Effect of chromium substitution on the dielectric properties of mixed Ni-Zn ferrite prepared by WOWS sol–gel technique

Materials Research Bulletin 79 (2016) 14–21

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Effect of chromium substitution on the dielectric properties of mixed Ni-Zn ferrite prepared by WOWS sol–gel technique M. Ashtara , A. Munirb , M. Anis-ur-Rehmanb , A. Maqsooda,* a b

Nano Scale Physics Laboratory, Department of Physics, Air University, PAF Complex E-9, Islamabad, Pakistan Applied Thermal Physics Laboratory, Department of Physics, COMSATS Institute of Information Technology, Islamabad 44000, Pakistan

A R T I C L E I N F O

A B S T R A C T

Article history: Received 1 June 2015 Received in revised form 18 January 2016 Accepted 28 February 2016 Available online 2 March 2016

Cr+3 doped Ni-Zn nanoferrite samples with composition Ni0.5Zn0.5Fe2-xCrxO4(x = 0.1, 0.2, 0.3, 0.4) were synthesized With Out Water and Surfactant (WOWS) sol-gel technique. The structural, morphological and dielectric properties of the samples were investigated. The lattice constant, crystallite size, theoretical density and porosity of each sample were obtained from X-ray diffraction (XRD) data. The specific surface area and specific surface area to volume ratio increased with the decrease in the size of Cr+3 doped Ni-Zn ferrite nanoparticles, as the concentration of Cr+3 increased. The SEM analysis revealed that the particles were of nano size and of spherical shape. The dielectric parameters such as dielectric constant (e0 ) and dielectric loss (tand) of all the samples as a function of frequency at room temperature were measured. The AC conductivity (sAC) was determined from the dielectric parameters, which showed increasing trend with the rise in frequency. ã 2016 Elsevier Ltd. All rights reserved.

Keywords: A. Sol-gel synthesis B. X-ray diffraction C. Nanostructures D. Dielectric properties

1. Introduction Spinel ferrites are technologically important class of materials and a subject of great research from several decades due to its enhanced electrical, magnetic, mechanical and optical properties [1]. The study of these materials at the nanoscale has become important recently, from both experimental and theoretical perspectives, due to the difference of its properties from bulk counterparts and possible potential application in various fields [2]. Nanocrystalline Ni-Zn ferrites are a category of magnetic materials having spinel crystal structure with the formula AB2O4, where A and B represent various metal cations, usually including Fe. The crystal structure of spinel ferrites usually consists of cubic close-pack oxides (O2) with A cations occupying 1/8th of the tetrahedral holes and B cations occupying half of the octahedral holes. If one eighth of the tetrahedral holes are occupied by B cations, then one fourth of the octahedral sites are occupied by A cations and the other one fourth by B cations, it is called the inverse spinel structure. Mixed spinel ferrites have the formula (M1-d2+ Fed3+)[Md2+Fe2-d3+]O4 where d is the degree of inversion. These are materials of interest due to significant physical properties like high electrical resistivity, high permeability at high frequency, low coercivity, mechanical hardness and chemical

* Corresponding author. E-mail address: [email protected] (A. Maqsood). http://dx.doi.org/10.1016/j.materresbull.2016.02.044 0025-5408/ ã 2016 Elsevier Ltd. All rights reserved.

stability [3–5]. These properties are responsible for the potential applications in magnetic storage devices, drug delivery, electronic devices, sensors, catalysts etc. [6–8]. The physical properties of these ferrites are very sensitive to the method of preparation, type and amount of substitution, sintering time and temperature [8]. Ni-Zn ferrite exists as mixed cubic spinel lattice in which tetrahedral sites are occupied by Zn2+and Fe3+ions and the octahedral sites are occupied by Ni2+and Fe3+. The change in composition of these ferrites causes the re-distribution of cations (divalent and trivalent) over the tetrahedral and octahedral sites, which alters the properties of these ferrites [1,9]. The nanoparticles formed with low temperature techniques show great effect on the distribution of cations in these sites [10]. These types of ferrites behave like semiconductors and are therefore referred as hopping semiconductors. In spinel ferrites, the basic conduction process is due to the transfer of charge carriers, like electrons and holes, between metal ions present in octahedral sites. The possibility of transfer of the charge carriers arise only when a metal ion exists in more than one valence state. In case of Ni-Zn ferrites, the p-type conductivity results due to the hopping of holes between Ni+2 and Ni+3, while n-type conductivity is due to the hopping of electrons between Fe+2 and Fe+3 [11]. The low losses in Ni-Zn ferrite is the reason for the potential application in high frequency devices. The grain size affects the losses in these materials [12]. Doping of nanocrystalline ferrites with various metals like Cr, Cu, Co, Zn etc. are generally used to improve electric and magnetic properties of these materials.

