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Ceramics International 40 (2014) 11205–11211www.elsevier.com/locate/ceramint

Doping magnesium ion to tune electrical and dielectricproperties of BaCo2 hexaferrites

Rafaqat Ali Khann, Sadullah Mir, Asad Muhammad Khan, Bushra Ismail, Abdur Rahman Khan

Applied and Analytical Chemistry Laboratory, Department of Chemistry, COMSATS Institute of Information Technology, Abbottabad 22060,Khyber Pakhtunkhwa, Pakistan

Received 18 February 2014; received in revised form 27 March 2014; accepted 27 March 2014Available online 4 April 2014

Abstract

Pure BaCo2 W-type hexaferrites i.e. BaCo2�xMgxFe16O27 (xr1) have been synthesized by the chemical co-precipitation method at 1223 Kand have been investigated for their structural, electrical and dielectric properties. The X-ray diffraction (XRD) patterns conformed to thestandard single phase of the hexagonal W-type structure. We examined the surface morphology of the prepared samples by the use of scanningelectron microscopy (SEM) analysis. The dc-electrical resistivity (ρ) measurements showed metal to semiconductor transition (TM–S), in atemperature range of 298–663 K. The maximum, at which the TM–S occurs, varies with the magnesium concentration in the samples.Crystallographic distribution of Mg2þ cations was responsible for the variation in the overall resistivity (ρ) pattern of BaCo2-W type hexaferrites.The dielectric constant (ε

0) and the dielectric loss (tan δ) measurements at room temperature showed normal behavior of hexaferrites. The

obtained results indicated that the Mg2þ ions played an important role in the tuning of the electric and the dielectric properties of BaCo2-Whexaferrites, which might have possible applications in transformer cores to reduce the eddy current losses.& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: A. Powders: chemical preparation; B. Grain boundaries; C. Dielectric properties; C. Electrical properties; D. Ferrites

1. Introduction

Electrical and electronic devices are the basic need of themodern age and therefore are very important in designing andoptimizing the performance of these devices and increasingtheir efficiency. Scientists have already been working in thesedirections, encountering problems/discrepancies, seeking solu-tions to these complications in different parts of the world.Ferrite, an important material from technological points ofview, possess both dielectric and magnetic characteristics.Among ferrites, BaCo2-W-type hexaferrites are structurallyand technically magnificent and vital compounds. In the lastfew years, W-type hexaferrites with their superior properties ascompared to their counterparts, have gained some importance.Recently, researchers tuned W-type hexaferrites for their

10.1016/j.ceramint.2014.03.16014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

g author. Tel.: þ92 992 3835916x368;34 41.ss: rafaqat2khan@yahoo.com (R.A. Khan).

improved electrical and magnetic characteristics, by dopingtheir hexagonal crystal structure with suitable ions either ontrivalent iron sites or on the divalent metallic sites. Ahmedet al. synthesized La doped BaZn0.5Mg0.5CoFe16O27 hexafer-rites using conventional ceramic route and evaluated thesematerials for their electrical and magnetic properties. Theyshowed that La doping on Ba site considerably affected bothdielectric and conducting parameters in the samples [1]. Inanother study, Ahmad et al. doped La ions on Fe site inBaZn2Fe16O27 hexaferrites using a chemical co-precipitationtechnique [2] and they observed decrease in saturation andremanent magnetizations and enhanced resistivity of dopedsamples. Rana and co-workers synthesized BaCo2Alx-Fe16�xO27 hexaferrites by sol–gel auto-combustion and char-acterized for magnetic, and microwave properties [3]. Due tomagnetic and attenuation characteristics of the synthesizedmaterials, the authors proposed the materials for suppression ofelectromagnetic interference and radar absorbing materials.Similarly, doping Ni and La in SrCo2Fe16O27 hexaferrite,

Fig. 1. TGA curve for un-annealed sample of proposed W-type hexaferrite.Rectangles show weight losses in different regions.

