Effect of Zn Dilution on the Electrical Properties of CO1-xZnxLayFe2-yO4 (0.1≤X≤0.9 and γ = 0.25)

Indian J. Phys. 11A (4), 359-364 (2(K)3)

5 I J PA \

E^ect of Zn dilution on the eiectrical properties of C0i-,Zn;,La,Fei^O4 (0.1<x<0.9 and^' = 0.25)

( | AIxicllatil

Department ol Physics. Faciilt| ol Science, Cairn Universily, Gi/a, F^^ypi E-m ail |nnala47 (g4totmaiI.com

Received 14 Aufiiist W 02, im cptcd IJ behruarv 2(X)d

A b stra c t The A C conductivity, dielectric constant n and diclcctiic loss lacior c ” were measiircil lor the leriile sam ples C’oi 7n,La,Fc2.,0.4 (0 I S \ < 0 4 and y = 0 2S), prepared by standard ceramic technique X-ray and IR analysis were earned out to assure the formation o f the spinel .samples The measurements were performed at dilfcrent temperatures from 300 K upto --'800 K The conductivity results were discussed on the basis o f hopping mechanism o f electrons and holes where the activation energy values are m the semiconducting region fhe electron exchiinge between the dilfcrent cations in the same equivalent lailice sites or in difterent sites, plays a role in the conductivity behaviour.

The ineasiiremenis ol the dielectric constant e ' reveals a dcuease with the increase of both ficqiicncy and Zii-content The optimum condition ol the critical concentration was \ = 0.7 The deci'ca.se in viscosity and lattice vibration as well as the phonon generation, help in increasing energy dissipation and in turn r "

K e y w o rd s . Ferrite .samples, electiical properties, cinnposition and temperature dependence

P A C S N os. 72 1,5 Bb, 75 50.Gg, 72.20 Ec

L Introduction

In the pa.st decades, many physisls and chemists were very interested to study the electrical properties ol’ ferrites and rare earth ferrites because of their technological importance.

They are widely used in daily life such as in transfonner cores, inductors, data storage and microwave devices. These applications demand materials with high electrical resistivity, high permeability and low losses at higher frequencies. The.se useful applications can be achieved by careful control of composition and microstructure.

The rare earth ferrites also have many important applications in modern telecommunications and electric devices. For this reason, engineers and scientists arc keenly interested in new methods for preparation of rare earth ferrite to detenuine their optimum characterizations 1 1].

It is w ell known that the transport properties of Icrrimagnelic materials are intrinsically determined by the

properties of the electron hand slrucliire. This is in contrast to those of non-niagnclic materials which arc characterized by the superposition ol a broad .v-orhital and a narrow r/-orbilal with common Fermi level |2,3|. The temperature- dependence of the clcclrical conductivity of the rare earth metal compound has been the subject of many studies both by theoreticians and experimentalists. For example, the influence of spin wave in the electrical conductivity of .some magnetic compounds, has been investigated theoretically and the results are in good agreement with the experimental data 14-8J.

D etailed investigation o f the com position and temperalurc-dependcncc of both electrical conductivity and dielectric con.stant e / were carried out in view of the Zn- dilution. Also the effect of ions as a constant content was discu,sscd in view of the hopping process that takes place between the iron ions of different valences on the same equivalent lattice site.

© 2003 lACS

2. Experimental

A n a la r g ra d e form o x id e s (B D H ); C o O , Z n O , L a 2 0 i and w e r e u s e d lo prepare the rare earth Tcrrite o f the g e n e ra l fo r m u la Coi-rZn^LUyFe^-^Oa. S to ic h io m e tr ic ratios o f th e se o x id e s are m ix e d and g ro u n d ed to g e th er u s in g a g a te m ortar for fo u r h o u rs an d then transferred in to b a llm ill for a n o th er fou r h o u rs. D o u b le sin te r in g c e r a m ic te ch n iq u e is u sed for p rep a rin g the s a m p le s ( 9 |. P rc sin tc rin g w a s carried ou t in free a tm o sp h e r e u s in g L e n lo n fu rn a ce U A F 16/5 (E n g la n d ) w ith m ic r o p r o c e s s o r to co n tr o l th e p rep aration c o n d itio n s at 8 5 0 '’C for 3 0 hrs w ith h e a tin g rate o f 6 X / m i n and then c o o le d to room tem p era tu re w ith the sa m e rate as that o t h e a lin g . 'Fhc s a m p le s w e r e fin e ly g r o u n d ed a g a in and p r e sse d in to p e lle t form u s in g p ressu re o f 5 x 10^ N/m ^.

looorr (k')

1000/T |K ' )

Final sintering was perfonned at 1100°C for 90 hrs with the same rate as that of pre-sintering.

