ARTICLE IN PRESS

Journal of Magnetism and Magnetic Materials 321 (2009) 3436–3441

Contents lists available at ScienceDirect

Journal of Magnetism and Magnetic Materials

0304-88

doi:10.1

� Corr

Faculty

E-m

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

Role of Cu2+ concentration on the structure and transport properties of Cr–Znferrites

M.A. Ahmed a,�, N. Okasha b

a Materials Science Lab (1), Physics Department, Faculty of Science, Cairo University, Giza, Egyptb Physics Department, Faculty of Girls, Ain Shams University, Cairo, Egypt

a r t i c l e i n f o

Article history:

Received 8 March 2009

Received in revised form

14 May 2009Available online 18 June 2009

Keywords:

Cu–Cr–Zn ferrites

X-ray and EDAX analyses

FTIR

SEM

Dielectric

ac conductivity

53/$ - see front matter & 2009 Elsevier B.V. A

016/j.jmmm.2009.06.041

esponding author at: Materials Science La

of Science, Cairo University, Giza, Egypt.

ail address: moala47@hotmail.com (M.A. Ahm

a b s t r a c t

The influence of Cu concentration on the transport and microstructure characteristics of

CuyZn1�yCr0.8Fe1.2O4 with 0.2ryr1 ferrite was studied. X-ray, energy dispersive X- ray (EDAX)

and infrared spectra (IR) were carried out to assure the formation of the sample in the proper

form. The dielectric constant (e0) and ac conductivity were measured at different frequencies ranging

from 600 kHz to 5 MHz from room temperature up to 800 K. The obtained data reveals that, a single

phase cubic spinel structure for all the concentrations. From the results of IR spectra, mainly two bands

were observed. The dielectric constant and the dielectric loss tangent decrease with increasing

frequency and Cu concentration. The dielectric constant shows a dispersion peak (e0max) which shifts to

higher frequency with increasing the temperature. The results are explained as due to the fact that the

dielectric polarization process is similar to that of conduction. The appearance of the dispersion peak is

related to the contribution of two types of charge carriers.

& 2009 Elsevier B.V. All rights reserved.

1. Introduction

The oxide spinel comprises a large group of ternary com-pounds, significant not only as naturally occurring minerals, butalso in many branches of solid-state science. Despite of theirsimple structure, many spinels exhibit complex disorderingphenomena involving the two cation sites, which have importantconsequence both for their thermochemical and for their physicalproperties.

Ferrimagnetic material technology has reached a very ad-vanced stage, so that the properties may be controlled to a largeextent by the design engineer to suit the particular purpose of hisdevice. The conduction mechanism in ferrite is described usingboth the band picture and hopping models, where the change ofthe mobility with temperature is considered to constitute theconduction current by jumping or hopping from one iron ion tothe next [1,2].

Copper ferrite shows variation in its magnetic propertiesdepending on the thermal history of the preparation [3,4] whichis attributed to the distribution of Cu2+ and Fe3+ among the twonon-equivalent sites, tetrahedral (A) and octahedral (B), providedby the spinel structure.

ll rights reserved.

b (1), Physics Department,

ed).

It is known that, the slowly cooled copper ferrite has atetragonal deformed spinel structure below 1170 K and cubicabove this temperature, where the tetragonal deformed phase isdue to the Jahn–Teller effect of Co2+ ions located at tetrahedral (B)sites [5,6]. This abnormal behavior of copper ferrite is due to theimproved homogenization and structural perfection of Cu1+ ion.The possibility of the reduction of Cu2+ to Cu1+ in the process ofpreparation of copper-containing ferrites could probably haveinfluenced the behavior of these compounds. The presence of Cu1+

enhances the appearance of p-type conductivity of these ferriteswhich pointed out, that the variation of the slopes in the lns vs. 1/T curve leads to the phase transition from tetragonal to the cubicphase. This fact determines the abnormal behavior of thedielectric parameters as a function of frequency and temperature.Microstructure and magnetic properties of CuZn ferrites arehighly sensitive to composition, sintering conditions, grain size,type and amount of additives, impurities and the preparationmethodology [7,8].