R.A. Khan et al. / Ceramics International 40 (2014) 11205–1121111206

yielded the magnetocrystalline anisotropy transition from in-plane to axial because of field anisotropy [4]. Besides these,one can find in the literature that the structural, electrical andmagnetic properties have been tuned using Mg2þ ions asdopant in the spinel and hexaferrites [5–8]. However, theliterature on doping Mg2þ in W-type hexaferrite is scarce. Inthe present work we have attempted to dope Mg2þ ions onCo2þ site in BaCo2Fe16O27. This will enable us to explore theeffect of substitution on structural and electrical properties ofW-type hexaferrite.

2. Experimental

To synthesize BaCo2�xMgxFe16O27 (xr1.0) hexaferrites,Co(CH3COO)2 � 4H2O (99%), NaOH (98%) and NaCO3 (98%)were purchased from Merck. Fe(NO3)3 � 9H2O (98%), Mg(NO3)2 (98%) and BaCl2 � 4H2O (99%) were supplied bySigma-Aldrich and Analar.

The chemical co-precipitation technique was adopted tosynthesize the ferrites samples of various compositions bymaking changes in the ratios of Co and Mg, ofBaCo2�xMgxFe16O27 (where x=0, 0.2, 0.4, 0.6, 0.8, and1.0). Fe(NO3)3 � 9H2O was dissolved in de-ionized waterhaving acidic pH to avoid precipitation of Fe(OH)3. Duringsynthesis, stoichiometric amounts of the chemicals weredissolved in de-ionized water to obtain a homogeneous mixin solution. The temperature of the solution mixture was raisedto 353 K under constant stirring and a mixture of NaOH andNa2CO3 in a 5:1 ratio was added drop wise to increase the pHto �12. The precipitates were washed, dried (at 373 K) in anoven and were finely ground in a mortar. Finally, the sampleswere annealed at 1223 K for 4 h in a temperature-programmedfurnace at a heating rate of 5 K min�1. Thermogravimetricanalysis was carried out with an increasing rate of 10 K min�1

in the range of 300–1250 K (Mettler-Toledo, TGA/SDTA851e). X-ray diffraction analysis was carried out using adiffractometer (SmartLab by Rigaku corp.) that employs CuKαas a radiation source. A scanning electron microscope (HitachiS-3400N) was used for the morphological studies. Electricalresistivity measurements were carried out by a typical twopoint probe method in a temperature range from 298 K to663 K using a source meter (Keithley 2400). Dielectricmeasurements were performed on silver-coated pellets usingan LCR meter bridge (Wayne Kerr LCR 4275) in a range of1000 Hz to 1 MHz.

3. Results and discussion

3.1. Thermogravimetric analysis

The autocatalytic combustion process of the co-precipitatedmetal hydroxides for the proposed structure of BaCo1.8Mg0.2-Fe16O27 hexaferrites was investigated by thermal analysis.Fig. 1 shows the result of a thermogravimetric analysiscombined with differential analysis conducted on hexaferritesample. The first weight loss of about 3% with a correspondingpeak in the DTG graph (360 K) appears due to the evaporation

of adsorbed water and water of crystallization in a mixture ofcomplex hydroxides. The second weight loss combined with apeak in DTG appears between 470 and 577 K which representsthe loss of weight due to the conversion of metal hydroxide tooxide form. Further, the weight losses between 900–1005 Kand 1091–1175 K most probably correspond to the decom-position of BaCO3 and MgCO3 respectively. The conversionof the metal precursors to their complex hydroxides/carbonatesand subsequently to the W-type hexaferrite particles is given inScheme 1.

