Journal of Magnetism and Magnetic Materials 323 (2011) 2137–2144

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Journal of Magnetism and Magnetic Materials

0304-88

doi:10.1

n Corr

E-m

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

Tailoring of structural, electrical and magnetic properties of BaCo2 W-typehexaferrites by doping with Zr–Mn binary mixtures for useful applications

Muhammad Javed Iqbal a,n, Rafaqat Ali Khan a, Shigemi Mizukami b, Terunobu Miyazaki b

a Surface and Solid State Chemistry Laboratory, Department of Chemistry, Quaid-I-Azam University, Islamabad 45320, Pakistanb WPI Advanced Institute for Material Research, Tohoku University, 2-1-1 Katahira, 980-8577 Sendai, Japan

a r t i c l e i n f o

Article history:

Received 28 October 2010

Received in revised form

4 March 2011Available online 15 March 2011

Keywords:

Hexaferrites

Co-precipitation

Electronic characterization

Magnetic properties

Mossbauer analysis

53/$ - see front matter & 2011 Elsevier B.V. A

016/j.jmmm.2011.03.009

esponding author. Tel.: þ92 51 90642143; fa

ail address: mjiqauchem@yahoo.com (M.J. Iqb

a b s t r a c t

Single-phase W-type hexaferrite, BaCo2Fe16�2x(ZrMn)xO27 (x¼0.0–1.0), has been synthesized by the

chemical co-precipitation technique. Mossbauer analysis indicates substitution of Zr ions on tetrahedral

(4e and 4fIV) sites and Mn ions on the octahedral ‘4fVI site’ at low-doped concentration when the

concentration is increased Mn ions but show preference for the octahedral ‘2b site’. The highest

enhancement in the value of the room temperature resistivity of 2.82�109 O cm has been obtained by

doping with Zr–Mn content of x¼0.6. The dissipation factor increases from 6.49�103 to 9.97�103 at

10 kHz with the addition of Zr–Mn dopants. Such materials are potentially suitable for electromagnetic

attenuation purposes, for microwave absorption and as radar absorbing material. High values of

saturation magnetization (67 emu/g) and remanent magnetization (34.7 emu/g) are obtained for

substitution level of x¼0.4 making them suitable for data processing devices.

& 2011 Elsevier B.V. All rights reserved.

1. Introduction

Ferrites are important magnetic materials that, find its applicationin almost 70–80% of the electronic materials such as home appliances,communication equipments and data processing devices [1,2]. Havinga high coercivity and moderate magnetization, hexaferrites arevaluable from technological point of view. W-type hexaferrites aresuitable for competing in the field of magnetism because of their highsaturation magnetization as compared to the most widely used othertypes of ferrites [3–7]. Furthermore, with large magneto-crystallineanisotropy in easy axis, W-type hexaferrites are applicable for bothlongitudinal and perpendicular recording media.

Owing to the crystallographic view, the cations with the exceptionof the Ba2þ ions in W-type ferrites occupy seven non-equivalent sub-lattices, i.e. 12k, 4fVI, 6g, 4f (octahedral coordination), 4e, 4fIV (tetra-hedral coordination) and 2d (bipyramidal coordination) [8,9]. How-ever, there are only five magnetically non-equivalent sub-latticesfrom magnetic perspective, i.e. 4e and 4fIV that merge into fIV,magnetic sub-lattice while 6g and 4f combine to 2b sub-lattice andcalled as b magnetic sub-lattice [10]. All these sub-lattices are presentin different number of R-blocks (Ba containing layer) and S-blocks(spinel layer) that construct W-type ferrites [11].

In the last few years variations in different properties of ferritessuch as cation distribution, electrical resistivity, saturation magneti-zation, coercivity and remanence, were made possible by substitutionof manganese ions. For example, Mn–Zn doped spinel ferrites,

ll rights reserved.

x: þ92 51 90642241.

al).

synthesized by combinatorial synthesis, show the effect of variousadditives on the formation of high performance materials by sinteringthem at lower temperature [12]. Synthesis of Mn–Zn doped spinelferrites with Fe-poor composition and investigation of their electro-magnetic properties found that the measured value of electricalresistivity (105 O cm) is significantly higher than that of theFe-rich Mn–Zn ferrite and the Curie temperature decreases withsubstitution of diamagnetic Zn ions [13]. Variations in the crystallitesizes were found to be due to Mn substitution in nickel ferrite, whichin turn affected the structural and magnetic properties [14].

