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Journal of Magnetism and Magnetic Materials 322 (2010) 1720–1726

Contents lists available at ScienceDirect

Journal of Magnetism and Magnetic Materials

0304-88

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/jmmm

Physical, electrical and dielectric properties of Ca-substituted strontiumhexaferrite (SrFe12O19) nanoparticles synthesized by co-precipitation method

Muhammad Javed Iqbal a,n, Muhammad Naeem Ashiq b, Iftikhar Hussain Gul c

a Surface and Solid State Chemistry Laboratory, Department of Chemistry, Quaid-i-Azam University Islamabad-45320, Pakistanb Department of Chemistry, Bahauddin Zakariya University, Multanc School of Chemical and Materials Engineering, National University of Science and Technology (NUST), Islamabad

a r t i c l e i n f o

Article history:

Received 24 July 2009

Received in revised form

18 November 2009Available online 23 December 2009

Keywords:

Dielectric properties

FTIR

TEM

Electrical properties

Calcium doped material

53/$ - see front matter & 2009 Elsevier B.V. A

016/j.jmmm.2009.12.013

esponding author. Tel.: +92 519 064 2143.

ail address: mjiqauchem@yahoo.com (M. Jave

a b s t r a c t

Calcium substituted strontium hexaferrite CaxSr1�xFe12O19 (x=0.0�0.6) nanoparticles are synthesized

by chemical co-precipitation method. The synthesized samples are characterized by Fourier Transform

Infrared (FTIR), X-ray diffraction (XRD), Scanning Electron Microscopy, Transmission Electron

Microscopy, DC electrical resistivity and dielectric measurements. FTIR data of uncalcined sample

shows that nitrate ions are present which disappeared on calcination at 920 1C. The XRD data shows

that a single hexagonal magnetoplumbite phase is formed in samples in which the calcium content, x, is

r0.20. However, a nonmagnetic phase (a-Fe2O3) in addition to the hexagonal phase is also present in

samples with x40.20. The average crystallite size is found between 17 and 29 nm. The DC electrical

resistivity increases with increase of calcium content up to level of x=0.2 but decreased on further

addition of calcium. The enhanced resistivity of the calcium doped material has potential applications

in microwave devices. The variations of dielectric constant and dielectric loss angle are explained on the

basis of Maxwell–Wagner and Koops models.

& 2009 Elsevier B.V. All rights reserved.

1. Introduction

Development of morphology-controlled synthesis methodolo-gies is of great interest in Materials Science [1–3]. Recently,synthesis of one-dimensional (1D) nanostructure materials is oneof the most exciting areas in materials science due to their uniquephysical properties and their potential applications in nanoscaledevices and has received considerable attention during the passeveral years [4–9]. The chemical route is considered an excellenttechnique for the synthesis of highly pure multi-componentoxides. Potential advantages of the wet chemical route over theconventional solid state reaction method include better homo-geneity, better compositional control and lower processingtemperatures [8]. Hexaferrites are still much used in permanentmagnets market because of their low price combined withreasonable magnetic performances. Ferrites and garnets areferromagnetic oxides with good electrical and dielectric proper-ties that are desirable for radiofrequency and microwaveapplications.

The high electrical resistivity of ferrites coupled with lowmagnetic losses is critical in maintaining low insertion loss inmicrowave devices. The trend of higher frequency electronic

ll rights reserved.

d Iqbal).

products promotes the application frequency of various compo-nents to GHz. It is a great challenge for chip-inductive component,one of the three most common components. Conventional softmagnetic materials, such as Ni, Cu, Zn, Mn and Mg spinel ferrite,cannot meet this need, because their crystal structure limits thecut-off frequency. Hexagonal ferrites have a cut-off frequency atGHz, about an order of magnitude higher than that of spinelferrites [10]. Hexagonal ferrites exhibit excellent magneticproperties in hyper frequency could meet the need of softmagnetic materials for chip components [11]. Strontium hex-aferrite has great importance due to its numerous technologicalapplications in fields such as recording devices, telecommunica-tion, magneto-optical, microwave devices and permanent mag-nets [12–14]. Study of magnetic properties of Sr-hexaferrite havebeen attempted in the past by several workers by replacing itsFe3 + and Sr2 + ions with Cr3 +, Al3 + , La3 +, Sm3 + etc. and variousbivalent–tetravalent cation combinations such as Ti–Co, Ti–Mn,Ti–Zn, Ir–Zn, Zr–Zn, Zr–Mn and Zr–Ni [15–20]. Most of theresearchers worked on Ba, Sr-hexaferrites and Sr-doped bariumhexaferrites. Ca also belongs to the same group of the periodictable and same electronic configuration as that of Sr and Ba.Moreover, no systematic studies available on Ca-doped hexafer-rites. So we have chosen the Ca as dopant and the other aim wasto study the effect of calcium on the electrical properties of thesynthesized material. Studies concerning DC electrical resistivity(r) and dielectric properties give valuable information about the

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conduction mechanism and their applications in the microwavedevices.