3.2. Structure analysis

Fig. 2 shows XRD patterns for the chemically synthesizedMg2þ doped BaCo2Fe16O27 hexaferrite samples. It is clearfrom the figure that after doping with Mg2þ ions, thecompound retained the hexagonal crystal structure and thepeaks are well matched with the standard pattern for W-typehexaferrites [ICSD 00-019-0098]. Moreover, Mg2þ ions withsmaller ionic size (0.70 Å) compared to the size of Co2þ ions(0.74 Å) slightly altered both the lattice parameters. In Fig. 3,both the lattice parameters ‘a’ and ‘c’ were calculated usingequation found elsewhere [9]. The lattice parameters ‘a’ and‘c’ have shown a decreasing trend with an increase in theMg2þ concentration. The resultant product was well in thenanoregime as shown in Table 1. The crystallite size for eachsample was calculated using the Gaussian fit from the mostintense peak of the XRD pattern. The crystallite sizes obtainedin this manner were found in the range of 39 to 45 nm.

3.3. Morphological studies

Fig. 4 shows the scanning electron micrographs for theselected synthesized samples on a scale of 5 mm and resolutionof 5000� . The micrographs show that the particles arecoagulated to form large grains. The phenomenon of coagula-tion is common in nanomaterials and is due to the high surfaceenergies that such types of materials have at nanometer scale.

BaCl2(aq) + 2-xCo(CH3COO)2(aq) + 16Fe(NO3)3(aq) + xMg(NO3)2(aq) + NaOH + Na2CO3

Bay/2Fe4(OH)12+y/2 + Co2-xFe4(OH)14-x + MgxFe4(OH)12+x + Bay/2CO3.Fe4(OH)12 + NaNO3 + NaCl + CH3COONa

BaCo2-xMgxFe16O27

353 K, pH=12

1223 K

Scheme 1. Formation of BaCo2�xMgxFe16O27 during the synthesis.

Fig. 2. X-ray diffraction patterns for BaCo2�xMgxFe16O27 hexaferrites.

Fig. 3. Lattice parameters ‘a’ and ‘c’ versus composition x ofBaCo2�xFe16O27.

Table 1Crystallite sizes and different electrical parameters obtained forBaCo2�xMgxFe16O27.

Parameters (x¼ ) 0.0 0.2 0.4 0.6 0.8 1.0

Crystallite sizes (Dx/nm) 40 41 43 39 45 44Debye temperature (θD/K) – – 916 896 1006 986Activation energy (Ea/eV) 0.80 0.56 – – – –

T0 (K�1/4) – – 233 147 143 118R (Å) – – 28.4 18.0 16.2 14.1N (cm�3) (� 1019) – – 1.04 4.07 5.54 8.47E (meV) – – 493 305 372 264Activation energy (Ea/eV) – region II – – 0.80 0.71 0.70 0.59

R.A. Khan et al. / Ceramics International 40 (2014) 11205–11211 11207

The Mg2þ ions substitution affected the morphology of theBaCo2Fe16O27 hexaferrites. It is clear from the micrographsthat by doping Mg2þ at a content level of x¼0.2 the particleschanged their shape from thin plate with sharp edges to theglobular shape (Fig. 4a and b). The resultant shape is welldepicted in Fig. 4c, where the size and shape become uniform,although coagulated due to high surface energies.

3.4. Electrical resistivity measurements

Resistivity versus temperature curves are shown in Fig. 5 forMg2þ doped BaCo2Fe16O27 hexaferrites nanoparticles. Themetal to semiconductor transition (TM–S) temperature is presentin all samples. According to Maltiessen's rule, the totalresistivity of a ferromagnetic material can be expressed asgiven in the following equation [10–12]:

ρðTÞ ¼ ρrþρphðTÞþρeðT ;ωÞ ð1Þwhere ρr is the residual resistivity due to impurities in samplesand lattice defects and is independent of temperature. ρph(T) isthe contribution of the electron–phonon interaction related tothe mobility of electric charge carriers and follows theArrhenius equation of small polaron hopping in the case offerrite materials and is given by the following equation:

ρphðTÞ ¼ ρoexpEa

KbT

� �ð2Þ

The term ρe(T,ω) is the contribution from electron spin wavescattering, which is related to the dielectric relaxation causedby localized electric charge carriers. In Fig. 5 the maximumtemperature at which transition occurs become lower withincrement of Mg2þ content. Several factors may stand equallyresponsible for the observed behavior of TM–S e.g. increase inthe resistivity due to the phonon–electron effect, the effect ofelastic and inelastic scattering due to Fe3þ and Co2þ metalions, residual part due to impurities, crystal defects and spincanting. Among these possibilities, the residual part due to thecrystal defects and spin canting are very important factors.Spin canting (especially the isotropic exchange) indeed is themost possible reason for the observed TM–S phenomenon in thesamples. Below TM–S in the samples, the electronic spins arecoupled strongly and with an increase in temperature above

Fig. 4. Scanning electron microscopy images for (a) BaCo2Fe16O27, (b) BaCo1.8Mg0.2Fe16O27 and (c) BaCoMgFe16O27 hexaferrite samples.

Fig. 5. Resistivity versus temperature for BaCo2�xMgxFe16O27 hexaferrite. Fig. 6. Non-linear behavior of log of resistivity versus temperature for samplesunder considerations.

R.A. Khan et al. / Ceramics International 40 (2014) 11205–1121111208

TM–S the charge carriers are free and start hopping. In Fig. 5,the amount of Mg2þ ions is changing the maximum tempera-ture at which the transition occurs, thus clearly supporting theidea of spin canting [13]. It will be clear from subsequentdiscussion that Mg2þ ions have partial preferences for theoctahedral sites and therefore replace the Co2þ or Fe3þ ions atthese sites. The low Mg2þ ion concentration leads to decreasein the number of spins occupying the octahedral B sites. As theMg2þ ion concentration is increased, the exchange interactions

are weakened and B spins are no longer held rigidly thusresulting in the lowering of the transition temperature in thestudied samples. Above TM–S the resistivity decrease with thetemperature showing the semiconducting nature of the samplesunder study. The non-linear behavior of the log of resistivityversus temperature shows temperature dependent electricalconduction. In Fig. 6, two distinct regions were exhibited bysamples with magnesium content of xZ0.4, which indicatedtwo different conduction mechanisms operating in the samples.

R.A. Khan et al. / Ceramics International 40 (2014) 11205–11211 11209

The temperature, at which the kink in the pattern is observed,can be referred to as (θD/2) where ‘θD’ is the characteristicDebye temperature given in Table 1 [14]. The behaviorobserved here was previously referred to as the Curie or Neeltemperature by many authors [15,16]. However, in our case thetransition temperature at this point could be ruled out forhexaferrites. The data above TM–S and below (θD/2) is fittedwith the Motts VRH model and above (θD/2) with the smallpolaron model.

Variable range hopping model is based on hopping of thecharge carriers that refers to tunneling transition from occupiedto non-occupied localized states through which energy gap ismaintained by absorption and emission of single or multiplephonons. For the 3-D system, as in our case, the data in regionI (TM–S-θD/2) can be fitted well with the Motts variable rangemodel given by the following equation [17,18]:

ρ ¼ ρ0 expT0

T

� �14

ð3Þ

The 3-D Mott's temperature can be represented by

T0 ¼λ

kBηðEFÞa3ð4Þ

where ‘a’ is the wave function localization radius with a valueof 1.5 Å, λ is a dimensionless constant with a value of 18.1, kBis the Boltzmann constant in eV K�1, ηEF is the density ofstates at the Fermi level. T0 is obtained from slope fitting ofEq. (3) in region-I (Fig. 7). For calculating the mean hoppingdistance (R), we made use of following equation:

9a8πkBTη EFð Þ

� �14

ð5Þ

The relation used for calculating mean density of hoppingcarriers (N) is

N ¼ 3

4πR3 ð6Þ

Fig. 7. Fitting of VRH model between TM–S and θD/2 for xZ0.4.

