Indian Journal of Pure & Applied Physics Vol. 41, September 2003, pp. 731-738

Effect of charge transfer behaviour on the dielectric and ac conductivity of Co-Zn ferrite doped with rare earth element

M A Ahmed & M H Wasfy*

Physics Department , Faculty of Science, Cairo University, Giza Egypt

*Physics Department, Faculty of Education , Suez Canal University, AI Ariesh , Egypt

Received 19 August 2002; revised 17 April 2003; accepted 2 July 2003

The dielectric constant E' and ac conductivity were measured fo r the ferrite samples Co 1 .,Zn,L~l.2 5Fe 1750~ ; O.l :Sx:S0.7 at different temperatures as a function of the applied frequency. The dielectric constant was interpreted on the basis o f changing the enthalpy as well as :he internal energy of the system. The orientational and rotational polarizations play a significant role in increasing the value of£' in the second temperature region . The conductivity results give more than one straight line, indicating the different conduction mechanisms. The electron hopping between the iron ions and the hole hopping between the cobalt ions are the main conduction processes in the investigated samples as it was enhanced from the drift mobility data. The values of the acti vation energy obtained, indicate the semiconducting behaviour of the samples.

[Keywords: Dielectric constant, ac Conductivity, Ferrite, Rare earth element]

1 Introduction

In the past few years, ferrites have attracted the attention of many physicists and chemists 1

·5

, due to their wide range of technical applications, especially at high frequency ranges . They are characterized by their high resistivity, high dielectric constants and low losses r, . Ferrites are used in most of the electronic equipments, telecommunication devices, and some of them, especially those with high absorption coefficients, can be used as radar absorbing paints.

The usefulness of ferrites is influenced by the physical and chemical properties of the materials and depends on many factors, including the preparation conditions, such as, sintering temperature, sintering time, rate of heating, cooling and grinding time. Though, one can control and improve the ferrite properties till they reach the optimu m conditions of preparation, which fit the required applications. ZnFe20~ is a normal spinel ferrite7

·9 while CoFe20 4 is an inverted spinel 10

•11

,

therefore, the mixed ferrite Co 1 .xZnxL~125Fe 17504 is an interesting materi al, which attracted many authors, to study their properties. The variation of the ioni c radii of the metal ions forming the spine l system will not vary the un it cell dimensions over a wide range, because the stability of the spinel

structure takes place only if the cations (divalent and trivalent) are of medium size.

Although, many applications of ferrites, as ceramic materials , require high denisty to achieve the desired properties, there are many applications for lower density ferrites , where high surface area is preferred . Lower temperature methods for ferrite preparation offer many potentially attracti ve features fo r synthesis of materials, particularly when the materials are used as simulated corrosion products. This method is not used in the present work.

The objectives of the present study is to investigate the effect of substitution of Zn 2

+ ions in place of Co2

+ ions (in the ferrite material), on the electrical properties of the samples. The role of La1

+

ions substitution on the octahedral site was also discussed .

2 Experimental Details

Analar grade ox ides-CoO, ZnO, La20 3 and Fe20 .1 were used to prepare the ferr ite Co 1.xZnxL<lt125Fe 1 750 4 . 0 .1 5ox5D.7. Stoichi ometric ratios were taken to prepare the samples us ing standard ceramic technique 12

• Good mi xing for the materials was carried out for four hours using agate mortar and then transferred to a ball -mill fo r another

732 INDIAN J PURE & APPL PHYS , VOL 4 1, SEPTEMBER 2003

Table I -Values of act ivation energy in eY in the low £ 1 and high £ 11 temperature regions, respectively

.f(kHz) x= 0.1 x=0.3 x=0.5 x= 0.7

£, Eu £, Eu £ , Eu £, En

100 0.456 0.630 0.565 0.983 0.430 0.398 0.240 0.364 200 0.465 0.598 0.594 0.932 0.438 0.270 0.265 0.243 410 0.494 0.523 0.597 0.881 0.454 0.334 0.297 0.266 600 0.478 0.4 13 0.613 0.815 0.424 0.299 0.260 0.346 800 0.443 0.398 0.628 0.830 0.469 0.3 18 0.276 0.365 1000 0.488 0.353 0.61 1 0.864 0.445 0.303 0.262 0.467 2000 0.473 0.363 0.657 0.899 0.386 0.320 0.286 0.532 3000 0.492 0.404 0.644 0.851 0.371 0.3 17 0.250 0.657 4000 0.662 0.406 0.833 1.050 0.597 0.324 0.333 0.632 5000 0.600 0.392 0.945 1.357 0.386 0.346 0.127 0.623

