Materials Chemistry and Physics 85 (2004) 104–112

Effect of diamagnetic substitution on the structural, electricaland magnetic properties of CoFe2O4

M.A. Gabala,∗, S.S. Ata-Allahba Chemistry Department, Faculty of Science, Benha University, Benha, Egypt

b Reactor and Neutron Physics Department, Nuclear Research Center, Atomic Energy Authority, Cairo, Egypt

Received 11 June 2003; received in revised form 7 December 2003; accepted 17 December 2003

Abstract

Polycrystalline samples with the general formula Co1−xCdxFe2O4 (0 ≤ x ≤ 1) were synthesized by calcining the respective oxalatesmixtures at 1000◦C for 5 h. Their structural, electrical and magnetic properties are studied using X-ray diffraction, Fourier transforminfrared and Mössbauer spectroscopy, and electrical conductivity and magnetic susceptibility techniques. With cadmium ion substitution,the lattice parameter, X-ray density, oxygen parameter, inversion factor and radii of tetrahedral and octahedral sites are calculated. TheFourier transform infrared spectra showed two dominant bands in the high- and low-frequency range which are assigned to the tetrahedral andoctahedral complexes, respectively. The relationship between bands position and cadmium content was also investigated. The Mössbauerspectroscopy was carried out to study the cation distribution in this system. It was found that ions at octahedral site moved to the tetrahedralsite, and that this system varied from an inverse to a normal spinel structure. The temperature variation of the conductivity showed a definitekink, except for the CdFe2O4 sample, which corresponds to the ferrimagnetic to paramagnetic transitions. The effective magnetic momentof the samples and their Curie temperature were observed to decrease by the substitution effect.© 2004 Elsevier B.V. All rights reserved.

Keywords: CoFe2O4; CdFe2O4; Mössbauer spectroscopy; Electrical conductivity; Magnetic properties

1. Introduction

Ferrites, i.e. ferrimagnetic cubic spinels possess the com-bined properties of magnetic materials and insulators. Theyhave been extensively investigated and being the subject ofgreat interest because of their importance in many techno-logical applications. The important structural, electrical andmagnetic properties of these spinels, responsible for theirapplications in various fields, are found to depend on themagnetic interaction and the distribution of cations amongthe two sublattices, tetrahedral (A) and octahedral (B) sites.

The compounds capable of crystallizing in the spinelsystem can present in either of two structure: the normalspinel or inverse spinel. In the normal spinel (such asCdFe2O4), cadmium ions are located in the tetrahedral siteand are symmetrically surrounded by four O2− ions. Inthe inverse spinel (like CoFe2O4), half of Fe3+ ions are

∗ Corresponding author. Present address: Institut für Mechanische Ver-fahrenstechnik und Mechanik, Gas-Partikel-Systeme, Universität Karl-sruhe, 76128 Karlsruhe, Germany. Tel.:+49-176-2001-9090;fax: +49-721-608-2405.E-mail address: mgabalabdo@yahoo.com (M.A. Gabal).

located in the tetrahedral positions whereas the other halfare in the octahedral positions. Therefore, a normal ferritecan be described as (M2+)[Fe2

3+]O4 and an inverse ferriteas (Fe3+)[M2+Fe3+]O4 [1]. X-ray diffraction and Möss-bauer spectroscopy techniques have been devised to studythe cation distribution in spinel ferrites. Generally[2], thecation distribution calculated from X-ray intensity dataagrees with the Mössbauer results.

Cobalt ferrite is ordinarily an almost inverse cubic spinelin which the degree of inversion is critically determined bythe preparation conditions[3]. The partial replacement ofdiamagnetic ions (e.g. zinc or cadmium) in such ferrites isexpected to weaken the magnetic coupling which may bereflected in a decrease of the magnetic hyperfine field aswell as the Curie temperature[2].

