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Journal of Magnetism and Magnetic Materials 316 (2007) 73–80

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Review

Influence of sintering time on structural, magnetic and electricalproperties of Si–Ca added Sr-hexa ferrites

Shahid Hussain, A. Maqsood�

Thermal Physics Laboratory, Department of Physics, Quaid-i-Azam University, Islamabad 45320, Pakistan

Received 23 November 2006; received in revised form 20 March 2007

Available online 11 April 2007

Abstract

Sr-hexa ferrites with the addition of Si (0.5wt%) and Ca (0.5wt%) have been prepared by solid-state reaction method with sintering

time variation ranging from 2 to 10 h. The structural characterization of the samples confirmed the major phase of Sr-hexa ferrite.

Average grain size was found within the range of 1–4mm. Vicker hardness increased from 512 to 1187Hv. The coercivity and remanence

had the ranges from 596 to 4255Oe and 324 to 516G, respectively. The DC electrical resistivity measurements were carried out by two-

probe method as a function of temperature from 303 to 723K. The room temperature DC resistivity increased from 1.67� 106 to

2.89� 108O cm in turn the activation energy also increased from 0.314 to 0.495 eV. The DC electrical resistivity decreased while drift

mobility increased with the rise in temperature, ensuring the semi-conducting behavior. Dielectric properties were studied as a function of

frequency in the range of 80Hz to 1MHz at room temperature.

r 2007 Elsevier B.V. All rights reserved.

Keywords: Coercivity; Dielectric constant; Electrical resistivity; Ferrite; Remanence; X-ray diffraction

1. Introduction

Hard ferrites of Ba or Sr-hexa ferrites are the majorpermanent magnetic materials [1]. The M-type hexa-ferritesMFe12O19 (M ¼ Ba, Sr or Pb) are important ferrimagneticoxides. Their magnetic properties make them potentialmaterials for use as permanent magnets, recording media,microwave and high-frequency devices [2] because of theirhigh intrinsic coercivity, fairly large crystal anisotropy andlow cost. Besides, these are very stable and have very highelectrical resistivity [3]. The mentioned family has tradi-tionally served as permanent magnets in applications fordielectric media. For many applications, permanentmagnets are the best choice because it provides a constantfield without continuous expenditure of electric power andwithout generation of heat [4]. As far as the bulk material isconcerned, Sr-ferrite is found to have better hard magneticproperties than Ba-ferrite [5]. If one requires pores free and

- see front matter r 2007 Elsevier B.V. All rights reserved.

/j.jmmm.2007.03.206

onding author. Tel.: +9251 2601014; fax: +92 51 90642240.

ddresses: shahidhussain77_qau@yahoo.com (S. Hussain),

a.net (A. Maqsood).

well-oriented very small grains for a permanent magnet ofhexa-ferrite, then small amount of additives are known tohave a profound influence on these ferrites. Studies alsoindicated that high-frequency magnetic and dielectricproperties of hexa-ferrites could be improved by dopingwith certain oxides such as SiO2 and Bi2O3 [6–8]. Cochardt[9] noticed that presence of low-melting secondary phaseis beneficial in the preparation of sintered compactBaFe12O19 and SrFe12O19 materials with improved mag-netic properties. Toker [10] using additives in the systemPbO–B2O3 and PbO–SiO2, concluded that liquid phasesintering was important in developing the microstructurerequired for high magnetic quality of PbFe12O19 sinteredmagnets. SiO2 was introduced as grain growth inhibitor inhexa-ferrites [11]. The mechanism of the action of SiO2 inferrite was studied by Haberey [12] and Kools [13]. Thesintering behavior and mechanism of grain boundarymotion hindrance was studied in detail by Kools [14,15]and new mechanism of reaction induced grain growthimpediment (RIGGI) has been purposed. Moreover, thereare some indications that the size of the additives mightalso play a crucial role in determining their effectiveness for