The synthesis of nanosized calcium-doped strontium hexafer-rite and the effect of calcium substitution on its structure, DCelectrical resistivity, dielectric constant (e0) and dielectric loss(tan d) are the focus of the present study.

2. Experimental

The chemicals used in the synthesis of samples were Fe(-NO3)3 �9H2O (Panreac Quimica SA, 98%), Sr(NO3)2 (FlukaZ99%),Ca(NO3)2 �4H2O (Merck, 98%) and NaOH (FlukaZ97%). Calciumsubstituted strontium hexaferrite (Sr1�xCaxFe12O19, wherex=0.0�0.6) samples were prepared by the co-precipitation method[21]. Aqueous solutions of iron nitrate and strontium nitrate in themolar ratio (Fe/Sr=11) with the required stoichiometric amount ofcalcium nitrate solution were mixed and heated slowly up to 70 1Cwith constant stirring. Sodium hydroxide (2M) was added to themixture until the solution pH reached a value of 11–12. The pH of thesolution was monitored by a pH meter. An intermediate precipitatesuspension appeared in the solution which was stirred for 3 h. Theprecipitates were washed with distilled water, dried at 100 1C in anoven for 5 h and calcined at five different temperatures of 100, 600,700, 800 and 920 1C for 1 h in a temperature programmed tubefurnace (Carbolite CTF 12/100) at a heating rate of 5 1C/min. Thecalcination temperature of 920 1C was optimized to obtain the singlehexagonal phase.

Fourier transform infra-red (FTIR) spectrophotometer (BioRadMerlin FTS 3000MX) was used to establish structural informationon the formation of calcium substituted strontium hexaferrite.The crystalline structural analysis was performed by an X-Raydiffractometer (JEOL JDX-60PX) using CuKa radiation source. Thecrystallite sizes were calculated by Scherrer formula [22].

D¼Kl

bCosyð1Þ

where l is the X-ray wavelength and is equal to 1.542 A, b thehalf-peak width, y the Bragg angle and K is the constant.

The values of X-ray density rX-ray, bulk density rm, porosity P

and the unit cell volumes Vcell were calculated by followingequations [23]:

1

d2¼

4

3

h2þhkþk2

a2

� �þ

l2

c2Vcell ¼ 0:8666a2c ð2Þ

rX-ray ¼2M

NAVcellð3Þ

rm ¼m

pr2hð4Þ

P¼ 1�rm

rX�ray

ð5Þ

where a and c are lattice constants, M is the molar mass, m themass of pellets, r radius of the pellets, NA Avogadro’s number andVcell the unit cell volume.

The energy dispersive X-ray fluorescence (ED-XRF) analysiswas carried out by Horiba MESA-500. The scanning electrommicscopy (SEM) was performed by field emission gun scanningelectron microscopy (Hitachi, FE-SEM S-800). The transmissionelectrom microscopy (TEM) was performed by JEOL JEM-1010).

DC electrical resistivity was measured by a two-point probemethod described previously [21]. The samples were used in theform of pellets of 13 mm diameter and of 3 mm thickness. Thepellets were prepared at room temperature by compressing at 8tons. The DC electrical resistivity of all the samples decreased

with increasing temperature in accordance with Arrheniusequation [21]

r¼ r0 expDE

kBT

� �ð6Þ

where kB is the Boltzmann constant, T is temperature and DE isthe activation energy, which is the energy needed to release anelectron from the ion for a jump to neighbouring ion, giving rise tothe electrical conductivity.

The dielectric constant (e0) measurements were carried out inthe frequency range from 600 Hz to 1 MHz at room temperatureusing LCR meter bridge (WK LCR 4275). The dielectric constantwas determined from the following formula

e0 ¼ Cd

eoAð7Þ

where C is the capacitance of the pellet in farad, d the thickness ofthe pellet in meter, A the cross-sectional area of the flat surface ofthe pellet and eo the constant of permittivity of free space.