Several authors [9–14] studied the dc electrical conductivityand the behavior of the dielectric parameters as a function offrequency, concentration and temperature on copper-containingferrites such as Cu–Cr [15], Cu–Zn–Al ferrite [16]. So, the earlierdiscussions assumed [17] that the (B) sites cations occupypositions at the exact center of the octahedral B-atom interstices.

The aim of the present work is to investigate the effect of Cusubstitution in Cr–Zn ferrites on microstructure, activation energyand the kind of charge carriers. The choice of Cu is based on theincrease of the densification and the decrease of the sintering

ARTICLE IN PRESS

M.A. Ahmed, N. Okasha / Journal of Magnetism and Magnetic Materials 321 (2009) 3436–3441 3437

temperature which increases the resistivity of the material. Anoptimum doping ratio of Cu could be successful in developing alow-power loss material operating in the MHz frequency region.

2. Experimental techniques

The measurements were carried out on polycrystalline samplesof the composition CuyZn1�yCr0.8Fe1.2O4; 0.2ryr1 which wereprepared using the standard ceramic technique [18], using analargrade form (BDH) oxides mixed stochiometrically. Grinding usingagate mortar for 3 h was carried out for each sample. After that,the powder was pressed into pellets and presintered at 800 1C for6 h. Finally, grinding again and some of this powder pressed topellets (10 mm diameter and 1 mm thickness) using the uniaxialhydraulic pressure of 1.9�108 N/cm2 and were finally sintered at1150 1C for 14 h followed by cooling with the same rate as that ofheating (4 1C/min) in Lenton furnace (16/5 UAF England). Theelemental analyses was performed by energy dispersive X- ray(EDAX) spectroscopy analyses (scanning electron micrographs(SEM) quant with silica detector) model, that confirmed thedifferent concentration of the Cu, Zn, Cr and Fe ions in oursamples. The completion of the reaction and production of singlephase material at room temperature was verified by Scintag (USA)X-ray diffractometer equipped with CuKa (l ¼ 1.5418 A) radiationsource with nickel filter. X-ray pattern indicated crystalline phasefor all concentrations, belong to the fcc system. The equipmentused for IR analysis is (FTIR) 1650 Perkin-Elmer to detect the IRabsorption spectra for the samples with different concentration ofcopper content. The particle microstructure was investigated by atransmission electron microscope (JEM-100S Japan). The twosurfaces of each pellet were coated with silver paste and checkedfor good electrical contact. The RLC Bridge (Hioki model 3530Japan) was used to measure the ac electrical resistivity of theinvestigated samples. The dielectric constant (e0), dielectric loss(e00) for the samples were measured from room temperature up to800 K at different frequencies from 600 to 5000 kHz. Thetemperature of the sample was measured using copper–constan-tan thermocouple connected to Digi-sense thermometer (USA)with junction in contact with the sample. The accuracy ofmeasuring temperature was better than 71 1C. The generalflowchart for the preparation process is shown in the figure.

3. Results and discussion

EDAX analyses were done to determine the chemical composi-tion of the surface of the sample to support our observations on

10 15 200

2000

4000

6000

8000

10000

12000

ZnO Zn

ZnCu

FeCr

FeCr

Cou

nts

Energy (keV)0 5

Fig. 1. The typical EDAX spectra of the composition Cu0.6Zn0.4Cr0.8Fe1.2O4.

the structure of the ferrite. EDAX measurements were carried outon the same point with electrons having accelerating voltage of20 keV to give the chemical composition of essentially the core ofthe particle. Results of EDAX analyses of a typical sample are givenin Fig. 1. It is clear that, the iron has very high concentration aswould be expected. The atomic weight percentages of variouscations in the investigated samples are found to be Cu (15.80%), Zn(15.07%), Cr (23.92%) and Fe (34.28%), which corresponds to acomposition ratio of approximately 1:1:2. These ratios areexpected by the preparation method. Consequently, the Cu–Znferrite obtained can be expressed as Cu0.6Zn0.4Cr0.8Fe1.2O4. Thesemixed ferrites are technologically relevant because of their goodelectrical properties, high resistivity and low microwaveabsorption loss.

