Role of grain boundaries in the conduction of Eu–Ni substitutedY-type hexaferrites

Irshad Ali a,n, M.U. Islam a, Muhammad Naeem Ashiq b,n, M. Asif Iqbal c, Hasan M. Khan a,G. Murtaza d

a Department of Physics, Bahauddin Zakariya University, Multan 60800, Pakistanb Institute of Chemical Sciences, Bahauddin Zakariya University, Multan 60800, Pakistanc National University of Sciences and Technology, EME College, Islamabad, Pakistand Centre for Advanced Studies in Physics, Government College University, Lahore 54000, Pakistan

a r t i c l e i n f o

Article history:Received 20 June 2013Received in revised form5 March 2014Available online 15 March 2014

Keywords:HexaferritesAC conductivityGrain boundariesConduction mechanismActivation energy

a b s t r a c t

Single phase nanostructured (Eu–Ni) substituted Y-type hexaferrites with nominal composition of Sr2Co2�xNixEuyFe12�yO22 (x¼0.0–1, y¼0.0–0.1) were synthesized by the microemulsion method. Tem-perature dependent DC electrical conductivity and drift mobility were found in good agreement witheach other, reflecting semiconducting behavior. The presence of Debye peaks in imaginary electricmodulus curves confirmed the existence of relaxation phenomena in given frequency range. The ACconductivity follows power law, with exponent (n) value, ranges from 0.81–0.97, indicating that themechanism is due to polaron hopping. In the present ferrite system, Cole–Cole plots were used toseparate the grain and grain boundary effects. Eu–Ni substitution leads to a remarkable rise of grainboundary resistance as compared to the grain resistance. As both AC conductivity and Cole–Cole plotsare the functions of concentration, they reveal the dominant contribution of grain boundaries in theconduction mechanism. It was also observed that the AC activation energy is lower than the DCactivation energy. Appreciable improved values of quality factor suggested the possible use of thesesynthesized materials for power applications and high frequency multilayer chip inductors.

& 2014 Elsevier B.V. All rights reserved.

1. Introduction

Nanoscale polycrystalline ferrites with unique electrical andmagnetic properties have always been of great interest for scien-tists and engineers [1–3]. Owing to their vital properties, ferritesare very effective as microwave absorbers since they have bothlow cost and permittivity loss. The high electrical resistivity andlow eddy current losses make them excellent core materials to beused in electronic and telecommunication applications [4]. Due torecent developments in the field of telecommunication, theoperational frequency of electronic gadgets is rapidly moving togigahertz range. Polycrystalline hexaferrites having low AC elec-trical conductivity and thereby have low dielectric losses. Theexperimental measurements for electrical conduction, thermo-electric power, charge carriers concentration, charge carriersmobility and activation energy give much information on thebehavior of the free and localized charge carriers. This systematic

information leads to good explanation to understand the con-duction mechanism in the studied hexagonal ferrite system.The dielectric behavior of polycrystalline ferrites is very importantto understand the polarization mechanism. Soft hexagonal ferriteswith a planar magnetic structure have an extra high cutofffrequency in gigahertz that can be used to meet the needs forprospect applications [5–9]. Y-type hexagonal ferrite exhibitsexcellent magnetic properties in the hyper-frequency range andhas a promising future in novel monolith multi-layer chip induc-tive components [8,9]. The electrical conductivity and dielectricproperties greatly depend on the method of preparation, sinteringconditions and doping of cations [10]. Therefore, by tailoring thecomposition and the structure of the Y-type hexaferrite, it ispossible to improve the properties required for specific applica-tions. The change in method of preparation and chemical compo-sition alters the physical properties to be used in a particularapplication. Because of this possibility to tailor the properties offerrites, they have been synthesized by various methods, includingsolid state reaction [11], hydrothermal [12], sol–gel [13], sol–gelauto-combustion [14], co-precipitation [15], citrate precursortechnique [16], thermal plasma [17], pulsed wire discharge [18],etc. In the present work, microemulsion method has been used,

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Journal of Magnetism and Magnetic Materials

http://dx.doi.org/10.1016/j.jmmm.2014.03.0220304-8853/& 2014 Elsevier B.V. All rights reserved.

n Corresponding authors.E-mail addresses: irshadalibzu@gmail.com (I. Ali),

naeemashiqqau@yahoo.com (M. Naeem Ashiq).