The dielectric tangent loss factor was calculated using therelation

tand¼1

2pf RpCpð8Þ

where d is the loss angle, f is the frequency, Rp is the equivalentparallel resistance and Cp is the equivalent parallel capacitance.

The dielectric loss (e00) was measured in terms of tangent lossfactor (tan d) defined by the relation [24]

e00 ¼ e0 tand ð9Þ

where e0 and tan d are defined above.

3. Results and discussion

The FTIR spectra of the calcined and uncalcined powderedsamples were recorded in 400–4000 cm�1 range are compared(shown in supplementary materials). The spectrum of uncalcinedsample has dominant peaks at 3430 and 1383 cm�1, representingO–H and N–O stretching vibrations of water and nitrate ions,respectively, indicating that NO3

� are present in the sample. Theuncalcined sample has also an absorption band at 617 cm�1 dueto the iron–oxygen bonds characteristics of g-Fe2O3[25]. Theabsorption band at 1383 cm�1 which is present in the uncalcinedsample disappeared on calcination, indicating the removal ofnitrate ions as a result of calcination at 600, 700 and 920 1C. In thesample calcined at 600 1C, three new peaks appears representingvibrations of Fe–O–Fe (at 1472 cm�1), the stretching mode of Fe–O–Fe group (at 1089 cm�1) and bending vibrations of Fe–O–H (at859 cm�1) [26]. The peak corresponding to the stretching mode ofFe–O–Fe is still present in the sample calcined at 700 oC whereasthe absorption bands at 1047 and 859 cm�1, corresponding to Fe–O–Fe stretching mode and bending vibrations of Fe–O–H,respectively, are found to disappear at 920 1C. The absorptionbands at 590, 554 and 436 cm�1 appeared for the sampleannealed at 920 1C which are identified as the metal oxygenstretching vibrations of hexaferrite [25].

The XRD patterns for the samples calcined at differenttemperatures (shown in supplementary materials) show thatthe uncalcined sample is amorphous and shows no crystallinity.At calcination temperature of 600 1C (and above) an additional a-Fe2O3 phase also appears in addition to the hexagonal phase. Asthe calcinating temperature was increased from 600 to 920 1C, theconcentration of the a-Fe2O3 phase decreased gradually and apure magnetopulmbite phase having no a-Fe2O3 phase wasobtained at 920 1C. Therefore, all doped samples were calcinedat the temperature of 920 1C.

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X-ray diffraction patterns of Sr1�xCaxFe12O19 (wherex=0.0�0.6) samples calcined for 1 h at 920 1C are shown inFig. 1. The samples containing calcium contents xr0.0–0.2 showa single magnetoplumbite phase. But the nonmagnetic a-Fe2O3

phase also appeares when the calcium content increases abovex=0.2. The intensity of the main peaks i.e. whose hkl values are(1 1 4) and (1 0 7), decreases by increasing the calcium contentabove x=0.2 indicating that the nonmagnetic phase increases atthe expense of magnetoplumbite phase as shown in Fig. 1. The ‘a’and ‘c’ lattice parameters as a function of calcium content, x, havebeen calculated from the XRD data using Eq. (2)[27]. It is seen thatthe value of ‘a’ remains almost constant but the value ‘c’ decreaseswith increase in the calcium content as shown in Table 1. This isdue to a slightly smaller ionic radius of Ca2 + (0.99 A) replacingSr2 + (1.13 A). In addition, the decrease in c-axis withconcentration also confirms that Ca2 + ions actually enter thehexaferrite [27].

Variation of the unit cell volume Vcell, porosity P, X-ray densityrX-ray and bulk density rm as a function of Ca concentration aretabulated in Table 1. X-ray density decreases with the addition ofcalcium because the molar mass of calcium is smaller ascompared to strontium. Values of bulk density and the porosity

Fig. 1. XRD patterns of Sr1�xCaxFe12O19 for different calcium contents (a)=0.0,

(b)=0.2, (c)=0.4, (d)=0.6, the peaks having the hkl values are for the strontium

hexaferrites. n corresponds to a-Fe2O3 phase.