ARTICLE IN PRESSS. Hussain, A. Maqsood / Journal of Magnetism and Magnetic Materials 316 (2007) 73–8074

microstructure control. Positive effects of fine-grainedsilica [16] and the uniform incorporation of Ca and Siutilizing a sol–gel route [17] were reported. For highremanence materials, it is essential to create a dense,anisotropic microstructure by firing at high temperature.On the other hand, grain growth is determined with respectto the coercivity because of the formation of multidomainparticles. A large coercivity requires small grains with a sizesmaller than the critical size for single domain particles ofabout 1 mm [18]. To produce magnet with large remaneceand coercivity, it is essential to precisely tailor the ceramicprocess which in turn controls the complex interplaybetween a dense microstructure and grain growth. Ferritepowder particles should have dimensions below 1 mm toallow some unavailable grain growth during sintering andlimit the grain size in the sintered magnets to few microns.Taguchi [19] and Schwarzer et al. [20] reported on sub-micron powders prepared by improved calcinations andmilling procedures, which allow the preparation of high-performance magnets with large remanence (Br ¼ 0.44T).

This paper reports the effect of sintering time onmicrostructural, control of the magnetic and electricalproperties of Sr-hexa ferrites (SrFe12O19) by simultaneousaddition of Si and Ca.

2. Experimental procedure

2.1. Sample preparation

All the samples of Sr-hexa ferrites (SrFe12O19) wereprepared by solid-state reaction method. High-purityreagent grade raw materials namely SrCO3, Fe2O3, SiO2

and CaCO3 were used for preparing the samples. Powderswith specific composition corresponding to SrFe12O19 wereground in an electric grinder (RM-100) for 2 h. Themixtures were calcined in a muffle furnace at 90075 1Cfor 12 h, fixing the heating rate 3 1C/min followed by thefurnace cooling. Some of the calcined composition (with-out Si–Ca) of namely SC-0 was palletized with the help ofpoly vinyl alcohol (PVA) binder by exerting the uni-axialpressure of 60 kN for 1–2min and was sintered at120075 1C for 2 h. Remaining calcined composition ofSrFe12O19 was added with 0.5wt% SiO2, CaCO3 andremixed for 5 h. The samples of names SC-1, SC-2, SC-3,SC-4 and SC-5 (with Si–Ca) were palletized and sintered at120075 1C for 2, 4, 6, 8 and 10 h, respectively, with heatingrate of 3 1C/min followed by the furnace cooling with therate of 60 1C/min.

2.2. Characterization

X-ray diffraction of the samples was done by RIGAKUGEIGER FLEX D-MAX III C diffractometer using CuKa (l ¼ 1.5406 A) radiation, which confirmed the forma-tion of hexagonal structure with a few peaks of Hematite(a-Fe2O3) and Ca-silicate as a second phase in a minutequantity. The measured density, dm and X-ray density, dx

were determined by using the formula

dm ¼m

pr2h, (1)

where ‘m’ is the mass, ‘r’ is the radius and ‘h’ is the heightof sample and

dx ¼2M

NAV, (2)

‘M’ being the molecular weight of the sample; ‘2’ ismultiplied due to the fact that elementary cell contains twomolecules [18], ‘NA’ is Avogadro’s number and ‘V’ isvolume of the unit cell of hexagonal system and is given as

V ¼ 0:866a2c, (3)

where ‘a’ and ‘c’ are the lattice constants. The porosity ‘P’of all the samples was then determined by employing therelation

P ¼ 1�dm

dx. (4)