X-ray diffraction pattern for the samples CuyZn1�yCr0.8Fe1.2O4;0.2ryr1 is illustrated in Fig. 2a. The data showed intense sharppeaks and reveal well-crystalline single phase spinel structure forall Cu content as compared with the corresponding ICDD card nos.[25-0283 and 43-0554]. The lattice parameter (a) was calculatedfrom the diffraction patterns for all samples and plotted vs. Cucontent Fig. 2b. The data clarify that, the value of (a) decreaseswith increasing Cu content, which is due to the substitution ofsmaller ionic radius of Cu2+ (0.72 A) ions on the expense of thelarger Zn2+ (0.74 A) ions. This causes a decrease in the size of theunit cell, where, the length of the edge of the unit cell decreasesfrom 8.493 to 8.216 A. At the same time, some of Fe cations fromtetrahedral site go to the octahedral site, to balance the relativeoccupancy given by the space group. In other words, the increaseof Cu content forced the structure of these compounds in directionto a normal spinel. This is clearly due to the tendency of Zn cationto go to its preference site is consistent where its electronicconfiguration has marked preference for the tetrahedral site, sincetheir 4s and 4p or 5s and 5p electrons can form covalent bondwith the 2p electron of oxygen ion. This means that, the size andthe valancy of the cations are the important factors to fulfill thistendency. Moreover, the large divalent cations (Cu and Zn) tend tooccupy the tetrahedral site as this is favored by polarizationeffects of the oxygen atoms intermediate between A and B sites[19]. This means that the tetrahedral sites are expanded by anequal displacement of the four oxygen ions outwards along thebody diagonal of the cube; at the same time the oxygen ionsconnected with the octahedral sites move in such away as toshrink the size of the octahedral site by the same amount as thetetrahedral site expands. This behavior lead to decrease of thelattice constant values. The theoretical density (Dx) was calculatedfrom the relation Dx ¼ 8M/Na3, where (N) is Avogadro’s number,(M) the molecular weight and (a) the lattice parameter. Theporosity (P) was calculated for each sample from the relation:P ¼ 1�D/Dx. The experimental density (D) and the porosity (P)were listed in Table 1. The decrease of density and the increase ofporosity with increasing Cu content are due to the increase ofoxygen vacancies which play a predominant role in acceleratingdensification [20]; i.e. the decrease in oxygen ion (anion) diffusionwould retard the densification. The presence of Cu2+ reduces thepopulation of Fe3+ in B sites resulting in the decrease of density aswell as the increase in porosity.

The values of the transmission bands of the infrared spectra(IR) for the investigated samples as a function of Cu contentare observed at Fig. 3. From the figure and the band position inTable 2, the presence of two strong absorption bands n1 and n2 arefound in the expected range. The high-frequency band n1 lies in therange 428–605 cm�1 which belongs to the tetrahedral sites and thelower frequency band n2 in the range 353–422 cm�1 which belongto the octahedral sites [21,22]. The change in the band positionis due to the change in the Fe3+–O2� internuclear distance forthe octahedral and tetrahedral sites. A small band n3 near n2

ARTICLE IN PRESS

Table 1Calculated values of lattice parameter (a), X-ray density Dx, porosity P% of

Zn1�yCuyCr0.8Fe1.2O4 system as a function of Cu content (y).

y a (A) Dx (gr/cm3) P%

0.2 8.493 5.57 0.67

0.4 8.341 5.38 3.16

0.6 8.297 5.27 2.28

0.8 8.218 5.22 3.45

1 8.235 5.16 3.88

Wavenumber (cm-1)