Table 1Lattice constants (a and c), unit cell volumes (Vcell), X-ray densities (dX-ray), bulk densitie

Calcium content a (A) c (A) Vcell (A3) rX-ray (g

0.00 5.89 23.219 699.01 5.05

0.20 5.89 23.203 698.05 5.01

0.40 5.89 23.199 696.75 4.97

0.60 5.89 23.194 697.31 4.92

of various samples remain almost constant. The average crystal-lite sizes found in different samples as calculated by Eq. (1) arefound in the range of 17-29 nm (Table 1). The crystallites of thissize are noted to be much smaller as compared to 83 nm reportedfor La–Zn substituted hexaferrites synthesized by the sol–gelmethod [28].

The energy dispersive x-ray fluorescence (ED-XRF) analysis isperformed in order to optimize the Fe/Sr ratio in the sampleSrFe12O19. The theoretical ratio of Fe/Sr=12.0 is obtained whenthe Fe/Sr ratio is kept 11.0 as shown in Table 2 and therefore, thesame ratio i.e. Fe/Sr=11 is kept to synthesize all other samples.This is due to the higher solubility of Fe than that of Sr and it isnecessary to keep the concentration of strontium slightly higher.A single hexagonal phase is found when Fe/Sr=11 whereas thesamples, in which the Fe/Sr molar ratio is above 11, an additionalnonmagnetic a-Fe2O3 phase is also appeared along withhexagonal phase. It is also observed that as the Fe/Sr molar ratiois decreased from 12 to 11, the percentage of the M-type phase isincreased while that of a-Fe2O3 decreased.

The results of the EDX analysis for calcium substituted samplesclearly demonstrates that by increasing calcium concentration ofthe sample the stoichiometric calcium content increases whilestrontium content decreases indicating that Ca2 +ions havereplaced Sr2 + in the hexaferrite structure as shown in Table 3.However, the stoichiometric calcium content in Sr0.4Ca0.6Fe12O19,is determined as 0.491 mol whereas the calcium content that wasadded in the synthesis mixture was 0.60 mol. This again is due tothe lower solubility of calcium as compared to iron and probablythis is the reason that a nonmagnetic phase appeared as thecontents of calcium increased.

The SEM and TEM micrographs of calcium substitutedstrontium hexaferrite samples containing calcium content of x =0.0 and 0.2 are shown in Figs. 2 and 3, respectively. It is clear fromthe figures that the particles have well-defined shape andboundaries. In view of the fact that the crystallite sizescalculated by the Scherrer formula are not considered to be veryaccurate therefore we have recorded the SEM and TEM images ofsome of the synthesized samples. The values of particle sizescalculated by TEM were found to be 30–60 and 15–25 nm and bySEM found to be 40–90 and 30–50 nm, respectively for thesamples with Ca content of x=0.0 and 0.2. The particle sizes forthe samples synthesized in the present study are much smaller ascompared to those of the reported earlier 83, 151–200, 70–100

s (rm), porosity, activation energy and crystallite sizes of Sr1�xCaxFe12O19 samples.

cm�3) rm (g cm�3) Porosity (%) Crystallite size (nm)

2.36 0.53 17.92

2.46 0.50 22.18

2.45 0.51 26.19

2.39 0.51 19.93

Table 2ED-XRF of the samples having different Fe/Sr ratio and the observed phases.

Fe/Sr ratio added Fe/Sr ratio observed

by ED-XRF

Phases observed

12.00 12.80 M-type, a-Fe2O3

11.80 12.74 M-type, a-Fe2O3

11.60 12.51 M-type, a-Fe2O3

11.40 12.36 M-type, a-Fe2O3

11.20 12.17 M-type, a-Fe2O3

11.00 11.96 M-type

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6

cm) 1

08

20

25

30

1 S-1

) 10-1

3

ρμd

M. Javed Iqbal et al. / Journal of Magnetism and Magnetic Materials 322 (2010) 1720–1726 1723

and 60–300 nm for the M-type hexaferrite [28–30]. These particlesizes are small enough to obtain the suitable signal-to-noise ratioin the high density recoding media. The synthesized samples canbe therefore used for application in recoding media.

The DC electrical resistivity was measured using two pointprobe method [21]. The electrical resistivity increases by increas-ing the calcium content up to xr0.2 and then decreases as shownin Fig. 4. The drift mobility decreases up to calcium contentx=0.0–0.2 then starts to increase at x40.2 (Fig. 4). The electricalconductivity in ferrite is mainly due to hopping of electronsbetween ions of the same element present in more than oneoxidation state, distributed randomly over crystallographicallyequivalent lattice sites. Hexaferrite structurally form hexagonalclose packed oxygen lattice with the cation at the octahedral,trigonal bipyramidal and the tetrahedral sites. The distance

Table 3EDX for the calcium substituted strontium hexaferrite nanoparticles.