A little work was found in the literature on mixed Co–Cdferrites. Ghani et al.[4] have been studied the magnetic be-havior of mixed Co–Cd ferrites. Vasambekar et al.[3] wereinterested in understanding the magnetic behavior of theCd2+ and Cr3+ substituted cobalt ferrites prepared by stan-dard ceramic method. Nikumbh et al.[5] have been preparedthe compounds of the system Cd1−xCoxFe2O4 (0 ≤ x ≤1) by the tartrate coprecipitation technique and attempted

0254-0584/$ – see front matter © 2004 Elsevier B.V. All rights reserved.doi:10.1016/j.matchemphys.2003.12.013

M.A. Gabal, S.S. Ata-Allah / Materials Chemistry and Physics 85 (2004) 104–112 105

to determine the effect of cadmium ferrite spinel formationand the role of cobalt content on its structural, magnetic andelectrical properties. However, there is no reported work inthe literature for the preparation of mixed Co–Cd ferritesthrough the thermal decomposition of mixed oxalates.

Accordingly, the present communication attempts to pre-pare the compounds of Co–Cd ferrites through the thermaldecomposition reactions of their corresponding metal ox-alates. The aim of the present study is to understand the effectof diamagnetic substitution of cadmium ions on structural,electrical and magnetic properties of cobalt ferrite spinelby means of X-ray diffraction (XRD), Fourier transform in-frared (FT-IR) and Mössbauer spectroscopy, and electricalconductivity and magnetic susceptibility measurements. TheMössbauer spectroscopy was also undertaken to investigatethe probable cation distribution of the ferrite samples.

2. Experimental procedure

2.1. Materials and preparation procedure

The starting materials were solids of CoC2O4·2H2O,CdC2O4·3H2O and FeC2O4·2H2O. These metal oxalateswere prepared by precipitation from aqueous solution oftheir salts sulfate using analytical grade oxalic acid. Forthe preparation of the mixed metal oxalates, seven mixturesof these respective metal oxalates with calculated mole ra-tios, equivalent to the formation of Co1−xCdxFe2O4 ferritesystem (x = 0.0, 0.2, 0.4, 0.5, 0.6, 0.8 and 1.0) after theirthermal decomposition, were prepared by the impregnationtechnique previously described[6].

Mixed Co–Cd ferrites, Co1−xCdxFe2O4 (0 ≤ x ≤ 1),were prepared by annealing the oxalates mixtures in a mufflefurnace at 1000◦C for 5 h under static air atmosphere. Then,the samples were quenched to room temperature and keptin desiccator.

2.2. Characterization techniques

Simultaneous differential thermal analysis-thermogravi-metry (DTA–TG) behavior of the oxalate mixtures wasinvestigated using a Shimadzu DT-60 thermal analyzer(Japan) in the temperature range from room temperature upto 1000◦C at a heating rate of 5 K min−1. The experimentswere proceeded in flowing air atmosphere (50 ml min−1).

XRD of the samples were performed at room temperatureusing a Philips PW 1370 diffractometer, with monochro-mated Cu K�1 radiation wavelength (λ = 1.5406 Å).FT-IR spectra were measured in the frequency range1000–200 cm−1 with KBr disc technique using a Jascomodel FT-IR 310 (Japan). The Mössbauer spectra were ob-tained at room temperature in transmission geometry witha source of 50 mCi57Co (Rh matrix) and a constant accel-eration transducer. Spectra were analyzed using the leastsquare fitting computer program (Mos-90)[7].

The temperature dependence of electrical properties asa function of the applied frequency (10–1000 kHz) weremeasured with a Hioki 3531 LCR bridge (Japan) using thetwo-probe method[8]. The samples were palletized using acylindrical dye of diameter 1 cm by applying a pressure of2 t cm−2. The thickness of the discs was about 1 mm. Pel-lets of the samples were then annealed at 900◦C for 2 hand cooled to room temperature. The two surfaces of eachpellet are polished, coated with silver paste and checkedfor good conduction. A K-type thermocouple connected toa digi-sense thermometer was used to measure the sampletemperature with an accuracy better than±1◦C. The tem-perature dependence of magnetic susceptibility as a functionof magnetic field intensities was performed using Faraday’smethod.