Journal of Magnetism and Magnetic Materials 362 (2014) 115–121

being more suitable to obtain the nano size materials as comparedto others. The surfactant used form the micelle which eventuallyacts as a nanoreactor to control the size and shape of particles.Moreover, it is well known that the microemulsion methodsynthesis has been proved to be one of the most effective routesto realize the low temperature sintering of ferrites. The chief focusof present experimental investigation is to prepare nanostructuredsamples so as to investigate the role of grain boundaries in theconduction mechanism, which subsequently leads to high resis-tivity and high quality factor.

2. Experimental

2.1. Chemicals

The analytical reagents Fe(NO3)3 �9H2O (Riedel-de Haen, 97%),Co(NO3)2 �6H2O (Merck, 499%), NiCl2 �6H2O (Merck, 99%),Sr(NO3)2 (Merck, 99%), Eu2O3 (Merck, 99%), (cetyltrimethyl ammo-nium bromide) CTAB (Merck, 97%) as a surfactant, NH3 (FisherScientific, 35%) as a precipitating agent and methanol (Merck, 99%)as washing agent were used to synthesize Sr2Co2�xNix Euy

Fe12�yO22.

2.2. Synthesis procedure

The Y-type hexaferrite samples with nominal compositionSr2Co2�xNix EuyFe12�yO22 (x¼0.0–1, y¼0.0–0.1) were preparedby the normal microemulsion method. The metallic salt solution ofthe required molarities were prepared in deionized water andthen mixed in a beaker. The CTAB was also added in metalssolutions with ratio 1:15 (metals:CTAB). The solution was stirredon a magnetic hot plate by maintaining the temperature at 60 1Cuntil it formed a clear solution. The 2 M ammonia solution wasadded drop wise to form the precipitates. There after the pre-cipitates were washed with deionized water and finally withmethanol. The precipitates were then dried in an oven at 150 1C.Final annealing was carried out at 1050 1C for 8 h using a boxfurnace (Heyaius, D-6450 Hanau, Germany).

2.3. Characterization

The DC electrical conductivity was measured by a simple two-probe method in the temperature range 300–573 K. A Keithlysource meter model-197 was used for the said purpose. The DCconductivity (sDC) was calculated using the following relation [19]

sDC ¼dRA

ð1Þ

where “R” is resistance of the sample, “A” is area of the samplepellet and “d” is thickness of the sample pellet.

Drift mobility (md) was calculated using relation [19]

μd ¼sDC

neð2Þ

where “e” is charge of the electron, “sDC” is DC conductivity and“n” is the concentration of charge carrier which can be calculatedfrom the well-known equation: [19]

n¼ NAdbPFe=M ð3Þwhere NA is the Avogadro's number, db is the measured bulkdensity of sample, PFe is the number of iron atoms in the chemicalformula of ferrites and M is the molecular weight of the samples.

The AC conductivity was calculated from dielectric constantand dielectric loss tangent (tan δ) assAC ¼ 2πfεoε0 tan δ ð4Þ

where “sAC” is the AC conductivity, “f” is the frequency, “εo” is thepermittivity of free space and “ε0” is the real part of dielectricconstant. The real and imaginary parts of the electrical modulusM0

and M″ respectively can be calculated as follows [20]

M' ¼ ε'=fðε'Þ2þðε''Þ2g ð5Þ

M'' ¼ ε''=fðε'Þ2þðε''Þ2g ð6Þwhere ε' and ε'' is the real part of the dielectric constant andimaginary part of dielectric constant, respectively. Dielectric dataand impedance were measured by using Agilent impedanceanalyzer model E4991ARF. Impedance measurements were per-formed in the frequency range from 1 MHz to 3 GHz at roomtemperature by taking the absolute value of impedance |Z| withvarying complex angle θZ. The real and imaginary parts ofimpedance can be written as

Z' ¼ R¼ Zj j cos θZ ð7Þ

Z'' ¼ X ¼ Zj j sin θZ ð8Þwhere R and X are the real and imaginary parts of impedance,respectively.

