ARTICLE IN PRESS

Journal of Magnetism and Magnetic Materials 321 (2009) 3967–3973

Contents lists available at ScienceDirect

Journal of Magnetism and Magnetic Materials

0304-88

doi:10.1

� Corr

E-m

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

Could Mg content control the conduction mechanism of Ba Co Zn-W-typehexagonal ferrites?

M.A. Ahmed a,�, N. Okasha b, R.M. Kershi c

a Material Science Laboratory (1), Physics Department, Faculty of Science, Cairo University, Giza, Egyptb Physics Department, Faculty of Girls, Ain Shams University, Cairo, Egyptc Physics Department, Faculty of Science, Ibb University, Ibb, Yemen

a r t i c l e i n f o

Article history:

Received 18 June 2008

Received in revised form

24 June 2009Available online 4 July 2009

Keywords:

W-type hexaferrite

XRD

Transition temperature

Conductivity

Dielectric property

53/$ - see front matter Crown Copyright & 2

016/j.jmmm.2009.07.002

esponding author.

ail address: Moala47@hotmail.com (M.A. Ahm

a b s t r a c t

Electrical properties as a function of composition, frequency and temperature for a series of W-type

hexagonal ferrites with the general formula BaCoZn1�xMgxFe16O27; 0rxr0.6 prepared using the

conventional ceramic method were studied. These samples are semiconductor-like materials, where the

ac conductivity increases with increasing temperature. The results show that the conduction

mechanism depends on the Mg2+ substitution. The transition temperature (Ts) increases with

increasing Mg content and gives a hump at x ¼ 0.5; after that Ts decreases again. Both the ac

conductivity and dielectric constant vary with Mg content and reach the highest value at x ¼ 0.5, due to

the highest value of the ratio of Fe2+/Fe3+ at x ¼ 0.5. The peak value of the dielectric constant depends on

the Mg content x.

Crown Copyright & 2009 Published by Elsevier B.V. All rights reserved.

1. Introduction

In the last decades ceramic magnetic materials, capable ofcombining a high resistivity and permeability, are found innumerous products used in our daily life such as home appliances,electronic devices, communication equipments and computers[1]. Ferrimagnetic materials can have one of three mainstructures: spinel, garnet and hexagonal [2]. For the hexagonalstructure, there are six possible different types designated M, W, Y,Z and U. The W-type hexagonal ferrites BaMe2Fe16O27 (where Meis any divalent element), have a crystalline structure built up as asuperposition of S and R blocks, where the S block has the formulaFe6O8 and R block has the formula BaFe6O11. The unit cell iscomposed of the sequence RSSR*S*S*. Between S and S*, andbetween RR*, a 1801 rotation about the c-axis occurs [3]. However,the presence of divalent and trivalent cations distributed amongvarious sublattices makes the W-type hexaferrites very interest-ing for basic studies and different technical applications, sincetheir characteristics may, in principle, are varied by substitution ofboth divalent and trivalent cations. Also, they have proved the wayfor the development of a wide variety of applications frommicrowave to radio frequencies. W-type barium ferrite is used forelectromagnetic wave absorber [4,5] and radar absorbing materi-als due to their relative complex permeability, high magnetizationand planar anisotropic behavior [6]. In addition, dielectric and

009 Published by Elsevier B.V. All

ed).

magnetic properties such as permeability, saturation magnetiza-tion, etc., can be controlled by substitution of divalent or trivalentions [7,8] as well as the preparation condition such as sinteringtemperature, time and rate of heating and cooling. Extensivestudies have been made for the frequency and temperaturedependence of the dielectric constant and permeability of W-typehexaferrite [9–11]. Few studies are available on the electricalproperties of W-type hexaferrite [12–17].

The electrical properties of hexaferrites are important due totheir applications at high frequencies. Rana et al. [18] and Abo El Ataet al. [19] have reported the electrical properties of W-typehexaferrites BaCu2�xZnxFe16O27 and BaCo2�xNixFe16O27, respectively.The results suggested that the conduction mechanism in bothsystems is due to the hopping of electron between Fe2+ and Fe3+ ionsalong with the hole transfer between Ni2+ and Ni3+. They found thatthe conductivity increases by the replacement of Co2+ by Ni2+ ions.

The aim of the present work is to study the effect of Mg2+

substitution on the dielectric constants, the electrical conductivityand magnetic characterization for BaCoZn1�xMgxFe16O27 hexafer-rite in order to reach a composition at which the samples found tobe an effective way of improving the quality of material forpermanent magnet device applications.

2. Experimental techniques

W-type hexagonal ferrite with the formula BaCoZn1�xMgx-

Fe16O27; 0rxr0.6 was prepared by the solid-state reaction using

rights reserved.

