 (x = 0.0, 0.05,

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0.1, 0.15 and 0.2) ferrites are shown in Figure 1, which demonstrates that the XRD patterns were sharp and intense. The patterns matched well with the characteristic reflections of the face centered cubic (FCC) single phase spinel structure with the Fd-3m space group and no extra peaks were identified. All of the compounds exhibited their maximum intensity at (311) and the presence of the strong diffraction peaks corresponding to the (111), (220), (311),

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(222), (422), (511), (440) and (533) planes indexed the hkl values and labels. The interplanar spacing values for all of the compounds are listed in Table 1. Figure 2 clearly shows the continuous shift in 2θ as the copper concentration increased where the angle shifted toward higher values, which was attributed to increases in the lattice parameter [11-13]. The lattice (a)

was

calculated

for

all

the

samples

using

the

standard

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constant

relationship: a = d h 2 + k 2 + l 2 , where d is the interplaner spacing and hkl are the Miller

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indices. The results obtained using unit cell software and the relationships given above are shown in Table 2. The value of the lattice constant (a) ranged from 8.3442 to 8.3693 Å, as found in previous studies of spinel ferrites. The values of the lattice constant versus the dopant concentration are shown in Figure 3, which demonstrates that the lattice constant tended to increase slowly with the dopant concentration despite the substitution with copper ions instead of cobalt ions, thereby obeying Vegard’s law [14-16]. The increase in the lattice constant was attributed to the difference in the ionic radii of Co2+ and Cu2+. Thus, the slight variation may have been due to the small difference in the ionic radii of the Co2+ (0.745 Å) and Cu2+ (0.73 Å) ions. The increase in the lattice constant may be explained by the migration of a proportional amount of the large cobalt ion to the octahedral (B) site, thereby leading to a 3

ACCEPTED MANUSCRIPT relative expansion of the B-site sub lattice, and thus an increase in the lattice constant. The changes in the lattice parameters also indicated variations in the distribution of cations[1719]. The average crystallite sizes were calculated for the synthesized ferrites using the Debye–Scherer formula: kλ , β cosθ

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D=

where k is a constant that depends on the shape of the particle, λ is the X-ray wavelength, β is the full width at half maximum of the most intense peak (311), and θ is the diffraction angle of the peak. The average crystallite size ranged from 10.92 to 12.61 nm at different dopant

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concentrations, which can be explained in terms of increased pore mobility due to the creation of excess cation vacancies.

The hopping lengths (LA and LB) between magnetic ions (the distance between the relationships. LA =

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ions) in the tetrahedral A-site and octahedral B-site were calculated using the following

a 3 a 2 and LB = 4 4

The hopping length values are listed in Table 5. The hopping length increased as a function of copper substitution, where the changes were similar to those in the lattice constant

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[20], which may be explained based on the variations in the lattice constant with the dopant concentration. Figure 3 clearly shows the increases in the LA and LB values.

Theoretical lattice constant

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In order to verify the cation distribution, the theoretical lattice parameter was

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calculated using the following relationship and compared with the experimental values:

 8  a th =  (rA − ro ) + 3 (rB + ro ) 3 3 ,

where rA is the tetrahedral site radius, rB is the octahedral site radius, and ro is the radius of the oxygen ion (1.32 Å). The ionic radii of the tetrahedral (rA) and octahedral (rB) sites were calculated using the following relationship: rA = [ x(Cu A2+ ).r (Cu A2+ ) + 2( Fe 3A+ ).r ( Fe 3A+ )] rB = [0.7( Ni A2+ ).r ( Ni A2+ ) + x(Cu B2+ ).r (Cu B2+ ) + 1 − x(CoB2+ ).r (CoB2+ ) + 2( FeB3+ ).r ( FeB3+ )] / 2 , where rCo2+, rNi2+, rCu2+, and rFe3+ are the cationic radii of Co, Ni, Cu, and Fe ions according to Shannon, respectively [21]. Table 1 shows clearly observed that the cation distribution estimated based on the X-ray intensity calculations agreed well with the actual distribution,

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ACCEPTED MANUSCRIPT where rA increased and rB decreased as the copper concentration increased. The theoretical and experimental lattice parameters had the same increasing trend, although the theoretical lattice constant values were slightly smaller than the experimentally determined values due to the copper ions occupying the A-site and replacing cobalt ions in the B-site [22]. The mean

1  rA =  u − a 3 − ro 4  5  rB =  − u a − ro 8  ,

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radii of the ions at the tetrahedral and octahedral sites are given by:

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where rA represents the radius of the tetrahedral (A) site cation, rB represents the radius of the tetrahedral (B) site cation, u is the oxygen positional parameter and ro represents the radius of oxygen anions. The oxygen parameter (u value) is determined using the following formula.