3. Results and discussion

3.1. X-ray diffraction analysis

Fig. 1 depicts the XRD patterns of Eu–Ni substituted Co2Sr2-Fe12O22 ferrites scanned in the selected 2θ range of 34.6–36.61. Thecomplete XRD patterns of these samples have already beenreported in our earlier work. The (1 1 9) peak of Sr2Co(2x)NixEuy-

Fe(12�y)O22 hexaferrites initially shifts slightly towards the lowerangles however with increasing Eu content gradually moves to thehigh angle region. This behavior of shifting peak is attributed tothe change in average ionic radius of Eu (0.94 Å) with Fe (0.64 Å)coupled with the redistribution of cations on A and B sub-lattices.It is also observed that with the increasing substitution of Eu, theintensity of the peak is improved. Improved intensity of peakclearly suggests that the substitution in the nominal range iscompletely dissolved in hexagonal lattice.

3.2. DC electrical conductivity

The DC electrical conductivity for Eu–Ni substituted Co2Sr2-Fe12O22 ferrites was measured by two probe method in the

34.6 35.1 35.6 36.1 36.6

1 1

9

2 (degree)

Intin

sity

(a. u

.)

Fig. 1. XRD patterns in a 2θ range of 34.6–36.61 of the (1 1 9) peak for (Eu–Ni)substituted Co2Sr2Fe12O22 hexaferrites.

I. Ali et al. / Journal of Magnetism and Magnetic Materials 362 (2014) 115–121116

temperature range of 293–667 K. The increasing trend of conduc-tivity with temperature is illustrated in Fig. 2. This behaviorresembles the semiconducting nature of the synthesized samples.The increase in conductivity with increase in temperature may bedue to the increase in drift mobility of the charge carriers. Twodistinct regions are observed in DC conductivity curves. Similarbehavior was examined by many researchers [21,22]. First regionappears in ferrite at low temperatures. At higher temperatures, thehopping of charge carriers from one site to the other is initiatedthereby increasing the conductivity. Low activation energies offirst region suggest that the conduction in this region may be dueto the free charge carriers. The second region appears at hightemperature, where the conductivity is attributed to the polaronhopping with high activation energy. Comparison of the composi-tional dependence for DC conductivity and activation energyshows a good correspondence. The values are listed in Table 1, i.e. samples having high conductivity have low activation energyand vice versa [23,24]. The DC electrical conductivity tends todecrease from 8.1�10�7 to 3.3�10�10 (Ω cm)�1. This trend maybe explained on the basis of cation distribution at various sites inthe hexagonal structure. The Ni2þ ions occupy octahedral siteswhile Co2þ and Fe3þ occupy both the octahedral (B) as well astetrahedral (A) sites [25]. It has also been reported that the Eu3þ

ion occupies the octahedral site [26]. The conductivity in ferritesdepends upon the hopping of electrons between Fe2þ and Fe3þ atthe hexagonal sites. Because Eu3þoccupy the octahedral B-site,this subsequently decreases the number of Fe ions at that sitethereby reducing the hopping of electrons between ferrous andferric ions. This eventually decreases the conductivity.

It is a well known fact that Ni2þ and Co2þ are oxidized toNi3þ and Co3þ during heat treatment and create positive holes.The operative mechanism of conductivity is subjected to thehopping of both electrons and positive holes. The conversion ofCo2þ to Co3þ is more easier than that of Ni2þ to Ni3þ because thethird ionization energy of cobalt (3232 kJ mol�1) is less than that

of nickel (3395 kJ mol�1) [20]. Because of this very reason, lessholes are produced in Ni2þ rich samples as compared to that ofCo2þ [27]. The other reason for decrease of DC conductivity isattributed to the difference in resistivity of cobalt and nickel asresistivity of cobalt (6.2�10�8 Ω m) is smaller than that of nickel(6.93�10�8Ωm).

3.3. Drift mobility

Using the experimental data of electrical conductivity, themobility for the charge carriers was calculated for Sr2NixCo(2�x)

EuyFe(12�y)O22 ferrites. The drift mobility is related to the tem-perature by the following relation

μd ¼ μ0 expð�Eμ=kBTÞ ð9Þwhere m0 is pre-exponential constant, kB is Boltzmann constantand Em is the activation energy for mobility of ions. The values ofthe charge carrier mobility for different compositions are alsoincluded in Table 2. The data of electrical conductivity and driftmobility are in good agreement with each other. It can be seenfrom the table that the mobility is maximum for unsubstitutedferrite. The values of mobility are found to be very low whencompared with those of typical semiconductors. However, suchlow values are not new as far as ferrite semiconductors areconcerned; such low mobility values have already been reportedby several researchers [28–31]. The variation of mobility withtemperature is shown in Fig. 3. It can be seen from the figure thatthe charge carrier mobility values increase continuously with theincrease of temperature. The increase in mobility with increasingtemperature suggests that the conduction in these ferrites is dueto the hopping mechanism of electrons from Fe2þ to Fe3þ andholes transfer from Co3þ to Co2þ and Ni3þ to Ni2þ . Similarbehavior has also been reported by many researchers [28–31].