In the present work, important modification of structural, elec-trical and magnetic properties of W-type hexaferrites is achieved bydoping with a relatively small amount of the transition metals. Themodification of the above-mentioned properties is suitable for theirapplications in various electrical devices employed for industrial andmilitary applications. The chemical co-precipitation technique [15]has been used to synthesize homogeneous nano-crystallites ofhexagonal ferrites because of the simplicity of this technique withrespect to both composition and morphology.

2. Experimental

2.1. Materials

The raw materials utilized in the present work are BaCl2 �4H2O(Analar 99%), Co(CH3COO)2 �4H2O (Merck 99%), Fe(NO3)3 �9H2O(Sigma-Aldrich 98%), ZrOCl2 �8H2O (BDH 96%), Mn(CH3COO)2 �4H2O(Merck 98%), NaOH (Merck 98%) and Na2CO3 (Merck 99%).

M.J. Iqbal et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 2137–21442138

2.2. Synthesis of BaCo2Fe16O27 nanoparticles

Samples of composition BaCo2Fe16�2x(ZrMn)xO27 (x¼0, 0.2, 0.4,0.6, 0.8 and 1) were prepared by chemical co-precipitation method.For this stoichiometric ratio, the corresponding chemicals weredissolved in distilled water and thoroughly mixed to form a homo-geneous mixture. Raising the temperature of the solution to 353 Kunder constant stirring, a mixture of NaOH and Na2CO3 in ratio of 5:1was added dropwise to increase the solution pH to 11 at whichprecipitates were formed. The precipitates were washed 3–4 timeswith distilled water and finally dried at 373 K. The dried precipitateswere ground and annealed at 1153 K in a temperature programmedtube furnace at a heating rate of 5 K min�1. Pellets of 13 mmdiameter and 2 mm thickness produced by compressing powderedsamples at 68.9 MN m�2 were employed for measurements of theirelectrical resistivity, dielectric behavior, etc. The powdered sampleswere used to carry out the magnetic measurements.

Fig. 1. Powder XRD patterns of BaCo2Fe16�2x(ZrMn)xO27.

Table 1Structural parameters for BaCo2(ZrMn)xFe16�xO27 (x¼0, 0.2, 0.4, 0.6, 0.8 and 1).

Dopant content (x) Crystallite size, D (nm) Cell volume, V (�A3)

0 42 984

0.2 52 995

0.4 35 1002

0.6 35 1017

0.8 42 1020

1 47 1023

2.3. Characterization techniques

X-ray diffraction analysis was carried out by a Phillips X’ PertPRO 3040/60 diffractometer employing CuKa as a radiation source.Scanning electron microscope (Hitachi S-3400N) was employed formorphological studies. Mossbauer analysis is carried out usingSEECo MSCI Mossbauer spectrometer with NRD-I 43-DMB, CN2cryostat, running in constant acceleration mode with a source of50 mCi 57Co in Rh matrix. The model of the Lorentzian multipletanalysis was used to analyze the data obtained. This model permitsto set several Lorentzian singlets, doublets or sextets, correspond-ing to paramagnetic sites with or without a quadrupole splitting. Itis also capable of identifying sites with a magnetic hyperfine fieldand the quadrupole shift within the first order perturbation limit.Furthermore, the model assumes that the hyperfine interactionsare static and the observed line shapes can be modeled asLorentzian lines. DC electrical resistivity was measured in atemperature range of 298–673 K using a well-known two-pointprobe method. Dielectric studies were performed on the silver-coated pellets of the size mentioned before, using LCR meter bridge(Wayne Kerr LCR 4275). We performed magnetic measurements atroom temperature with an external field up to 15 kOe usingvibrating sample magnetometer (Model: VSM-5-15 AUTO)equipped with electromagnets (TEM-WF86R-153). The ac suscept-ibility of the samples was measured with a laboratory-built acsusceptometer using a lock-in amplifier (Stanford Research SR 830,Sunnyvale, CA).