3. Results and discussion

3.1. Thermal decomposition studies of the oxalate mixtures

Fig. 1 shows a typical DTA–TG curves for the thermaldecomposition of CoC2O4·2H2O–CdC2O4·3H2O–FeC2O4·2H2O (0.5: 0.5: 2 molar ratios) in air. The DTA curve impliesa sharp endothermic peak at 103◦C, closely corresponds tothe first weight loss step (4.3%) observed in the TG curve.This step is attributed to the dehydration of CdC2O4·3H2Oin the mixture (expected weight loss= 4.5%). Two over-lapped TG steps occurred in the range from 155 to 240◦Cand are accompanied by 15.8 and 22.2% weight loss, i.e.a total weight loss of 38.0%. The first one is characterizedby an endothermic DTA peak at 202◦C and is attributer tothe loss of two water molecules from hydrated cobalt ox-alate and four water molecules from hydrated ferrous oxalate(theoretically 5H2O is equivalent to 15.5% of the molec-ular weight of the mixture) with the formation of anhy-drous oxalates mixture. The endothermic DTA peak wasswamped by a sharp exothermic DTA peak maximized at226◦C and is ascribed to the oxidative decomposition ofFeC2O4–Fe2O3 (calculated weight loss= 22.1%). The twosuccessive weight loss steps follows immediately after thedecomposition of FeC2O4 are attributed to the oxidative de-composition of CoC2O4 and CdC2O4 contents to Co3O4and CdO, respectively. The two steps exhibit a weight lossof 5.5 and 6.6% (closely corresponds to theoretically cal-culated weight loss of 5.8 and 6.5%) and accompanied bytwo exothermic DTA peaks maximized at 280 and 365◦C,respectively. Upon raising the temperature a further verysmall weight loss (0.5%) characterized by a very small en-dothermic DTA peak at 935◦C was obtained. This step isattributed to the reduction of Co3O4–CoO (expected weightloss= 0.5%). The total weight loss after complete decom-position of the mixture, was calculated to be 54.9% of themolecular mass of the mixture which closely corresponds tothe formation of Co0.5Cd0.5Fe2O4 ferrite. Thus, the mini-mum temperature needed for the calcination process (which

106 M.A. Gabal, S.S. Ata-Allah / Materials Chemistry and Physics 85 (2004) 104–112

Fig. 1. DTA–TG curves of a CoC2O4·2H2O–CdC2O4·3H2O–FeC2O4·2H2O (0.5:0.5:2 molar ratios) mixture in air at a specified heating rate of 5 K min−1.

is enough to obtain the corresponding ferrite) was selectedat 1000◦C on the basis of this decomposition course.

From the thermal decomposition course of the metal ox-alates in their mixture, it is appeared that the decompositionbehavior of the metal oxalates is not much affected by theirpresence in a mixture since each one behaves as it is presentalone[8,9]. This behavior can be attributed to their presencein a solid state mixture.

3.2. Characterization of Co1−xCdxFe2O4

3.2.1. X-ray powder diffractionFig. 2 shows the X-ray powder diffraction patterns of

the investigated samples. The diffraction peaks are fitted bymodified Gaussian functions, and the lattice parameters areobtained and reported inTable 1by fitting at least sevendiffraction peaks using standard least-square methods. Theresults of indexing X-ray diffraction patterns showed thatthe nominal composition structure with different cobalt con-centrations are single-phase cubic with no additional linescorresponding to any other phases, which provide clear evi-dence of the formation of a series of solid solutions betweenCdFe2O4 and CoFe2O4.