The mean energy (E) necessary for hopping is calculatedusing the following equation:

E¼ N

ηðEFÞð7Þ

The data obtained using the above equations is given inTable 1. As can be seen from the table the value ofcharacteristic Mott's temperature i.e. T0 decrease with anincrease in the dopant content showing that the conductivityincrease because of the localized state become weaker. There-fore, with the increase in Mg2þ content, low energy is requiredfor hopping as shown in Table 1. The obtained values of ‘N’ inthe order of 1019 cm�3 indicate an adiabatic hopping conduc-tion in the low temperature region. Although the values fortypical oxides semiconductor fall in the range of 1017 cm�3 to1018 cm�3; however, in our case the high values are due to thehigh conduction of these ferrites compared to other semicon-ductor oxides with small polaron conduction mechanism. Thecurves (not shown) above θD/2 (region-II), are fitted with themost famous Arrhenius equation and the values of activationenergies are reported in Table 1. The activation energies, inboth cases i.e. fitting using Mott's variable range model and theArrhenius equation, decrease with an increase in Mg content,since part of the substitution takes place at the tetrahedral siteswhich may therefore force iron ions to migrate to octahedralsites and result in the increment of hoping probabilities amongFe2þ and Fe3þ thereby lowering the overall resistivity of thesamples under study. According to the literature [19,20] thecations, except large divalent ions, in W-type hexaferrites aredistributed on seven magnetically distinguishable sublatticeswithin the crystal structure. These include 12k, 4fVI, 6g, 4f withoctahedral coordination, 4e, 4fIV with tetrahedral coordinationand 2d having bi-pyramidal geometry. Moreover, in the case ofMg2þ addition in W-type crystal structure, it was anticipatedthat the ions had preference to the 4fVI (octahedral), 6g(octahedral), 4e (tetrahedral) and 4fIV (tetrahedral) sub-lattices. Co2þ had a probability of going to 12k (octahedral),4fVI (octahedral), 6g (octahedral), 4e (tetrahedral) and 4fIV(tetrahedral) sub-lattices [21,22]. As can be seen from Fig. 6,the overall resistivity value of the samples under studyexhibited random distribution. The distribution of data canbe attributed to the Mg2þ ions preference to the bothoctahedral and tetrahedral sites, which compete with thedistribution of Co2þ ions at these sites.

3.5. Dielectric characterization

The dielectric properties are very important for many highfrequency applications of hexaferrites particularly if theybecome integrated with chip components. The room tempera-ture dielectric constant for the Mg2þ doped BaCo2Fe16O27

hexaferrites was calculated using the following equation:

ε0 ¼ Cd

ε0Að8Þ

where ‘C’ is the capacitance, ‘d’ is the thickness, ‘ε0’ is thedielectric permittivity of air, ‘A’ is the surface area of the

Fig. 8. Room temperature dielectric constant measurements for BaCo2�xMgxFe16O27 hexaferrites.

Fig. 9. Plot of dielectric loss (tan δ) as a function of logarithm offrequency (F).

R.A. Khan et al. / Ceramics International 40 (2014) 11205–1121111210

device under test. The measured values for the dielectricconstant as a function of log of frequency are plotted inFig. 8. The room temperature measured values of dielectricconstant decrease with increase in applied frequency showingtypical trends for hexaferrites. The lowering of dielectricconstant at low frequency is steeper than that at the highfrequency. Therefore, it represents the dielectric structure as aninhomogeneous medium of two layers of the Maxwell–Wagner type i.e. well conducting grains and poorly conductinggrain boundaries. At low frequency, the grain boundaries aremore effective in contribution while at high frequency thecontribution to the property is by grains. Moreover, the metalcations and oxygen anions form the dipoles, creating anintrinsic dielectric polarization. In addition, the dielectricbehavior of hexaferrites can be explained by consideringbipolar and interfacial polarization due to surface effects. Atlow frequency, both dipolar and interfacial polarization con-tribute to the value of dielectric constant, but at higherfrequencies, only the electronic polarization becomes signifi-cant. Hence, based on the above discussion, the value ofdielectric constant is the maximum at low frequency whileminimum at high frequency. Fig. 9 depicts the variation of losstangent versus log of frequency denoting phenomenon ofrelaxation in sample with Mg content of x¼0, 0.8, and 1.0.The relaxation peak in dielectric loss tangent (tan δ) isconsidered due to the presence of non-equivalent sites. Adistribution height in the crystal lattice exists because of thepresence of ions with different valence states in the B-sites.Mobile electrons would pile up at sites with high frequencybarrier heights, which correspond to Maxwell–Wagner polar-ization due to inhomogeneities in the sample. The possiblesource of inhomogeneity is the presence of oxygen deficiencyin polycrystalline ferrite matrix, possibly created due toannealing at high temperatures. Beside, the tan δ relaxationcan be attributed to the hopping motion of small polarons andgrain boundary effects. Since the dielectric polarization of