3. Results and discussion

3.1. Structural analysis

X-ray diffraction patterns for BaCo2Fe16�2x(ZrMn)xO27 (x¼0,0.2, 0.4, 0.6, 0.8 and 1) hexaferrites along with the standard pattern(00-019-0098) are shown in Fig. 1. Pure W-type hexagonal ferritesare generally obtained by sintering at temperature Z1273 K butwe are able to synthesize a single W-hexaferrite phase at relativelylow annealing temperature of 1193 K. The X-ray density (rx), cellvolume (V) and crystallite sizes (D) are calculated using the well-known equations [15–17] and are shown in Table 1. The observedvalues of the lattice parameters ‘a’ and ‘c’ are in close agreementwith the reported values for hexagonal ferrites [16]. The latticeparameters ‘a’ and ‘c’ as a function of Zr–Mn content are shown inFig. 2, indicating that both ‘a’ and ‘c’ increase with increase inconcentration of the dopant. The phenomenon relates to the ionicradii of the substituted ions. Since the ionic radius of Fe3þ (0.67

�A)

is smaller than the ionic radii of Mn2þ and Zr4þ (0.80�A) ions, the

increase in the lattice parameters is due to the larger ionic radii ofthe substituted ions. For this reason by increasing the concentra-tion of the doped metal ions, the lattice parameters also increase.The Fig. 2 also shows that the ratio of the parameters c/a remainsconstant for all the doped samples indicating that the relativeincrease in both the lattice parameters ‘a’ and ‘c’ is equal.

X-ray density, rx (gm�3) Bulk density, rx (gm�3) Porosity, P (%)

5.33 4.57 14.2

5.30 4.41 16.8

5.29 4.29 18.9

5.23 4.17 20.3

5.24 4.11 21.6

5.24 3.97 24.2

0.05.685.705.725.745.765.785.805.825.845.865.885.905.92

Content, x

a

c

c/a33.30

33.35

33.40

33.45

33.50

33.55

33.60

33.65

33.70

33.75

0.2 0.4 0.6 0.8 1.0

Fig. 2. Lattice parameters ‘a’, ‘c’ and ‘c/a’ ratio versus composition x of

BaCo2Fe16�2x(ZrMn)xO27.

Fig. 3. SEM photographs of (a) BaCo2Fe16O27, (b) BaCo2(ZrMn)0.2Fe15.8O27 and

(c) BaCo2Zr1Mn1Fe15O27 hexaferrites.

M.J. Iqbal et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 2137–2144 2139

Deviations from Vegard’s rule [18] are not noted and the depen-dence of cell dimension on substitution is linear as ratio of theparameters c/a remains constant for all of the doped samples. Thecell volume (V) that is directly dependent on the lattice parametersincreases with increase in the concentration of Zr4þ-Mn2þ con-tents. The bulk density (rm), as deduced from the Archimedesprinciple [19], increases with increase in the dopant concentrationlevel while X-ray density remains almost constant. The crystallitesize (D) calculated by Scherrer formula [20] is found to be in therange of 35–52 nm.

Fig. 3 depicts the scanning electron microscopic images of thesamples annealed. It is evident from Fig. 3a that the undopedcompound is homogeneous but the crystallite coagulation occursat a doping level of x¼0.2 (Fig. 3b), which leads to grain growth.The grain growth phenomenon increases up to the dopantconcentration of x¼1 (Fig. 3c). Manganese ions are reported toaccelerate the process of grain growth and favors densifica-tion [21]. The porosity of material is also increased for largergrains as fewer number of grain boundaries are present.