M. Ashtar et al. / Materials Research Bulletin 79 (2016) 14–21

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In the present research, we report fabrication of Ni0.5Zn0.5Fe2(0.1, 0.2, 0.3, and 0.4) nanoferrites With Out Water and Surfactant (WOWS) sol-gel technique, along with structural, morphological and dielectric properties in detail.

where 0.9 is the symmetry constant, l is wavelength of the source, b is the width of peak at half maximum in radian and u is the Bragg’s angle. The X-ray density rx for the samples was calculated from XRD data using the relation [11]

2. Experimental

rx ¼

2.1. Synthesis

where M denotes molecular weight, N is Avogadro’s number and a3 is the volume of the cubic unit cell. The measured density rm was calculated for each sample from the pellets dimensions using the relation [11]

xCrxO4

Ferrite samples with general formula Ni0.5Zn0.5Fe2-xCrxO4 (where x = 0.1, 0.2, 0.3, 0.4) were prepared using WOWS sol-gel technique [13]. The samples with different composition were prepared using stoichiometric amounts of Cr(NO3)3.9H2O, Fe (NO3)3.9H2O, Ni(NO3)2.6H2O and Zn(NO3)2.6H2O. The precursors were dissolved in ethylene glycol. The chemicals used were of analytical grade. In order to dissolve the metal salts homogeneously in ethylene glycol, the molar ratio of the salts to ethylene glycol was kept 1:14.The solution was stirred for 30 min at room temperature to get a homogeneous solution, the temperature was then increased to 80C with continuously stirring until a thick gel of metal nitrates was obtained. The temperature of the gel was then increased to 300 C with continuous stirring which resulted in the drying out of the gel that formed a soft product by the redox reaction of the gel where the nitrates ions act as the oxidant and ethylene glycol acts as the reducing agent. This process was repeated for the formation of each sample under identical experimental conditions. The product was grinded to convert it into fine powder. The powders were then pelletized in cylindrical disks of 13 mm diameter and thickness of about 2.2 mm by applying a uniaxial load of 1000 psi for 10 min. The pellets were sintered at 550  C.The powder samples were used for structural characterization and pellets for dielectric measurements. 2.2. Characterization techniques The structural properties of the as prepared as well as the sintered samples were determined using X-ray diffraction (XRD) technique. The XRD patterns were recorded using CuKa radiation at room temperature. The average crystallite size (t) was calculated from the line width of the (3 11) XRD peak for all the samples using Scherrer formula [14] 0:9l t¼ bCosu

ð1Þ

8M Na3

rm ¼

ð2Þ

m

ð3Þ

pr2 h

where m is the mass of the pellet, r is its radius and h is the height of the cylindrical disc like pellet. The calculated values of these densities are given in Table 1. The porosity of each pellet was calculated with the help of following relation [11] P¼

rx rm rx

ð4Þ

where rx and rm are x-ray and measured densities respectively. For spherical grains the number of the particles can be obtained from the relation [15] N¼

Volume of 1 gram 4 1 3p 2the

ð5Þ


3 average grain size

where N represents the number of particles per gram of each sample. This helps in calculating the surface area with the help of following formula [15]
ð6Þ S ¼ N: 4pr2 where r is 1/2 of the average grain size. The morphology of the samples was examined using scanning electron microscope (SEM). The EDX of the samples was also undertaken. Dielectric measurements were carried out using WANE KERR LCR meter 6440B in the frequency range from 25 Hz to 3 MHz at room temperature. The samples were placed in a specially 0 designed sample holder. The dielectric constant (e ) was determined by the relation [12]

e0 ¼

Cp d e0 A

ð7Þ

where Cp is the parallel plate capacitor in Farad, d is the thickness of the pallets in m, A is the area of the surface of the pallet in m2 and