200 500 800

y=0.2

y=0.4

y=0.6

y=0.8T%

Fig. 3. IR spectra of the composition Cu0.6Zn0.4Cr0.8Fe1.2O4.

y=0.2

0

200

400

600

y=0.8

25 35 45 55 65 75 852θ

(533)

Intensity %

(553)(311)

(222)

(511)

(440)

8.1

8.25

8.4

8.55

0 0.2 0.4 0.6 0.8 1 1.2

Cu content (y)

Latti

ce p

aram

etr a

(A°)

Fig. 2. (a) XRD Patterns of CuyZn1�yCr0.8Fe1.2O4 ferrites. (b) Variation of lattice parameter as a function of copper content (y).

M.A. Ahmed, N. Okasha / Journal of Magnetism and Magnetic Materials 321 (2009) 3436–34413438

was attributed to the divalent octahedral metal ion–oxygen ioncomplexes [22]. And with the highest concentration of Cu, theband n3 could be seen clearly. Also, the appearance of the band n4

in the range 229–291 cm�1 depends on the mass of the divalenttetrahedral cations and it is assigned to the lattice vibrations ofthe system [22]. Moreover, the spectra reveal that the absorptionband n1 does not show any splitting or shoulder, while there is aweak splitting around n2 which may be due to the presence ofCu2+ ions ( which is a Jahn–Taller ion) on the B sites. Also, theincrease of n3 and n4 intensity near n2 with increasing Cu2+

concentration agrees well with this interpretation. The ratio of thehigh-frequency band positions n1/n2 ¼ (Kt/Ko)O2 [22]; Kt and Ko

represented the force constants associated with the unit celldisplacement of a cation–anion in A and B sites, respectively. Thisratio is equal to nearly 0.88 for all copper contents which indicatesthat, the expansion of the tetrahedral sites is not equallycompensated by the same amount of the shrinkage of theoctahedral sites which lead to a slight decrease in the latticevolume.

Another effect of increasing copper substitution on theinvestigated samples is the enhancement of the grain growth as

seen from the scanning electron micrographs (SEM) in Fig. 4.Uniform grains are progressively increased with increasing Cucontent (x) and the ferrite samples exhibit an aggregated

ARTICLE IN PRESS

Table 2The transmission band position of Zn1�yCuyCr0.8Fe1.2O4 system as a function of Cu

content (y).

y n1 (cm�1) n2 (cm�1) n3 (cm�1) n4 (cm�1)

0.2 579 387 – 251

0.4 582 391 325 263

0.6 432 353 – 283

0.8 605 422 317 291

1 428 368 288 229

Fig. 4. SEM of the compositi

y=

5

35

65

300 400 500

600 kH800 kH1 MH2 MH3 MH5 MH

y=0.4

0

50

100

300 400 500

600 kH800 kH1 MH2 MH3 MH5 MH

y=0.

0

30

60

300 400 500

600 kH800 kH1 MH2 MH3 MH5 MH

ε'x105

Fig. 5. The variation of dielectric constant (e0) with absolute tempera

M.A. Ahmed, N. Okasha / Journal of Magnetism and Magnetic Materials 321 (2009) 3436–3441 3439

continuous grain growth with grains containing some finepores.

Fig. 5: a–c shows a typical curve clarify the variation of the realpart of dielectric constant e0 with absolute temperature as afunction of the applied frequencies. The data reveal that, there is adispersion peak in the dielectric constant. The values of thedielectric constant increases with temperature up to the peakvalue and then decrease again. The dielectric peak (e0max) isshifted towards higher temperature with increasing Cu content. Atthe same time, the values of dielectric constant decreases with

on CuxZn1�xCr0.8Fe1.2O4.

0.2

600 700 800

600 700 800

6

600 700 800T(K)

ture as a function of frequency ranging from 600 kHz to 5 MHz.