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1 1  u = (rA + ro ) +  3a 4  

In spinel structures, the oxygen positional parameter has a value of about 0.375 Å, where the arrangement of the O2– ions has a cubic closed packing. However, we found that the u value varied from 0.3610 to 0.3616 Å, which is smaller than the ideal value (u =

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0.375Å), where this may have been attributable to the history of the samples, as well as experimental or measurement errors. In addition, small displacements of anions from the ideal situation may have formed extended tetrahedral interstices [23-24].

Interionic distances

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The interionic distances comprising the tetrahedral and octahedral bond lengths dAL and dBL, tetrahedral edge, and shared and unshared octahedral edge lengths (dAE, dBE, and

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dBEU) were calculated using the experimental values obtained for the lattice parameter a and oxygen positional parameter u according to the following equations, and the values are listed in Table 5.

d AL = a 3 (u − 0.25)

d BL = a 3u 2 −

11 43 u+ 4 64

d AE = a 2 (2u − 0.5) d BE = a 2 (1 − 2u )

d BEU = a 4u 2 − 3u +

11 16 5

ACCEPTED MANUSCRIPT Clearly, that the values of dAL, dBL, dAE, dBE, and dBEU increase with the copper concentration, probably due to the substitution of cations and the cation distribution.

Proposed cation distribution Determining the cation distribution can provide very useful information to facilitate the development of materials with desirable properties. The cation distribution in spinel

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ferrites can be obtained by analyzing the XRD pattern using the Bertaut method, where this method selects a few pairs of reflections according to the following expression. hkl hkl R = ∑ I obs − I cal hkl

The best information about the cation distribution is obtained by comparing the

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experimental and calculated intensity ratios for reflections with intensities: (i) That is almost independent of the oxygen parameter;

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(ii) That vary with the cation distribution in an opposite manner; and (iii) That does not differ significantly.

In the present study, the (220), (400) and (440) reflection planes were used to calculate the theoretical and experimental intensity ratios. These reflection planes were assumed to be sensitive to the cation distribution. For the comparison, the intensities of the crystal planes were extracted from the diffraction pattern as the observed intensity and the

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theoretical intensity was calculated by:

I hkl = F hkl pL p 2

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1 + cos 2 2θ Lp = sin 2 θ . cos θ ,

where F is the structure factor, p is the multiplicity factor, and Lp is the Lorentz polarization

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factor.

Table 4 shows that the Ni2+ ions occupy the B-sites whereas Co2+ ions occupy the

tetrahedral A-site. Cu2+ ions preferably replace Co2+ from the tetrahedral site. The proposed cation distribution and the intensities are shown in Table 4.

SEM

Figure 4 shows that the particles were spherical and their size tended to decrease as the copper concentration increased. The average grain size was approximately 10 µm and it was slightly larger than the average crystallite size determined by XRD because every particle comprised a number of crystallites or grains. There was a direct connection between the crystallite size according to XRD and the grain size determined by SEM, thereby 6

ACCEPTED MANUSCRIPT indicating that the particles formed via the agglomeration of crystallites. SEM images of the Ni0.7Co0.3-xCuxFe2O4 (x = 0.0, 0.05, 0.1, 0.15, and 0.2) nanocrystalline ferrites are shown in Figures 4(a) to 4(f), which demonstrate that the grain sizes and shapes varied significantly with the dopant concentration. In addition, intergranular pores were not present on the surfaces of the grains.

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Energy-dispersive X-ray spectroscopy (EDS) and image mapping The elemental compositions of the samples were determined by EDS and using SEM. To confirm the presence of the substituted cations, EDS analysis was conducted using the synthesized ferrites, as shown in Figures 5(a) to 5(e). The results obtained by EDS analysis

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confirmed the presence of all the elements i.e., Ni, Fe, Co, Cu and O ions, in the synthesized samples. According to the accuracy of EDS, no traces of impurity elements were found in the nanocrystalline ferrites, which confirmed that the final compositions of the samples were the

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same as the starting compositions without any impurity elements. The almost uniform cation concentration distribution was clearly observed in the micrographs. The chemical compositions of the samples were as expected. It should be noted that EDS is a semiquantitative analytical technique and thus the exact amounts of the cations could not be detected, but we did determine consistent results in terms of the atomic percentages of Ni, Fe,

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Co, Cu and O in the synthesized nanoparticles. After introducing copper into the chemical composition of nickel ferrite, the corresponding micrographs obtained are shown in Figures 5(b) to 5(e). The intensity of the copper-related peaks increased with the substitution content. In the whole series of synthesized nanoparticles, the results obtained by EDS and the

FT-IR spectra

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expected chemical compositions were consistent [25-26].

