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Ceramics International 40 www.elsevier.com/locate/ceramint

Doping effect of Zr–Zn binary mixture on the structural and electricalproperties of SrCo2-W type hexaferrites

Rafaqat Ali Khann, 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 28 March 2014; received in revised form 7 May 2014; accepted 9 May 2014Available online 16 May 2014

Abstract

SrCo2Fe16�2x(Zr–Zn)xO27 hexaferrite nanocrystallites of average crystallite sizes in the range of 38–43 nm were synthesized by the chemicalco-precipitation method and their structural and electrical properties were determined. Single hexaferrite phase was established by X-raydiffraction (XRD) analysis of the synthesized samples and various parameters have been calculated using XRD data. Doping Zr–Zn binarymixture in SrCo2Fe16O27 resulted in a considerable enhancement of the room temperature resistivity (109 Ω cm), which make it suitable forapplications at higher frequencies and for the minimizing eddy current losses in the transformers cores. Of all the synthesized samples, the lowestvalue of the reflection loss is obtained for the dopant contents of x¼0.2 and x¼0.8. Materials with high absorption characteristics may find theirapplications in the microwave and radar absorption as well as for the electromagnetic attenuations.& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: A. Calcination; C. Electrical conductivity; C. Dielectric properties; D. Ferrites; E. Capacitors

1. Introduction

Hexaferrites, the highly resistive magnetic semiconductors,are very important and indispensable materials from thetechnological point of view. They are widely used in theelectrical and electronic devices to increase their efficiency [1].Electromagnetic interference (EMI) causing interruptions inthe electronically controlled systems, is one of the majorproblems that the electronic materials have today. Moreover,EMI can cause device malfunctioning, generate false images,and reduce performance of various devices due to the system-to-system coupling. Therefore, for all these problems causedby EMI, the use of hexaferrites with optimum electrical andmagnetic characteristics can be a remedy.

Sr-hexaferrites are attractive materials for high frequencycircuits and are widely used as permanent magnets, high-

10.1016/j.ceramint.2014.05.03414 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

g author. Tel.: þ92 992 3835916x368;3441.ss: rafaqat2khan@yahoo.com (R.A. Khan).

density magnetic recording media and microwave devices [2].Co2-W type hexagonal ferrites are superior to most of thespinel ferrites for their applications in the microwave and indifferent electromagnetic devices operating in the radio fre-quency region [3]. Recently, Pr3þ doped BaCoNiFe16O27

showed paramagnetism and strong magnetization. The realparts of the permittivity and the permeability decreased whilethe corresponding imaginary parts increased with the doping ofPr3þ and resulted in considerable improvements of themicrowave absorption [4]. High frequency characterizationsfor rare earth (RE¼La, Nd, Sm) ions doped BaCo2Fe16O27 hasshown improved static magnetic properties and enhanced highfrequency absorption [5]. Substitution of Ti in BaNi2Fe16O27

hexaferrites substantially increased magnetization and theremanence due to the replacement of Ti ions for spin downsub-lattices [6]. Low coercivity values of Ga dopedSrZn2Fe16O27 hexaferrites made them fit for applications inthe electromagnetic materials. In addition, some compositionsof these hexaferrites showed a minimum value for thereflection loss [7]. Another study showed that Sr doping in

Fig. 1. Powder XRD patterns for doped SrCo2Fe16�2xZrxZnxO27 hexaferritessamples (x¼0.2, 0.6, 1.0).

R.A. Khan et al. / Ceramics International 40 (2014) 13257–1326213258

BaCo2AlFe15O27 with epoxy could considerably improve thereflection loss [8].

In this study, we have tried to explore the impact ofnonmagnetic ions on the structural, and electrical behavior ofSrCo2Fe16�2xZrxZnxO27 (where x¼0.0–1.0) hexaferrites. Weselected Zr, and Zn because these ions have exciting effects onthe crystalline anisotropy. This presents us with a possibility ofobserving some interesting changes in the high frequencymagnetic properties of these compounds.