In order to find out the hardness tests, WilsonWOLPERT Micro Vicker 401 MVA was employed. Thephysical and mechanical properties are strongly influencedby their microstructure, so their studies are essential tounderstand the relationship between the processing para-meters as well as the behavior of the materials when used inpractical application. In the present study, a JEOL 2000CXscanning electron microscope (SEM) was employed toexamine the microstructural features such as grain size andporosity. The line intercept method [21] is used to measurethe grain size. For the purpose of more accuracy, at least 10grains were measured at different sections of each sample.To study the magnetic properties of the samples, avibrating sample magnetometer was used at room tem-perature with maximum applied field of 10 kOe to find outcoercivity Hc, and remanence Br. As ferrites have very highelectrical resistivity, so two probes method was used tostudy DC electrical resistivity of the Sr-ferrite samples inthe temperature range 300–723K. For the temperaturevariation of resistivity, the sample holder containing thesample was kept in a furnace that maintained at a desiredtemperature within 72K error with the help of tempera-ture controller. The temperature was also measured withPt-100 thermometer. Disc shape samples of diameter8–9mm and thickness 3–5mm were used for the measure-ments. The dielectric properties of the samples weremeasured using LCR meter (WAYNE KERR 4275) withinthe frequencies ranging from 80Hz to 1MHz at roomtemperature.

3. Results and discussion

3.1. Structure

The structure of these ferrites may be considered toconsist of alternating spinel (S ¼ Fe6O8

2+) and hexagonal(R ¼MFe6O11

2�) blocks. The O2� ions form close-packed

ARTICLE IN PRESSS. Hussain, A. Maqsood / Journal of Magnetism and Magnetic Materials 316 (2007) 73–80 75

layers with M2+ substituting for an O2� in the hexagonalblock. The 12 Fe3+ ions are distributed in the interstitialspaces of the close-packed layers. Three of Fe3+ sites areoctahederally coordinated (12k, 4f2, and 2a); one istetrahedreally coordinated (4f1), one is pentacoordinatedwithin a trigonal bipyramid (2b). Fig. 1 shows the indexedXRD pattern of all the studied samples. All the peakswere indexed using ICDD cards [22–24]. Lattice para-meters were calculated by indexing the XRD patterns andresults showed the formation of major phase of Sr-hexaferrites, with hematite (a-Fe2O3) and Ca-silicate as secondphases and similar observation was reported in W-typeferrites [25].

X-ray density decreases from 5.14 g/cm3 (SC-0) to5.10 g/cm3 (SC-1) and then remains almost constant asshown in Fig. 2 and Table 1. The measured density alsoincreases from 4.52 to 4.80 g/cm3 (SC-0, SC-5) [17].

From Table 1, it is clear that porosity has decreasingtrend from 12 to 5.88% for the samples SC-0 to SC-5,respectively. This decrease in porosity is attributed to theaddition of Si and Ca primarily and by increasing thesintering time secondarily [25,26]. The effects of Si and Caadditives on the microstructure of the sintered SrO � 6Fe2O3

samples were investigated as shown in Fig. 3(a) shows theSEM micrographs of the sample without additives (SC-0)having abnormal grain growth with enlarged aciculargrains. Compared to the microstructural characteristicsof the sample without additives, the added sample(SC-1) remarkably improved the characteristics as shownin Fig. 3(b), where it shows microstructure containing0.5wt% of Si and 0.5wt% Ca sintered at same temperature(1200 1C) and time (2 h) like sample SC-0 (pure Sr-ferrite).The incorporation of additives showed apparent suppres-sion of abnormal grain growth, and a uniform distributionof grain size with less porosity was obtained by adoptingthis addition method. An average grain size of the samplewas estimated ranging from 1 to 4 mm [17,27]. So the role ofSiO2 and CaCO3 is forming the Ca-silicate secondary phaseat grain boundaries and grain junctions.

Vicker’s hardness increases from 512 to 1187Hv ofsamples SC-0 to SC-5, respectively, as shown in Fig. 4. Thisincrease in hardness could be attributed to the decrease inporosity due to the addition of silica and calcium or it maybe due to prolong sintering effect.