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The FT-IR spectra provided information about the positions of ions in the crystal and the cation distribution in the spinel structure. The FT-IR spectra for Ni0.7Co0.3-xCuxFe2O4 (x = 0.0, 0.05, 0.1, 0.15 and 0.2) nanocrystalline ferrites in the wave number range of 1300–400 cm–1 are shown in Figure 6, which indicate the presence of two main absorption peaks at wave numbers ν1 and ν2, where these were characteristic of the spinel structure in all the samples. The stretching bands (ν2) at a higher wave number indicated the vibration of the octahedral site and the bands at a lower wave number show denoted the vibration of the tetrahedral site. The higher frequency band ν1 was at 667.39 cm–1 and the lower frequency band ν2 was at 419.53 cm–1. The difference in the band positions was expected because of the different interionic distances for the octahedral and tetrahedral coordinates. The slight shift of 7

ACCEPTED MANUSCRIPT the ν1 bands toward a low frequency was also expected because an increase in the site radius reduced the fundamental frequency, and thus the central frequency of the bands shifted toward the lower frequency side and vice versa. The vibrational frequency depends on the cation mass, cation to oxygen distance, and bonding force. Similar FT-IR spectra were also obtained for the copper substituted nanocrystalline ferrites. The position of the bands in FT-

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IR spectra obtained for all of the synthesized materials confirmed the formation of spinel ferrite structures. Enlarged views of ν1 and ν2 are shown in Figure 7.

According to Waldron, the force constants for tetrahedral (Kt) and octahedral (Ko) sites are given by:

K o = 10.62 ×

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K t = 7.62 × M 1 ×ν 12 × 10 −3 dyne/cm

M2 ×ν 22 × 10 −3 dyne/cm, 2

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where M1 and M2 are the molecular weights of the cations on the A and B sites, respectively. The calculated Kt and Ko values are listed in Table 6, which clearly shows that the force constant Kt and Ko values decreased with the dopant concentration. This behavior can be attributed to the cation–oxygen bond lengths at the A and B sites. The decrease in the force constant may be attributed to the increase in bond lengths rA and rB at the tetrahedral and

Dielectric constant

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octahedral sites, respectively [27-29].

The frequency dependence of the dielectric constant for the Ni0.7Co0.3-xCuxFe2O4 (x = 0.0, 0.05, 0.1, 0.15 and 0.2) nanocrystalline ferrites at room temperature is shown in Figure 8,

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which demonstrates that the dielectric constant decreased as the frequency increased where this is the normal dielectric behavior for spinel ferrites. The dielectric constant decreased as the concentration of Cu increased, probably due to the migration of some Fe3+ ions from B-

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sites to A-sites, thereby decreasing the hopping between Fe3+-Fe2+ ions in the octahedral Bsite. Therefore, the polarization decreased and the dielectric constant was lower. The high dielectric constants at low frequencies may have been due to dislocations, voids, and defects present in the crystal structure of the ferrites because of electron exchange between the Fe3+ and Fe2+ ions, which can be explained using Koop’s theory by considering the dielectric structure as an inhomogeneous medium with two Maxwell–Wagner layers. According to Maxwell–Wagner theory, the two dielectric parameters comprising the dielectric constant and tanδ have inverse relationships with frequency. Very low dielectric constants values were observed for these samples, so they are appropriate for use at higher frequencies. At room 8

ACCEPTED MANUSCRIPT temperature, the dominant mode of conduction was electron hopping between Fe2+ and Fe3+, and the number of Fe2+ ions decreased as the dopant concentration increased. Decreases in the ferrous ions were responsible for polarization, and a reduction in the dielectric constant was expected with the dopant concentration [30-32].

Dielectric loss tangent (tanδ)

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The variations in the dielectric loss tangent (tan δ) as a function of frequency for Ni0.7Co0.3-xCuxFe2O4 (x = 0.0, 0.05, 0.1, 0.15 and 0.2) nanocrystalline ferrites are shown in Figure 9, which demonstrates that in a similar manner to the dielectric constant, the dielectric loss tangent decreased exponentially as the frequency increased. The decrease in the

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dielectric loss tangent as the frequency increase was due to the strong association between the dielectric behavior of ferrites and the conduction mechanism. Furthermore, the decrease in tanδ as the frequency increased occurred because the hopping frequency of the charge

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carriers did not follow the changes in the externally applied electric field beyond a certain frequency limit. Materials with high conductivity usually have high dielectric losses whereas those with low conductivity usually have low dielectric losses. The results obtained were in agreement with those obtained in previous studies [33-36].

DC resistivity

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Figure 10 shows the variations in the DC resistivity with frequency for all of the synthesized samples at room temperature. The DC resistivity increased in a linear manner with the frequency up to 6 MHz and then decreased gradually, where this variation can be explained by the locations of the cations in the spinel ferrite. The variation in the DC

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resistivity was almost linear up to 6 MHz, after which a break occurred that indicated a change in the magnetic ordering from ferrimagnetism to paramagnetism. The resistivity increased as the dopant concentration increased. The resistivity of ferrites depends on various

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factors such as the porosity, microstructure, chemical composition, and amount of Fe2+ ions present in B-sites. A small amount of Fe3+ was converted into Fe2+ during sintering at high temperature. The electrical conduction phenomena in ferrites can be explained based on the hopping mechanism, i.e., the exchange of charges between ions of the same element in different valance states. The cation distribution studies confirmed that copper ions occupied the octahedral site according to their ionic radius [37-39]. The increase in Cu2+ ions at the Bsite led to the replacement of Fe3+ ions, thereby decreasing the number of ferrous ions formed. The copper ions did not participate in the conduction mechanism, but they limited the degree of Fe2+ ↔ Fe3+ transfer, thereby obstructing electron hopping and increasing the 9

ACCEPTED MANUSCRIPT resistivity. In addition, the hopping rates of electron transfer decreased as the amount of Fe3+ ions decreased. Thus, the DC resistivity increased with the doping concentration. The resistivity results obtained at room temperature agreed well with those reported previously [36-38].