The obtained values of the lattice parameter (a) agreeswell with those reported in[3,5] and exhibit a linear contentdependence, thus obeying the Vegard’s law[10] as shownin Fig. 3a. Such a remarkable change is due to the fact thatthe Pauling ionic radius of Cd2+ ions (1.03 Å) is higherthan that of Co2+ ions (0.72 Å)[5], consequently, the re-placement of cobalt by cadmium leads to an increase in thelattice parameter. The X-ray density (DX) calculated usingthe formulaDX = 8M/Na3 [10], whereM is the molecularweight, N the Avogadro’s number, anda is the lattice pa-rameter, increases with the increasing the cadmium contentas can be seen fromTable 1. Woleska et al.[11] analyzed

Table 1Data of the Co1−xCdxFe2O4 system

x Lattice parameter (Å) DX (gm cm−3) ν1 (cm−1) ν2 (cm−1)

0.0 8.329 5.394 586 3890.1 8.414 5.470 585 3880.3 8.467 5.612 577 3870.5 8.497 5.660 576 3820.7 8.536 5.698 563 3840.9 8.573 5.849 561 3781.0 8.614 5.986 541 376

M.A. Gabal, S.S. Ata-Allah / Materials Chemistry and Physics 85 (2004) 104–112 107

Fig. 2. Characteristic parts of the XRD patterns of the Co1−xCdxFe2O4 system.

Fig. 3. (a) Variation of the lattice parameter as a function of Cd content. (b) The ratio of observed intensities of X-ray lineI2 2 0/I2 2 2 and I4 2 2/I2 2 2 as afunction of Cd content. (c) Variation ofν1 FT-IR band with composition. (d) Variation ofν2 FT-IR band with composition.

108 M.A. Gabal, S.S. Ata-Allah / Materials Chemistry and Physics 85 (2004) 104–112

Table 2Mössbauer parameters of the Co1−xCdxFe2O4 system

x Site Area (%) δ (mm s−1) EQ

(mm s−1)Γ (mm s−1) Hn (kOe)

0.0 A 70 0.39 0.02 0.58 493B 30 0.68 0.08 0.68 502

0.2 A 50 0.43 0.05 0.65 411B 50 0.46 0.07 0.72 463

0.4 B – 0.43 0.07 2.27 3640.5 – – 0.58 0.28 7.08 1920.6 – – 0.44 0.83 0.56 –0.8 – – 0.44 0.84 0.42 –1.0 – – 0.44 0.79 0.36 –

the intensity ratios of the (2 2 0) and (4 2 2) X-ray diffractionlines, dependently on the tetrahedral site, to the weak (2 2 2)lines, dependently only on the cations on the octahedral site,because these ratios are very sensitive to the residence ofcadmium ions in the tetrahedral site. The observed valuesof these intensity ratios are represented inFig. 3c. It is ob-served that these ratios are increased asx increases since theelectronic configuration of Cd2+ ions have a special prefer-ence for tetrahedral coordination. This observation is similarto the lattice parameter as a function ofx.

3.2.2. FT-IR spectroscopyThe results of FT-IR spectroscopic measurements ob-

tained in the present work are summarized inTable 1. Thetwo dominant bands observed at 586 and 389 cm−1 for theCoFe2O4 sample(x = 0) are entirely consistent with thosereported in[9]. The change in the bands position on goingfrom one concentration to another may be due to the changein the internuclear distance of Fe3+–O2− in the equivalentlattice sites. The bandν1 is attributed to the stretching vi-bration of Fe3+–O2− in the tetrahedral complexes and theν2 to that of the octahedral complexes[13]. FromTable 1, itis clear that on going to the CdFe2O4 side the position ofν1band changes with composition while that ofν2 band slightlychanged. The two bands appeared at 541 and 367 cm−1 forthe sample withx = 1 agrees well with those reported forCdFe2O4 [14]. The variation of theν1 and ν2 bands withcadmium concentration is shown inFig. 3c and d.

3.2.3. Mössbauer spectral studiesThe representative Mössbauer spectra for Co–Cd ferrites

at room temperature are shown inFig. 4. The spectra werefitted with Lorentzian-shaped lines by the method of leastsquares. The Mössbauer spectral data, including isomer shift(δ), quadrupole splitting ( EQ), line width (Γ ) and hyperfinemagnetic field (Hn), are summarized inTable 2.