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150

x = 0.6

0

50

100

150

(008

)

(110

)

(001

2)

(216

)

(21

9)

(211

1) (211

4)

(222

)(116

)

(201

)

(209

)

M.A. Ahmed et al. / Journal of Magnetism and Magnetic Materials 321 (2009) 3967–39733968

high-purity cobalt, Zinc, Magnesium and ferric oxides, in additionto barium carbonate. The conditions of preparation were men-tioned elsewhere [20]. X-ray diffraction was carried out usingScintag (USA) diffractometer with CuKa radiation, the details ofthe X-ray parameters were mentioned elsewhere [20]. The twosurfaces of the sample were good polished to obtain uniformthickness and coated by sliver paste and checked for goodconduction to be ready for measuring the dielectric propertiesand the ac conductivity (sac) using Hioki LCR Hi tester type 3531(Japan) as a function of temperature from 300 to 750 K andfrequencies from 100 kHz to 4 MHz. The temperature wasmeasured using T-type thermo-couple with junction in contactwith the sample. The accuracy of measuring temperature wasbetter than 71 1C.

Int

ensi

ty

x = 0.1

0

40

80

120

x = 0.4

0

40

80

120

x = 0.3

0

50

100

x = 0

0

40

80

120

20

.

x = 0.5

0

50

100

2θ

30 40 50 60 70

Fig. 1. X-ray diffraction patterns for ferrite BaCoZn1�xMgxFe16O27; 0rxr0.6.

3. Results and discussion

Fig. 1 shows the X-ray diffraction patterns at room temperaturefor the investigated samples. The patterns show that all sampleshave a single phase W-type hexagonal ferrite without notableresiduals of the original constituent oxides. The average values ofthe lattice parameters were calculated from the diffractogramsand it was found that W-type structure has an averagelattice parameter of (a ¼ 5.94 A) which agree well with otherauthors [21].

Fig. 2 illustrates the variation of with temperature at differentfrequencies for the investigated samples. It can be seen from thefigure that in the low-temperature region the effect oftemperature on the dispersion of is weak. After that,increases with temperature until reaching a maximum value.The dispersion of at low frequencies is much greater than that athigh frequencies. After maximum value decreases rapidly withincreasing temperature until reaching a minimum value afterwhich it increases again. This behavior of with temperature canbe explained as follows: at relatively low temperature, the electricdipoles cannot orient themselves with respect to the direction ofthe applied field; therefore, they possess a weak contribution tothe polarization as well as to the dielectric constant . As thetemperature increases, most of electric dipoles get enoughexciting thermal energy to be able to follow up the change inthe external field. This enhances the contribution of the dipoles tothe polarization mechanism leading to an increase in thedielectric constant . The decrease in after its maximum valueis due to: the overcoming the thermal energy on the field effectand decrease the internal viscosity i.e. decrease . The increase inafter minimum value means the participation of another type ofpolarization such as Maxwell Wagner [22,23]. It was expectedthat, the size of the conducting grains increases and the isolatinggrain boundary decreases with increasing temperature [24]. Thisin turns helps in the migration of free charge carriers through thecrystal under the influence of the applied field and they may betrapped by vacancies which were caused by a non-uniformdistribution of oxygen ions during sintering [25,26]. Therefore, alocalized accumulation of charge is created. In other words, thelocalized accumulation of carriers will induce its image on theelectrode rising the dipole moment as well as [27]. Decreases ofwith increasing frequency are due to the decrease in the numberof the electric dipoles which follow the field variation.

Fig. 3(a, b) correlates the dependence of the transitiontemperature obtained from and the peak transition value ofon the Mg content, respectively, as a function of frequency. Thefigure showed that, the dielectric constant of the sample withMg content ¼ 0.5 has higher values than the rest of theinvestigated samples. This may be due to the highest Fe2+/Fe3+

ratio on octahedral and tetrahedral sites at x ¼ 0.5, so the highest

value of the polarizability can be obtained at the same Mgcontent.

Fig. 4 illustrates the ac conductivity (lnsac) and the reciprocalof absolute temperature (1000/T) as function of the appliedfrequency in the range (100 kHz–4 MHz). The data show thesemiconducting like behavior of BaCoZn1�xMgxFe16O27

hexaferrites where the value of activation energy lie in the small

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x = 0.1

00.20.40.60.8

11.21.41.61.8

2

x= 0.0

0

0.6

1.2

1.8

300

100 kHz200 kHz400 kHz800 kHz 1 MHz 2 MHz 3 MHz 4 MHz

x = 0.3

0

1

2

3

4

x = 0.5

0

1

2

3

4

x = 0.6

0

0.1

0.2

0.3

T (K)

100 kHz200 kHz400 kHz800 kHz 1 MHz 2 MHz 3 MHz 4 MHz

T (K)

ε ×

105

400 500 600 700 800

300 400 500 600 700 800

300 400 500 600 700 800

300 400 500 600 700 800

300 400 500 600 700 800

Fig. 2. The real part of dielectric constant ( ) a function of temperature T (K) (300–800 K) at different applied frequency (100 kHz–4 MHZ).