Initial permeability

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The initial permeability measurements for toroid samples were made using a Wayne– Kerr high frequency LCR meter model 65120 in the frequency range of 20 Hz to 8 MHz at room temperature. The permeability of ferrite is due mainly to spin rotation and domain wall displacement at microwave frequencies. The frequency dependence of the initial permeability

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with the dopant concentration is shown in Figure 11, which demonstrates that the initial permeability decreased gradually frequency increased. This result is in accordance with Snoek’s law:

γM s 3π ( µ − 1) ,

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f =

where f is the resonant frequency, Ms is the saturation magnetization, and γ is the gyromagnetic ratio. According to the formula, the initial permeability and saturation magnetization are interdependent terms. In addition, this law indicates that µi will be high

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when the resonant frequency is low. The value of µi depends significantly on the saturation magnetization, grain size, and magneto-crystalline anisotropy. The relationship between grain size and permeability is linear only when the grain growth is normal, i.e., if all grains grow at the same time and at the same rate [40-41].

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ESR analysis

The X-band ESR spectra for Ni0.7Co0.3-xCuxFe2O4 (x = 0.0, 0.05, 0.1, 0.15 and 0.2)

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nano-crystalline ferrites at room temperature are shown in Figure 12, which indicates the high spin–spin relaxation nature due to its high line width. The broadness of the ESR resonance signal was due to the random orientation of ferromagnetism, thereby indicating strong super exchange interactions among the cations via the oxygen ions present in Ni0.7Co0.3-xCuxFe2O4. The peak to peak line width increased with the copper concentration due to the strengthening of the dipolar interactions among cations via oxygen. All of the spectra were analyzed using the Lorentzian distribution function to obtain the values of various parameters such as the gvalue, line width, spin concentration, and relaxation time. The g-value is a constant representing the proportionality between the frequency and the field, where it is a function of

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ACCEPTED MANUSCRIPT the molecular motion and the paramagnetic properties. The effective g-factor is determined by g=

hυ βH ,

where h is Planck’s constant, υ is the frequency of the microwave,

is the magnetic field,

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and β is the Bohr magneton. We observed that the -value decreased in a linear manner as the magnetic field and dopant concentration increased. The decrease in the

-value may have

been due to super-exchange interactions between Ni2+ and Fe3+ ions via nonmagnetic O2− ions. The super-exchange interaction in ferrite is responsible for the magnetic ordering within

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each sub lattice. The interaction between octahedral and tetrahedral sites is strongest in ferrites. The interaction between octahedral and tetrahedral sites predominates, so the spins of the ions in ferrites are opposite and the resulting magnetic moment is equal to the difference

364.144 mT to 350.121 mT [42-44].

Conclusion

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between those of the octahedral and tetrahedral site ions. The resonance field was about

In this study, nanocrystalline Ni0.7Co0.3-xCuxFe2O4 (x = 0.0, 0.05, 0.1, 0.15 and 0.2) ferrites were successfully prepared using the sol–gel method with citric acid as the fuel and

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analytical grade metal nitrates. The structural, magnetic, electric, and dielectric properties of the synthesized ferrites were analyzed. The patterns matched well with the characteristic reflections of the FCC single phase spinel structure with the Fd3m space group, and no extra peaks were identified. The lattice constant determined based on XRD data increased as the

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copper concentration increased due to the difference in the ionic radii of Co2+ and Cu2+. The cation distribution was studied based on the intensity ratio calculations. The FT-IR spectra

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indicated the presence of two absorption bands located near 400 cm–1 and 600 cm–1, which are characteristics of a spinel ferrite. The grain size determined by SEM was 2–4 µm. The EDS data suggested that all of the elements present in the synthesized ferrites maintained their stoichiometricy properties. The DC resistivity increased as the dopant concentration increased. The observed

-value determined by ESR decreased in a linear manner as the

magnetic field and dopant concentration increased. The results of this study clearly indicate that Cu substituted Ni0.7Co0.3-xCuxFe2O4 ferrites are promising candidates for applications in high frequency devices.

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distribution of nanocrystalline CoxFe3-xO4 ferrites, J. Magn. Magn. Mater. 378 (2015) 246-252. 13. Zein K. Heiba, Mohamed Bakr Mohamed, L. Ard, N. Dogan, Cation distribution correlated with magnetic properties of nanocrystalline gadolinium substituted nickel ferrite, J. Magn. Magn. Mater. 391 (2015) 195-202.