S. no. Sample formula Fe (mol) Sr (mol) Ca (mol)

01 SrFe12O19 12.001 0.996 0.000

02 Sr0.8Ca0.2Fe12O19 12.007 0.804 0.197

03 Sr0.6Ca0.4Fe12O19 12.058 0.629 0.370

04 Sr0.4Ca0.6Fe12O19 12.118 0.576 0.491

Fig. 2. Scanning electron micrographs (SEM) o

Fig. 3. Transmission electron micrographs (TEM)

between two ferric ions at octahedral sites is less than thedistance between two metals ions at octahedral and tetrahedralsite, therefore the hopping between tetrahedral and octahedralhas very small probability compared with that for octahedral–octahedral hopping. The hopping between tetrahedral–tetrahedral sites does not exist, due to that there are only Fe3 +

ions at tetrahedral sites and any Fe2 + ions formed during sinteringprocess preferentially occupy octahedral sites only [28]. The

f (A=SrFe12O19) and (B=Ca0.2Sr0.8Fe12O19).

of (A=SrFe12O19) and (B=Ca0.2Sr0.8Fe12O19).

0

2

0Calcium content (x)

ρ (o

hm

0

5

10

15

μ d (c

m2 V

-

0.2 0.4 0.6

Fig. 4. Variation of electrical resistivity and drift mobility as a function of calcium

content.

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conduction mechanism in hexaferrites can be explained on thebasis of hopping of electrons between Fe3 + and Fe2 + at octahedralsites and the number and mobility of holes by following reactions.

Fe3þþe�-Fe2þ

ð10Þ

O2�þ2e�-O ð11Þ

By accommodating calcium, the crystal structure becomesmore compact due to smaller ionic radius of the doped cation andthe concentration and mobility of holes which are generatedaccording to Eq. (1) and (11) would be decreased causing increasein the DC electrical resistivity as a result. As the concentration ofCa increases above xo0.2 then it does not replace the Sr in theproper quantity as we have substituted as confirmed by the EDXanalysis as shown in Table 2 and this is the reason that anadditional phase appeared in the samples. The decrease in DCelectrical resistivity at x40.2 may be due to the presence ofnonmagnetic (a-Fe2O3) phase. The observed decrease in the DC

6

0

1x104

2x104

3x104

4x104

5x104

6x104

Die

lect

ric c

onst

ant (

ε/ )

7 8 9

Fig. 6. Dielectric constant (e0) as a function of ln f (Hz) fo

13

14

15

16

17

18

1.4

1000/T (K-1)

ln

0.00.20.40.6

1.5 1.6 1.7 1.8 1.9

Fig. 5. Variation of DC electrical resistivity (lnr) with inverse temperature (1000/

T) for Sr1�xCaxFe12O19 hexaferrites.

electrical resistivity on raising the temperature as shown in Fig. 5is a typical behavior of semiconductors.

The variation of dielectric constant with frequency is shown inFig. 6. It is seen that the dielectric constant decreases rapidly withincreasing frequency and reaches a constant value beyond certainfrequency. According to Rabinkin and Novikova [31], thepolarization in ferrites is through a mechanism similar to theconduction process. The polarization decreases with increase infrequency and then reaches a constant value due to the fact thatbeyond a certain frequency of external field, the electronexchange between Fe2 + and Fe3 + cannot follow the alternatingfield. The large value of ‘e0’ at lower frequency is due to thepredominance of species like Fe2 + ions, interfacial dislocationspile ups, oxygen vacancies, grain boundary defects, etc. [32],However the decrease in ‘e

0

’ with frequency is natural because ofthe fact that any species contributing to polarizability lag behindthe applied field at higher and higher frequencies. Substitution ofCa content up to xr0.2 causes decrease in the number and themobility of holes and hence results in a decrease in the values ofboth DC electrical resistivity and the dielectric constant up to thisconcentration. According to the Maxwell–Wagner model [33], thedielectric materials with heterogeneous structure can beimagined to contain well conducting grains separated by highresistive thin layers (grain boundaries). In this case, the appliedvoltage on the sample drops mainly across the grain boundariesand a space charge polarization is built up at the grain boundaries.The space charge polarization is governed by the available freecharges on the grain boundary and the conductivity of the sample.Koops [34] proposed that the effect of grain boundaries ispredominant at lower frequencies i.e. higher dielectric constantfor thinner grain boundaries. High dielectric constants decreasethe penetration depth of the electromagnetic waves by increasingthe skin effect. Hence, the much lower dielectric constantsobtained for the ferrites warrant their application at highfrequencies.

