Structural, electrical, and optomagnetic tweaking of Zn doped CoFe2−xZnxO4−δ nanoparticles

Structural, electrical, and optomagnetic tweaking of Zn doped CoFe2−xZnxO4−δ nanoparticles

Journal of Magnetism and Magnetic Materials 414 (2016) 144–152 Contents lists available at ScienceDirect Journal of Magnetism and Magnetic Materials...

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Journal of Magnetism and Magnetic Materials 414 (2016) 144–152

Contents lists available at ScienceDirect

Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm

Structural, electrical, and optomagnetic tweaking of Zn doped CoFe2  xZnxO4  δ nanoparticles Shraddha Agrawal, Azra Parveen n, Ameer Azam Department of Applied Physics, Z.H. College of Engineering & Technology, Aligarh Muslim University, Aligarh 202002, India

art ic l e i nf o

a b s t r a c t

Article history: Received 7 January 2016 Received in revised form 6 April 2016 Accepted 17 April 2016 Available online 20 April 2016

Nanoparticles of pure and Zn doped CoFe2O4 of the composition CoFe2  xZnxO4  δ (x ¼0, 0.05, 0.10, 0.15) have been successfully synthesized by microwave gel combustion. The microstructural and compositional analyses were carried out by X-ray diffraction, Scanning Electron Microscopy. The crystallite size was found to increase with the increase in the Zn content. The dielectric constant ( ε ) and A.C. conductivity were studied as a function of frequency and temperature and were explained on the basis of Maxwell–Wagner model and electron hopping respectively. The energy band gap was found to decrease gradually with Zn doping. The magnetic measurements, depicts an increase in magnetization with the increase in Zn concentration, which in turn shows a strong dependency on the particle size. The magnetic hysteresis loop confirms the ferromagnetic nature. & 2016 Elsevier B.V. All rights reserved.

Keywords: X-ray diffraction Electron microscopy Optical properties Magnetic properties Dielectric Properties

1. Introduction Magnetic nanoparticles are newly explored materials due to its large surface to volume ratio, smaller grain size, quantum confinement effect, large uniaxial anisotropy, and super-paramagnetism. Nanomaterials of spinel ferrites constitute an important class of magnetic materials having several technological applications like magnetic diagnostic, magnetic drug delivery, high density magnetic information storage devices, biomedical applications, microwave devices, electrical generators, etc. [1]. The optical, electrical, and magnetic properties of nanomaterials changes due to variation in the band gap, electrical conductivity and saturation magnetization which makes them suitable for optoelectronics as well as optomagnetic devices [2,3]. Cobalt ferrite (CFO), a ferromagnetic ceramic among the spinel ferrites possesses unique properties like high curie temperature, high coercivity (∼5400 Oe), high magneto crystalline anisotropy, moderate saturation magnetization (∼80 emu/g), excellent chemical stability, mechanical hardness, large Kerr effect and Faraday rotation [4–7] which makes them suitable for magneto-optical recording media [8,9]. Many physical properties, such as electrical conductivity, optical band gap, dielectric constant, magnetization and defect structure are greatly affected by the amount of impurities added [10–12]. Thus, doping is an effective method to regulate the properties of nanomaterials. Saafan et al. [13] studied the magnetic n

Corresponding author. E-mail address: [email protected] (A. Parveen).

http://dx.doi.org/10.1016/j.jmmm.2016.04.059 0304-8853/& 2016 Elsevier B.V. All rights reserved.

and electrical properties of Ca substituted cobalt ferrite at Co site and found that magnetization and the coercivity decreases whereas the AC conductivity and the dielectric constant increases. The effect of magnetic (Mn2 þ , Ni2 þ , etc.) and nonmagnetic (Zn2 þ , Cd2 þ , La3 þ ,etc.) substitution in spinel ferrite have been studied which shows the change in their structural, optical, and magnetic behavior [14], due to distribution of cations in between the available A- and B- sites [15,16]. Silva et al. [17] reported the effect of heat treatment on cobalt ferrite ceramic powders. Therefore, the information of different types of cation substitution plays an important role in the development of new materials which may be useful in the industrial applications [5]. The present study attempts to investigate the structural, optomagnetic and electrical properties of Zn2 þ doping on Fe site of cobalt spinel ferrites synthesized by microwave gel combustion method using citric acid as a fuel. The microwave gel combustion offers effective and simple method for rapid synthesis of cobalt ferrite yielding high crystallinity, purity and homogeneous nanomaterials.