14. Mohamed Bakr Mohamed, Adel Maher Wahba, M. Yehia, Structural and magnetic properties of CoFe2−xMoxO4 nanocrystalline ferrites Mater. Sci. Eng. B 90 (2014) 52-58.

15. Adel Maher Wahba, Mohamed Bakr Mohamed, Structural, magnetic, and dielectric properties of nanocrystalline Cr-substituted Co0.8Ni0.2Fe2O4 ferrite Ceram. Int. 40 (2014) 6127–6135. 16. Mohamed Bakr Mohamed, M. Yehia, Cation distribution and magnetic properties of nanocrystalline gallium substituted cobalt ferrite, J. Alloy. Compd. 615 (2014) 181-187. 17. Erum Pervaiz, I.H. Gul, High frequency AC response, DC resistivity and magnetic studies of holmium substituted Ni-ferrite: A novel electromagnetic material, J. Magn. Magn. Mater. 349 (2014) 27–34.

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ACCEPTED MANUSCRIPT 18. Hugh St. C. O’Neill, Alexandra Navrotsky, Simple spinels: crystallographic parameters, cation radii, lattice energies, and cation distribution, American Mineralogist, 68 (1983) 181-194. 19. A.H. Ashour, O.M. Hemeda, Z.K. Heiba, S.M. Al-Zahrani, Electrical and thermal behavior of PS/ferrite composite, J. Magn. Magn. Mater. 369 (2014) 260-267. 20. Adel Maher Wahba, Mohamed Bakr Mohamed, Structural and magnetic characterization and cation distribution of nanocrystalline CoxFe3xO4 ferrites, J. Magn. Magn. Mater. 378 (2015) 246-252.

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21. R. D. Shannon, Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides, Acta Crystallogr. A 32 (1976) 751-767.

22. N.Y. Mostafa, Z.I. Zaki, Z.K. Heiba, Structural and magnetic properties of cadmium substituted manganese ferrites prepared by hydrothermal route, J. Magn. Magn. Mater. 329 (2013) 71-76. 23. K.J. Standley, Oxide magnetic materials, Second ed., Clarendon Press, Oxford, 1972.

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24. V.M. Khot, A.B. Salunkhe, M.R. Phadatare, N.D. Thorat, S.H. Pawar, Low-temperature synthesis of MnxMg1−xFe2O4(x = 0-1) nanoparticles: cation distribution, structural and magnetic properties, J. Phys. D: Appl. Phys. 46 (2013) 055303(1-8).

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25. Ali Ghasemi, Compositional dependence of magnetization reversal mechanism, magnetic interaction and Curie temperature of Co1−xSrxFe2O4 spinel thin film, J. Alloy. Compds. 645 (2015) 467–477. 26. K. Maaz, W. Khalid, A. Mumtaz, S.K. Hasanain, J. Liu, J.L. Duan, Magnetic characterization of Co1-xNixFe2O4 (0⩽x⩽1) nanoparticles prepared by co-precipitation route, Physica E 41 (2009) 593– 599.

27. P. Laokul, S. Maensiri, J. Optoelectron. Aloe vera solution synthesis and magnetic properties of Ni-CuZn ferrite nanopowders, Adv. Mater. 11 (2009) 857-862.

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28. S.A. Saafan, T.M. Meaz, E.H. El-Ghazzawy, M.K. El-Nimr, M.M. Ayad, M. Bakr, A.C. and D.C. conductivity of NiZn ferrite nanoparticles in wet and dry conditions, J. Magn. Magn. Mater. 322 (2010) 2369-2374.

29. M.A. Amer, T.M. Meaz, S.S. Attalah, A.I. Ghoneim, Structural and magnetic characterization of the Mg0.2-xSrxMn0.8Fe2O4 nanoparticles, J. Magn. Magn. Mater. 363 (2014) 60–65. Maxwell,

Electricity

and

EP

30. J.C

Magnetism,

2,

Oxford

University

Press,

New

York,

1973, section 828.

AC C

31. K.W. Wagner, The Distribution of Relaxation Times in Typical Dielectrics. Annals of Physics, 40 (1973) 817-855.

32. C.G. Koop, On the Dispersion of Resistivity and Dielectric Constant of Some Semiconductors at Audio frequencies, Physical Review 83 (1951) 121–124.

33. M.A. Ahmed, M.K.E.L. Nimr, A. Tawfik, A.M. El. Hasab, Resistivity and thermoelectric power of NiAl ferrites, Physica Status Solidi (A) 123 (1991) 501-506.

34. V.T. Zaspalis, E. Antoniadis, E. Papazoglou, V. Tsakaloudi, L. Nalbandian, C.A. Sikalidis, The effect of Nb2O5 dopant on the structural and magnetic properties of MnZn-ferrites, J. Magn. Magn. Mater. 250 (2002) 98-109. 35. S.A. Kanade, Vijaya Puri, Composition dependent resistivity of thick film Ni(1−x)CoxMn2O4: (0 ≤ x ≤ 1) NTC thermistors Mater. Lett. 60 (2006) 1428-1431.