polycrystalline ferrites is affected by grain boundary thereforethe relaxation peaks are also expected to be associated withgrain boundaries effects [23]. Another possibility of therelaxation phenomenon could be the potential accumulationof Co-ions on octahedral sites due to the preference of someMg-ions for tetrahedral sites and the hopping of holes betweenCo3þ and Co2þ .

4. Conclusion

We synthesized W-type hexaferrite nanoparticles with anominal composition of BaCo2�xMgxFe16O27 (x¼0.0–1.0)through chemical co-precipitation route. Resistivity measure-ment showed TM–S in all samples that decreased with theMg2þ content. TM–S samples showed two types of conductionmechanisms operating which we tried to explain with the helpof Mott's variable range model and small polaron model. Thedielectric constant showed normal behavior characteristics ofhexaferrites while tan δ versus frequency curve showeddispersion behavior due to the presence of non-equivalentsites in the crystal structure of W-type hexaferrites. In short,the doping of Mg2þ ions in BaCo2Fe16O27 has a considerableeffect on morphology, electrical and dielectric properties.

Acknowledgments

The authors are thankful to Centralized Resource Laboratory(CRL), University of Peshawar, Pakistan for carrying out SEManalysis.

References

[1] M.A. Ahmed, N. Okasha, R.M. Kershi, Dramatic effect of rare earth ionon the electrical and magnetic properties of W-type barium hexaferrites,Physica B 405 (2010) 3223–3233.

R.A. Khan et al. / Ceramics International 40 (2014) 11205–11211 11211

[2] M. Ahmad, Faiza Aen, M.U. Islam, S.B. Niazi, M.U. Rana, Structural,physical, magnetic and electrical properties of La-substituted W-typehexagonal ferrites, Ceram. Int. 37 (2011) 3691–3696.

[3] M. Ahmad, R. Grossinger, M. Kriegisch, F. Kubel, M.U. Rana, Magneticand microwave attenuation behavior of Al-substituted Co2W hexaferritessynthesized by sol–gel autocombustion process, Curr. Appl. Phys. 12(2012) 1413–1420.

[4] C. Stergiou, G. Litsardakis, Preparation and magnetic characterization ofCo2-W strontium hexaferrites doped with Ni and La, J. Magn. Magn.Mater. 323 (2011) 2362–2368.

[5] M. Jazirehpour, M.H. Shams, O. Khani, Modified sol–gel synthesis ofnanosized magnesium titanium substituted barium hexaferrite and inves-tigation of the effect of high substitution levels on the magneticproperties, J. Alloys Compd. 545 (2012) 32–40.

[6] M.A. Khan, M.U. Islam, M. Ishaque, I.Z. Rahman, Magnetic anddielectric behavior of terbium substituted Mg1�xTbxFe2O4 ferrites, J.Alloys Compd. 519 (2012) 156–160.

[7] C. Sujatha, K.V. Reddy, K.S. Babu, A.R.C. Reddy, K.H. Rao, Structuraland magnetic properties of Mg substituted NiCuZn nano ferrites, PhysicaB 407 (2012) 1232–1237.