ARTICLE IN PRESS

y=0.2

-14

-12

-10

-8

-6

1.1 1.6 2.1 2.6 3.1

lnσσ

((ΩΩ−1

cm−1

))

600 kH800 kH1 MH2 MH3 MH5 MH

y=0.4

-11

-9

-7

-5

-3

1.1 1.6 2.1 2.6 3.1

600 kH800 kH1 MH2 MH3 MH5 MH

y=0.6

-12

-8

-4

1.1 1.6 2.1 2.6 3.1

1000/T(K-1)

600 kH800 kH1 MH2 MH3 MH5 MH

Fig. 6. The variation of lns vs. 1000/TK�1 as a function of frequency.

Table 3Activation energy values in the low (E1) and high (E11) regions at different Cu

content (y) of Zn1�yCuyCr0.8Fe1.2O4.

Cu content 600 kHz 800 kHz 1000 kHz 2000 kHz 3000 kHz 5000 kHz

(y) E1 E11 E1 E11 E1 E11 E1 E11 E1 E11 E1 E11

0.2 0.13 0.86 0.14 0.79 0.15 0.36 0.18 0.31 0.28 0.38 0.09 0.21

0.4 0.09 0.72 0.11 0.65 0.11 0.56 0.13 0.53 0.17 0.49 0.19 0.38

0.6 0.18 0.52 0.19 0.43 0.21 0.31 0.23 0.26 0.24 0.21 0.26 0.16

0.8 0.15 0.83 0.17 0.78 0.21 0.69 0.23 0.61 0.26 0.57 0.28 0.37

1 0.14 0.91 0.15 0.86 0.15 0.82 0.17 0.65 0.18 0.55 0.21 0.34

0

30

60

90

0 0.2 0.4 0.6 0.8 1 1.2

Cu content (y)ε'

800kHz

3000kHz

ε'

y=0.2

2

3

4

1000 3000 5000 7000

log f (kHz)

0

1

2

3

2.5 3.0 3.5 4.0

log f

tan

δ

y=0.2y=0.4y=0.6y=0.8y=1

Fig. 7. (a) Cu content (y) dependence of e0 at f ¼ 800, 3000 kHz. (b) Frequency

dependence of e0 at y ¼ 0.2 and f ¼ 670 kHz. (c) Variation of tand with log f for

different Cu content (y).

M.A. Ahmed, N. Okasha / Journal of Magnetism and Magnetic Materials 321 (2009) 3436–34413440

increasing frequency due to the electron exchange which cannotfollow the variation field. Moreover, this behavior can beexplained according to the assumption that the mechanism ofdielectric polarization is similar to that of conduction process inferrites [23,24], i.e. the electronic exchange interactionFe2+2Fe3++e� results in a local displacement of the electrons inthe direction of the electric field which determines thepolarization of ferrites. However, some of iron ions decrease inoctahedral sites leading to a decrease in the electronic exchangeinteraction Fe2+2Fe3+ and, hence, the polarization decreases withincreasing Cu content.

In general, all the compositions of Cu–Zn–Cr system exhibit anabnormal behavior in (e0) and tan d. This behavior in thepolarization process of ferrites containing copper is due tothe existence of two types of charge carriers (n and p) and theappearance of p-carriers is due to Cu2+2Cu1+ exchange. On theother hand, the preference of Cr3+ on octahedral sites (B) causesthe reduction of Fe3+ due to the exchange interaction process[25,26].

Fig. 6: a–c correlates the value of lns vs. the reciprocal ofabsolute temperature as a function of the applied frequency for theinvestigated samples. From this plots and Table 3, it is seen that allsamples show two conductivity regions (E1, E11) with changingslope indicating the different conduction mechanisms. Theactivation energy E1 at low temperature (ferrimagnetic region)seems to be frequency dependent; while, the activation energy inthe high-temperature region E11 is slightly frequency independent.This could be related to the disordered state in the paramagneticregion (of higher activation energy and lower conductivity) incomparison to the ordered state of ferrimagnetic region (for loweractivation energy and higher conductivity). This result is inaccordance with those observed for Mn–Zn ferrites [26], Li–Tiferrites [27] and Ni–Mg ferrites [28].