3.4. AC conductivity

The dependence of AC conductivity on frequency can be expressedby the following equation [32]

stotðωÞ ¼ sDCþAωn ð10Þwhere A is a pre-exponential factor with electrical conductivityunits, n is the frequency exponent with dimensionless value, and ωis the angular frequency. It is observed that AC conductivityincreases with increasing frequency of the applied field as shownin Fig. 4. Since the frequency increases it enhances the hoppingfrequency of the charge carriers between Fe2þ and Fe3þ whichsubsequently increases AC conductivity. This behavior of AC con-ductivity can be explained on the basis of Maxwell–Wagner modeland Koop's phenomenological theory according to which theferrites are imagined to act as a multilayer capacitor in which theferrite samples are characterized by a microstructure consisting ofconducting grains separated by highly resistive thin layers (grainboundaries). According to this model our results of AC conductivityat low frequencies describe the grain boundary behavior, while the

1.5

0.045

0.050

0.055

0.060

0.065

0.070

0.075

0.080

0.085

0.090

0.095

0.100 x= 0.00 y=0.0 x=0.02 y= 0.2 x=0.04 y=0.4 x=0.06 y=0.6 x=0.08 y=0.8 x=0.10 y=1.0

Log

Dc(

-cm

)-1

1000/(T)K3.53.02.52.0

Fig. 2. Change in DC electrical conductivity with temperature for (Eu–Ni)substituted Co2Sr2Fe12O22 hexaferrites.

Table 1Compresses the DC activation energy, exponential n and AC activation energy of (Eu–Ni) substituted hexaferrites, Sr2Co(2�x)NixEuyFe(12�y)O22, (x¼0.00–1.00; y¼0.00–0.10).

Compositional formula sDC (Ω cm)�1 EDC1 (eV) EDC2 (eV) (EDC1þEDC2)/2 n EAC

Sr2Co2Fe12O22 8.1�10�7 0.352 0.51 0.431 0.818 0.078Sr2Co1.8Ni0.2Eu0.02Fe11.98O22 9.7�10�8 0.360 0.526 0.443 0.924 0.033Sr2Co1.6Ni0.4Eu0.04Fe11.96O22 2.5�10�8 0.368 0.564 0.466 0.989 0.004Sr2Co1.4Ni0.6Eu0.06Fe11.94O22 6.9�10�9 0.370 0.568 0.469 0.958 0.019Sr2Co1.2Ni0.8Eu0.08Fe11.92O22 2.1�10�9 0.372 0.576 0.474 0.974 0.012Sr2Co1Ni1Eu0.1Fe11.90O22 3.3�10�10 0.376 0.594 0.484 0.976 0.011

I. Ali et al. / Journal of Magnetism and Magnetic Materials 362 (2014) 115–121 117

dispersion at high frequency may be attributed to the conductivityof grains [33]. The AC conductivity decreases with increasing (Eu–Ni) contents and is listed in Table 2.

Fig. 5 illustrates a typical log–log demonstration of the fre-quency dependence of electrical conductivity of Eu–Ni substitutedin Sr2Co2Fe12O22 ferrites. It is clear from the figure that ACconductivity shows very minute variations at low frequencies;however at relatively high frequencies AC conductivity follows thepower law by following the empirical expression s0(ω)pωn,where n is a fractional exponent (0rnr1), associated with thedynamic of hopping ions [34]; thus, the value of n progressivelyincreases with increasing interactions among mobile ions and vice

versa at very low values i.e. n¼0, completely independent offrequency. In the present experimental findings, the values ofexponents vary between 0.81–0.97 and are listed in Table 1. Highvalues of n (0rnr1) suggest that the conduction phenomena inthe studied samples follow hopping mechanism [35].