4888,-------------------------------, [coun-t.sJ

3588

Fig. I - XRD patterns for the samples with x = 0.3 , 0.5, 0.7

two hours, and then the samples were inserted into a muffle furnace type Lenton UAF 16/5 (UK), using a platinum crucible. Pre-sintering was carried out at 850 °C, for 30 h, at a heating rate of 4 "C/min, and then cooled down to room temperature, at the same rate as that of heating. The samples were ground again and pressed into pellet form of thickness :::::: 1.5 mm and diameter :::::: 0.9 em, using a pressure of 5 x I o~ N/m\ and then sin tered at II 00 °C, for 90 h with the same rate as that of pre-sintering.

IR and X-ray analysis were carried out to assure that, the samples were formed in the spinel phase. The two surfaces of each pellet were polished well to remove the surface layer, from which the zinc ions were evaporated, then coated with a thin film of a si lver paste and checked for good conduct ion.

The dielectric constant as we ll as the ac resistivity measurements were performed using HIOKI LCR tester model 353 1 (Japan). The measurement accuracy was ±1 %. Reproducibility of

AHMED W ASFY: EFFECT OF CHARGE TRANSFER ON Co-Zn FERRITE 733

the data was also checked to assure that , the crystallinity of the samples do not change with heat treatment during the col lect ion of the experimental data. The temperature was measured using a T-type thermocouple with its junction in contact with the samples, to avoid the temperature gradient. The accuracy of measuring the temperature was better

N 15 0

• 100kHz D 200kHz A400 kHz • 600 kHz X 800kHz

>< e 1 MHz

-~ 10 I +2 M~~ Hz

5

6 '0 'K -~

JOO

•100kHz D 200kHz A 400 kHz X 500 kHz X 800kHz

e1 M~z +2 MHz - 3 MHz

- 4 MH.t • s MHZ

450 600 750

T(K)

...... • •• • ••

450 600 750 T(K)

than ±1 °C.

3 Results and Discussion

Fig. I represents the XRD patterns performed for the three samples having the compositions, Co 1.,ZnxLa0 _25Fe w 0 4 , where 0.3:Sx:S0.7, al l in one separate figure. The chart shows the phase spinel

20

J •100kHz (b) X= 0 .3 11200 kHz

N 15 ~~~~ ~ X800kHz >< e1 MHz

-w10 ~; ~:

N 6 0 -)(

4

-4 MHz 0 5 MHZ

450 600 750 .T(K)

=~~~ (d) X =0 .7 ••••

A400 ""Hz .•• a X6CXlkHz • m

!F~~~ ••• • _.r#J +2 MHz • !J

- 4 MHz ..... ~ 05 MHz :O:''E - ~ MHz ~· .,;oD /LA

2!8~~ ·o

0 ~----------~------------------------~~ JOO 450 600 7ff.J

T (K)

Fig. 2-- (a-d): Dependence of the dielectric constant E' on the absolute temperature, Jt d ifferent frequencies for

Co1_,ZnxLao.25Fe1750 4; 0.1 :Sx :5 0.7

~rr~~~--------------------.,OOkHz l (a)x=0.1

10

5

• 200kHz

~~~~~~ X BOO kHz e 1MHz + 2MHz - 3MH:z

- 4MHz 0 SMHz

400

B rr.~1;;00;:;kH~z~­.200kHz

6 · A 400 kHz )( 600kHz X BOO kHz e1 MHz +2 MHz - 3 MHZ

- 4 MHZ • S MHz

500 600

(C) X= 0 .5

.... . ~ ... .. "'". • •

700

••• • •

• • • • ..... __. . • ••

•

• .• AA ...

800

T(K)

. ~ o -~----------....... ~llii~&iii~iii·L-1 300 <50 600 750

T(K)

15 ------------

JOO

•100 kHz D 20UkHz A 400kHz e sookHz X GOO kHz e 1 MHz +2 MHz -3 MHz

·-4 MHz O S MHz

0 D-., 300

450

.. 450

-----·---------(b) X= 0 .3

600 750 T(K)

(d) x= 0 .7 ---- •• •~

•• ID I •• • .. ·.. I

~L J 600 750

T(K)

Fi g. 3-- (a-d): Dependence or the d ie lec tric Loss factor E" on the absolu te temperature, at d i ITen.:n t frequenci es for Co 1_

xZn ,Lao.2sFe l.7s04. at 0. 1 :5 X :5 0.7

734 INDIAN J PURE & APPL PHYS, VOL 41, SEPTEMBER 2003

Fig. 4 - (a-b): Dependence of the dielectric constant r ' on the applied frequency , at different tixed temperatures for the sample CoJ .xZnxLan.2sFeus0 4 at x = 0.1, 0.3

.... ----·-----

X" 0.7

-8