2. Experimental procedure

Different chemicals used for the synthesis were Sr(NO3)3(Analar 99%), Co(CH3COO)2 � 4H2O (Merck 99%), Fe(NO3)3 �9H2O (Sigma-Aldrich 98%), ZrOCl2 � 8H2O (BDH 96%), Zn(CH3COO)2 � 4H2O (Merck 99%), NH4OH (Merck 99%). Thechemical co-precipitation process was adopted for synthesizingSrCo2Fe16�2xZrxZnxO27 hexaferrites powders. By dissolvingstoichiometric amounts of the corresponding chemicals in waterat 353 K under constant stirring, a homogeneous mixture ofsolution resulted. NH4OH was added dropwise to increase thepH of the solution to 12 at which the precipitates were formed.After washing, the precipitates were dried at a temperature of373 K, finely ground, and afterwards annealed at 1250 K in atemperature programmed muffle furnace at a pre-calibratedheating rate of 5 K min�1. Circular pellets of 13 mm diameterand 2 mm thickness were produced by compressing thepowdered samples at 68947.6 kN m�2 and used for electricaland high frequency measurements. X-ray diffraction analysiswas carried out by a diffractometer (Smart Lab by Rigakucorp.) having CuKα as a radiation source. DC electricalresistivity measurements were carried out by a home builtapparatus in a temperature range of 400–673 K using a well-known two-point probe method [9]. Complex permittivity andpermeability measurements were carried out with RF impe-dance/materials analyzer (Agilent E4991A) over a range from1 MHz to 1 GHz. For the permittivity measurements, silver-coated pellets (thickness, 2 mm and diameter 13 mm) wereused in 16453A test fixture. Agilent E4991A uses the induc-tance method for measuring relative permeability [10]. Forthat purpose, 2 mm thick toroidal shaped samples with innerand outer diameters of 6 mm and 13 mm respectively, wereutilized.

3. Results and discussion

3.1. Structural analysis

XRD analysis confirmed the formation of W-type hexagonalferrites. The spectra for the representative samples from thesynthesized series are shown in Fig. 1. The XRD spectra didnot exhibit any extra peak except those for the pure W-typehexaferrites, which perfectly matched the standard pattern(00-054-0106). Lattice parameters ‘a’ and ‘c’, shown in theTable 1, were calculated from the XRD data. A regularincrease in the lattice parameters with the Zr–Zn content wasjustified by the comparatively higher ionic radii of zirconium

(0.80 Å) and Zinc (0.74 Å) than the ionic radius of Fe3þ

(0.67 Å). We observed a regular increase in the cell volume(Vcell) (Table 1) with the dopant level in the substitutedsamples, due to the increase in the lattice parameters ‘a’ and‘c’. The average crystallite sizes of all the samples werecalculated from the full width at half maxima (FWHM) of thefirst four reflections in the XRD patterns, using the well knownDebye Scherrer formula D¼ kλ=β cos θB where k is the shapeconstant, β is the broadening of diffraction line measured athalf width of maximum intensity and λ is the X-ray wave-length used for the analysis. The crystallite sizes shown in theTable 1 were in the range of 38–43 nm.

3.2. Electrical resistivity measurements

Plots of resistivity versus temperature in a temperature rangeof 400–663 K (Fig. 2) showed semiconducting behavior. Theelectrical conduction in ferrites is due to the electron hoppingwhich involves the exchange of electron between the ions ofthe same element present in different valence states and aredistributed randomly over the crystallographic equivalentlattice sites [11]. The conduction occurs at octahedral sitesbecause of the smallest energy difference in the d-orbitals ofthe transition metal ions present at these sites [12]. In the lowtemperature region, the electron densities of the neighboringd-orbitals are far apart from each other but as the tempera-ture is increased, the increase in the vibration of interactingd-orbitals followed. Table 1 shows the room temperatureresistivity values for different samples of the series.The room temperature resistivity was found to increases

with the dopant concentration of up to x¼0.6. In the samplesunder investigation, the replacement of iron ions by zirconiumand zinc ions decreases the overall concentration of both Fe2þ

and Fe3þ ions to lower the hoping frequency between the twoions that in turn is responsible for the observed enhancedelectrical resistivity. Fig. 3 shows Arrhenius-type plots of the

Table 1Structural and electrical parameters for BaCo2Fe16�2xZrxCdxO27.