For vibration of ions, in their hexagonal conduction sitescoupled with hopping to immediate sites through barriers ofenergy EAC, following equation has been applied [35]:

τ0ðTÞ ¼ τ1 exp ðEAC=kTÞ ð11Þ

where τ1 is the reciprocal of the attempted frequency of ions andτ0 is the relaxation time for independent ion-hopping. Usually theenergy barrier (EAC activation energy) will be lesser than that ofactivation energy for the DC conductivity and is given by therelation [35]

EDC ¼ EDC=ð1�nÞ ð12Þ

Enhanced interactions between the mobile ions result in highervalues of “n”. Furthermore higher values of “n” are a measure ofhigher degree of cooperativity in the ion-hopping process [36,37].

In fact, by using the experimental values, obtained for EDC (DCactivation energy) and n, the activation energy EAC for the barrierthat ions must overcome to hopp (independently) betweenneighboring vacant hexagonal sites in the Eu–Ni substitutedSr2Co2Fe12O22 ferrites can thus be calculated by Eq. (12). A valueEAC is found which is dependent of (Eu–Ni) concentration and islisted in Table 1. Higher degree of structural disorder is produceddue to high rare earth-contents [35] which is accredited to thedifference in size of dopant and host ions at various hexagonalconduction sites. Enhanced ion–ion interactions are expected andsubsequently higher values of the exponents n. The higher value ofn increases the energy penalty that these correlations impose onlong-range or DC ionic conductivity. This elucidates the increasingdifference found between EDC and EAC (larger value of n).

Table 2Real and imaginary parts of electric modulus and impedance at frequency of 1 MHz, drift mobility and AC conductivity (at 1 MHz) of Eu–Ni substituted hexaferrites,Sr2Co(2�x)NixEuyFe(12�y)O22, (x¼0.00–1.00; y¼0.00–0.10).

Compositional formula M0 Mʺ Z0 Zʺ md (cm2 v�1 s�1) rAC (Ω cm)�1

Sr2Co2Fe12O22 0.061 0.0089 32,762 577 3.3�10�12 1.3�10�4

Sr2Co1.8Ni0.2Eu0.02Fe11.98O22 0.067 0.0092 36,143 771 4.1�10�13 1.1�10�4

Sr2Co1.6Ni0.4Eu0.04Fe11.96O22 0.073 0.0095 44,669 971 1.1�10�13 9.6�10�5

Sr2Co1.4Ni0.6Eu0.06Fe11.94O22 0.077 0.0087 62,660 819 2.9�10�14 8.0�10�5

Sr2Co1.2Ni0.8Eu0.08Fe11.92O22 0.082 0.0080 89,319 886 9.1�10�15 6.5�10�5

Sr2Co1Ni1.0Eu0.1Fe11.90O22 0.087 0.0074 96,183 1141 2.9�10�15 5.4�10�5

250

0.0

2.0x10-11

4.0x10-11

6.0x10-11

8.0x10-11

1.0x10-10

1.2x10-10

1.4x10-10

1.6x10-10

1.8x10-10

x= 0.00 y=0.0 x=0.02 y= 0.2 x=0.04 y=0.4 x=0.06 y=0.6 x=0.08 y=0.8 x=0.10 y=1.0

Mob

ility

(cm

2 v-1s-1

)

T(K)700650600550500450400350300

Fig. 3. Change in drift mobility with temperature for (Eu–Ni) substituted Co2Sr2-Fe12O22 hexaferrites.

0.0 5.0x108 1.0x109 1.5x109 2.0x109 2.5x109 3.0x109

0.00

0.05

0.10

0.15

0.20

0.25

0.30

x= 0.0 y=0.0 x=0.2 y= 0.02 x=0.4 y=0.04 x=0.6 y=0.06 x=0.8 y=0.08 x=1.0 y=0.1A

C(

-cm

)-1

Frequency (Hz)

Fig. 4. Variation in AC conductivity with frequency of (Eu–Ni) substituted Co2Sr2-Fe12O22 hexaferrites at room temperature.

6.5-4.5

-4.0

-3.5

-3.0

-2.5

-2.0

-1.5

-1.0

x= 0.0 y=0.0 x=0.2 y= 0.02 x=0.4 y=0.04 x=0.6 y=0.06 x=0.8 y=0.08 x=1.0 y=0.1

Log

AC(

-cm

)-1Log( )

10.510.09.59.08.58.07.57.0

Fig. 5. Variation in log sAC with log ω of (Eu–Ni) substituted Co2Sr2Fe12O22

hexaferrites.