Fig. 3. (a) The change in the position (transition temperature) of the dielectric constant ( ) peak with Mg content (x) at different frequency. (b) Values of dielectric constant

( ) (peak value) as a function of Mg content (x) at different frequency.

M.A. Ahmed et al. / Journal of Magnetism and Magnetic Materials 321 (2009) 3967–3973 3969

ARTICLE IN PRESS

x = 0.0

-14

-12

-10

-8

1

x = 0.1

-14

-13

-12

-11

-10

-9

x = 0.3

-14

-13

-12

-11

-10

-9

-8

100 kHz200 kHz400 kHz800 kHz 1 MHz 2 MHz 3 MHz 4 MHz

x = 0.5

-15

-12

-9

-6

x = 0.6

-17

-14

-11

-8

100 kHZ200 kHZ400 kHz800 kHz 1 MHz 2 MHz 3 Hz 4 MHz

ln σ

(σ =

Ω-1

.cm

-1)

1.5 2 2.5 3 1 1.5 2 2.5 3

1 1.5 2 2.5 31 1.5 2 2.5 3

1 1.5 2 2.5 31000/T (K-1)

1000/T (K-1)

Fig. 4. Correlation between the electrical conductivity (lns) and reciprocal of absolute temperature 1000/T.

M.A. Ahmed et al. / Journal of Magnetism and Magnetic Materials 321 (2009) 3967–39733970

range as that of semiconductor, where the increase in the acconductivity with increasing temperature, means that, thetrapped charges are liberated, which increase the exchange ofelectrons between Fe2+ and Fe3+ ions. The dispersion of s wasappeared in the low-temperature region and relatively highfrequency. This dispersion in s decreases with increasing thetemperature for all samples. In the high-temperature region, the svalues seem to be slightly frequency independent. This result canbe explained on the basis of the relation [26] sac(o,T) ¼ sdc(T)+s(o, T), where the first term is a temperature-dependent term representing the dc electrical conductivitywhich is related to the drift mobility of the free charge carriers,and the second term is a frequency and temperature-dependentterm which is related to the dielectric relaxation of the boundcharger carriers. The first term is predominant at low frequenciesand high temperature, while the second term is predominant athigh frequencies and low temperatures. The frequencydependence of the second term can be written as [28,29] s(o,T) ¼ B(T)os(T) , the exponent s can be calculated as a functionof temperature for each sample by plotting lns versus lno Fig. 5.

Fig. 6(a, b) shows the variation of the parameter s with thetemperature for all the studied samples. It is shown that s isapproximately temperature independent for sample with x ¼ 0.1s

decreases with increasing the temperature for samples withx ¼ 0.0, 0.5 and 0.6 and for x ¼ 0.3 s increases with increasing

temperature up to about 400 K, then decreases with increasingtemperature. These results of s (T) means that the quantummechanical tunneling mechanism [30,31] is the most probableconduction mechanism for the sample with x ¼ 0. For the sampleswith x ¼ 0.0, 0.5, 0.6, the behavior of s with the temperature maybe explained on the light of the correlated barrier hoppingconduction mechanism [32]. In the case of the sample withx ¼ 0.3, the small polaron tunneling model is the predominantone in the low-temperature region but the hopping conductionmechanism is more pronounced in the higher temperature region[33]. Below the transition temperature (Ts where a break in lnsvs. 1000/T is obtained), both the field and applied frequencyaligned the charge carriers in the same direction of the field, sothe conductivity in this case increases with temperature. While byincreasing temperature above Ts the disturbance of the chargecarriers in the different directions takes place, through theconductivity is decreased as shown in Fig. 4. Table 1 illustratesthe values of the activation energies EI, EII and EIII as a function ofMg content (x). From the data, EI is smaller than EII and both EI andEII increase with Mg content up to x ¼ 0.5 after which theydecrease. The activation energy after transition temperature (EIII)decreases, which may be due to the decrease in the ultra-thininsulating increasing of the conducting grains and decreasingthe size layers [24]. Consequently, the number and velocities ofthe free charge carriers increase under the applied field through

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ln σ

(σ =

Ω-1

.cm

-1)

x = 0.0

-16

-14

-12

-10

-8

6

300 K350 K400 K450 K500 K550 K600 K

x = 0.1

-13

-12

-11

-10

-9

350 K400 K450 K500 K550 K600 K

x = 0.3

-14

-12

-10

-8

300 K350 K400 K450 K500 K550 K600 k

x = 0.5

-14

-12

-10

-8

300 K350 K400 K450 K500 K550 K600 K

x = 0.6

-15

-14

-13

-12

-11

-10

300 K350 K400 K450 K500 K

7 8 9 10 11 12

6 7 8 9 10 11 12

6 7 8 9 10 11 12

6 7 8 9 10 11 12

6 7 8 9 10 11 12

ln ω (ω = Hz)

ln ω (ω = Hz)

Fig. 5. Correlation between the electrical conductivity (lnsac) for the investigated samples versus lno.