X-ray diffractograms and IR analysis were carried out using Diano corporation of target CoK,, (A = 1.79026

A)

The two surfaces of each pellet were polished and coated with silver paste (BDH) and checked for good conductU)ti RLC Hioki tester model 3531 (Japan) is used to mca.surc the dielectric properties and AC conductivity at diffcrcni frequencies. The temperature of the sample was mcasural using T-type thcnnocoupic with junction just in contact with the sample to prevent any temperature gradient. The accuracy of measuring the temperature is better than ± r C and that of measuring electrical properties is 1 %. Reproducibility of

1000/T <K ’ )

— ---1—

6

E

-10

c ;

100 kH/

O 400 kHz

C *14 ^{800 KHZ}

1 MHz - 1 6 -

- H l h - 2 MHz 3MHz

-18

— 4— 4MHz

— 5 MHz

1000/T (K ’ )

1000/T (K ’ )

J'igure l(a-c). Typical curve correlates In a venws the reciprocal of absolute temperature as a function of the applied frequency (100 kHz - 5 MHz)

Effect of Zn dilution on the electrical properties etc 361

ihc experimental data was carried out to assure that the crystallinity of the sample is kept constant.

3. Results and discussion

Figure l(a~e) correlates the AC conductivity, a , and the reciprocal o f absolute temperature for the samplics Coi-^ZnALayFc2-y04 as a function of the applied iVcquciKO'.

From the figure, it is clear that more than one straight liiic is obtained indicating the different conduction mechahisnis.

The data in the figure, reveals an increase in a values w |h increasing temperature and frequency. From a closer loik to the data, one can find that two or three distinct rcgiojjs arc observed. The first one represent the data at temperaturfes ranging from room temperature upto 570 K in which t|lc conductivity can be assigned as AC because its val|*c

\arics drastically from one frequency to another. Otter phenomenon is observed in this region for all samples where the conductivity increases with increasing frequency upto certain value which was rca.sonably expected because the hopping process between Co-"*^ and Co^"^ as well as Fe’"*^ and

is increased,

C o “-» ^ C o ^ ^ + c ; C()2^ ^ C o ' » - + F e - » -

The increase in the ratio of Fe^'^/Fe^^ is accompanied by an increase in the conductivity of the sample.

The second region of temperature (570-690 K) shows a decrease in hopping process as well as the conductivity.

This is due to the relatively high thciTnal energy given to the .sample which causes an electron lattice scattering.

In the third region of temperature (above 690 K), no noticeable variation in the conductivity ((jy ) with frequency IS observed. However, the total conductivity value can be considered as the sum of AC part (C7^) and DC part (<jy).

The presence of Co^^ ions on the octahedral sites, plays a significant role in the conductivity mechanism. This is

because the variation of the divalent Co ions to the trivaicnt ones increases the number o f electrons as well as conductivity. The presence of I .a^^ ions with small proportion in the octahedral site replaces the Fe^* ions in the .same site.

But since the atomic radius of La (2.74 A ) is larger than that oi Fe (1.72 A) and Co (1.67 ^A), therefore microstrain in the bulk ot the sample is produced. Accordingly, the holes are generated, though the conduction process in the investigated samples can be ascribed as due l(^ both electrons and holes. The calculated values of the activation energy (Table I) enhance our expectation about the semiconducting behaviour lor the investigated samples. In either words, the electrical conductivity m CO|_,Zn^LavFe>_,04 is explained on the basis of the transport phenomena of charge carriers through cation vacancies present on the octahedral site. All the above results arc in good agreement with the previous work |1()|. Generally, the results in the present work are explained on Ihc assumption that the sub.stituling Zn~"^ instead of Co-^ ions in the composition cau.scs the movement of Fe^^ ions from the tetrahedral to octahedral sites. This changes Fc^^ to ions during sintering process which in turn, changes the si/e of the crystal. 1‘his is due to the fact that the radius of the octahedral site is larger when it is occupied by Fe^*^ than by Co^"^ ions.

Figure 2(a-c) is a typical curve correlates the real part of the dielectric constant e ' and the absolute tcmpcraiure (from room temperature to -4 6 0 K as a function of frequencies from 100 kHz upto 5 M il/. From the figure, il is clear that the general trend of r' is the increase in its value as the temperature increases at each separate frequency.

The data in the figure can be divided into three different regions. Region I from room temperature upto about 500 K, region II above 500 K upto the transition temperature, and region III above the transition temperature. In region I, the values of e ' is nearly the same and temperature-independent in most of the investigated samples except x = 0.7 and 0.9.

Table 1. Values of the activation energy in the low (E\) and high (£n) temperature regions for the Coi ,Zn,LaT^2 0 I ^ i s 0 9 at dillcrcnt frequencies.