The Mössbauer spectra of Co1−xCdxFe2O4 with x ≤ 0.2exhibit normal Zeeman split sextets, one due to Fe3+ ions atthe tetrahedral site and the other due to Fe3+ ions at the oc-tahedral site. The compound withx = 0.4 showed a relaxedspectrum which was fitted with one sextet. On the other hand,the compound withx ≥ 0.5 shows paramagnetic doublet,i.e. no distinct evaluation of the hyperfine field observed.

Fig. 4. The Mössbauer spectra at room temperature of the Co1−xCdxFe2O4

system.

The isomer shift is a useful parameter in identifying thespectra due to tetrahedral and octahedral site Fe3+ ions.Because of the difference in Fe3+–O2− distance for the twosites, isomer shift is expected to be different. The Zeemanpattern with the smaller isomer shift exhibits the smaller

M.A. Gabal, S.S. Ata-Allah / Materials Chemistry and Physics 85 (2004) 104–112 109

hyperfine field (in agreement with the generally acceptedcorrelation between isomer shifts and hyperfine fields inferrites [16]) and is attributed to Fe3+ ions at tetrahedralsite. On this base, the sextet obtained for the compound withx = 0.4 is assigned to the presence of Fe3+ ions in theoctahedral site.

On a closer look toTable 2, it can be observed that theisomer shift obtained at both sites changed with Cd2+ con-tent in the range 0≤ x ≤ 0.4, indicating that the s-electrondistribution of Fe3+ ions is influenced by Cd2+ substitution.The observed decrease in the hyperfine fields obtained atdifferent sites with increasingx demonstrate a reduction inferrimagnetic behavior by the addition of cadmium. Fromthe table it is also apparent that compounds with highercadmium content show high values of quadrupole splitting,while compounds withx ≤ 0.4 have practically ver smallor negligible quadrupole splitting, which suggests the co-existence of chemical disorder and overall cubic symmetrycauses no net quadrupole splitting. This behavior agrees wellwith the results obtained by Nikumbh et al.[5].

The fraction of iron ions at tetrahedral and octahedralsites were determined using the areas of the Mössbauer sub-spectra. For stoichiometric ferrite like CoFe2O4 it is easy toestimate the cation distribution, but it becomes rather diffi-cult for mixed ferrites, since they contain a mixture of morethan one cation other than iron. However, if the metal ionshave an exclusive preference for any particular site in thespinel, then it is possible to estimate the cation distribu-tion in the mixed ferrites. In the present work it is foundthat CoFe2O4 is a partially inverse spinel with Co2+ ions(70%) on the octahedral site and exhibits a cation distri-bution: (Co0.3Fe0.7)[Co0.7Fe1.3]O4. Since it is well knownthat Cd2+ ions have a strong tetrahedral site preference[1], Cd2+ will cause some migration of Fe3+ ions fromtetrahedral to octahedral sites and as a result, the expectedcation distribution for the sample withx = 0.2 will be(Cd0.2Co0.3Fe0.5)[Co0.5Fe1.5]O4 which agrees well with thecalculated ratio of Fe3+ ions at tetrahedral and octahedralsites (Table 2). On this basis and considering values ofthe subspectra areas obtained inTable 2, the cation distri-bution formula which can be used to describe the systemis (CdxCoyFe1−x−y)[Co1−x−yFe1+x+y]O4, wherex denotesthe cadmium content andy denotes the normalcy of cobalt

Table 3Cation distribution, oxygen parameter, inversion parameter and cation–anion distance of the Co1−xCdxFe2O4 system