3.2. Mossbauer analysis

Mossbauer spectra of BaCo2Fe16�2x(ZrMn)xO27 (x¼0.0–1) areshown in Fig. 4, confirming the pattern of pure W-type hexafer-rite [8,22]. The spectra were analyzed in terms of superposition ofsix Zeeman sextets. The assignments of sub-lattices for differentpeaks were made based on the scheme adopted for W-typehexagonal ferrites [8,10] demonstrating different positions of ironmagnetic sub-lattices in the structure. Mossbauer spectrum of theundoped sample shows that the k1 sub-lattice is noticeablydistinguishable and is distinct. Adjacent to it, another sextet ofk2 amid a quadrupole splitting and an isomer shift (Table 2)appears and can be ascribed as Fe3þ ions in the 12k lattice sites.Similar results were also obtained in case of BaMg2Fe16O27, [23]and for BaZn2�xCoxFe16O27 [24]. Although at higher substituentconcentration, the peak splitting of 12k sub-lattice could not bedifferentiated and appear to have merged in a single peak withlow intensity and sufficient broadening (Table 2). This may be theresult of change in the surroundings of the Fe3þ ions on the 12ksite due to dopant substitution. However, as the dopant concen-tration level increases the Heff for both k1 and k2 decreasessomewhat compared to the k1 and k2 values of the undopedsamples, as shown in Fig. 5(a).

The intensity of each sextet is directly proportional to thenumber of iron atoms and thus gives an estimate of the number

of dopant cations that are substituted for iron at a particular site.Analysis of the area ratios of the Mossbauer spectra (Fig. 5b)confirms that the dopant ions enter the fIV and 4fVI sites and thereis corresponding increase in the areas of both 2b and 12k sites.There is also a decrease in the area of the trigonal bipyramidal 2dpattern, suggesting substitution at this site. This result is in

Fig. 4. Mossbauer spectra for polycrystalline BaCo2Fe16�2x(ZrMn)xO27.

Table 2Mossbauer parameters obtained for various sub-lattices using Lorentzian line

fitting procedure, where DE is the quardrupole splitting, d is the chemical shift and

! is the line width.

Content (x) Sublattice DE (mm s�1) d (mm s�1) ! (mm s�1)

0 4fVI 0.018 0.310 0.133

0.2 �0.064 0.256 0.171

0.4 �0.079 0.340 0.097

0.6 �0.145 0.187 0.128

0.8 �0.113 0.266 0.129

1 �0.103 0.232 0.257

0 fIV (4e and 4fIV) �0.029 0.104 0.167

0.2 0.031 0.203 0.169

0.4 �0.112 0.213 0.097

0.6 0.211 0.316 0.152

0.8 �0.170 0.178 0.178

1 0.025 0.172 0.205

0 2b (6g and 4f) 0.045 0.122 0.162

0.2 0.049 0.145 0.226

0.4 �0.004 0.171 0.241

0.6 �0.027 0.083 0.285

0.250 (2b0) 0.147 (2b0) 0.526 (2b0)

0.8 0.181 0.256 0.205

0.703 (2b0) 0.977 (2b0) 0.147 (2b0)

1 0.048 0.206 0.204

0.247 (2b0) 0.79 (2b0) 0.378 (2b0)

0 2d 0.033 0.176 0.236

0.2 �0.169 0.162 0.138

0.4 0.333 0.243 0.231

0.6 0.002 0.191 0.198

0.8 �0.085 0.160 0.097

1 0.062 0.163 0.202

0 k1 0.191 0.224 0.159

0.2 0.177 0.224 0.146

0.4 0.205 0.263 0.165

0.6 0.163 0.228 0.193

0.8 0.147 0.188 0.212

1 0.157 0.241 0.236

0 k2 0.192 0.224 0.209

0.2 0.131 0.079 0.097

0.4 0.231 0.286 0.285

0.6 – – –

0.8 – – –

1 – – –

M.J. Iqbal et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 2137–21442140

accordance with the previously reported results for the Zr sub-stituted M-type hexaferrites [25]. At higher substitution level, themanganese ions occupy the octahedral (6g and 4f) site, i.e. 2b site.Due to occupation of manganese ions at octahedral 2b site, a fewmanganese ions may have been converted to the trivalent statethat in turn results in the formation of divalent ferrous ions whoseconcentration is enough for detection by the Mossbauer spectro-meter. In fact the 2b sub-lattice is split into two sextets and ofthese two, one may be attributed to the Fe2þ ions occupying the2b0 site. We postulated that some of the Mn ions at the octahedralsite are present in the trivalent state, which is in completeagreement with the results obtained by the same authors in caseof M-type hexaferrite [26]. Furthermore, this assumption supportsthe resistivity trend, i.e. room temperature resistivity of thesamples is decreased at a dopant concentration level of x¼0.8and onward. Our results for Mn occupation are in contradictionwith the assumption made by other authors using neutrondiffraction studies [5].