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ACCEPTED MANUSCRIPT 36. N. Popandian, P. Balay, Narayanasamy, Electrical conductivity and dielectric behaviour of nanocrystalline NiFe2O4 spinel, J. Phys. Condens. Matter 14 (2002) 3221-3237. 37. K.F. Niessen, Curie temperature of nickel-zinc ferrites as a function of the nickel-zinc ratio, Physica 17 (1951) 1033-1049. 38. I.H. Gul, A. Maqsood, Structural, magnetic and electrical properties of cobalt ferrites prepared by the sol–gel route, J. Alloys Compd. 465 (2008) 227-231.

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39. A.A. Kadam, S.S. Shinde, S.P. Yadav, P.S. Patil, K.Y. Rajpure, Structural, morphological, electrical and magnetic properties of Dy doped Ni–Co substitutional spinel ferrite, J. Magn. Magn. Mater. 329 (2013) 59-64.

40. R. Laishram, C. Prakash, Magnetic properties of Cr3+ substituted Li–Sb ferrites, J. Magn. Magn. Mater. 305 (2006) 35-39.

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41. Cao Jun-Gang, Li Jian-Jun, Duan Hai-Feng, Lin Ying-Jie, Synthesis and Characterization of Manganese-copper Spinel Ferrite Powders, Chem. Res. Chin. Univ. 28 (2012) 590-593. 42. Ashok

Kumar,

Parmod

Kumar,

Geeta

Rana,

M.

S.

Yadav,

R.

P.

Pant,

Appl. Sci. Lett. 1(2) (2015) 33-36.

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A study on structural and magnetic properties of NixZn1-xFe2O4 (0 ≤ x ≤ 0.6) ferrite nanoparticles, 43. K. Jalaiah, K. Vijaya Babu, Structural, magnetic and electrical properties of nickel doped Mn-Zn spinel ferrite synthesized by sol-gel method, J. Magn. Magn. Mater. 423 (2017) 275-280. 44. Kannipamula Vijaya Babu, Matangi Ravi Chandra, Gondu Venkata Santosh Kumar, Kantamsetti Jagadeesh, Effect of cobalt substitution on structural, electrical and magnetic properties of NiFe2O4

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Process. Appl. Ceram. 11 (2017) 60-66.

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ACCEPTED MANUSCRIPT Table 1: Inter planer spacing (d) of Ni0.7Co0.3-xCuxFe2O4 (x=0.0, 0.05, 0.1, 0.15 and 0.2) Interplanar distance (Å)

Plane x=0

x=0.05

x=0.10

x=0.15

x=2.0

111

4.8172

4.8161

4.8185

4.8143

4.8126

220

2.9512

2.9505

2.9513

2.9498

2.9631

311

2.5171

2.5168

2.5172

2.5160

2.5262

222

2.4097

2.4096

2.4102

2.4094

2.4178

422

2.0872

2.0872

2.0874

2.0867

511

1.7044

1.7045

1.7048

1.7042

440

1.6070

1.6072

1.6072

1.6068

533

1.4762

1.4726

1.4725

1.4718

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hkl

2.0933 1.7081 1.6063

SC

1.4751

Table 2: Lattice constant, cell volume, X-ray density, bulk density, porosity and crystallite size of Ni0.7Co0.3-xCuxFe2O4 (x=0.0, 0.05, 0.1, 0.15 and 0.2) nanocrystalline ferrites

8.3444

0.10

8.3447

0.15

8.3485

0.20

8.3693

x-ray density (g.cm-3) 5.3612

Crystallite size (nm) 12.6115

581.012

5.3659

17.7247

581.085

5.3705

10.9239

581.888

5.3683

10.9451

586.237

5.3337

12.5846

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0.05

Volume (Å)3 580.99

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0

Lattice constant (Å) 8.3442

x-value

Table 3: Ionic radii of tetrahedral A-site (rA), octahedral B-site (rB), theoretically lattice constant (ath) and

rA (Å)

rB (Å)

ath (Å)

u-value (Å)

0.4865

0.7660

8.3350

0.3610

EP

oxygen positional parameter (u) of Ni0.7Co0.3-xCuxFe2O4 (x=0.0, 0.05, 0.1, 0.15 and 0.2) x-value 0

0.4866

0.7661

8.3452

0.3611

0.10

0.4866

0.7661

8.3495

0.3611

0.15

0.4875

0.7671

8.3520

0.3611

0.4920

0.7723

8.3752

0.3616

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0.05

0.20

Table 4: Cation distribution and intensity ratios of Ni0.7Co0.3-xCuxFe2O4 (x=0.0, 0.05, 0.1, 0.15 and 0.2) x-value

Cation distribution

I(220)/I(400)

I(422)/I(440)

I(220)/I(440)

Obs.