~~~itr1 E luhu. + u 0 • g_ -to 0 - 0

0 0 0 .: 0 0 0 - .o + + 0 ~ 0 0 0 0 0 0 0 0

b 0 0 - - f • ---:•, - .: c: :; + t - - • - " -' - - • ll

+

0 + ~ -: • • oo!l·e+ + + I + I • + • ... • - 12 ... D f I • +

I • ... ! ... i • ll ... •

- - • ' I ll ...

• ll • • • • A * • • * I '"'

... • I ... • t • • ll • • •'"' I I • I

ll • • + • •tOO kHz iii • • • ll ... • • • • B200kH.z I I •

- 14 • +

A 400kHz • • + + • I • e 600kHz

JC BOO kHz • • • • • • e l MHZ • +2

- 16 MHZ

•3 MHz

- 4MHz 0 5 MHz

- 18

1.3 1 5 1.7 1.9 2 .1 2.3 2 .5 2.7 2.9 3.1

1000 IT (k"' )

Fi g. 5 - Vari ation of the ac conductivity with the reciprocal of absolute temperatu re, at different frequencies

formation.

' Fig. 2(a-d) correlates the real part of the dielectric constant E' and the absolute temperature as a function of the applied frequency which ranges from I 00 to 5 MHz at different Zn concentrations. From Fig. 2 it is clear that, the general behaviour of the data is nearly the same, but the peak position and height vary, depending on Zn content in the

sample and the applied frequency . Two main regions were obtained, the first one, in which E' is nearly temperature-independent and varies slightly with frequency . The second region in which E' varies dramatically with both temperature and frequ ency. The first region ends at about 450, 475 , 500, 600 K, for 0.1 $x.:S0.7, respectively. More than one type of polarization is expected to partic ipate in the

AHMED WASFY: EFFECT OF CHARGE TRANSFER ON Co-Zn FERRITE 735

die lectric process, the electronic polarization is the most predominant, in the first temperature region . Orientational, rotational and Maxwell-Wagner polarizations are expected to play a role in the second temperature region . Also, in this region, the thermal energy is too small to release the charge carriers from their localized positions, giving ri se to small va lues of£' at all Zn concentrations.

-5 ,----

T = 700K

-6

)i g -7

I? ...J

-8

-9 +--------~--01 0.2 0.3

In the high temperature region, the thermal energy is quite sufficient to free more charge carriers and the field accompanied wi th the applied frequency orients them in its direction , through increases in polarization as well as £' . From the point of view of frequency effect on £', one can say that, the normal behaviour of a dielectric is due to the fact that, beyond a certain high frequency of

0 .4 0 .5 0 .6 0.7 0 .8

Zinc Concentration (xl

j: [ ,..

Fig. 6- Dependence of Ln cr on the Zn Concentration in the ferrite Co 1 _,Zn,L~125Fe~.7,04 ; 0 .1 :5 x :5 0 .7

-1 X= 0 . 3

t .E -06 6 400 kH~

)( 6(X) kHz

X 800kHz

1.E-06 ...... 0 + +2 ......

-3 MH<

"' 1.E..Q6 -· ...... O S ...... + I

lS a ... B.E-07

)( : A

6 .E-07 . "" ><

. . -

4 E-07

... . s I >< )( A

2E-07

O.E.OO a a ~6 a a a n " n a I D 0 1 o I

X

0 10 A : •

i i ! . <>

I . . . . . 310 410 510 6 10

T(K)

Fig. 7 - Variation of the drift mobility 11 with the absolute temperature, as a function o f the app li ed frequency at x = 0. 3 for the sample Co 1._.Zn .. L~1.2sFeu504

736 INDIAN J PURE & APPL PHYS, VOL 41 , SEPTEMBER 2003

app lied ac electric field , the electron exchange between Fe"• and Fe1+ ions and hole transfer between Co1+ and Co2+ ions cannot fo llow up the alternating frequency of the applied field 11

•

Above the trans ition point, the dielectric

constant r.' decreases at all frequencies . Thi s can be attributed to the increase in the lattice vibrations, the increase in the e lectron lattice scattering, the decrease of the in terml viscosity, and the increase in the internal friction of the dipoles, as well as the thermal energy dissipati on inside the sample. In other words, the general behaviour of r.' can be explained on the basis of Koops theoryl 1 for the inhomogenous double-layer dielectric stmcture . This dielectric stmcture was supposed to be formed of two conducting layers separated by a thin nonconduct ing layer1w (Maxwell -Wagner type). The apperance of the low frequency re laxation is related mainly, to the grain boundaries and the decrease in r.' at lower frequenc ies is large enough to display a noticeable peak in dielec tric loss (Fig. 3).