M.A. Ahmed et al. / Journal of Magnetism and Magnetic Materials 321 (2009) 3967–3973 3971

the crystal and in turns increases the conductivity of thesample.

The transition temperature (Ts) of the investigated samplesfrom ferrimagnetic state to paramagnetic state varies with Mgcontent as in Fig. 7. The results in the figure show an increase in Tswith increasing Mg content up to a hump at x ¼ 0.5 after that itsdecreases again. This means that the substitution of Mg2+ insteadof Zn2+ ions with xr0.5 allow some of Mg2+ ions to migrate andoccupy octahedral (B) sites in S blocks [33,34]. Accordingly, Fe3+

ions are transferred from octahedral sites to tetrahedral (A) sitesin blocks S instead of Zn2+ ions which lead to an increase inFe3+–Fe3+ (AA) and Fe3+–O–Fe2+ (AB) hopping as well as Ts.Further increase of Mg2+ content after x ¼ 0.5 leads to the inversemigration of ferric ions. This will decrease the AA and AB hoppingas well as Ts. The data in Table 1 gives lower values of Ts thanthose of TC, this is in agreement with previously studies [13,35,36].This is because we expect that the ac field acts in cooperation with

thermal energy to accelerate the change from ordered todisordered state. Also, the transition temperature Ts wasascribed to the change in the conduction mechanism and not tothe magnetic transition [13]. The values of the transitiontemperature Ts for the investigated samples are higher thanthose of the previously reported data for the samplesBaZn2�xMgxFe16O27 [13]. This may be due to the existence ofCo2+ ions with magnetic moment equal to 5.2 B.M [37] in theinvestigated samples.

Fig. 8 (a–c) illustrates the dependence of ac conductivity (lns)on Mg content (x) as a function of frequency and temperature.From the figure, it can be seen that the ac conductivity variesslightly with Mg content and increases at x ¼ 0.4 to reach highestvalue at x ¼ 0.5 at all frequencies and temperatures. This may beattributed to highest value of the hopping electrons between Fe3+

and Fe2+ ions in tetrahedral and octahedral sites at suchconcentration.

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s (P

aram

eter

)

T (K)

0

0.1

0.2

0.3

0.4

0.5

0.6

x = 0.0x = 0.6x = 0.5

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

200

x = 0.1x = 0.3

300 400 500 600 700

Fig. 6. (a, b) The variation of the parameter s with the absolute temperature T (K).

Table 1Values of Curie temperature (TC), transition temperature (Ts) and activation

energies (EI, EII) as a function for Mg substitution.

T (K) 100 kHz 1 MHz

x TC (K) Tr (K) EI (eV) EII (eV) EI (eV) EII (eV)

0 790 634 0.14 0.43 0.12 0.300.1 722 568 0.04 0.73 0.03 0.340.3 753 578 0.12 0.60 0.10 0.340.5 800 613 0.12 0.86 0.12 0.430.6 783 549 0.10 0.27 0.08 0.25

0 0.1 0.3 0.5 0.6

500

520

540

560

580

600

620

640

Mg (content)

T σ (K

)Fig. 7. Variation of transition temperature with Mg content (x).

800 kHz

-14

-13

-12

-11

-10

-9

-8

400 K500 K600 K

ln σ

(σ =

Ω-1

.cm

-1)

200 kHz

-14

-13

-12

-11

-10

-9

-8

400 K500 K600 K

M.A. Ahmed et al. / Journal of Magnetism and Magnetic Materials 321 (2009) 3967–39733972

4. Conclusion

4MHz-8

�

-9

The BaCoZn1�xMgxFe16O27 hexaferrites system has semicon-ducting like behavior, where the ac conductivity increases withincreasing temperature.

� -10 The transition temperatures (Ts) of the investigated samples

vary with the Mg content and the highest value was obtainedat the critical concentration x ¼ 0.5.

-11400 K

�

-12

500 K600 K

The values of the activation energies EI and EII below thetransition temperatures vary with Mg content, where both EI

and EII values increase with Mg content up to x ¼ 0.5 thendecrease.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 �

Mg content (x)

The maximum value of ac conductivity and the dielectricconstant ( ) was obtained at x ¼ 0.5.

Fig. 8. (a–c): The change of lns with Mg content (x) at selected frequencies and

� temperatures.

The transition temperature in dielectric constant measure-ments depends strongly on the Mg content.

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M.A. Ahmed et al. / Journal of Magnetism and Magnetic Materials 321 (2009) 3967–3973 3973

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