A plot of hyperfine interactions versus dopant concentrationlevel shown in Fig. 5(a) suggests that hyperfine interactions for fIV

site decreases from 491 to 471.6 mm/s, which further confirmsthe substitution of non-magnetic ion on this site, i.e. the Zr4þ

ions. This occupation of diamagnetic zirconium ions at fIV site

results in the aforementioned broadening of 12k sub-lattice. Ascan be seen in the case of x¼0.8 the isomer shift value (Table 2) isabout 0.97 mm/s, which definitely shows the presence of Fe2þ

ions in the lattice and it can be attributed to 2b site. This isbecause of greater concentration of Zr4þ at the tetrahedral site sothat the iron ions migrate towards the octahedral site. However,some of the Fe3þ ions are converted to the Fe2þ ions due tocharge imbalance in the lattice.

3.3. Electrical properties

3.3.1. Electrical resistivity measurements

Fig. 6 shows the plots of dc resistivity versus temperature fordifferent samples. The plots show that the resistivity increasesas the temperature increases up to 380 K, indicating metal tosemiconductor transition temperature (TM–S). Similar behaviorwas also observed in case of hexaferrites [17,27] and in the caseof spinel ferrites [28]. Several factors may stand responsible forthe observed phenomenon such as phase transition, cationmigrations, cation reordering, magneto-transport effect, etc. How-ever, these factors can be considered only at higher temperatures.Therefore, for the case under consideration the particular beha-vior can be due to the spin canting [28] because this is the onlyproperty that can be thought to occur in the temperature range of

300

350

400

450

500

content, x

Hef

fkO

e

4fVIfIV2b2dk1k22b'

0.00

10

20

30

40

50 4fVIfIV2b2dk1k22b'

rela

tive

area

, %

content, x0.2 0.4 0.6 0.8 1.0

0.0 0.2 0.4 0.6 0.8 1.0

Fig. 5. (a) Hyperfine magnetic fields at iron nuclei and (b) relative area of various

sublattices for polycrystalline BaCo2Fe16�2x(ZrMn)xO27.

300

05

101520253035404550556065707580

Temperature/ K

Res

istiv

ity 1

08 / Ω

cm

x = 0x = 0.2x = 0.4x = 0.6x = 0.8x = 1

350 400 450 500 550 600 650 700

Fig. 6. Variation in resistivity with temperature for different hexaferrite samples. 1.412

13

14

15

16

17

18

19

20

21

22

23

1000/T (K-1)

ln ρ

x = 0x = 0.2x = 0.4x = 0.6x = 0.8x = 1

1.6 1.8 2.0 2.2 2.4 2.6 2.8

Fig. 7. Arhenious plots of electrical resistivity of BaCo2Fe16�2x(ZrMn)xO27

hexaferrites.

M.J. Iqbal et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 2137–2144 2141

298–383 [29]. Hexaferrites form a closed packed oxygen ionlattice with metal ions situated at both the tetrahedral (A) andthe octahedral (B) sites. These metal ions are presumably isolated

from each other and the charge carriers could be localized ateither the ionic or vacant sites [30]. Due to lattice vibrations, theions occasionally come closer to each other and the transfer ofcharge carriers takes place to induce conduction. Above 373 K, theresistivity rapidly decreases with temperature, due to enhance-ment in lattice vibrations. Furthermore, the reason for theexponential decrease in resistivity with increase in temperature,representing a semi-conducting type behavior, is due to enhancedelectron exchange between the Fe2þ and Fe3þ ions occupying theB-site. The electronic hopping between octahedral–octahedralsites becomes a dominant mode of conduction at highertemperatures [31].

The values for activation energies (Ea) for electron hoppingcalculated from the Arrhenius type plots of log r versus 1/T(Fig. 7) are given in Table 3. It is evident from Table 3 that theroom temperature electrical resistivity (rRT) increases to a value of109�O cm for a dopant level of x¼0.6; it however decreases withfurther increase in the dopant concentration. According to theresults obtained using Mossbauer spectroscopy the Mn2þ ionsexhibit preference for octahedral site (4fVI and 2b) and hence theywould replace the Fe3þ and Fe2þ ions [32]. The activation energy forelectron hopping between Mn2þ–Fe3þ and Mn2þ–Mn3þ ions ishigh; therefore the resistivity increases with the dopant concentra-tion. However, beyond certain concentration level there is a possi-bility of the presence of Fe2þ ions in enough concentration that mayresult in the lowering of room temperature resistivity. The increasein room temperature resistivity guarantees the material’s diminu-tion of eddy current losses and can be used in radio frequencycircuits, high quality filters, rod antennas and transformer cores.