The dependence of the dielectric loss factor r." on the absolute temperatu re as a function of the applied frequency is shown in F ig. 3(a-d). From Fig. 3, it is clear that, two distinct regions are obtained, the first one from room temperature, up to 450 K, for x=O.l and 0.3, in which, r." is frequency and temperatu re-dependent, while for x=0.5 and 0 .7, the first region extends to 550 K. The second region is frequency and temperature-dependent, in which r." reaches a maximum value and then decreases. From a closer look of Fig. 3, one can find that, wi th increase in the frequency , r." is decreased. A lso, r." decreases with increasing cobalt content, except for x=0.7. The peak was shifted towards higher temperature with increasing frequency. This means that , in the first temperature region, the dielectric loss factor is small , while at high temperature region , the change in enthalpy b.H, increases wi th increasing temperature. In other words , the internal viscos ity of the system is decreased with heating, leading to more degrees of freedom for the d ipoles, wi th the result of increasing fr iction as well as r." . Si milar behaviour has been reported 11

,.,7

. The increase in b.H , is believed to be attributed to the fas t relaxation effect of Co2+ ions on octahedral site, wh ich is based on their crystal fie ld stabilization energy. A lso, the observed increase and decrease in

r." can be explained by considering the effect of Co2+ on the an isotropy of the material and enhancement of spin lattice relaxation rates, leadi ng to a larger value of b.H. The decrease in r." after the peak value can be ascribed to the participation of another type of polarization , at this high temperature region , such as, Maxwell-Wagner polarization. Fig. 4(a-b) en hances the expectations about the dependence of the relaxation process on both temperature and frequency.

Fig. 5 is a typical curve which correlates the ac conductivity In a and the reciprocal of absolute temperatu re for the ferrite sample Co ,.xZn,Lao25Fe 1750 .), at x=0.7. From Fig. 5, it is c lear that, three distinct regions are obtained and that, they obey the well known Arheniu s re lation cr=cr.. e _E,kT' where cr is the coefficient of the

e lectrical conducti vity , cr .. is the exponent constant , E is the activation energy, k is Boltzman constant, and T is the abso lute temperature. The first region from room temperature up to 520 K, in which no noticeable variation of 0 with temperature is observed (metallic behaviour region) . This region is frequency- dependent. The second reg ion which extends from 520 up to 606 K is frequency and temperature- dependent. The third region covers the rest of the temperature range. This region is temperature- dependent and slightly frquency­dependent. More than one straight line is obtained in the conductivity data, indicat ing the different conduct ion mechanisms.

The hopping mode l is the most predominant one . The hopping process may be either electron hoppi ng between ions of diffe rent valences ( Fe2+ f.-+

Fe'+ + e-), or hole hopping between cobalt ions where Cn2

+ is changed into Co'+, (Co3+ f.-+ Co'+ + e+).

The replacement of La'+ ions of larger radius (rL/+ = 1.154 A) than Fe1+ ions (rFc.l+ = 0.641 A) on octahedral sites helps in initiating some defects which act as trapping centers for the charge carriers . By increasing the temperature, these trapping centers become sources of charge carriers. Though one expects an increase in the electrical conductivity

· with increas ing temperature after the metallic behaviour region. The data in the figure enhances such expectation . The values of the calculated activation energy in both low and high tempreature regions indicate the semiconducting behaviour of the investigated samples. The results of Ramana 1x

enhances the results of the authors, where the

AHMED WASFY: EFFECT OF CHARGE TRANSFER ON Co-Zn FERRITE 737

dilution of cobalt by zmc tons decreases the conducti vty.

Fig. 6 correlates the variation of Ina and zinc concentration, from which a continuous decrease in cr with increasing zinc content is observed. This observation is in good agreement with the results reported by Rezlescu et al.19

, who found that, the resistivity of Li-Zn ferrites increases by increasing the zinc content.