Dielectric tangent loss factor is an important part of the totalcore loss in ferrites. Hence for low core loss, low dielectric lossesare desirable. Dielectric loss factor tan d represents the energy ofdissipation in the dielectric system. The variation of tan d as a

ln f

x=0.0x=0.2x=0.4x=0.6

10 11 12 13 14

r Sr1�xCaxFe12O19 hexaferrites at room temperature.

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6

0.0

5.0x104

1.0x105

1.5x105

2.0x105

Die

lect

ric lo

ss (ε

// )

ln f

x=0.2x=0.4x=0.6x=0.6

7 8 9 10 11 12 13 14

Fig. 8. Dielectric loss (e00) as a function of ln f (Hz) for Sr1�xCaxFe12O19 hexaferrites at room temperature.

6

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

Die

lect

ric lo

ss fa

ctor

(tan

δ)

ln f

x=0.0x=0.2x=0.4x=0.4

7 8 9 10 11 12 13 14

Fig. 7. Dielectric loss factor (tan d) as a function of ln f (Hz) for Sr1�xCaxFe12O19 hexaferrites at room temperature.

M. Javed Iqbal et al. / Journal of Magnetism and Magnetic Materials 322 (2010) 1720–1726 1725

function of frequency at room temperature is shown in Fig. 7. It isseen that for all the samples, it decreases continuously withincreasing frequency. Thus the dielectric loss factor of the hexa-ferrite nanoparticles is expected to decrease approximatelyinversely to the frequency. In the low frequency region whichcorresponds to high resistivity (due to grain boundaries), moreenergy is required for electron exchange between Fe3 + and Fe2 +

ions, so that the energy loss is high. In the high-frequencyrange, which corresponds to low resistivity (due to grain), a smallenergy is needed for electron transfer between Fe3 + and Fe2 + inthe grains and hence the energy loss is small. According to Eq. (9)tan d is proportional to the imaginary part of dielectric constant.

The dielectric loss was determined using Eq. (9). Dielectric lossis an important part of the total core loss in ferrites [35]. Hence forlow core loss, low dielectric losses are desirable. The dielectricloss as a function of frequency for all the compositions is depicted

in Fig. 8. The dielectric loss profiles are similar to those of the realpart of dielectric constant. The increase in hopping electronsresults in a local displacement in the direction of the extentelectric field, causes an increase in electric polarization and thusenhances dielectric loss. Hudson [36] has shown that, thedielectric losses in ferrite is generally reflected in theconductivity measurements where the materials of highlyconductivity exhibiting high losses and vice versa.

4. Conclusion

The calcium-substituted strontium hexaferrite was synthesized bythe co-precipitation method. XRD analysis showed that the materialhas a single magnetoplumbite phase containing calcium content inthe range 0.0–0.2. An increase in the stoichiometric amount of

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calcium results in a nonmagnetic a-Fe2O3 phase as an impurity alongwith the magnetoplumbite phase. The Fe/Sr ratio was optimized byEDX analysis and was found to be 11. The crystallites size was foundto 15–50 and 17–29 nm calculated by TEM and Scherrer equation,respectively. The particle size is small enough to obtain a suitablesignal to noise ratio in the magnetic recording media. The DCelectrical resistivity increases up to content of x=0.2 and thendecreases. The drift mobility increases with the decrease in resistivityand vice versa. The dielectric constant and dielectric loss factordecrease with frequency and are in good agreement with the DCelectrical resistivity data. The increase in DC electrical resistivitysuggests that the synthesized materials can be used for applications inmicrowave devices.

Acknowledgement

Research support under the indigenous scholarship program byHigher Education Commission (HEC), Pakistan is gratefully acknowl-edged. The author (M. N. Ashiq) is also thankful to BahauddinZakariya University (BZU), Multan for financial support for a part ofthis.

Appendix A. Supplementary material

Supplementary data associated with this article can be foundin the online version at doi:10.1016/j.jmmm.2009.12.013.

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