Cal.

Obs.

Cal.

Obs.

Cal.

[Co0.3Fe0.5][(Ni0.7Fe1.5]O4

1.1252

1.1338

0.8700

0.8376

1.5373

1.4755

0.05

[Co0.25Fe0.5Cu0.05][Ni0.7Fe1.5]O4

1.0464

1.1423

0.9475

0.8915

1.4781

1.4455

0.10

[Co0.20Fe0.5Cu0.10][(Ni0.7Fe1.5]O4

1.0800

1.1201

0.9425

0.8999

1.4693

1.5217

0.15

[Co0.15Fe0.5Cu0.15][Ni0.7Fe1.5]O4

1.0131

1.0180

0.9357

1.1690

1.4684

1.9018

0.20

[Co0.10Fe0.5Cu0.20] [Ni0.7Fe1.5]O4

0.9892

1.2761

1.1678

1.1865

1.3310

1.2788

0

ACCEPTED MANUSCRIPT Table 5: Hopping length LA, LB, Tetrahedral bond (dAL), octahedral bond (dBL), tetra edge (dAE) and octahedral edge (dBE) (shared and unshared) of Ni0.7Co0.3-xCuxFe2O4 (x=0.0, 0.05, 0.1, 0.15 and 0.2) nanocrystalline ferrites LA (Å)

LB (Å)

dAL

dBL

dAE

dBE

dBEU

0

3.6131

2.9501

1.6042

2.2090

2.6197

3.2805

2.9593

0.05

3.6132

2.9501

1.6057

2.2081

2.6221

3.2782

2.9592

0.10

3.6133

2.9502

1.6057

2.2082

2.6222

3.2783

2.9594

0.15

3.6150

2.9516

1.6065

2.2092

2.6234

3.2798

2.9607

0.20

3.6240

2.9589

1.177

2.2101

2.6417

3.2761

2.9674

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x-value

ferrites

Kt x 105 (dyne/cm)

ν1

ν2

0

419.50

667.39

1.555

0.05

418.57

669.32

1.551

0.10

419.53

667.39

0.15

418.57

668.36

0.20

418.57

667.39

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Ko x 105 (dyne/cm)

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x-value

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Table 6: FTIR transmittance bands for Ni0.7Co0.3-xCuxFe2O4 (x=0.0, 0.05, 0.1, 0.15 and 0.2) nanocrystalline

1.442

1.443

1.555

1.442

1.551

1.443

1.551

1.442

440

x=0.20

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x=0.15

20000 15000 10000 5000 0

x=0.10

15000 10000 5000 0

SC

Intensity (cps)

15000 10000 5000 0

511

422

400

222

111

15000 10000 5000 0

220

311

ACCEPTED MANUSCRIPT

10

20

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20000 15000 10000 5000 0

30

40

50

60

x=0.05

x=0

70

2θ

18000 16000

EP

x=0 x=0.05 x=0.10 x=0.15 x=0.20

12000 10000

AC C

Intensity (cps)

14000

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Figure 1: XRD patterns of Ni0.7Co0.3-xCuxFe2O4 (x=0.0, 0.05, 0.1, 0.15 and 0.2)

8000 6000 4000 2000

35.4

35.5

35.6

35.7

35.8

2θ

Figure 2: Enlarged view of 311 peak for Ni0.7Co0.3-xCuxFe2O4 (x=0.0, 0.05, 0.1, 0.15 and 0.2)

ACCEPTED MANUSCRIPT 587 8.370 586

lattice constant unit cell volume

8.365

8.360

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584

unit cell volume

lattice constant

585

8.355

583

8.350

582

8.345

8.340 0.00

0.05

0.10

0.15

580

0.20

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dopant concentration

SC

581

EP

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Figure 3: Variation of lattice constant and unit cell volume with dopant concentration

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Figure 4(a): Scanning electron microscopy images of Ni0.7Co0.3-xCuxFe2O4 (x=0.0) nano-crystalline ferrites

Figure 4(b): Scanning electron microscopy images of Ni0.7Co0.3-xCuxFe2O4 (x=0.05) nano-crystalline ferrites

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ACCEPTED MANUSCRIPT

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Figure 4 (c): Scanning electron microscopy images of Ni0.7Co0.3-xCuxFe2O4 (x=0.1) nano-crystalline ferrites

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Figure 4 (d): Scanning electron microscopy images of Ni0.7Co0.3-xCuxFe2O4 (x=0.15) nano-crystalline ferrites

Figure 4 (f): Scanning electron microscopy images of Ni0.7Co0.3-xCuxFe2O4 (x=0.2) nano-crystalline ferrites

ACCEPTED MANUSCRIPT Quantitative results

Weight%

1.5

1.0

0.5

0.0 Fe

Co

Ni

SC

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O

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Figure 5 (a): EDS map analysis of Ni0.7Co0.3Fe2O4 nano-crystalline ferrites