Table 1 Lattice constant (a), crystallite size (t(311)), x-ray density (rx), measured density (rm), porosity (P) and specific surface area (S) of the as-prepared and sintered samples of Cr doped Ni-Zn ferrites. Compositions (x)

x = 0.1

x = 0.2

x = 0.3

x = 0.4

As-prepared samples Lattice constant a (Å) Crystallite size (t(311)) (nm) X-ray density (rx) (g/cm3) Measured density (rm)(g/cm3) Porosity (P)% Specific surface area (S) (m2/g)

8.400  0.011 23 – – – –

8.393  0.011 35 – – – –

8.379  0.014 29 – – – –

8.385  0.012 11 – – – –

Sintered samples Lattice constant a (Å) Crystallite size (t(311)) (nm) X-ray density rx (g/cm3) Measured density (rm)(g/cm3) Porosity (P)% Specific surface area (S) (m2/g)

8.412  0.015 56 5.34 2.45 54 43.7

8.402  0.011 46 5.29 2.39 55 45.5

8.385  0.013 28 5.33 2.27 57 79.4

8.389  0.001 12 5.32 2.15 60 232.6

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M. Ashtar et al. / Materials Research Bulletin 79 (2016) 14–21

Fig. 1. Indexed X-ray diffraction patterns for the as-prepared (a) and sintered (b) Ni0.5Zn0.5Fe2xCrxO4 samples at room temperature.

eo is the permittivity of free space, equal to 8.85  1012F/m. The dielectric loss tangent (tand) which is the loss of energy from the applied field into the sample can be determined in terms of the real (e0 ) and imaginary parts (e00 ) of the dielectric constant as e0 e0

ð8Þ

The AC electrical conductivity (sAC) of the samples was calculated from tangent loss and dielectric constant using the relation [16]

sAC ¼ 2pye0 e0 tand

ð9Þ

where y is the applied frequency;e0,e and tand are defined already. 0

3.1. Structural properties The x-ray diffraction (XRD) patterns of the as-prepared as well as sintered Ni0.5Zn0.5Fe2-xCrxO4 (x = 0.1, 0.2, 0.3 and 0.4) ferrite samples obtained at room temperature are shown in Fig. 1 (a and b). The peaks of all the patterns are well defined and are indexed by comparing with standard pattern of Ni0.5Zn0.5Fe2-xCrxO4 reported in JCPDS card# (01-072-6799), no extra peak is observed in the patterns, indicating the formation of pure single phase. The peak (3 11) is the most prominent peak in all the patterns, which is the major characteristic of the spinel ferrites [17]. The XRD pattern of the sintered samples indicated that the width of the peaks had

250

50

S S/V 200

100 20

28

30

2

S (m /g)

150

/mg

40

S / V *10

tand ¼ D ¼

3. Results and discussion

50 10 0 10

20

30

40

50

60

Crystallite size (nm) Fig. 2. Variation of the specific surface area and specific surface area to volume ratio of Ni0.5Zn0.5Fe2-xCrxO4 samples with crystallite size.

M. Ashtar et al. / Materials Research Bulletin 79 (2016) 14–21

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Fig. 3. Scanning electron microscopic images for the sintered samples: (a) x = 0.1, (b) x = 0.2, (c) x = 0.3, (d) x = 0.4.

reduced and became more intense because of the elimination of stresses, strains and dislocations from the structures [18]. The lattice constants (a) of all the samples were calculated from the XRD data and are listed in Table 1. The lattice constants are almost same with in error bar due to the same values of ionic radii of Fe3+ and Cr3+ which is about 900 pm. The sintering temperature produced no effect on the lattice constant of the samples. This shows that the internal phase structure has not changed with sintering temperature. The crystallite size of each sample was calculated employing Scherrer formula (Eq. (1)) taking the most prominent peak (3 11). The crystallite size first increased with introducing Cr inNi0.5Zn0.5Fe2-xCrxO4 ferrite and then decreased

with further addition of Cr contents. The crystallite size ranges from 11 to 35 nm for the as-prepared and 12–56 nm for the sintered samples and results are tabulated in Table 1. The decreasing trend in the crystallite size indicates that the addition of the Cr hinders the crystal growth. At the surface of the grain, the surface temperature influence the molecular concentration, which leads to decrease the crystal growth [19]. In sintering process, the small particles of the sample collide and fuse with one another to form a larger particle. This process depends upon the available energy of the system and temperature, that’s why, with increasing temperature, grain size increases. It can be observed that the grain size for the samples x = 0.3 and x = 0.4 did not change after sintering. With

Fig. 4. Dielectric constants (e0 ) as a function of natural log of frequency (lny) for Ni0.5Zn0.5Fe2-xCrxO4 nanoferrites.