3.2. Magnetic properties

The magnetic properties of hexa-ferrites depend on itschemistry and microstructure. The magnetic structuregiven by the Gorter model is the ferrimagnetic with fivedifferent sub-lattices, three parallel (12k, 2a, and 2b) andtwo antiparallel (4f1 and 4f2), which are coupled by superexchange interactions through the O2� ions. In order thatthe magnets have a high remanence (Br), the powdermaterial should be sintered to a high density; whereas ahigh coercive force is obtained by keeping the grain sizesmall. In practice, good permanent magnetic properties are

obtained by sintering with an excess of MeO (Me ¼ Sr, Ba,Pb) which promotes sintering [28].Fig. 5 and also Table 1 represent the magnetic properties

of pure and Si–Ca added, sintered for various timeSrFe12O19 samples. There is sharp increase in coercivityHc from sample SC-0 to SC-1 sintered at same time (2 h)and temperature (1200 1C). The increase in coercivity iscertainly associated with silica addition due to control ofmicrostructure by suppressing abnormal grain growth[15,29]. This can be attributed mainly to uniformlydistributed fine grains, responsible for the creation of alarge number of impediments to domain wall motion[17,25]. The decrease in coercivity from the samples SC-2 toSC-5 is due to longer annealing time, which can beinterpreted on the basis of microstructure changes. Grainsize increases with increasing annealing time, one can see inFig. 3. In those large crystallites, the multidomain structureis possible to occur and the magnetization by the domainwall displacement is easier than by spin rotation [30].Similar behavior was observed with remanence Br, asreported in literature [17,26].

3.3. Electrical properties

In general, the electrical properties of the ferritematerials depend upon chemical composition, methods ofpreparation, sintering temperature and grain size. Therelationship between resistivity and temperature may beexpressed as [18]

r ¼ r0 e

DE

kTB . (5)

Here ‘r’ is the resistivity at temperature ‘T’, ‘r0’ is the pre-exponential constant which equals the resistivity atinfinitely high temperature; ‘kB’ is the Boltzman’s constantand ‘DE’ is the activation energy. The Arhenius plot ofeach sample is shown in Fig. 6(a). This plot shows decreasein resistivity with the rise in temperature, ensuring thesemiconductor behavior of ferrites [31–33]. The measuredDC electrical resistivity of the studied ferrites was found tovary from 1.67� 106O cm (SC-0) to 2.89� 108O cm (SC-1)at 303K as given in Table 1. The increase in resistivity maybe due to the fact that Si (r ¼ 0.1O cm at 273K) is moreresistive than that of Fe (9.71� 10�6O cm at 293K) and Sr(23� 10�6O cm at 300K) [34]. A similar behavior wasobserved in SiO2 added Ni–Zn ferrites [35] and silicasubstituted Cu-ferrites [32]. Further decrease in resistivityfrom 1.5� 108O cm (SC-2) to 2.21� 105O cm (SC-5) maybe explained on the basis of microstructure. As sinteringtime increases the crystal growth is enhanced, due to theincreased grain–grain contact area for the electrons to flowand therefore the electrical resistivity decreases [36,37].Conduction in ferrites may be explained by Verwey’shopping mechanism [38]. According to Verwey, theelectronic conduction in ferrites is mainly due to hoppingof electrons between ions of same element present in more

ARTICLE IN PRESS

Fig. 1. X-ray diffraction patterns of all the samples: ‘�’ corresponds to peaks of hematite and ‘]’ corresponds to Ca-silicate.

S. Hussain, A. Maqsood / Journal of Magnetism and Magnetic Materials 316 (2007) 73–8076

than one valance state, distributed randomly over crystal-lographically equivalent lattice sites. The M-type ferritecrystallizes in a hexagonal structure with 64 ions per unitcell on 11 different symmetry sites. The 24 Fe3+ atomsare distributed over five distinct sites. Three octahedral(B) sites (12k, 2a, and 4f2), one tetrahedral (A) site (4f1) and

one a new type of interstitial (C) site (2b) than the spinelstructure and is surrounded by five oxygen ions constitut-ing a trigonal bipyramid. In the hexagonal structure, twotetrahedral sites are adjacent to each other and for thesetwo only one metal ion is available. This metal ion nowoccupies position halfway between them, amidst the three

ARTICLE IN PRESSS. Hussain, A. Maqsood / Journal of Magnetism and Magnetic Materials 316 (2007) 73–80 77

oxygen ions. The distance between two metal ions at(B) site is smaller than the distance between a metal ion at(B) site and another metal ion at (A) site. The electronhopping between (A) and (B) sites under normal conditionstherefore has a very small probability compared with thatfor (B)–(B) hopping. Hopping between (A) and (A) sitesdoes not exist for the simple reason that there areonly Fe3+ ions at (A) site and any Fe2+ ions formedduring processing preferentially occupy (B) sites only. Thehopping probability depends upon the separation betweenthe ions involved and the activation energy [39].