Quantitative results

10

0 Fe

Co

Ni

Cu

EP

O

TE D

5

AC C

Weight%

15

Figure 5 (b): EDS map analysis of Ni0.7Co0.3-xCuxFe2O4 (x=0.05) nano-crystalline ferrites

ACCEPTED MANUSCRIPT Quantitative results

Weight%

15

10

5

0 Fe

Co

Ni

Cu

SC

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O

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Figure 5 (c): EDS map analysis of Ni0.7Co0.3-xCuxFe2O4 (x=0.1) nano-crystalline ferrites

Quantitative results 10

6

2 0 Fe

Co

Ni

Cu

EP

O

TE D

4

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Weight%

8

Figure 5 (d): EDS map analysis of Ni0.7Co0.3-xCuxFe2O4 (x=0.15) nano-crystalline ferrites

ACCEPTED MANUSCRIPT Quantitative results

10

5

0 Fe

Co

Ni

Cu

SC

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O

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Figure 5 (e): EDS map analysis of Ni0.7Co0.3-xCuxFe2O4 (x=0.2) nano-crystalline ferrites

400 350

250

AC C

100

EP

200 150

x=0 x=0.05 x=0.10 x=0.15 x=0.20

TE D

300

Transmittance (%)

Weight%

15

50

0 400

500

600

700

800

900

1000

1100

1200

-1

wave number (cm )

Figure 6: FTIR spectra of Ni0.7Co0.3-xCuxFe2O4 (x=0.0, 0.05, 0.1, 0.15 and 0.2)

1300

ACCEPTED MANUSCRIPT x=0 x=0.05 x=0.10 x=0.15 x=0.20

180

200 180

Transmittance (%)

140 120 100 80

160 140 x=0 x=0.05 x=0.10 x=0.15 x=0.20

120 100 80

60

60 40

40 415

420

425

630

430

640

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Transmittance (%)

160

220

650

660

670

680

690

700

-1

-1

wave number (cm )

SC

wave number (cm )

Figure 7: Enlarged view of ν1 and ν2 bands of Ni0.7Co0.3-xCuxFe2O4 (x=0.0, 0.05, 0.1, 0.15 and 0.2) nano-

M AN U

x=0 x=0.05 x=0.10 x=0.15 x=0.20

EP

TE D

8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0

AC C

Dielectric Constant (εr)

crystalline ferrites

6.2

6.4

6.6

6.8

7.0

7.2

7.4

7.6

7.8

log f

Figure 8: Variation of dielectric constant with frequency of Ni0.7Co0.3-xCuxFe2O4 (x=0.0, 0.05, 0.1, 0.15 and 0.2) nano-crystalline ferrites

ACCEPTED MANUSCRIPT 2.4 2.2 2.0 1.8 x=0 x=0.05 x=0.10 x=0.15 x=0.20

1.6

RI PT

tan δ

1.4 1.2 1.0 0.8 0.6 0.4

SC

0.2 0.0 7.40

7.45

7.50

7.55

7.60

7.65

7.75

7.80

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log f

7.70

Figure 9: Variation of dielectric loss tangent with frequency of Ni0.7Co0.3-xCuxFe2O4 (x=0.0, 0.05, 0.1, 0.15 and 0.2) nano-crystalline ferrites

TE D

5

4

EP

2

x=0 x=0.05 x=0.10 x=0.15 x=0.20

AC C

log ρ

3

1

0

6.2

6.4

6.6

6.8

7.0

7.2

7.4

7.6

7.8

8.0

8.2

log f

Figure 10: Variation of log ρ with frequency of Ni0.7Co0.3-xCuxFe2O4 (x=0.0, 0.05, 0.1, 0.15 and 0.2) nano-crystalline ferrites

ACCEPTED MANUSCRIPT

50 x=0 x=0.05 x=0.10 x=0.15 x=0.20

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30

20

10

SC

Initial Pemeability (µi)

40

7.2

7.4

M AN U

0 7.6

7.8

8.0

8.2

log f

Figure11: Variation of initial permeability with frequency of Ni0.7Co0.3-xCuxFe2O4 (x=0.0, 0.05, 0.1, 0.15 and

TE D

0.2) nano-crystalline ferrites

2000 1500

EP

500 0

AC C

ESR signal

1000

x=0 x=0.05 x=0.10 x=0.15 x=0.20

-500

-1000 -1500 -2000

0

100

200

300

400

500

600

Magnetic field (Gauss)

Figure 12: ESR spectra for Ni0.7Co0.3-xCuxFe2O4 (x=0.0, 0.05, 0.1, 0.15 and 0.2) nano-crystalline ferrites

ACCEPTED MANUSCRIPT Ni0.7Co0.3-xCuxFe2O4 nano-ferrite materials prepared by sol-gel auto combustion. Compounds characterized based on XRD, SEM, ESR and initial permeability. XRD indicated the formation of a single phase with no impurities.

AC C

EP

TE D

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SC

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Observed g-value determined by ESR approximately equal to the standard value.

1