I. Ali et al. / Journal of Magnetism and Magnetic Materials 362 (2014) 115–121118

3.5. Impedance analysis

3.5.1. ImpedanceThe complex impedance may be written in terms of real and

imaginary components of impedance and follows the relation

Zj j ¼ Z'þ jZ'' ð13ÞThe values of both resistive (Z0) and reactive (Z″) components ofimpedance are listed in Table 2. It has been found that the valuesof impedance and its components increase with (Eu–Ni) substitu-tion which is very much consistent with compositional depen-dence of AC conductivity, i.e. increase in impedance results indecrease in AC conductivity. The values of AC conductivity arelisted in Table 2. Fig. 6 and inset show the variation of theimpedance (Z) with frequency. It is found that the magnitude ofZ decreases with the increase of frequency, indicating increase inAC conductivity. It also indicates the semiconducting type beha-vior in these samples.

3.5.2. Cole–Cole plotsThe appearance of a broad Debye peak in these plots shows

the existence of relaxation process in all the studied samples.To separate the grain and grain boundary contributions, compleximpedance plane plots (Cole–Cole plots) have been investigated.The total electrical conductivity is governed by the grain and grainboundary contributions for ferrites. The impedance measurementsdata gives both resistive (real) and reactive (imaginary) compo-nents for a material. It can be demonstrated in terms of any of thefour complex variables i.e. permittivity(εn), admittance (Yn), impe-dance (Zn), electric modulus (Mn) and dielectric loss (tan δ) in acomplex plane plot (Nyquist plot). Their relation to one another isas follows [38,39]

tan δ¼ ε''=ε' ¼ Y ''=Y ' ¼ Z''=Z' ¼M''=M' ð14Þwhere Y ' and Y '' are the real and imaginary parts of admittance.The nanocrystalline samples are characterized by small grain sizeand large number of grain boundaries. The electrical modulus (M)was used, in order to study the frequency dependences of theinterfacial polarization effect, which generates electric chargeaccumulation around the ceramic particles by displacing relaxa-tion peaks and can be written in term of both resistive (real) andreactive (imaginary) components as given below

M¼ 1=εn ¼ 1=ðε'� jε''Þ ¼M'� jM'' ð15Þ

Fig. 7(a and b) shows the variation of both real and imaginaryparts of electric modulus against frequency. The appearance of losspeaks in imaginary parts of electric modulus against frequency forthe present investigated samples show the relaxation processoccurred with the change in frequency in the polycrystalline(Eu–Ni) substituted Sr2Co2Fe12O22 hexaferrites. The peak isobserved when the jumping frequency of charge carriers approxi-mately becomes equal to external applied AC field [39].The Maxwell–Wagner model provides to assess the behavior ofcomplex conductivity in heterogeneous systems having two ormore phases [20,40]. In a heterogeneous system, if the region ofgrain boundary occupies a large volume, the graph of the modulus(Mn¼1/εn) M″ versus M0 provides better information about thesemicircles. It suggests that there is a probable relationshipbetween the behavior of grain boundary and the appearance ofthe peaks of M″ as a function of frequency. In the second case, ifthe region of continuity of the grain boundary occupies a smallvolume, the spectrum of impedance (Z″ versus Z0) provides bettervisualization of the semi circles in the plane. Since there is aprobable relationship between the behavior of grain boundary andthe appearance of the peaks in Z″ as a function of frequency,former case is in great agreement with our present experimentalresults.

0.0 5.0x108 1.0x109 1.5x109 2.0x109 2.5x109 3.0x109

0

20000

40000

60000

80000

100000

120000Im

pide

nce

()

Ferequency (Hz)

x= 0.0 y=0.0 x=0.2 y=0.02 x=0.4 y=0.04 x=0.6 y=0.06 x=0.8 y=0.08 x=1.0 y=0.1

Fig. 6. Variation in impedance with frequency of (Eu–Ni) substituted Co2Sr2Fe12O22

hexaferrites at room temperature.

14

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.20

0.22

0.24

x= 0.0 y=0.0 x=0.2 y=0.02 x=0.4 y=0.04 x=0.6 y=0.06 x=0.8 y=0.08 x=1.0 y=0.1

M

lnf (Hz)

14

0.00

0.01

0.02

0.03

0.04

0.05

0.06

x= 0.0 y=0.0 x=0.2 y=0.02 x=0.4 y=0.04 x=0.6 y=0.06 x=0.8 y=0.08 x=1.0 y=0.1

lnf(Hz)

22201816

22201816

Fig.7. (a) Variation in real part of electric modulus (M0) with frequency of (Eu–Ni)substituted Co2Sr2Fe12O22 hexaferrites at room temperature and (b) variation inimaginary part of electric modulus (M″) with frequency of (Eu–Ni) substitutedCo2Sr2Fe12O22 hexaferrites at room temperature.