Parameters x¼0 x¼0.2 x¼0.4 x¼0.6 x¼0.8 x¼1.0

Crystallite size (D/nm) 38 42 35 38 40 43Lattice constant (a/Ǻ) 5.838 5.841 5.843 5.874 5.881 5.887Lattice constant (c/Ǻ) 33.34 33.39 33.55 33.67 33.78 33.84Cell volume (V/Ǻ3) 984 987 992 1006 1012 1016Resistivity (ρ/108 Ω cm @ 298 K) 2.96 9.62 33.3 65.39 12.36 5.05Activation energies, E (eV) 0.54 0.56 0.67 0.72 0.59 0.57Matching thickness tm (mm) 3.0 2.9 2.8 2.4 2.8 3.3

Fig. 2. Variation of resistivity with temperature for SrCo2Fe16�2xZrxZnxO27

hexaferrites.

Fig. 3. Arrhenius plots of electrical resistivity of SrCo2Fe16�2xZrxZnxO27

hexaferrites.

Fig. 4. Real parts of permittivity ðε0Þ versus frequency for zirconium and zincdoped SrCo2 hexaferrites samples.

R.A. Khan et al. / Ceramics International 40 (2014) 13257–13262 13259

logarithms of the resistivity versus temperature in the semi-conductor region. Table 1 also reports the activation energyvalues calculated from these plots and show gradual enhance-ment of the electrical resistivity with the dopant substitutionlevel. The increase in the room temperature resistivity up to avalue of 109 Ω cm guarantees the material's diminution of eddy

current losses and can be used in the radio frequency circuits,high quality filters, antenna, and transformers cores [13,14].

3.3. High frequency attenuations

3.3.1. Permitivity (1 MHz–1 GHz)The complex permittivity have been measured using an Agilent

E4991A impedance/materials analyzer over 1 MHz–1 GHz withthe 16453A (permittivity) test fixture. The relative dielectricpermittivity for samples was calculated using the standard relationas follows:

εr ¼Ct

εoAð1Þ

where ‘C’ is the capacitance of the pellet in farad, ‘t’ is thethickness of the pellet in millimeters, ‘A’ is the cross-sectionalarea of the surface of the pellet and ‘εo’ is the permittivity of freespace. The imaginary part of permittivity was measured using thefollowing equation:

ε″ ¼ t

2πf εoARpð2Þ

where ‘t’ is the thickness of the sample, ‘Rp’ is the parallelresistance obtained during measurements.Fig. 4 shows variation in the real part of permittivity as

a function of frequency in the range from 1 MHz to 1 GHz.

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The slight decrease in value of relative permittivity is typical ofthe ferrites. The value for the dielectric constant is normallyhigh at low frequencies but its value decreases with theincrease in frequency [15]. Interfacial polarization due tosurface effect can possibly be a reason for observed dielectricbehavior of hexaferrites [16]. At low frequency, both thedipolar and interfacial polarization contribute to the value ofdielectric constant but at higher frequency only the electronicpolarization becomes significant. Moreover, it is alreadyestablished that the ferrites consist of well conducting grainsand poorly conducting grain boundaries. At lower frequencies,the electron exchange between the ions Fe2þ and Fe3þ followthe alternating field. However, when the intensity of theapplied frequency is increased, the hoping of electronsbetween two ions cannot follow the alternating field and theconduction lags behind. Therefore, the value of relativepermittivity is small at high frequencies and vice versa.Fig. 4 shows that the overall values for real part of permittivitydepend upon the concentration of Zr–Zn. As both electricalconductivity and dielectric behavior are transport propertieswhose variations are in proportionality to the sample composi-tion, which make it a possibility that the mechanismsresponsible for these two phenomena could be similar to eachother. As the Zr–Zn substitutes the iron ions in the W-typecrystal, the hoping probability between the Fe2þ and Fe3þ