The activation energy of each sample in the measuredtemperature range can be determined from the slope of thelinear plots shown in Fig. 6(a) of DC electrical resistivity.The values of activation energies range from 0.294 to0.495 eV. It is noticed that the activation energy obtainedin the present work for these ferrites is greater thanapproximately 0.3 eV, which according to Klinger [40]

SC-0 SC-1 SC-2 SC-3 SC-4 SC-5

4.5

4.6

4.7

4.8

4.9

5.0

5.1

5.2

Den

sit

y (

g/c

m3)

Studied samples

Po

rosit

y

6

8

10

12dx

dm

P

Fig. 2. X-ray density, measured density and porosity of all samples.

Table 1

X-ray density (dx), measured density (dm), porosity (P), average grain size

correlation coefficient (R), drift mobility (md), dielectric constant (e0) and loss

Parameters SC-0 SC-1 SC

dx (g/cm3)70.01 5.14 5.10 5.

dm (g/cm3)70.01 4.52 4.69 4.

P (%) 12.00 8.04 7.

S (mm)70.001 2.273 1.384 —

Hv at 25 gm710 512 702 98

Br (G) 516.1 529.4 50

Hc (Oe) 596.5 4255 33

r at 303K (O cm) 1.67� 106 2.89� 108 1.

r at 723K (O cm) 1.51� 103 3.16� 103 2.

DE (eV)70.001 0.314 0.495 0.

R 0.997 0.999 0.

md (cm2/(V s)) at 723K 1.29� 10�7 5.97� 10�8 6.

e0 at 1 kHz 4.80 82.49 15

e0at 100kHz 1.477 26.74 48

e0 at 1MHz 1.41 20.02 32

tan d at 1 kHz 0.952 0.355 0.

tan d at 100 kHz 0.058 0.220 0.

tan d at 1MHz 0.004 0.352 0.

suggests that the conduction is due to polaron hopping.The hopping depends upon the activation energy, which isassociated with the electrical energy barrier experienced bythe electrons during hopping. In addition to the above saidconsiderations, the activation energy is influenced by thegrain size. Bigger grain size increased grain–grain contactarea for the electron to flow and therefore, a lower barrierheight [37]. The drift mobility (md) of all the samples hasbeen calculated using the well-known relation [41]

md ¼1

ner, (6)

where ‘e’ is charge on electron, ‘r’ is defined above, ‘n’ isthe concentration of charge carriers, which can becalculated from the following relation:

n ¼NAdmPFe

M, (7)

where ‘M’ is molecular weight, ‘NA’ is Avogadro’s number,‘dm’ is defined already, and ‘PFe’ is the number of ironatoms in the chemical formula of the Sr-ferrites.Table 1 shows that by the addition of Si–Ca in the pure

Sr-ferrite (SC-1), drift mobility decreased due to increase inresistivity, but further increase in drift mobility of the restof samples (SC-2 to SC-5) is due to change in micro-structure by increasing the annealing time, that can also beobserved in Fig. 6(c).