2. Experimental In the present work, microwave gel combustion method was used for the synthesis of pure and Zn doped cobalt ferrite nanoparticles. All chemicals were of analytical grade purchased from Sigma Aldrich and were used without further purification. Cobalt nitrate [Co(NO3)2  6H2O], Zinc nitrate Zn(NO3)2  6H2O and Iron were weighed according to nitrate [Fe(NO3)3  9H2O]

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stoichiometric amount and dissolved in de-ionized water (100 ml) to make a solution and then 0.1 M Citric acid (C6H8O7) was added to the solution to serve as chelating agent. The aqueous solution of the precursor was then stirred in a beaker for 20 min at room temperature in order to mix the solution uniformly and then placed inside microwave oven for evaporation through microwave irradiation for 15 min at 800 W output power and 2.45 GHz. When the water was completely evaporated, the solution got converted into gel form. The viscous gel was then washed well with the ethanol and dried at 80 °C. Thus obtained ash was crushed manually to obtain a very fine powder. This powder was further ground for 60 min and annealed at 600 °C for 5 h to improve the ordering. The calcined nano powders of pure and Zn doped Cobalt ferrite were characterized for crystal phase identification by X-ray diffraction XRD (Rigaku Miniflex II) using Cu-Kα radiations (λ ¼ 1.5406 Å) operated at a voltage of 30 kV and current of 15 mA in 2θ range from 20° to 80°. All XRD patterns were indexed by powder X software (Institute of Physics, Chinese Academy of Science). Microstructural and compositional analysis of the prepared samples was done by field emission electron microscope FESEM (Ultra Plus, ZEISS) and energy dispersive X-ray spectroscopy EDAX attached with the FESEM equipment. The morphological details of the prepared samples were also probed by transmission electron microscope TEM (JEOL, JEM-2010). The UV–Visible spectra of Zn doped Cobalt ferrite nano powders were performed in the range of 200–800 nm by UV–Visible spectrophotometer (Perkin Elmer, Lambda 35). Dielectric and impedance spectroscopy measurements were carried out in the frequency range of 100 kHz to 4.5 MHz using LCR meter (Agilent, 4285A). Temperature dependence of dielectric measurements were carried out in the temperature range of (100–350) °C. The pellets obtained from the pure and Zn doped nano powders were coated on the adjacent faces with silver paste to obtain metallic contact on both sides, thereby forming geometry of parallel plate capacitor. Magnetic hysteresis loops were measured at room temperature using a vibrating sample magnetometer VSM (PAR, 155).

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parameter and volume of the unit cell can be easily explained on the basis of Vegard’s law [18], which may be attributed to the larger ionic radius of Zn2 þ (0.74 Å) in comparison to Fe3 þ (0.64 Å). Changes in FWHM are in accord with the particle size which was estimated using the Debye–Scherrer’s formula [19],

D=

0. 9λ β cos θ

(3)

where λ is the wavelength of X-ray radiation (Cu Kα ¼1.5406 Å), β is the full width at half-maximum of the most intense peak at the diffraction angle θ. The crystallite size of pure CFO was found to be 12.38 nm and increased in size to 15.79 nm for 15% Zn doped CFO as summarized in the Table 1. To further verify the formation of self-assembled pure and doped nanostructures and to estimate particle size, TEM measurement was performed, Fig. 1(c). The TEM image reveals that synthesized sample was highly crystalline in nature and in agglomerated form of NPs. The average particle size of pure and 5%, 10%, 15% doped samples was found to be 17, 19, 22, and 27 nm respectively as shown in the insets of Fig. 1(c).Variation of crystalline size, lattice parameter and density with respect to Zn concentration are also shown in Fig. 1(b). 3.2. FESEM and EDAX analysis The micro structures of Zn doped cobalt ferrite-nanoparticles (NPs) were observed by FESEM as exhibited from Figs. 2(a) and 3 (a) respectively which reveals the surface morphology for pure and 15% Zn doped Cobalt ferrite. The FESEM analysis shows that the pure and Zn doped cobalt ferrite – NPs are randomly agglomerated. In order to confirm the elemental structure in pure and Zn doped Cobalt ferrite – NPs EDAX spectroscopy analysis was performed. Fig. 2(b) shows the EDAX spectra of pure Cobalt ferrite sample. Three main peaks of Co, O, and Fe atoms were observed in the EDAX analysis. Fig. 3(b) exhibits the EDAX spectra of 15% Zn doped Cobalt ferrite sample with four main peaks of Co, Zn, O, and Fe atoms. The EDAX results are in fair agreement with the expected elemental composition as given in Table 2.