decreases and as a result the frequency dependent permittivityvalue become lower up to Zr–Zn content of x¼0.6. Moreover,there is somehow randomness in overall values such as in thecase for Zr–Zn content of x¼0.2, 0.8 and 1.0. The behaviorcan be possibly attributed to the increased distribution of Co2þ

ions at octahedral sites in hexagonal crystal structure becauseof the incoming dopants. Therefore, the variations in permit-tivity values for these samples may be contributed by thepolarized cobalt ions.

Fig. 5 shows the dielectric loss tangent i.e. the imaginarypart ðε″Þ, indicates decreasing trend with the applied fre-quency, a similar behavior to that of real part of permittivity

Fig. 5. Imaginary parts of permittivity ðε″Þ versus frequency for zirconium andzinc doped SrCo2 hexaferrites samples.

values. The overall values of ðε″Þ follow the same trend ascounter real part with the maximum values exhibited by thesamples with Zr–Zn content of x¼0.8, 1.0. A maximum valueof ðε″Þ at a certain frequency reflects a minimum stored energyat that frequency. The maximum values for these samplesare due to the increased concentration of iron and cobaltions at the octahedral site because Zr–Zn ions prefer mostlytetrahedral sites.

3.3.2. Permeability (1 MHz–1GHz)The inductance method to measure the relative permeability

involves a sample (toroidal core) wrapped with a wire thuscalculating relative permeability at the end of the core. Therelation used for the determination of the complex relativepermeability of the ring is given as [17] follows:

μr ¼2πðL�LsÞμot lnðc=bÞ

þ1 ð3Þ

where ‘L’ is the inductance of test fixture 16454A with sampleand ‘Ls’ is the inductance of the same test fixture withoutsample. mo is the relative permeability of air, t, b, and c, are thethickness, internal and outer diameters of the ring shapedsample respectively. The inductance of 16454A test fixture iscalculated using the following relation:

Ls ¼μo2π

ho ln e=a ð4Þ

where ho, e, and a, are respectively the height, external andinner diameters of the 16454A test fixture. The followingrelation calculates the imaginary part of permeability:

ðμ″Þ ¼ μ0Dμ ð5Þwhere ‘μ″’ and ‘μ0’ are the imaginary and real parts ofpermeability. Dm is the magnetic loss.Fig. 6 shows variation in the real part of permeability for

SrCo2Fe16�2xZrxZnxO27 (x¼0.0–1.0) in a range from 1 MHzto 1 GHz. It is clear that the real part of permeability remainsalmost constant in the measured range while showing compo-sition dependence in different samples. The overall values for

Fig. 6. Real parts of permeability ðμ0Þ versus frequency for zirconium and zincdoped SrCo2 hexaferrites samples.

Fig. 7. Imaginary parts of permeability ðμ″Þ versus frequency for zirconiumand zinc doped SrCo2 hexaferrites samples.

R.A. Khan et al. / Ceramics International 40 (2014) 13257–13262 13261

permeability increase with the dopant content up to concentra-tion of x¼0.6. The minimum value of permeability wasobserved for hexaferrites with Zr–Zn contents of 0.8 and 1.0.

The crystal structure of W-type hexaferrite consists of sevenmagnetic non-equivalent sub-lattices distributed over octahe-dral and tetrahedral sites. The iron and cobalt ions aredistributed randomly on these sub-lattices with the electronicspin either along or opposite to the easy c-axis in the crystalstructure. Zirconium and zinc ions were reported earlier tooccupy the tetrahedral sites while cobalt ions prefer octahedralsites [18–20]. In the present case, it can be assumed thatsubstituted ions prefer these sites due to which the values ofreal part of permeability increase up to a content of x=0.6.Beyond this content, because of the diamagnetic nature ofzirconium and zinc ions, the super exchange interactionsamong tetrahedral-A and octahedral-B sites decrease. Hencethe permeability values decrease at a content level of x=0.8and x=1.0. Fig. 7 shows variations in imaginary parts ofpermeability with frequency, showing a dispersion phenom-enon at high frequencies. The slight dispersions in theimaginary part of permeability above 800 MHz are due tothe natural resonance and wall resonance characteristics ofpolycrystalline ferrites.