Cation distribution u γ rA Rb Tet. bond Oct. bond Tet. edge Oct. edge

Shared Unshared

(Co0.3 Fe0.7)[Co0.7Fe1.3]O4 0.381 0.7 0.511 1.360 1.889 2.033 3.086 2.803 2.946(Cd0.2Co0.3Fe0.5)[Co0.5Fe1.5]O4 0.383 0.5 0.569 1.340 1.938 2.038 3.165 2.784 2.978(Cd0.4Co0.3Fe0.3)[Co0.3Fe1.7]O4 0.387 0.3 0.627 1.320 2.009 2.02 3.281 2.706 3.000(Cd0.5Co0.3Fe0.2)[Co0.2Fe1.8]O4 0.388 0.2 0.656 1.310 2.031 2.02 3.292 2.692 3.012(Cd0.6Co0.3Fe0.1)[Co0.1Fe1.9]O4 0.389 0.1 0.685 1.300 2.055 2.021 3.356 2.680 3.027(Cd0.8Co0.2)[Fe2]O4 0392 0.0 0.736 1.290 2.108 2.008 3.443 2.618 3.045(Cd)[Fe2]O4 0.394 0.0 0.780 1.290 2.149 2.003 3.509 2.583 3.063

rA = tet. radius,rB = oct. radius, tet. = tetrahedral, oct. = octahedral.

ions and the inversion parameter(γ) = 1 − x − y. The de-duced values of the cation distribution of the system aregiven inTable 3.

Now the values of the oxygen parameter (u) and the de-gree of inversion (γ) can be calculated taking into consid-eration the cation distribution revealed from the Mössbauerexperiments (Table 3). Table 3exhibits the calculated valuesas a function of cadmium content. In most oxidic spinels theoxygen ions are apparently larger than the metallic ions, andin spinel like structure the oxygen parameter has a values ofabout 0.375 for which the arrangement of O2− ions equalsexactly a cubic closed packing, but in actual spinel lattice.This ideal pattern is slightly deformed. The higher values ofoxygen parameter obtained in our work(u > 0.375) may beattributed to the small displacement of anions due to the ex-pansion of the tetrahedral interstices[12]. Intercationic dis-tances were calculated using experimental values of latticeconstant and oxygen parameter for Co1−xCdxFe2O4 sys-tem using equations from[10]. The results are reported inTable 3.

The cation distributions obtained from the Mössbauer ex-periments show that the Cd2+ ions entirely occupy the tetra-hedral site and force the Fe3+ ions into octahedral site withthe increasing of the cadmium content. Therefore, becauseof the radius of Cd2+ (tetrahedral: 0.78 Å) is larger than thatof Fe3+ (tetrahedral: 0.49 Å) while the Co2+ (octahedral:0.745 Å) is compared to that of Fe3+ (octahedral: 0.645 Å)[15], the radius of tetrahedral becomes larger and that ofthe octahedral slightly changed with the content. This ex-plain why ν1 FT-IR band changes with the cadmium con-tent, whileν2 band slightly changed (Fig. 3c and d).

3.2.4. Electrical conductivity and its dependence oncomposition

Fig. 5a–c show typical curves of the alternative currentconductivity versus reciprocal absolute temperature as afunction of the applied frequency for the Co1−xCdxFe2O4system. It can be concluded that the conductivity for all thesamples is frequency independent at all temperature regions(except for the low temperature region) since its valuesslightly changed with changing frequency. On the otherhand, the conductivity of all samples was found to be tem-perature dependent. The curve shows that three regions are

110 M.A. Gabal, S.S. Ata-Allah / Materials Chemistry and Physics 85 (2004) 104–112

Fig. 5. (a)–(c) Relation between lnσ and reciprocal absolute temperature at different Cd contents as a function of the applied frequency for theCo1−xCdxFe2O4 system.

obtained by increasing the temperature. In the first region,the electrical conductivity is hardly changed with temper-ature which give rise to a metallic behavior. After that theconductivity was found to increase with increasing temper-ature giving a kink in each curve, at temperature which cor-responds to ferrimagnetic (ordered state) to paramagnetictransition (disorder state). No such a magnetic transforma-tion was observed for CdFe2O4 sample, suggesting that thissample is paramagnetic at room temperature. Irkhin andTurov [17] have proposed a theoretical explanation for theexistence of kinks at the ferrimagnetic–paramagnetic tran-sition temperature (TC). They conclude that the activationenergy and the effective mass of current-carrying excitonsin ferrimagnetic semiconductors depend on the spontaneousmagnetization because of a “magnetizing” exchange inter-action between the outer and inner electrons. This leadsto an additional temperature dependence of the electricalresistance which is particularly strong nearTC.