3. Results and discussion 3.3. Dielectric constant 3.1. X-ray diffraction analysis The X-ray diffraction patterns of pure and Zn doped Cobalt ferrite samples are shown in Fig. 1(a). The XRD pattern shows that all samples consist of single crystalline phase with cubic spinel structure and no impurity phases such as Fe2O3, CoO, ZnO, Co2O3 or Co3O4 were present in the samples. The unmodified spinel structure obtained by the addition of Zn ion into the CFO matrix indicates that the Zn dopant ought to be incorporated into the lattice as substitution ion. The powder patterns are in good agreement with the standard JCPDS Card no. (22-1086).The lattice parameter (a) and density of the unit cell (ρ) listed in Table 1were calculated by using the formula for the most intense peak.

dhkl=

a (√(h2 + k2 + l2)

nM ρ= V *NA

(1)

(2)

where dhkl is inter planar spacing for the most intense peak at (311), n ¼No. of atoms per unit cell (8), NA ¼ Avogadro number (6.023*1023 mol  1), M ¼Molecular weight and V¼Volume of the unit cell. It can be noticed from the Fig.1(a) that as we increase the Zn concentration from zero to 15% the lattice parameters of CFO increase from 8.35 nm to 8.58 nm.This increase in lattice

The most important property of dielectric constant is given in terms of frequency dependent dielectric permittivity. The complex dielectric constant is given by the following equation;

ε=ε′−jε′′

(4)

where, ε′ is the real part of dielectric constant representing stored energy and ε′′ is its imaginary part representing the dissipated energy. The method of synthesis, cation distribution, grain size, density of material, porosity, sintering, structural, compositional, and the electrical properties of solid materials are the various physical parameters which can affect the dielectric properties of ferrite materials [20]. The real part of dielectric constant is given by

ε′=

Cp d ε0 A

(5)

where Cp is the capacitance of the specimen in Farad (F), d is thickness of pellet, ε0 is the permittivity of free space (8.854  10  12 F/m) and A is the area of cross section of circular pellet. The effects of temperature on dielectric constant and dielectric loss at different frequencies 100 kHz, 500 kHz,1.0 MHz, and 4.5 MHz for pure and Zn doped Cobalt ferrite are shown in Figs. 4(a–d) and 5(a–d) respectively. Figures clearly show that the dielectric constant decreases with increase in frequency and

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Fig. 1. (a) X-ray diffraction patterns of Zn doped CoFe2  xZnxO4  δ (x ¼0, 0.05, 0.10, 0.15) powder. (b) Variation of crystallite size, lattice parameter and density with Zn concentration. (c) TEM Images of Zn doped CoFe2  xZnxO4  δ (x¼ 0, 0.05, 0.10, 0.15) powder.

Table 1 Variation of crystallite size, lattice parameters and band gap for all samples. Samples

Crystallite size (nm)

Lattice parameters (Å) (a¼ b¼ c)

Density (gm/ cm3)

Band gap (eV)

CoFe2O4 CoZn0.05Fe1.95O4 CoZn0.10Fe1.90O4 CoZn0.15Fe1.85O4

12.38 13.50 15.75 15.79

8.351 8.552 8.565 8.585

5.070 5.000 4.987 4.963

4.13 4.09 4.06 3.97

becomes almost constant at higher frequencies. This behavior can be explained on the basis of Maxwell–Wagner interfacial model [21,22] where a dielectric medium is assumed to consists of two

layers, the well conducting grains (First layer) separated by poorly conducting grain boundaries (Second layer). The conductivity of the grain is considered to be relatively higher than the grain boundary. The effect of grain boundaries are considered relatively higher in lower frequency regions whereas the grains are effective in the higher frequency region. As a result, the accumulation of charge carriers take place at the separating boundaries and the dielectric constant value is highly raised. Under the application of external electric field the charge carriers can easily migrate across the grains but are hindered at the grain boundaries. The grain boundaries are formed during the sintering process due to the surface reduction or oxidation of crystallites in a porous material because of their direct contact with the firing atmosphere [23]. The mechanism of polarization in ferrite is due to the electron

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Fig. 2. (a) FESEM images of pure Cobalt ferrite nanoparticles. (b) EDAX of pure Cobalt ferrite nanoparticles.