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M. Ashtar et al. / Materials Research Bulletin 79 (2016) 14–21

Fig. 5. Variation of dielectric loss (tand) as a function of natural log of frequency (lny) for Ni0.5Zn0.5Fe2-xCrxO4 nanoferrites at room temperature, the inset represents the resonance peaks in magnified version.

these compositions, the sintering temperature prevents the crystal growth. High sintering temperature not only decrease the crystallite size but also improve the crystallinity of the nanoparticles [20]. Long time of sintering and/or higher temperature give volatilization of the samples that affect both micro-structural and structural factors. It can be observed form Fig. 1 that all the respective diffraction peaks have almost the same position (Brag’s angle) in the diffraction patterns. This indicates that lattice parameters do not changes with Cr+3 doping. The measured density, X-ray density and porosity for all the sintered samples were calculated using the equations mentioned above and are listed in Table 1.The porosity of the sample was noted to decrease systematically due to the decrease of molecular weight of the samples with increase in Cr substitution. The calculated values of the specific surface area for each sample are tabulated in Table 1.The variation of specific surface

area (S) and the specific surface area to volume (S/V) as a function of average crystallite size are shown in Fig. 2. It is evident from the figure that both S and S/V decrease with the increase of the average crystallite size and the S/V ratio is of the order of 10 [28]. Scanning electron microscopy (SEM) of the ferrite samples was done and the SEM images obtained are depicted in Fig. 3 (a–d) at 50,000 magnification. These images reveal that the ferrite nanoparticles consist of spherical morphology. Soft agglomeration of the particles [21] in the images shows that the particles are magnetic in nature and are uniformly distributed over the whole mass. In some micrographs well defined boundary between the neighboring particles are also observed. The particle size distribution is not uniform but has a narrow range. Similar observation is reported by Marital [22] Energy dispersive X-ray (EDX) spectroscopy of Ni0.5Zn0.5Fe2xCrxO4 ferrite samples was also done. The EDX patterns confirmed

Fig. 6. Variation of AC conductivity (sAC) as a function of natural log of angular frequency (lnv) for Ni0.5Zn0.5Fe2-xCrxO4 nanoferrites at room temperature.

M. Ashtar et al. / Materials Research Bulletin 79 (2016) 14–21 Table 2 Dielectric constant (e0 ), dielectric loss tangent (tan (d)), AC conductivity (sAC), AC resistivity (rAC), product of dielectric constant and square root of AC p resistivity (e0 rAC) and slope (s1 and s2) of AC conductivity graphs for Ni0.5Zn0.5Fe2-xCrxO4 nanoferrites. Composition

x = 0.1

x = 0.2

x = 0.3

x = 0.4

e0 at 20 Hz e0 at 100 kHz e0 at 3 MHz tan (d) at 20 Hz tan (d) at 100 kHz tan (d) at 3 MHz sAC  (108 S/m) at 20 Hz sAC  (105 S/m) at 3 MHz rAC  (106 V m) at 20 Hz rAC  (104 V m) at 100 kHz rAC  (103 V m) at 3 MHz p e0 rAC  (104) at 20 Hz 0p e rAC  (102) at 100 kHz p e0 rAC  (102) at 3 MHz

163.42 9.51 5.30 5.46 0.38 0.17 99.26 15.22 1.01 4.54 6.57 16.42 20.26 1.36 0.61 0.34

158.85 7.70 5.66 4.69 0.34 0.08 82.83 8.29 1.21 6.91 12.06 17.47 20.24 1.96 0.47 0.33

96.78 7.56 6.31 3.28 0.25 0.05 31.22 5.40 3.20 9.68 18.52 17.31 23.52 2.72 0 .43 0.47