3.3.2. Dielectric studies.

Figs. 8 and 9 depict variations in the dielectric constant (e0) andloss tangent (tan d) with frequency (f), respectively. The dielectricconstant is high at low frequencies and its value decreases as thefrequency of the applied field increases, representing a normaldielectric behavior of ferrites. At lower frequency, the dipolar andinterfacial polarization contributes significantly to dielectric con-stant but at higher frequency only the electronic rather than thedipolar polarization becomes significant. As a result, the value ofdielectric constant is high at low frequencies and its valuedecreases as the frequency shifts to higher values. For dielectricloss in ferrites, the primary cause is the presence of multivalent

Table 3Electrical and magnetic parameters for BaCo2(ZrMn)xFe16�xO27 (x¼0, 0.2, 0.4, 0.6, 0.8 and 1).

Dopant

content

(x)

Resistivity, r(108 O cm at 298 K)

Activation

energies

(eV)

Dielectric

constant (e0)at 10 kHz

Dissipation

factor

at 10 kHz

Saturation

magnetization

(emu/g)

Remenance

magnetization

(emu/g)

Curie

temperature,

TC (K)

0 1.22 0.49 2115 6492 62.7 32.9 700

0.2 2.96 0.52 2067 835 64.9 31.8 678

0.4 10.7 0.56 1739 9973 67.0 34.7 655

0.6 28.2 0.59 700 8880 61.6 30.9 633

0.8 7.42 0.55 2143 6013 55.7 25.7 610

1 0.89 0.46 2278 6272 53.0 26.8 574

6-2

0

2

4

6

8

10

12

14

16

18

20

ln F

ε'/10

3

x = 0x = 0.2x = 0.4x = 0.6x = 0.8x = 1

8 10 12 14

Fig. 8. Plot of dielectric constant (e0) of Zr–Mn substituted BaCo2 hexaferrite

samples versus applied frequency (F).

60

2

4

6

8

10

12

14

16

ln F

tan

δ

x = 0x = 0.2x = 0.4x = 0.6x = 0.8x = 1

7 8 9 10 11 12 13 14

Fig. 9. Plot of the dielectric loss (tan d) as a function of logarithm of frequency (F).

0.00

500

1000

1500

2000

2500

-2024681012141618202224262830

Res

istiv

ity 1

08 / Ω

cm

Die

lect

ric c

onst

ant (

ε')

Content, x

Dielectric constant at 2000 Hz

0.2 0.4 0.6 0.8 1.0

Fig. 10. Variations in dielectric constant (e0) at frequency 10 kHz and room tempera-

ture resistivity (r) versus concentration level x for BaCo2Fe16�2x(ZrMn)xO27 (x¼0, 0.2,

0.4, 0.6, 0.8 and 1).

M.J. Iqbal et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 2137–21442142

iron ions, Fe3þ and Fe2þ , which permits excess of electrons in thematerial to jump from one valence state to another [33]. Thisprovides a means of electrical conduction and hence poor dielec-tric losses. The presence of Co2þ and Co3þ ions on octahedral(B) sites give rise to p-type carriers’ contribution to net polariza-tion in addition to n-type carriers as can be seen from the

relaxation peaks in the loss tangent (tan d) with frequency, whichis strongly dependent on the polarization process.

Fig. 9 shows that in case of un-doped sample, the relaxationpeak for loss tangent is intense, which represents the presence ofboth iron ions and cobalt ions in the sample. However, relaxationpeak intensity decreases with increase in the dopant concentra-tion level because the substitution of impurity atoms reduces thenumber of iron ions in the sample. Furthermore, the substitutionof diamagnetic ions has been reported [34] to reduce tan d; in thiscase, zirconium is a diamagnetic ion. As both electrical conduc-tivity and dielectric behavior are transport properties whosevariations are proportional to the sample composition, similarmechanism may be responsible for these two phenomena. Thevariations in dielectric constant e0 and the room temperatureresistivity (rRT) with changes in sample composition at a fre-quency of 10 kHz are shown in Fig. 10. The dielectric constantdecreases with the dopant concentration level of xo0.6 butincreases on further increase in dopant concentration. It can beinferred from these observations that electrical resistivity isaffected by the sample composition in a manner opposite to thatof dielectric constant.