Fig. 3. (a) FESEM images of 15% Zn doped Cobalt ferrite nanoparticles. (b) EDAX of 15 % Zn doped Cobalt ferrite nanoparticles.

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Table 2 Composition of the synthesize nanoparticles measured from EDS. Samples

Co (at%)

Fe (at%)

Zn (at%)

Fe/Co

Zn/Co

O

x ¼0.0 x ¼0.15

5.01 17.58

9.61 28.30

– 2.14

1.92 1.61

– 0.122

39.68 51.99

hopping between the ions of same element in different oxidation states such as electronic exchange between Co2 þ –Co3 þ and Fe2 þ –Fe3 þ ions [24]. As the field frequency increases the electronic exchange between Fe2 þ –Fe3 þ and Co3 þ –Co2 þ cannot follow the rapid field variation as a result of which the dielectric constant starts decreasing gradually. At low temperatures, (100–150) °C the dielectric constant are observed to be independent of temperature whereas at higher temperature they are found to increase with temperatures (175–350) °C at all frequencies. This may be attributed to the fact that at higher temperatures the mobility of charge carrier enhances because of extra thermal energy supplied by the temperature resulting in enhancement of hopping rate, while at lower temperature the thermal energy is not sufficient for the rise in mobility of charge carriers. The relative permittivity is directly related to the four types of polarizations, namely, electronic, ionic, interfacial, and dipolar (orientational) polarization [25]. The rapid increase in dielectric constant at lower frequencies is the result of interfacial and dipolar polarizations. These polarizations show a strong dependence on frequency and temperature. Introduction of small amount of Zn ions (x ¼0.05, 0.10 and 0.15) results in increase in the value of dielectric constant which can be explained on the

basis of the fact that polarization is the size dependent property. From the XRD analysis we found that the particle size increases from 12 to 16 nm with Zn ions and thereby increasing the dielectric polarization. The temperature dependence of dielectric loss at selected frequencies are illustrated in Fig. 5(a–d) which shows a gradual diminution with the Zn doping. The dielectric loss (tan δ) in ferrites is a measure of lag in the polarization with respect to the alternating field [26]. 3.4. AC conductivity The ac conductivity of pure and Zn doped CFO NPs has been measured as a function of temperature in the range (100–350) °C as shown in Fig. 6(a–d). The electrical conductivity of samples is mainly due to hopping of electron between ions of the same element present in more than one valence state. The electrical conductivity of a dielectric material is a summation of two terms [27], given by

σtotal=σ 0 (T ) + σ (ω, T )

(6)

The first term is frequency independent or dc conductivity which is related to the drifting of charge carriers whereas the second term is pure ac conductivity and is related to the dielectric relaxation caused by the localized electric charge carriers which obeys the ac power law [28]

σ (ω) = Cωn

(7)

where the constants C and n are both temperature and

Fig. 4. Variation of dielectric constant with temperature for all samples.

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149

Fig. 5. Variation of dielectric loss with temperature for all samples.