55.47 5.47 4.75 1.26 0.16 0.05 6.46 3.82 15.48 16.80 26.82 21.8 22.42 2.46 0.47 0.66

s1 (25–30 kHz) s2 (30kHz–3 MHz)

the presence of Fe, Cr, Ni, Zn and O. The calculated percentages had almost the same ratio of the above mentioned elements, when compared with the starting composition of the samples. 3.2. Dielectric properties 3.2.1. Dielectric constant The variation of dielectric constant as a function of frequency for Ni0.5Zn0.5Fe2-xCrxO4 samples at room temperature are depicted in Fig. 4. The dielectric constant decreased with increase in frequency, showing dielectric behavior as reported in the literature. The decrease in the dielectric constant is rapid at lower frequencies and became almost constant at higher frequencies. The frequency dependent variation in the dielectric constant can be explained on the basis of Maxwell-Wagner interfacial polarization [23] and Koop’s theory [24]. The dielectric constant of the samples decreased with the chromium increment, decreasing the grain size [25]. Further the cations which have strong preference for octahedral [B] sites lead to replace Fe+3 ions. The Cr+3 ions do not take part in conduction process [26], reducing the electron hopping, leads to decrease in dielectric constant. The coulomb field of the Cr+3 also localized the hopping electrons within its vicinity and hence reduced the hopping probability. As the conduction process in these materials is due to the hopping of electrons between Fe+2 and Fe+3, therefore the close values of the dielectric constant of the first three samples in frequency range of about 1–3 kHz can be explained as: the first (x = 0.1) sample has larger grain size therefore it shows greater value of dielectric constant because of the availability of more Fe+3 at octahedral sites. As the frequency of the field increases beyond 1 kHz the hopping electrons do not follow the field frequency and start decreasing. But due to the small grain size of the x = 0.2 sample more hopping electrons reach to the grain boundary. The x = 0.3 has small grain size as compared to the first two samples, therefore even at relatively high value of frequency, the hopping electron probability increases and hence the dielectric constant value is about the same as the first two samples. The sample for x = 0.4 has low grain size but due to the increase of Cr+3 content, greater coulomb force produced by Cr+3 and small amount of Fe+3 ions reduce the hopping probability, which reduced the dielectric constant to very small value. The values of the dielectric constants prepared by WOWS sol-gel technique are very low. Our results agree with Nasir et al. [13] and Munir et al. [27]. This may be due to the high resistivity and comparatively smaller crystallite

19

size of the synthesized samples. Low dielectric constant materials are very suitable for high frequency applications. This is due the fact that the low value of dielectric constant increases the penetration depth of the electromagnetic waves and decreases skin effects [13]. 3.2.2. Dielectric loss The variation of dielectric loss tangent as a function of frequency at room temperature for all the samples is shown in Fig. 5. The behavior of the curves can be understood from the Debye equation [26] for dielectric loss tangent i.e. tan d = e00 /e0 = 1/vt, where v is angular frequency and t is the relaxation time of electrons. It can be observed from Fig. 5 that the dielectric loss decreases as the frequency of the applied external field increases followed by the appearance of small resonance peaks. Many factors such as structural in homogeneity, non-stoichiometric, Fe+2 contents etc. influence the dielectric loss tangent, which itself depends on the preparation technique and composition of the samples [13]. The initial decrease of dielectric loss with increasing frequency can be explained on the basis of Koop’s model [24]. The resonance peaks appearing in the graph (Fig. 5) are due to more than one equilibrium position of the metallic ions in the structure, such as two adjacent octahedral sites in spinel structure. These sites have same potential energy and are separated by potential barrier, create a probability for the ions to jump from one equilibrium site to another. Under this probability, ions change their positions between the two equilibrium states with a certain critical frequency called natural frequency of jump ym between that states. When the alternating electric field frequency becomes equal to the natural frequency of jumping ions, the oscillating ions get maximum electrical energy and the power loss shoots up, causing resonance peaks [29,30]. The dielectric loss tangent decreases with increase in Cr+3 concentrations, pointing the fact that the larger grain causes greater tangent loss value. No resonance peak appeared for x = 0.1 sample while all other samples showed a shift towards lower frequencies with increase in Cr concentration. The peaks shifted due to the decrease in thermal mobility of the samples (observed in temperature dependent part and will be published). 3.2.3. AC conductivity The variation of AC electrical conductivity for all the samples as a function of frequency at room temperature is depicted in Fig. 6. All compositions show increase in AC conductivity (sAC) with increasing frequency and follow the dynamical law [31] s ðvÞ ¼ Avs , where A is a parameter with the units of conductivity and s is the slope of the linear plot of frequency dependent conductivity. The value of s lies between two extreme values 0 and 1 and may never exceed these limits [32]. Many low mobility crystalline and amorphous materials are found to hold the above equation [33]. The frequency dependent conduction processes can be explained on the basis of Maxwell–Wagner model [23]. The conductivity of the samples increases with increase in Cr+3concentration. The Cr+3ions are known to have strong preference for the octahedral [B] sites, due to which it replaced Fe+3ions when substituted. The reduction of Fe+3reduce the probability of the ion exchange between Fe+2 and Fe+3, resulting into decrease of conductivity. In addition, the Cr+3 ions also have its own coulomb field which localized the hopping electrons in its vicinity causing reduction in hopping probability of electrons. The AC conductivity in the frequency region 1 kHz to 3 kHz for the samples with x = 0.1, 0.2 and 0.3 overlap each other which can be explained on the basis of the grain boundary and the conductivity of the grains. According to the Maxwell- Wagner model [23] the