Therefore, in the paramagnetic region of disordered state,the increase of the conductivity with increasing temperaturecorresponds to thermally activated mobility of charge carriers andthe activation energy is nearly constant (frequency independent).Based on this, the conductivity behavior on passing through thetransition temperature (Ts) may be explained by assuming thatthe super exchange interaction takes place between Fe2+ and Fe3+

at the B-sublattice [29]. Thus, one has encountered a relationbetween electrical conductivity and magnetism, namely, thelining up of the spins of adjacent incomplete d-shells of themetallic ions will be accompanied by an increase in the rate ofmigration of the charge carriers, and hence by an increase in theelectrical conductivity.

Fig. 7a correlates the value of dielectric constant (e0) with Cucontent (y) as a function of selected frequency (800, 3000 kHz) attemperature (610 K). The data indicates an increase in (e0) withincreasing (y) up to the critical concentration (y ¼ 0.4), afterwhich (e0) tends to decrease at all frequencies. The data clarifiesthat, the cooperation of more than one type of polarization is

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M.A. Ahmed, N. Okasha / Journal of Magnetism and Magnetic Materials 321 (2009) 3436–3441 3441

similar to that of conduction in ferrites due to the electronicexchange interaction Fe2+2Fe3+ as mentioned before. Also, thepresence of peak in the dielectric constant with increasing Cucontent showed an abnormal behavior of the dielectric constantdue to the reduction of Cu2+2Cu1+ in the thermal process. Thevariation of the dielectric constant as a function of frequency isshown in Fig. 7b. The represented data indicates that, the decreasein (e0) with increasing frequency which is a general trend indielectric materials due to scattering of charge carriers at higherfrequencies as well as the fast variation of the electric fieldaccompanied with the applied frequency. This process leads torandom orientation of the dipole moments which accordinglydecreases the value of (e0). Also, the decrease of polarization withincreasing frequency is due to the fact that beyond a certainfrequency of the electrical field, the electronic exchangeFe2+2Fe3+ cannot follow the alternating field variation.Moreover, the electron hopping Fe2+2Fe3+or Cu2+2Cu1+ occursby electron transfer between adjacent octahedral sites (B) in thespinel lattice [30,31]. Thus, by the electronic exchange Fe2++Cu2+2Fe3++Cu+, one obtained local displacements of electrons inthe direction of the applied electric field, these displacementswhich reduces the field inside the medium leading to a decreasein dielectric constant with increasing frequency. Fig. 6c shows thedependence of the loss tangent (tand) on the frequency (log f) atdifferent Cu content (y). It is clear that, the values of loss tangentdecreases with increasing frequency. This is due to thereplacement of Cr3+ ions instead of Fe3+ ion in the B sites whichleads to a decrease in the number of Fe2+ and Fe3+ ions betweenthe hopping conduction mechanisms as mentioned before.

4. Conclusions

1.

X-ray diffractograms assured the cubic spinel structure for allthe investigated samples.

2.

With increasing Cu content the lattice parameter (a) wasincreased.

3.

The four fundamental bands were appearing in the IR spectra. 4. The presence of Cu ions in the ferrite under investigation

causes an abnormal behavior in the dielectric constant (e0)with a critical concentration at y ¼ 0.4. This behavior agreeswill with another ferrites containing copper.

5.

The ac conductivity of the investigated composition Zn1�yCuy

Cr0.8Fe1.2O4 increases with increasing temperature, which isthe characteristic nature of semiconductor ferrites.

6.

The dielectric constant (e0) for all samples decreases withincreasing frequency.

7.

The activation energy for electrical conduction in theparamagnetic region is higher than that in the ferrimagneticregion.

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