composition dependent parameters; n is dimensionless whereas C has the dimension of electrical conductivity and ω is the angular frequency. In ferrites the conduction mechanism is explained on the basis of hopping of electron between Fe2 þ and Fe3 þ ions at tetrahedral sites. The hopping frequency of electron enhances due to pumping force of applied frequency which in turn causes an increase in the mobility of charge carriers between the different localized states as well as liberating the trapped charges from the different trapping centers. Thus, we observe a gradual increase in electrical conductivity with frequency. The ac conductivity increases with temperature for all frequencies (100 kHz, 500 kHz, 1.0 MHz, and 4.5 MHz). This increase being more rapid at higher frequencies may be explained in terms of increased drift mobility and hopping frequency of the charge carrier. The conductivity increases because of the rate of charge hopping between Fe2 þ -Fe3 þ and also movement of excited or detached charge carriers from different ion centers. The conductivity decreases with the increase in Zn concentration because doping of Zn ions decreases the Fe number thereby isolating them, which in turn decreases the probability of charge hopping. The further increase in Zn doping introduces defects ions which have a tendency to segregate at the grain boundaries, due to the diffusion process resulting from sintering and cooling processes producing grain boundary defect barrier, leading to the blockage to flow of charge carriers [29,30]. 3.5. Optical properties The optical properties of synthesized nanoparticles were studied by UV–Visible absorption spectroscopy. The absorbance generally depends on several factors such as oxygen deficiency, band gap, impurity centers, grain size, lattice strain and surface

roughness [31]. The absorption spectra are shown in Fig. 7(a). The absorption coefficient α , of the nanoparticles has been calculated using the relation [32]. I = I0 e−α * t where, the absorption coefficient ( α )

α=

2. 303*A t

(8)

where A is the absorbance and t is the thickness of the sample. The optical band gap of the samples was calculated using the Tauc relationship as given by [33]

αhν=(hν − Eg )n

(9)

where n ¼½ for direct band gap semiconductors. An extrapolation of the linear region of the plot of hν vs (αhν)2 gives the value of the optical band gap Eg (Fig. 7(b)). The calculated band gaps are listed in Table 1. The table depicts that the energy band gap decreases as the particle size increases with the increase of Zn doping concentration from 0 to 15%. This may be due to the Brass’s effective mass model [34,35], where the measured band gap Eg can be expressed as a function of particle size as

Eg*=Eg bulk+

ћ2π 2 ⎛ 1 1 ⎞ 1. 8e2 + ⎜ ⎟− 2er 2 ⎝ me mh ⎠ 4πεε0 r

(10)

Eg bulk

where is the bulk energy gap, r is the particle size, me is the effective mass of electrons, mh is the effective mass of holes, ε is the relative permittivity, ε0 is the permittivity of free space, ћ is h/2 π and e is electron charge. 3.6. Magnetic properties Magnetic hysteresis loops for pure and Zn-doped Cobalt ferrite

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Fig. 6. Variation of conductivity with temperature for all samples.

samples synthesized by microwave gel combustion with various doping concentrations studied at room temperature are illustrated in Fig. 8 and the measured value of coercivity (Hc), remanant magnetization (Mr), saturation magnetization (Ms), and ratio of Mr/Ms are listed in Table 3. Magnetic properties of Cobalt ferrites are strongly affected by the site preference of cation in the spinel lattice [20]. There are three types of exchange interactions between the magnetic ions lying on the two different sub lattices of ferrite, i.e., AA interactions, BB interactions, and AB interactions. AB interactions dominate over AA or BB interactions. The net magnetic moment is the difference in moments of B and A sub lattices, i.e., M¼ MB  MA. Magnetic properties of Cobalt ferrite depend not only on the chemical composition which determines the intrinsic properties but also on various extrinsic factors, such as grain sizes, porosity and density [36]. The magnetic moment (nB) can be expressed as a function of saturation magnetization ( Ms) (Table 3)

nB =

Ms*Mol. Wt. (emu/gm) 5585

(11)

It was observed that the saturation magnetization of CoFe2O4, annealed at 600 °C, is 73 emu/g (Table 3), which agrees fairly well with the values reported by other workers [5]. However, the

saturation magnetization, Ms increases from 73.0 emu/g to 129.0 emu/g with the Zn ion concentration on Fe site. A similar trend in saturation magnetization was also observed in mixed ferrites (MO. Fe2O3; M ¼Mn , Co, Ni) with addition of nonmagnetic Zn2 þ [37]. Somaiah et al. [38] reported in the Zn doped Cobalt ferrite synthesized by auto combustion method of the composition CoFe2  xZnxO4 (x¼ 0, 0.1, 0.2, and 0.3) the Ms increases from 82 to 87 (emu/gm) followed by a subsequent decrease in Ms up to 81 (emu/gm) with increasing Zn content. Topkaya et al. [39] reported the decrease in the Ms from 75 (emu/gm) to 30 (emu/gm) at room temperature for Zn doped Cobalt ferrite at Co site. Varshney et al. [40] have also reported a decrease in saturation magnetization from 85 to 81 (emu/gm) at room temperature for Zn doped Cobalt ferrite at Co site. The formation of single-phase spinel CoFe2  xZnxO4  δ samples may contain some defects, due to interaction of microwave energy to the precursor’s materials and transformed into gel within a few minutes. Increase in saturation magnetization with Zn concentration is due to the microstructural defects in the doped system along with cationic stoichiometry and its occupancy in specific sites [41]. The increase in saturation magnetization with Zn concentration is the result of increase in particle size [42]. The values of the coercivity (Hc) of the Zn doped cobalt ferrites decrease with the increase in particle size. In spinel