[8] V.V. Soman, V.M. Nanoti, D.K. Kulkarni, Dielectric and magnetic propertiesof Mg–Ti substituted barium hexaferrite, Ceram. Int. 39 (2013) 5713–5723.

[9] M.J. Iqbal, R.A. Khan, Enhancement of electrical and dielectric proper-ties of Cr doped BaZn2 W-type hexaferrite for potential applications inhigh frequency devices, J. Alloys Compd. 478 (2009) 847–852.

[10] B.P. Jacob, S. Thankachan, S. Xavier, E.M. Mohammed, Dielectricbehavior and AC conductivity of Tb3þ doped Ni0.4Zn0.6Fe2O4 nanopar-ticles, J. Alloys Compd. 541 (2012) 29–35.

[11] V. Verma, V. Pandey, V.N. Shukta, S. Amapoomi, R.K. Kotlana, Remarkableinfluence on the dielectric and magnetic properties of lithium ferrite by Ti andZn substitution, Solid State Commun. 149 (2009) 1726–1730.

[12] M.A. Ahmed, S.T. Bishtay, G. Abdelatif, Effect of ytterbium on theelectrical properties of Li–Co ferrite, J. Phys. Chem. Solids 62 (2001)1039–1046.

[13] P.G. Bercoff, H.R. Bertorello, Localized canting effect in Zn-substitutedNi ferrites, J. Magn. Magn. Mater. 213 (2000) 56–62.

[14] A. Banerjee, S. Pal, E. Rozenberg, B.K. Chaudhuri, Adiabatic and non-adiabatic small-polaron hopping conduction in La1�xPbxMnO3þδ

(0.0rxr0.5)-type oxides above the metal–semiconductor transition, J.Phys.: Condens. Matter 13 (2001) 9489–9504.

[15] S.A. Mazen, H.M. Zaki, Ti4þ and Ge4þ ionic substitution in Cu-ferrite,electrical conductivity and thermoelectric power, J. Magn. Magn. Mater.248 (2002) 200–215.

[16] B.P. Jacob, S. Thankachan, S. Xavier, E.M. Mohammed, Effect of Tb3þ

substitution on structural, electrical and magnetic properties of sol–gelsynthesized nanocrystalline nickel ferrite, J. Alloys Compd. 578 (2013)314–319.

[17] D. Nandi, A.K. Ghosh, K. Gupta, A. De, P. Sen, A. Duttachowdhury, U.C. Ghosh, Polypyrrole–titanium(IV) doped iron(III) oxide nanocompo-sites: synthesis, characterization with tunable electrical and electroche-mical properties, Mater. Res. Bull. 47 (2012) 2095–2103.

[18] G. Huo, D. Song, Q. Yang, F. Dong, Structure, magnetic and electricaltransport properties of the perovskites Sm1�xCaxFe1�xMnxO3, Ceram.Int. 34 (2008) 497–503.

[19] G. Albanese, M. Carbucicchio, G. Asti, Spin-order and magneticproperties of BaZn2Fe16O27 (Zn2-W) hexagonal ferrite, Appl. Phys. 11(1976) 81–88.

[20] A.P. Lilot, A. Gerard, F. Grandjean, Analysis of the superexchangeinteractions paths in the W-hexagonal ferrites, IEEE Trans. Magn. Magn.18 (1982) 1463–1465.

[21] G. Albanese, E. Calabrese, A. Deriu, Mossbauer investigation of W-typehexaferrite of composition BaZn2�xCoxFe16O27, Hyperfine Interact. 28(1986) 487–489.

[22] A. Collomb, P. Wolfers, X. Obradors, Neutron diffraction studies of somehexagonal ferrites: BaFe12O19, BaMg2-W and BaCo2-W, J. Magn. Magn.Mater. 62 (1986) 57–67.

[23] W.H. Jong, J.H. Lee, J.H. Sohn, H.D. Nam, S.H. Cho, Dielectric lossanomaly in Ba(Fe1/2Ta1/2)O3 ceramics, Mater. Lett. 56 (2002) 334–338.