I. Ali et al. / Journal of Magnetism and Magnetic Materials 362 (2014) 115–121 119

The complex impedance (Cole–Cole) plots using M0 and M″ astwo variants are plotted in Fig. 8. A semi-circle was obtained for allthe samples except at x¼0.02 and y¼0.02. The incomplete arcobtained in case of x, y¼0.02 is mainly due to large grain boundaryresistance and some additional relaxation phenomena that mighthave occurred beyond the measured frequency range [41,42]. Itwas observed from these plots that the major contribution in theconduction is due to grain boundary density. At lower frequencyi.e. left side of the semicircle is a result of grain resistance [43],while the intermediate frequencies represent grain boundarycontribution [44]. Taking into account the higher frequency region,extreme right side represents the whole resistance of both grainand grain boundaries [43]. Substitution of (Eu–Ni) suppresses thegrain growth and hence the grain boundaries density increaseswhich in turn leads to a remarkable rise of grain boundaryresistance. Therefore, the conduction mechanism, observed incomplex impedance measurement, is well in agreement with theAC conductivity phenomenon, mentioned earlier. However, criti-cally speaking, as compared to the grain contribution, only grainboundary contribution is clearly observed from Cole–Cole plots.Moreover the resistance of the grain boundary increases withincreasing Eu contents. Increasing substitution level of Eu at theexpanse of Fe, restrained the electron transfer between Fe2þ and

Fe3þ thereby enhancing grain boundary resistance and subse-quently conductivity was impeded. The high resistance of thegrain boundary contributes in the resistivity and dielectricproperties.

0.080

0.010

0.012

0.014x=0.0, y= 0.0

M

M

M

M

M0.11

0.009

0.010

0.011

0.012

x=0.4, y= 0.04

M

MM

M

M0.150

0.0075

0.0080

0.0085

0.0090 x=0.6, y= 0.06

0.190

0.0100

0.0105

0.0110

0.0115

0.0120x=0.8, y= 0.08

0.240.22

0.030

0.035

0.040

0.045

0.050

0.055

0.060 x=1.0, y= 0.10

0.088 0.130.12

0.1650.1600.155 0.2150.2100.2050.2000.195

Fig. 8. Cole–Cole plots of (Eu–Ni) substituted Co2Sr2Fe12O22 hexaferrites at room temperature.

0.0 5.0x108 1.0x109 1.5x109 2.0x109 2.5x109 3.0x109

0

1000

2000

3000

4000

5000

6000

7000

8000

x= 0.0 y=0.0 x=0.2 y= 0.02 x=0.4 y=0.04 x=0.6 y=0.06 x=0.8 y=0.08 x=1.0 y=0.1

Q fa

ctor

Frequency Hz

Fig. 9. Variation of Q values with frequency of (Eu–Ni) substituted Co2Sr2Fe12O22

hexaferrites.

I. Ali et al. / Journal of Magnetism and Magnetic Materials 362 (2014) 115–121120

3.6. Q values

Fig. 9 shows the variation in Q values with change in frequencyfor Eu–Ni substituted Co2Sr2Fe12O22 ferrites. It is observed that thevalues of quality factor (Q) are maximum at higher frequencyregions. Thus high Q values and a resonance frequency above2 GHz clearly suggests that these materials can be used in highfrequency multilayer chip inductors [45].

4. Conclusions

� The observed mechanism of conductivity is mainly due to thehopping of holes and electrons.

� DC electrical conductivity decreased due to the cations dis-tribution at various sites in the hexagonal structure.

� AC conductivity at low frequency region describes the grainboundary behavior, while the dispersion at high frequency canbe attributed to the conductivity of grains.

� The complex impedance plane plots showed a single semicir-cle, indicating the capacitive and resistive properties of thematerials, due to contribution of grains and grain boundaries inthe samples.

� Appreciable improved values of quality factor suggested thepossible use of these synthesized material for power applica-tions and high frequency multilayer chip inductors.

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