The loss factor D is the loss tangent (tan d) times the dielectricconstant (e0) [35] and its magnitude provides a measure of theenergy dissipated in a dielectric when subjected to an oscillatingfield. The plot of dielectric loss factor (D) with frequency is shownin Fig. 11. Its value decreases with an increase in the appliedfrequency. The dissipation factor directly depends on the tan dvalue; therefore as the frequency of applied field approaches therelaxation frequency, the polarization response increasingly lagsbehind the applied field. The reorientation of each dipole isopposed by the internal friction, which leads to heating in thesample and power loss. This loss may be important at electric

-15000-80

-60

-40

-20

0

20

40

60

80

H (Oe)

M (e

mu/

g)

x = 0 x = 0.2 x = 0.4

-10000 -5000 5000 10000 150000

Fig. 12. Hysteresis loops for BaCo2Fe16�2x(ZrMn)xO27 hexaferrite nanoparticles.

0.0

34

36

38

40

42

44

46

48

50

52

54

Content, x

Cry

stal

lite

size

, D

D

Hc

800

1000

1200

1400

1600

1800

2000

Coe

rciv

ity, H

c

0.2 0.4 0.6 0.8 1.0

Fig. 13. Correlation between crystallite sizes and coercivity of BaCo2Fe16�2x

(ZrMn)xO27.

70

10

20

30

40

50

60

70

80

90

Loss

fact

or/1

03

ln F

x = 0x = 0.2x = 0.4x = 0.6x = 0.8x = 1

8 9 10 11 12 13 14

Fig. 11. Plot of loss factor (D) of Zr–Mn substituted BaCo2 hexaferrite samples

versus applied frequency (F).

M.J. Iqbal et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 2137–2144 2143

field frequencies near the vicinity of relaxation frequencies foreach of the operative dipole types for a specific material. Table 3shows the values of dissipation factor at a frequency of 10 kHz forall the synthesized samples. The highest values of D is obtainedfor a dopant level of x¼0.4 and 0.6 rendering the material to beused for absorption of microwave radiation, radar absorbingpurposes and electromagnetic interference attenuations.

3.4. Magnetic measurements

3.4.1. Hysteresis loops

M–H loops for some selected substituted W-type hexaferritesare shown in Fig. 12. The dependence of coercivity (Hc) on dopantcontent x is given in Fig. 13 that exhibit random behavior. It isassumed that the content of Zr–Mn has little effect on thecoercivity but more or less depends on the particles size of thematerials, i. e. for smaller particles the value of coercivity is largeand vice versa. This assumption is well supported when wecompare the crystallite sizes of the samples given in the samefigure. Previously it was noted that the coercivity values for cobaltsubstituted hexaferrites determined by various researchers lie inthe range of 50–200 Oe [36,37] but in our case we have obtainedthe Hc value of up to 1862 Oe, which is much higher than that ofthe previously reported values. For nanocrystalline materials, thecoercivity is closely related to the grain size ‘D’ of the materialsdepending on the exchange length Lex. For ‘D’ greater than Lex, Hc

varies inversely with the grain size (1/D) and for Fe based alloys,Lex is estimated to be 35 nm [38]. It is reported before in case ofBaCoZnFe16O27 [36] that Lex may be much smaller than 35 due tothe small values of exchange constant and large value of mag-neto-crystalline anisotropy constant. Consequently, the sameassumption holds for substituted BaCo2Fe16O27 with the grainsizes in the range of 35–41, which are larger than the exchangelength. Hence, it could be implicit that in our case the variationsin the coercivity are due to the particle size of the hexaferritenanoparticles.