The electrical conductivity-temperature behavior obeysWilson’s law [18]:

σ = σ0 exp

(− E

kT

)

indicating the semiconducting nature of all the com-pounds under investigation.σ0 is the pre-exponential factor,

contains several constants, including the vibrational fre-quency of the potentially mobile ions,E the activationenergy,k is Boltzmann’s constant, andT is the absolutetemperature. The activation energies were calculated for thetwo regions around the kink, firstly for the ferrimagnetic(low-temperature region:EI ) and secondly for the paramag-netic (high-temperature region:EII ). The calculated valuesof the activation energy are listed inTable 4. It is interestingto observe here that the activation energy in the paramag-netic region is higher than that in the ferrimagnetic one.The behavior of the activation energy on passing throughTC may be explained by the double-exchange mechanism

Table 4Values of the electrical conductivityσ and the activation energy at anapplied frequency of 200 kHz of the Co1−xCdxFe2O4 system

x σ at 500 K (�−1 cm−1) Activation energy (eV)

EI EII

0.0 2.25× 10−5 0.581 0.8250.2 1.37× 10−6 0.540 0.8100.4 9.18× 10−7 0.502 0.7720.5 6.32× 10−7 0.473 0.7150.6 5.60× 10−9 0.442 0.6900.8 1.25× 10−9 0.412 0.6711.0 2.30× 10−10 – 0.581

M.A. Gabal, S.S. Ata-Allah / Materials Chemistry and Physics 85 (2004) 104–112 111

Fig. 6. (a)–(d) Relation between molar magnetic susceptibility and the absolute temperature as a function of different magnetic field intensities for theCo1−xCdxFe2O4 system.

[17]. Hence, the electrical conduction in the system underinvestigation is due to the electron hopping in the sublatticesbetween Fe2+ and Fe3+ ions and results in an increasingof the activation energy in the paramagnetic region. Also,the electron hopping between Co2+ and Co3+ cannot beneglected.

The decrease in activation energy calculated for bothregions with increasingx can be interpreted as follows[5]:for CoFe2O4 spinel, the ratio of Co2+ and Fe3+ ions in theoctahedral site was found to be nearly equal. As hoppingbetween atoms of different metals on the octahedral sub-lattices needs a higher activation energy than for ions ofthe same metals[19]. Thus, conduction activation energyfor CoFe2O4 is higher. As can be seen fromTable 3, the

addition of Cd2+ ions leads to an increase of Fe3+ ions inthe octahedral site and a simultaneous decrease of Co2+ions present at the same site. This will increase ions of thesame metal (Fe3+ ions) and, consequently, decrease of theconduction activation energy.

Also, it can be seen that the conductivity values (takenat 500 K and a frequency of 200 kHz) decrease with in-creasing cadmium content (Table 4). Since the conductiv-ity in this temperature range is attributed to the electronhopping between iron ions (Fe2+ and Fe3+), cobalt ions(Co2+ and Co3+) or between the both, thus, the replace-ment of cadmium on the expense of cobalt will decreasethis hopping processes and, consequently, decrease of theconductivity.

112 M.A. Gabal, S.S. Ata-Allah / Materials Chemistry and Physics 85 (2004) 104–112

Table 5Magnetic data of the Co1−xCdxFe2O4 system at a magnetic field intensityof 1280 G

x TC (K) µ (BM)

Observed Calculated

0.0 800 5.66 5.630.2 630 5.12 6.960.4 455 4.68 8.280.5 390 4.02 8.95

3.2.5. Magnetic studiesFig. 6 correlates the molar magnetic susceptibility (χM)

and absolute temperature as a function of different mag-netic field intensities for the system Co1−xCdxFe2O4 with0 ≤ x ≤ 0.5. It can be observed that, by increasing eithertemperature or magnetic field, the molar magnetic suscepti-bility decreases. The magnetic parameters at a field intensityof 1280 G for samples with different cadmium content werecalculated by plotting the reciprocal molar magnetic suscep-tibility versus absolute temperature (Table 4). Two regionscharacterized by two straight lines are obtained (the ferri-magnetic and the paramagnetic) passing through the CurietemperatureTC.