3.4. Dielectric properties

The dielectric constant is the property of the dielectrics,which determines the electrostatic energy stored per unitvolume for unit potential gradient. It describes the material’scapacity to store charge when it is used as a capacitordielectric. In other words, dielectric constant is the ratio of

(S), hardness (Hv), DC electrical resistivity (r), activation energy (DE),

factor (tan d) of Si–Ca added SrFe12O19 ferrites

-2 SC-3 SC-4 SC-5

12 5.11 5.13 5.15

70 4.71 4.75 4.80

84 7.65 6.86 5.88

1.875 — 3.785

1 1018 1141 1187

3.8 450.7 361.8 324.1

88 2117 2396 958.4

50� 108 2.42� 107 2.89� 105 2.21� 105

9 4� 103 1.74� 103 5.09� 102 4.15� 102

486 0.449 0.313 0.294

999 0.997 0.998 0.996

40� 10�8 1.08� 10�7 3.66� 10�7 4.44� 10�7

8.54 271.22 297.69 462.63

.43 56.27 65.71 111.94

.51 37.29 40.23 60.93

343 0.474 0.421 0.403

292 0.310 0.350 0.453

555 0.556 0.663 0.931

ARTICLE IN PRESS

Fig. 3. SEM micrographs of selected samples: (a) SC-0, (b) SC-1, (c) SC-3 and (d) SC-5.

300

350

400

450

500

550

Br

500

1000

1500

2000

2500

3000

3500

4000

4500

Re

ma

ne

nc

e B

r (G

)

Co

erc

ivit

y H

c (

Oe)

Hc

All Samples

SC-0 SC-1 SC-2 SC-3 SC-4 SC-5

Fig. 5. Coercivity and remanence of all samples.

500

600

700

800

900

1000

1100

1200 Hv

Ha

rdn

es

s (

Vic

ke

r)

SC-0 SC-1

All Samples

SC-2 SC-3 SC-4 SC-5

Fig. 4. Vicker hardness of studied samples.

S. Hussain, A. Maqsood / Journal of Magnetism and Magnetic Materials 316 (2007) 73–8078

the charge that would be stored with free space as storedwith the material in question. Dielectric constant (e0) inferrites is also contributed by several structural andmicrostructural factors and was calculated using the relation

�0 ¼Ct

�0A, (8)

where ‘C’ is the capacitance of the pellet in farad, ‘t’ is thethickness of the pellet in meter, ‘A’ the cross-sectional areaof the flat surface of pellet and ‘e0’ is the permittivityconstant of free space.Fig. 7(a) shows the dielectric constants versus frequency

plots of studied samples. It is seen that all the samples showthe frequency-dependent phenomenon, i.e. the dielectricconstant decreases with increasing frequency and thenreaches a constant value. The observed dispersion of thedielectric constant can be explained on the basis of spacecharge polarization and hopping conduction between Fe3+

and Fe2+. According to the two-layer model of Maxwell[42] and Wagner [43], the space charge polarization arisesdue to the inhomogeneous dielectric structure of thematerial. In ferrites, it is well known that the samplesconsist of well-conducting grains separated by poorlyconducting phase (Ca-silicate) at inter-grain boundaries[44]. The electron reaches the grain boundary throughhopping and if the resistance of the grain boundary is highenough, electrons pile up at the grain boundaries andproduce polarization. However, as the frequency of theapplied field is increased, the electrons reverse theirdirection of motion more often. This decrease the prob-ability of electrons reaching the grain boundary and asa result the polarization decreased. Therefore, the dielectric

ARTICLE IN PRESS

15 20 25 30 35 40

6

8

10

12

14

16

18

20 SC-0

SC-1

SC-2

SC-3

SC-4

SC-5

lnρ

(Ω-c

m)

0

50

100

150

200

250

300

ρ at

723K

(k

Ω-c

m)

ρ at

303K

(M

Ω-c

m)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

μd

Dri

ft m

ob

ilit

y μ

d

(cm

2/ V

-Sec)

0.30

0.35

0.40

0.45

0.50

ΔE

1/kBT (eV)-1 All Samples

SC-0 SC-1 SC-2 SC-3 SC-4 SC-5

All Samples

SC-0 SC-1 SC-2 SC-3 SC-4 SC-5

Acti

vati

on

En

erg

y

ΔE (

eV

)

ρ at 723K

ρ at 303 K

Fig. 6. (a) Temperature dependence of the DC electrical resistivity for all samples. (b) Resistivity at 303 and 723K of all samples. (c) Drift mobilities and

activation energies of the samples.