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well conducting grains are separated by a thin layer of poorly conducting grain boundary. As the grain size increases the formation of the oxygen rich layer on the grain boundary is possible [34].The larger size grains contribute more to the conduction process at high frequency due to the regular decrease in activation energy barrier (formed by grain boundaries) with frequency. In the present case, as the grain size of the sintered samples decreased for these samples (Table 1), the AC conductivity also decreased. But the activation energy barrier for x = 0.1 composition is relatively high possibly due to accumulation of oxygen layer, therefore the slope of the sample for x = 0.1 composition does not show strong variation with increase in frequency. The compositions with x = 0.2 and x = 0.3 have low activation energy barriers, the hopping electrons follow the field frequency and the slopes of the conductivity line increases respectively, causing overlapping of the conductivity lines. Sample for x = 0.4 also behaves in the similar manner but does not intersect other lines because the replacement of more Cr+3 ion causes greater coulomb field in its vicinity and reduce Fe+3 ions in octahedral sites (responsible for electron hopping) showing a smaller value of conductivity. 3.2.4. Relation between AC resistivity and dielectric constant p The calculated values of AC resistivity (rAC) and e0 rAC are given 0 in Table 2. It is clear from Table that e is approximately inversely p p proportional to rAC such that their product e0 rAC is nearly constant at some fixed frequency for all the samples. An analogous relation between these two parameters was observed by Koops [24] for Ni0.4 Zn0.6Fe2O4 ferrite sample, Ravinder [35] for the mixed Li-Cd system and Mohan et al. [36] in case of polycrystalline mixed Ni-Zn ferrite sample. It has been shown by Hudson [37] that the loss tangent in ferrites is reflected in the measurement of AC resistivity. The ferrite materials with high resistivity show low dielectric loss tangent and vice versa. Our observations are in good agreement with the previous reports for Ni-Zn-Cr system as tabulated in Table 2. 4. Conclusions The nanocrystalline Ni-Zn-Cr ferrite samples have been synthesized by WOWS sol-gel technique successfully. The structural, morphological and dielectric measurements are performed and results are explained in detail. It has been observed that the lattice constant of the samples have same values within the error bar. The crystallite size of the as prepared samples first increased and then decreased systematically, while the sintered samples showed a decreasing trend with Cr addition, causing increase in specific surface area and specific surface area to volume ratio. Scanning electron microscopy showed spherical morphology with soft agglomeration and narrow size distribution of the nanoparticles. The dielectric parameters as a function of frequency at room temperature were studied, which agreed well with the Maxwell-Wigner model. The results suggest that the AC electrical conductivity is directly proportional to the frequency. It shows an increase in conductivity with increase in frequency for all the samples. AC conductivity increases with frequency in all the samples. The Cr contents had significant influence on the dielectric properties, like dielectric constant, dielectric loss and AC conductivity. It is also observed that the dielectric constant is inversely proportional to the square root of AC resistivity at fixed frequency for all the samples. The dielectric constant and dielectric loss of our samples had minimum values when compared with the reported values in the literature for similar compositions. Low dielectric constant substances are very important for application in high frequency devices, to minimize dielectric losses and skin effect. The implementation of these materials is one of several