Table 3 reports the saturation magnetization versus composi-tion at room temperature for the Zr–Mn substituted BaCo2

W-type hexaferrites. Being intrinsic in nature, saturation magne-tization depends mainly on phase and chemical composition.The substitution of Zr–Mn for iron initially leads to an increase inthe saturation magnetization that exhibits a maximum at x¼0.

4 having a value of 67.0 emu/g, which further decreases to thevalue of 53 emu/g when substitution is complete. This appears tobe due to the statistical distribution of Zr, Mn and Co ions in sevendifferent sub-lattices in W-type hexaferrites. Previously it wasreported that Co ions are located at 4fIV site with spin down and6g site with spin up direction [39]. Moreover, we pointed outearlier in the Mossbauer section that at lower concentration Zrand Mn ions occupy the tetrahedral sites, which has downwardspin direction. Although the Mn ions have a magnetic moment of5mB, which is comparable to that of Feþ3, but in case of Zr, it isdiamagnetic and therefore reduces the downward spin and theupward resultant spin is enhanced. The decrease in the magne-tization value above some dopant level of x is due to theaccumulation of non-magnetic Zr ions at the tetrahedral site,which results in the weakening of super-exchange interactionbetween tetrahedral A site and octahedral B site giving rise torandom canting of magnetic moment [29]. Remanent magnetiza-tion has a trend like that of saturation magnetization. Thesevariations in magnetization, coercivity and remanent magnetiza-tion make them suitable for data processing devices.

300

10

12

14

16

18

20

1/χ

(a. u

.)

Temperature K

x = 0x = 0.2x = 0.4x = 0.6x = 0.8x = 1

400 500 600 700

Fig. 14. Plot of magnetic susceptibility (w) of different Zr–Mn-doped samples,

BaCo2(ZrMn)xFe16�2xO27 (x¼0, 0.2, 0.4, 0.6, 0.8 and 1) versus temperature.

M.J. Iqbal et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 2137–21442144

3.4.2. Magnetic susceptibility measurements

The plot of inverse of magnetic susceptibility versus tempera-ture is shown in Fig. 14. The Curie transition temperature (TC) iscalculated by drawing slope through the paramagnetic region ofthe plots and intercepting at the x-axis. The values of TC calculatedfrom the magnetic susceptibility data are shown in Table 3. It canbe noticed that the transition temperature decreases with anincrease in concentration level of the dopant. The decrease in thevalue of the transition temperature can be explained on the basisof magnetic moments of the substituted ions. The decrease in theCurie temperature with Zr–Mn content is due to the weakening ofsuper-exchange interaction with inclusion of non-magnetic Zrions in the lattice. Since the magnetic moment of Zr4þ ions is(0mB) as compared to the magnetic moment of Fe3þ ion (5mB).

4. Conclusions

Zr–Mn doped BaCo2 W-type hexaferrites with a nominalcomposition can be synthesized by chemical co-precipitationtechnique at a temperature of 1153 K that is comparatively muchlower for W-type hexaferrites than that reported in literature. TheXRD analysis conforms to the single-phase W-type hexaferriteand the SEM images show that the crystallites are more or lesscoagulated and the grain growth increased with increase in thedopant concentration. The lattice parameters increase withincrease in the Zr–Mn content obeying Vegard’s law. It isconcluded using the Mossbauer analysis that the Zr ions sub-stituted at the tetrahedral 4e and 4fIV sites while manganese ionsprefer octahedral site (4fVI) at low-doped concentration and whenthe concentration of the ions increases the manganese ions showpreference for octahedral 2b site. The room temperature resistiv-ity of 2.82�109 O cm is obtained for the dopant concentrationlevel of dopant x¼0.6, showing suitability of this material forreduction in the eddy current losses in radio frequency circuits,high quality filters and transformer cores. Dielectric constant, losstangent and dissipation factors for the synthesized samples weredetermined and the result shows that the material can be used formicrowave absorption, radar absorbing and electromagnetic

attenuation purposes. High values of saturation magnetization(67.0 emu/g), remanent magnetization (34.7 emu/g) and coerciv-ity (1861 Oe) make these materials to be utilized as data proces-sing devices. The magnetic susceptibility measurements revealedthat the transition temperature decreases with increase in zirco-nium ion concentration.

Acknowledgment

Higher education commission of Pakistan (HEC) supportedthis work.

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