The dependence of the drift mobility (~), on the absolute temperature at different frequencies as the typical curve at x=0.3 is shown in Fig. 7, as ~ was calculated from the relation cr=ne~, where n is the number of the charge carriers per unit volume, and is given by the relation, n = (NA.p)IM, where NA, is Avogadro ' s number, p is the density, and M is the molecular weight of the sample, e is the electronic charge, and it equals 1.6x I o-19 C. From the figure it is clear that, the mobility is nearly constant from room temprature, up to 51 0 K, which corresponds to the metallic behaviour of Fig. 5. After 510 K, the mobility increases with increasing temperature. Thi s means that, the variation of the conductivity samples is due to thermally activated mobility and not to thermally created charge carriers. Also, the presence of cobalt on the octahedral sites of the spinel results in the conduction mechani sm 211 Co2++ Fe3+ ~ Co3+ + Fe2+, which is predominant in Co-Zn ferrite.

Generally, Fig. 5 shows that, the kinks in the neighbourhood of the Curie point has been explained by the theory given by Irkin et a U 1

•

Theoretically, it was shown that , on passing through the Curie point, a change must occur in the gradient of the straight line of the conductivity and the amount of change depends on the exchange interaction between the outer and inner electrons22

,

which in turn alter the Curie point. On a closer look to the activation energy values, Table I shows that, their values in the paramagnetic region is larger than those of the ferrimagnetic region . This result agrees with those of Irkin et a/. 21

•

In conclusion, one can state that, at the Curie point, the slope of conductivity curves is changed . In addition, increasing the zinc content in the sample increases its resistivity. Also, the increase in the conductivity is due to thermally activated mobility and not to thermally created charge

carriers. The presence of lanthanum with fixed concentration helps in initiating vacancies, which in turn helps the .:onduction process of the system, at a certain temperature region . The hopping mechanism, either by electrons or holes is the main process of conduction in the investigated samples .

References

Patil A N, Mahajan R P , Patankar K K. Gh atage A K. Mathe V L & Patil S A, Indian J Pure & Appl Phys. 38 (2000) 651 .

2 Koshy J, Kuri an J, Komar K S, Saj it h P K, Asha M J & Jose R, Indian J Pure & Appl Phvs , 34 ( 1996 ) 698.

3 Hegde M S, Sahana M & Satyal akshmi K M, Indian .I Pure & Appl Phys , 34 ( 1996) 674.

4 Bellad S S, Lokande C D & Bhosale C H, Indian .I Pure & Appl Phys, 35 ( 1997) 565.

5 Modi K B, Joshi H H & Kulkarni R G. Indian .I Pure & Appl Phys, 34 ( 1996) 92.

6 Goldman Alex, Modem f errite technology (Marcel Dekker, New York), 1993.

7 Ok H N & Evens B J , Phys Rev B. 14 ( 1976) 2956.

8 Dobson D C, Linnet J W & Rahman M M, .I Phys Chem Solids, 3 1 ( 1970) 2727.

9 Tang H, Du Y W, Qiu Z Q & Wal ker J C , .I Appl Phys, 63 ( 1988) 4105.

10 Vanoon H M, Marteens J W D & Peeters W L, Mater Res Bull, 20 ( 1985) 4 1.

II Murthy V R & Sobhanadir J, Phys Sta tus Solidi (a ). 36 ( 1976) K 133.

12 Ahmed M A, Bi shay S T & El-Khawas E H, .I Mat Sci Lett, 19 (2000) 791 .

13 KoopsCG, Phys Rev,82 ( 1951 ) 121 .

14 Haberey F F & Wijin H P, Phys Status Solidi (a) , 26 ( 1968) 231.

15 Maxwell J, Electricity & magnetism (O xford University Press, London), 1873, Section 328.

16 Van Hook H J & Di onne G F, AI P Con{ Proc, 24 ( 1975) 487 .

17 Dionne G F, .I Appl Phys , 57 ( 1985) 3727 .

738 INDIAN J PURE & APPL PHYS, VOL 41, SEPTEMBER 2003

I ~ Raman a Reddy A V, Ranga Mohan G, Boyanov B S, Ravinder D, Mater Lei/ , 39 ( 1999) !53.

19 Rezlescu , Condurach D, Petrariu P & Luca E, J Am Cerwn Soc, 57 ( 1974) 40.

20 Parker R, Griffiths B A & Elwell D, Br J Appl Phys, 17

(1966) 1269.

2 1 lrkin Y P & Turnov E A, Sov Phys JETP, 33 ( 1957) 673.

22 Smit J, Wijn H P J, Ferrite.1· (Phili ps Techni cal Library. London) 1959, p. 234.