strategies used to allow continued decrease in the size of microelectronic devices. The conducting parts in digital electronic circuits can be separated from one another using insulating dielectrics. In scaling the circuits, the size of the circuit components have reduced and transistors have gotten closer together, the low dielectrics insulator have thinned to the point where charge build up, that produce undesirable effect and effect the device performance. Acknowledgements The financial support of Pakistan Academy of Sciences (PAS) is greatly acknowledged. Thanks to Hania Hasan for improving this manuscript. References [1] M.A. Gabal, R.M. El-Shishtawy, Y.M. Al-Angari, Structural and magnetic properties of nano-crystalline Ni–Zn ferrites synthesized using egg-white precursor, J. Magn. Magn. Mater. 324 (2012) 2258–2264. [2] M.S. Al-Qubaisi, A. Rasedee, M.H. Flaifel, S.H.J. Ahmad, S.H. Al-Ali, M.Z. Hussein, E.E.M. Eid, Z. Zainal, M. Saeed, M. Ilowefah, S. Fakurazi, N.M. Isa, M. El. Ezzat, Zowalaty, Cytotoxicity of nickel zinc ferrite nanoparticles on cancer cells of epithelial origin, Inter. J. Nanomed. 8 (2013) 2497–2508. [3] J.D. Adam, L.E. Davis, G.F. Dionne, E.F. Schloemann, S.N. Stitzer, Ferrite devices and materials, IEEE Trans. Microw. Theory Tech. 50 (2002) 721–737. [4] Ü. Özgür, Y. Alivov, H. Morkoç, Microwave ferrites: part 2: passive components and electrical tuning, J. Mater. Sci. Mater. Electron. 20 (2009) 911–952. [5] S.K. Pradhan, S. Bid, M. Gateshki, V. Petkov, Microstructure characterization and cation distribution of nanocrystalline magnesium ferrite prepared by Ball milling Mater, Chem. Phys. 93 (2005) 224–230. [6] M. Muzaffari, S. Manouchehri, M.H. Yousefi, J. Amighian, The effect of solution temperature on crystallite size and magnetic properties of Zn substituted Co ferrite nanoparticles, J. Magn. Magn. Mater. 322 (2010) 383–388. [7] L.W. Yeary, J.W. Moon, C. Rawn, L.J. Lane, A. Rondinone, J.R. Tompson, B.C. Chakoumakis, T.J. Phelps, Magnetic properties of bio-synthesized zinc ferrite nanoparticles, J. Magn. Magn. Mater. 323 (2011) 3043–3048. [8] J. Slama, A. Gruskova, M. Usakova, E. Usak, J. Subrt, J. Lukac, Substituted Ni-Zn ferrites for passive sensor applications, J. Electr. Eng. 57 (2006) 159–162. [9] A.C.F.M. Costa, V.J. Silva, D.R. Cornejo, M.R. Morelli, R.H.G.A. Kiminami, L. Gama, Magnetic and structural properties of NiFe2O4 ferrite nanopowder doped with Zn2+, J. Magn. Magn. Mater. 320 (2008) e370–e372. [10] G.S. Shahane, A. Kumar, M. Arora, R.P. Pant, K. Lal, Synthesis and characterization of Ni–Zn ferrite nanoparticles, J. Magn. Magn. Mater. 322 (2010) 1015. [11] A. Sukta, G. Mezinskis, A. Lusis, Electric and dielectric properties of nanostructured stoichiometric and excess-iron Ni–Zn ferrites, Phys. Scr. 87 (2013) 025601 (7pp). [12] I.H. Gul, W. Ahmed, A. Maqsood, Electrical and magnetic characterization of nanocrystalline Ni–Zn ferrite synthesis by co-precipitation route, J. Magn. Magn. Mater. 320 (2008) 270–275. [13] S. Nasir, M. Anis-ur-Rehman, Structural, electrical and magnetic studies of nickel–zinc nanoferrites prepared by simplified sol–gel and co-precipitation methods, J. Phys. Scr. 84 (2011) 025603. [14] K. Bhattacharjee, C.K. Ghosh, M.K. Mitra, G.C. Das, S. Mukherjee, K.K. Chsttopadhyay, Novel synthesis of NixZn1-x Fe2O4 (0 x 1) nanoparticles and their dielectric properties, J. Nanopart. Res. 13 (2011) 739–750. [15] W.M. Keely, X-ray diffraction technique for rapid surface area determination, Anal. Chem. 38 (1966) 147–148. [16] E.M.A. Jamal, D.S. Kumar, M.R. Anantharaman, On structural, optical and dielectric properties of zinc aluminate nanoparticles, Bull. Mater. Sci. 34 (2011) 251–259. [17] N. Rezlescu, C. Dorpftei, E. Rezlescu, P.D. Popa, Structure and humidity sensitive electrical properties of the Sn4+ and/or Mo6+ substituted Mg ferrite, Sens. Actuat. B 115 (2006) 589–595. [18] T. Ungar, Microstructural parameters from X-ray diffraction peak broadening, Mater. Scripta 51 (2004) 777–781. [19] M. Younas, M. Atif, M. Nadeem, M. Siddique, M. Idrees, R. Grossinger, Colossal resistivity with diminished tangent loss in Zn–Ni ferrite nanoparticles, J. Phys. D: Appl. Phys. 44 (2011) 345402 (8pp). [20] F.A. Humánez, O. Almanza, C.V. Hernández, Effect of sintering temperature on the structure and mean crystallite size of Zn1-xCoxO (x = 0.01  0.05) samples, J. Superf. Mater. 27 (2) (2014) 43–48. [21] M. Tan, Y. koseoglu, F. Alan, E. Senturk, Overlapping large polaron tunneling conductivity and giant dielectric constant in Ni0.5Zn0.5Fe1.5Cr0.5O4 nanoparticles (NPs), J. Alloys Compd. 509 (2011) 9399–9405. [22] J. Nan, D. Han, M. Cui, M. Yang, L. Pan, Recycling spent zinc manganese dioxide batteries through synthesizing Zn–Mn ferrite magnetic materials, J. Hazard. Mater. 33 (2006) 257–261. [23] K.W. Wagner, Dielectric relaxation in distributed dielectric layers, Ann. Phys. 40 (1913) 817–855.