/ ^{X =} O.I X = 0.3 ^.X 0.5 ^{X ^} 0 7 ^.X=: 0 9

(Mllz) CitcV) £ii(eV) EiieW) ^{/ii,(e V ) El (eV)} Eii(cV) Ei(vV) /Tn ( fV ) A',(cV) / a d e V )

0 1 0 438 0.438 0.784 0.857 0.553 0.664 0 147 0 270 0 168 0.534

0 4 0.438 0.438 0.514 0.773 0.129 0.210 0 154 0 404 0.134 0.492

0 8 0.438 0.427 0.478 0 668 0 L55 0.225 0 101 0 376 0.141 0.462

1 0 0.374 0..399 0.485 0.626 0.159 0.223 0.114 0 327 0.038 0,148

2 0 0.314 0.327 0.401 0.637 0.194 0.233 . 0.083 0 337 0 120 0.410

30 0.549 0 928 0.534 0.682 0.211 0.249 0.113 0.139 0 099 0 397

4 0 0.419 0.643 0.593 0.740 0,334 0.517 0,185 0 440 0,094 0.469

5.0 0.391 0.655 0.717 1.043 0 426 0.541 0.210 0.393 0.239 0.397

This may be altributed to the direct elTcct of Zn dilution above x - 0.7 as well as the participation of electronic polarization which is tcmpcraturc-indcf^cndcnl. This means that X = 0.7 is considered as a critical concentration which will be discussed later in the AC conductivity results. Also the small variation of the dielectric constant e ' in this region is due to localization of the dipoles where the thermal energy is noi sufficient enough to make them free.

In the vSecond region of temperature, the dramatic increase in with temperature is due to the large thermal encru\

which is quite sufficient to liberate more dipoles and the field accompanied with the applied frequency aligned them in its direction, though increasing the polarizability as well as the dielectric constant e'. In other words, one can cxpeei that the participation of both orientational and rotational polarization play role in this region.

300 4 0 0

175

125

5 0 0 6 0 0

T ( K )

700

3 2 0 4 2 0 5 2 0 6 2 0

T(K)

300 4 0 0 SOO 6 0 0

T(K)

7 0 0 800

2000

160 0 -

10 0 0

5 0 0

♦ 100 kHz

■ 400 kHz A 800 kHz X 1MHz X 2MHz

# 3MHz

^ 4MHz - 5MHz

X = 0 7

720 3 2 0 420

T(K)

3 0 0 4 0 0 5 0 0 6 0 0 7 0 0

Figure The variation of {e ') with absolute temperature as a function of frequency at different Zn contant ix) : (a) .x = 0.1; (b) x ~ 0.3, I = 0.5; (d) A* « 0.7; (e) jc = 0.9.

Effect ofZn dilution on the electrical properties etc 363

In this third region (above transition), the lattice vibration and the generation of phonons becomes predominant, leading lo disordering o f the dipoles in this paracicctric region;

accordingly,

e'

Figure 3 correlates the dielectric constant f'an d Zn contain as a function of the applied frequency. The data of t ' gives minimum value at x = 0.5 and maximum at x = (>;7.

1’his means that the maximum polarizability is obtained at 0.7 and the minimum at 0.5. RczJcscu and Re/lescu ll^ll .suidicd the com position, frequency and lernperatut^- dependence ol'mixed ferrites and explained the composilic^- dcpcndcncc of e ' by using the assumption that the mechaniim oldiclcclric polarization is similar to that of conduction. Titc electron exchange between the Fe-^ and Fe'"*^ results in a lo^il displacement o f the electrons in the direction of the ciccttic licld. Similar interpretation was proposed [12 14). It is wcllknown that when the ferrite samples arc cooled from elevated temperature in an oxid izin g atm osphere, a considerable amount of oxygen is absorbed and the divalent ions are changed into trivalent ones. The formation of very small amount of F'c"^ ions owing to volalizalion of Zn during sintering process |I 5 | may be expected. Those ions preferentially occupy the octahedral sites. In this case, the conduction takes place thiough the valence exchange between M-'^ and (M = metal) ions in the oxygeivrich region and and ions in the oxygen-poor region | I6|. Similar behaviour is observed in the investigated .sample. The presence ol maximum dielectric constant at .v = 0.7 means that this value is critical. This result is in good agreement with that of Ahmed e f a l1171. From the point of view ol frequency effect,

It IS clear that c ' decrea.ses with increasing frequency due lo decrease in polarizability.

Co~Zn ferrite is wcllknown to be inverted spinel, therefore some of the Fe’*^ ions occupy the octahedral site in addition to another expected cations such as and the Zn-'^ cations

Zn concentration

l‘*Kurc 3. Variation of the dielectric coastant c ' as a function of the Zn vonicni (a) at different frcqucncic.s and fixed temperature of 500 K.

prefer the tetrahedral site. Accordingly, the main source of polarization in the investigated .sample is the electron Exchange between ferrite ions of different valences on the same equivalent lattice site (octahedral site) /.e. -ft-'.