2 3 4 5 6

0

100

200

300

400

500 SC-0 SC-1 SC-2 SC-3 SC-4 SC-5

Die

lectr

ic c

on

sta

nt

ε′

2 3 4 5 6

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

tan

δ

SC-0 SC-1 SC-2 SC-4

0

20

40

60

80

100

120

ε′ at 1MHz

Die

lectr

ic c

on

sta

nt

ε′

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0tan δ at 100kHz

Log f (Hz) Log f (Hz)

All Samples

SC-3 SC-5

Lo

ss f

acto

r ta

n δ

SC-0 SC-1 SC-2 SC-3 SC-4 SC-5

ε′ at 100kHztan δ at 1MHz

Fig. 7. (a) Frequency-dependent dielectric constants of all samples. (b) Frequency-dependent loss factors of the samples. (c) Dielectric constants and loss

factors at 100 kHz and 1MHz of all samples.

S. Hussain, A. Maqsood / Journal of Magnetism and Magnetic Materials 316 (2007) 73–80 79

constant decreases with increasing frequency of the appliedfield. The increase in dielectric constant (e0) with theincrease in sintering time from sample SC-1 to SC-5 inTable 1 may be attributed to the hopping between Fe3+

and Fe2+ ions due to decrease in resistance of the big

grains by grain growth phenomenon. As a result, thepolarization and hence the dielectric constant increases asreported in literature [44]. High dielectric constantdecreases the penetration depth of the electromagneticwaves by increasing the skin effect.

ARTICLE IN PRESSS. Hussain, A. Maqsood / Journal of Magnetism and Magnetic Materials 316 (2007) 73–8080

Fig. 7(b) depicts the dispersion of the loss tan d for thestudied samples at the frequency range 80Hz–1MHz.The dielectric loss has initially decreasing trend with theincrease in frequency but at higher frequencies, i.e.100 kHz–1MHz, it shows the increasing behavior exceptsample SC-0 (pure Sr-ferrite). This maximum in tan dversus frequency appears when the frequency of hoppingcharge carrier coincides with the frequency of the appliedalternating field [45]. However, the decrease of tan d withincrease in frequency is attributed to the fact that thehopping frequency of the charge carrier cannot follow thechanges of polarity of the external field beyond a certainfrequency [45].

4. Conclusions

Addition of Si and Ca strongly affect the structural,magnetic and electrical properties of Sr-hexa ferrites. Thisaddition suppresses the abnormal grain growth and producessub-micron grains of uniform size that causes denserstructure. The average grain size decreases from 2.273to 1.384mm and measured density increases from 4.52 to4.69 g/cm3 for pure and Si–Ca added Sr-hexa ferrites,respectively. The Vicker hardness increases from 512 to702Hv, respectively. Due to these structural and morpholo-gical changes, some improvement in magnetic and electricalproperties has been noticed. The coercivity Hc increases from596 to 4255Oe and remanence Br from 516 to 529G for pureand Si–Ca added Sr-ferrites, respectively. The DC electricalresistivity increases from 1.67� 106 to 2.89� 108O cm andactivation energy from 0.314 to 0.495 eV at room tempera-ture for SC-0 and SC-1, respectively. Similar trend can beseen with the dielectric properties at frequency ranging from80Hz to 1MHz. However, the electrical and magneticproperties were lost for long sintering time (SC-2 to SC-5).The sample SC-1 containing 0.5–0.5wt% Si–Ca, sintered at1200 1C for 2h showed the best performance regardingstructural, magnetic and electrical properties.

Acknowledgments

Mr. Shahid Hussain is thankful to HEC for the grantof Ph.D. scholarship under the 200 Merit ScholarshipsScheme. Dr. M. Anis-ur-Rehman and Mr. M.S. Awan ofthe Department of Physics, CIIT, Islamabad, Pakistan,are acknowledged for providing some of the experimentalfacilities and fruitful discussion.

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