M. Ashtar et al. / Materials Research Bulletin 79 (2016) 14–21 [24] C.G. Koop, On the dispersion of resistivity and dielectric constant of some semiconductors at audiofrequencies, Phys. Rev. 83 (1951) 121. [25] A.M. Sankpal, S.S. Suryavanshi, S.V. Kakatkar, G.G. Tengshe, R.S. Patil, N.D. Chaudhri, S.R. Sawant, Magnetization studies on aluminum and chromium substituted Ni–Zn ferrites, J. Magn. Mater. 186 (1998) 349–356. [26] A.M. El-Sayed, Electrical conductivity of nickel–zinc and Cr substituted nickel– zinc ferrites, J. Mater. Chem. Phys. 82 (2003) 583–587. [27] A. Munir, F. Ahmed, M. Saqib, M. Anis-ur-Rehman, Electrical properties of NiZn ferrite nanoparticles prepared by simplified sol-gel method, J. Supercond. Novel Magn. (2014) 983–987. [28] A.S. Fauzi, A.D. Sheikh, V.L. Mathe, Structural, dielectric properties and AC conductivity of Ni(1x)ZnxFe2O4 spinel ferrites, J. Alloys Compd. 502 (2010) 231–237. [29] V.R.K. Murthy, J. Sobhanadari, Dielectric properties of some nickel-zinc ferrites at Radio frequency, J. Phys. Status Solidi 36 (1976) K133–K135. [30] A.K. Singh, T.C. Goel, R.G. Mendiratta, O.P. Thakur, C. Prakash, Dielectric properties of Mn-substituted Ni-Zn ferrites, J. Appl. Phys. 91 (2002) 6626– 6629.

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[31] A. Mansingh, V.K. Dhawan, AC conductivity of V2O5-TeO2 glasses, J. Phys. C Solid State Phys. 16 (1983) 1675–1683. [32] S.S. Ata-Allah, M. Kaiser, Semiconductor-to-metallic transition in Cusubstituted Ni–Mn ferrite J, Phys. Status Solidi 201 (2004) 3157–3165. [33] N.F. Mott, E.A. Davis, Electronic Processes in Non Crystalline Materials, 2nd edn., Taylor & Francis, 1979. [34] S. Sindhu, M.R. Anantharaman, B.P. Thampi, K.A. Malini, P. Kurain, Evaluation of a.c. conductivity of rubber ferrite composites from dielectric measurements, Bull. Mater. Sci. 25 (2002) 599. [35] D. Ravinder, Effect of sintering temperature on electrical conductivity of mixed lithium–cadmium ferrites, Phys. Status Solidi A 129 (1992) 549. [36] G.R. Mohan, D. Ravinder, A.V. Ramana Reddy, B.S. Boyanov, Dielectric properties of polycrystalline mixed nickel–zinc ferrites, Mater. Lett. 40 (1999) 39–45. [37] S.A. Olofa, Oscillographic study of the dielectric polarization of Cu-doped NiZn ferrite, J. Magn. Magn. Mater. 131 (1994) 103–106.