Fig. 8. Reflection loss, R (dB) calculated at optimum thickness for dopedsamples of SrCo2 hexaferrites.

3.3.3. Absorption characteristicsThe reflection coefficient, R (dB) as a function of normal-

ized input impedance at the surface of a single layer materialbacked by conductor is given by the following equation [21]

R ðdBÞ ¼ �20 log10Zin�Z0

ZinþZ0

� �ð6Þ

where ‘Zin’ is the input impedance at air absorber interfaceobtained using equation:

Zin ¼ Z0

ffiffiffiffiffiμrεr

rtan h � j

2πf tc

� � ffiffiffiffiffiffiffiffiμrεr

p� �ð7Þ

where μr ¼ μ0 � jμ″ and εr ¼ ε0 � jε″are the complex relativepermeability and permittivity respectively for the absorber

medium, f is the frequency, c is the velocity of light and ‘t’ isthe thickness of samples. Z0 denotes the impedance of freespace given as

Z0 ¼ffiffiffiffiffiμ0ε0

rð8Þ

where ‘mo’ and ‘εo’ are the permeability and permittivity offree space, respectively. The absorbing material can beclassified in three categories based on the absorption mechan-ism, namely dielectric loss, conductive loss and magnetic loss[13,22]. With the development of modern science and technol-ogy, ferrite nanoparticles have attracted attention as usefulsubstances for microwave absorption due to their dielectric andmagnetic losses. Table 1 shows the calculated reflection lossesfor all samples at optimum matching thicknesses (Fig. 8). Theminimum reflection loss of �18.9 dB is obtained for thesample with dopant content of x¼0.8 at optimum thickness of2.8 mm. Although the value of magnetic loss for this sample iscomparable to that of other doped samples, the minimumreflection can be attributed to the high value of dielectric lossconverting the incoming radiations to heat. Fig. 8 showscomposition dependent variation of frequency at which max-imum absorption occurs. Therefore, the tuning of thesematerials is a possibility for obtaining required losses byvarying the concentration of Zr and Zn ions in the W-typehexaferrites.Electromagnetic absorbers are in wide use to minimize

electromagnetic interference and to ensure electromagneticcompatibility. Enhancements in the values of reflection lossesbecause of Zr–Zn doping establish their potential applicationsas electromagnetic wave absorber layers deposited on aconductive substrate to reduce the reflection of electromagneticwaves. Additionally, the same materials can be useful inelectronic circuits or other devices to minimize environmentalelectromagnetic interferences, to lower the noise level and toensure electromagnetic compatibility [20].

R.A. Khan et al. / Ceramics International 40 (2014) 13257–1326213262

4. Conclusions

Zr–Zn doped SrCo2-W hexaferrites were synthesized by thechemical co-precipitation technique at a temperature of1250 K. The XRD analysis corresponds to a single-phaseW-type hexaferrites. Increase in Zr–Zn content tends to expandthe cell volume based on the lattice parameters calculation.There was a 10 times enhancement in the room temperatureresistivity (109 Ω cm) of the synthesized materials by theaddition of dopants. Materials with such high values of roomtemperature resistivity (ρRT) are suitable for use in thereduction of eddy current losses in radio frequency circuits,high quality filters and transformer cores. The observed lowervalues of reflection losses for the synthesized samples haveshown that such materials might have applications in micro-wave absorption and electromagnetic attenuation. The powdersof such ferrite materials are ideal for the use in the high qualityfilters for the development of low electromagnetic radiationreflecting materials because of their high dielectric andmagnetic losses, and resistivity.

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