This will result in a local displacement of electrons in the direction of the field which determine the polarization of the ferrite samples. Similar behaviour is obtained in the dependence of the composition parameters uplo x = 0,5. In other words, the large field accompanied with the large applied frequency disturbs the polarization and the charge carriers can not follow up the field variation These results are in good agreement with the previous work 118J.

Safely, one can .say that, all .samples except x = 0.5 show an increase in the t:'values after a decreasing in the third temperature region. This may be due lo the participation of another type of polarization such as MaxwelFWagnar one which acts on the interface between the conducting grains and the non-conducting layers. The non-conducting layer will be decreased and a chance is given to the dipoles to tunnel between the conducting grains. The polarizability and dielectric constant increase as a result of this process.

Figure 4(a, h) shows the typical curve correlating the dielectric loss factor e " and absolute temperature as a function of the applied frequency (1 (K) kHz to 5 MHz). The data reveals a .stable region, and then increases giving a hump al =360 K with a small shift depending on the applied frequency. After peak, f''d ecr ea ses and give a stable values from -3 8 0 -4 6 0 K except at 100 kHz which begins a stability region .starling from «410 K. In the last region, e " increases dramatically. To discuss this behaviour, one can imagine that the thermal energy in the first region is not quite sufficient lo

7M

600 i *' 4S0 300 ISO

... ... ; - *

- • - 4 0 0 kHx

T wi i

" 1 • H 6 - 2 M H Z

t

L

^1

x«0 5

370 420

420 470 S20

T(K)

Figure 4(a, b). Variation of the dielectric lass factor (e " ) with absolute temperature as a function of the applied frequency : (a) a: = 0.5; (b) .v « 0.7

increase i'rietion helween the dipoles, therefore the dielectric loss factor e" is kept constant. With continuous incrca.se in tcinpcruturc, the internal viscosity of the sample decreases, and the dipole motion increases, resulting in an increase in the dipole fraction. However, the thermal energy dissipation increases as well as e". The expected increase in the electron hopping after hump will com pensate the decrease in polariyiation and the result is the decrea.se in e".

In the second temperature region, the expected participation of another types of polarization relatively keeps

e" at its constant value upto »46() K. Above 460 K, the

sudden increase in dielectric loss may he due to more than one factor, such as the large dccFca.se in internal viscosity as well as charge carrier hopping between the metal ions of different valences and disappearance of certain types of polari/alion such as the electronic one. Comparing the two parts of Figure 4, one can find that the resonance process which is strongly dependent on the rotational and vibrational motion of the localized dipoles, varies with temperature. At high frequency, the thermal energy dissipation inercattes due to increasing friction giving rise to an increase in f ". The decrease in e" after its incrca.se means that the Zn ions still play a role in the dispersion process. A comparison of the dispersion curves for the samples with increasing Zn content shows that the Zn ions affect drastically on both c'and e"

where e' increases from about 0.44 at Jr = 0.5 to 1228 at .V = 0.7 and .320 K. Table 2 enhances our expectation about the different regions of temperature and the increase in the dielectric loss with increasing temperature.

Tnhic 2. Values of c " at dirferent fixed temperatures and frequency of

ItX) kHz.

V

O-S 0.7

^20 K e "

420 K c "

0.44 1228.20

2.7.S 689.41

480 K t “ 22.96 2198.98

4 . C on clu sk M i

From the siu'tfy 1^1' thtf electrical • properties o f C o|.,Z n,L avFc2_v0 4, wc can conclude the follow ing remarks :

(i) I'hc hopping mechanism is the most dominant one in discussing the AC conductivity where this hopping takes place between the different ions o f different , valences. Also hole-hopping participates as^a result of

"replacement o f La’* ions instead o f Pc’* ions on

octahedral site. The miernstrain produced m this process, play.s a significant role in hole-hopping, fh^

reported activation energy values indicate Uic semiconducting behaviour of the investigated samples, (ii) More than one type of polarization participates m

relaxation process depending on temperature region In the low temperature region, the clccironk polarization plays a role. In the other regions, uihd types of polarization such as orientational, rotational, space charge and Maxwcll-Wagner polarization lake place.

(iii) The dielectric loss factor e" also varies depending on Zn-content in the sample. The change in internal viscosity of the sample varies the amount ol thermal energy dissipation and decreases e".

A c k n o w le d g m e n t

The author gratefully acknowledges Prof. M A Ahmed.

Professor of Experimental Solid State Physics, Faculty ol Science, Cairo University for his help.

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*■ » 'C