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151

Fig. 7. (a) UV Visible absorption spectra for all samples. (b) Band gap plotting for all samples.

cobalt ferrites decreases sharply from 890 to 365 G as a consequence of decrease in anisotropy with the Zn content [37].

4. Conclusions

Fig. 8. Hysteresis loop of CoFe2  xZnxO4  δ (x¼ 0, 0.05, 0.10, 0.15) at room temperature.

Table 3 Variation of coercivity (Hc), remanant magnetization (Mr) and saturation magnetization (Ms) of samples. Sample

Hc (G) Mr (emu/gm) Calculated magnetic moment (nB )

CoFe2O4 890 CoZn0.05Fe1.95O4 211 CoZn0.10Fe1.90O4 183 CoZn0.15Fe1.85O4 365

37.37 21.84 19.31 46.51

3.07 3.20 3.71 5.45

Ms (emu/gm) Mr/ Ms (%)

73 76 88 129

51.12 28.74 21.94 36.05

ferrites the coercivity (Hc) depends on grains (D) as 1/D [43]. In our materials we see a variation of Hc with D, however possibility due to variation in Zn content, which also influences Hc [37], no clear dependence could be established. The coercivity of Zn doped

In summary, we have successfully synthesized pure and Zn doped CFO nanoparticles of the composition CoFe2  xZnxO4  δ (x ¼0, 0.05, 0.1, 0.15) by microwave gel combustion method. The confirmation of nanoparticles structure of pure and doped cobalt ferrite was made by using XRD technique which shows the presence of all the main peaks. Crystalline size calculated from Debye Scherrer relation varies from 12.38 nm for pure CoFe2O4 to 15.79 nm for 15% Zn doped CFO. The spherical-shape in the TEM micrograph shows that the nanostructure makes efforts to arrange itself in minimum potential energy state. The FESEM micrograph exhibits the formation of pure and 15% Zn doped CFO nanoparticles and the EDAX spectroscopic analysis confirms the presence of Co, Fe, and Zn elements in the 15% Zn doped cobalt ferrite matrix. The electrical behavior of CoFe2  xZnxO4 (x ¼0, 0.05, 0.10, 0.15) have been studied over a wide range of frequencies and temperatures by using a.c. technique of complex impedance spectroscopy. The variation of dielectric constant and loss with temperature shows frequency dependent physical characteristics due to the electron hopping between the ions which plays prevailing role in the dielectric behavior. The variation of ac conductivity with temperature shows the frequency dependent behavior because of drift mobility and hopping frequency of the charge carrier. The optical band gap was determined 4.13 eV of pure CoFe2O4 that decreases with doping concentration to a value of 3.97 eV for 15% Zn doped Cobalt ferrite. From VSM measurements we can conclude that magnetization increases with the increase in concentration of Zn in CoFe2O4 nanoparticles. It was observed that the substitution of Zn2 þ ions in CoFe2O4 synthesized by microwave gel combustion plays an important role in the alteration and development of crystal structure, morphologies,

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dielectric, and opto-magnetic characteristics of the spinel CoFe2O4 nanoparticles.

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Acknowledgments Authors are grateful to the Council of Science & Technology (CST), Govt. of UP, India for financial support in the form of Center of Excellence in Materials Science (Nanomaterials) (D.O. No. F.19-33/2006). We need to convey our thanks to Mr. Shiv Kumar (Technical Assistant IIT Roorkee) for the FESEM and VSM characterizations. We are also thankful to Prof. Afzal Ahmad for his kind support.

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