Since the proposed cation distribution of the sys-tem estimated from the Mössbauer data is given by(CdxCoyFe1−x−y)[Co1−x−yFe1+x+y]O4. The magneticmoment (µ) of the system can be calculated as[5]:[3.87(1−x−y)+5.92(1+x+y)]−[3.87y+5.92(1−x−y)],where 3.87 and 5.92 are the magnetic moments of Co2+and Fe3+ ions in BM.

The observed magnetic moment at different cadmiumconcentrations was compared with that calculated using theabove equation (Table 5). It has been observed that in caseof x = 0 (i.e. CoFe2O4) the observed and calculated mag-netic moments are very similar to each other. The observedvalues for the other samples are lower than the calculatedvalues. This can be explained in terms of the non-collinearspin arrangement, i.e. the presence of a small canting of theoctahedral site moment with respect to the direction of thetetrahedral site moment[5,20].

In agreement with the Mössbauer results, the observedmagnetic moment was found to decrease with increasingcadmium content which demonstrates a reduction in the fer-rimagnetic behavior.

4. Conclusions

1. Due to the larger ionic radius of Cd compared with thatof Co, the replacement of Co with Cd in the investigatedsystem increases the lattice parameter.

2. The cation distribution was estimated using Mössbauerspectroscopy. It was found that the system varied froman inverse to normal spinel structure by the additionof Cd.

3. The FT-IR spectra showed two fundamental bandsν1andν2, corresponding to the tetrahedral and octahedralcomplexes, respectively. The bandν1 shifts towards thelow frequency site while the bandν2 remain constantwith increasing the Cd ions which can be interpretedon the base of the cation distribution calculated by theMössbauer spectroscopy.

4. The temperature variation of the electrical conductiv-ity of all the samples except for the CdFe2O4 sam-ple shows a definite break, which corresponds toferrimagnetic–paramagnetic transition. The activationenergy in the paramagnetic region is higher than in theferrimagnetic region.

5. The Curie temperature and the effective magneticmoment were found to decrease with increasing Cdcontent.

Acknowledgements

The authors would like to thank Dr. A.A. El-Bellihi,Chemistry Department, Faculty of Science, King AbdulAziz University, Jeddah, KSA, for measuring the DTA–TGexperiments.

References

[1] A. Goldman, Modern Ferrite Technology, Marcel Dekker, New York,1993.

[2] B.S. Trivedi, N.N. Jani, H.H. Joshi, R.G. Kulkarni, J. Mater. Sci. 35(2000) 5523.

[3] P.N. Vasambekar, C.B. Kolekar, A.S. Vaingankar, J. Mater. Chem.Phys. 601 (1999) 282.

[4] A.A. Ghani, A.A. Sattar, J. Pierre, J. Mag. Mag. Mater. 97 (1991)141.

[5] A.K. Nikumbh, A.V. Nagawade, V.B. Tadke, P.P. Bakare, J. Mater.Sci. 36 (2001) 653.

[6] M.A. Gabal, J. Phys. Chem. Solids 64 (2003) 1375.[7] G. Grosse, Mos-90, Version 2.2, Oskar-Maria-Graf-Ring, München,

1992.[8] El.-H.M. Diefallah, M.A. Gabal, A.A. El-Bellihi, N.A. Eissa,

Thermochim. Acta 376 (2001) 43.[9] M.A. Gabal, A.A. El-Bellihi, S.S. Ata-Allah, J. Mater. Chem. Phys.

81 (2003) 84.[10] Q.M. Wei, J.B. Li, Y.J. Chen, J. Mater. Sci. 36 (2001) 5115.[11] E. Woleska, E. Riedel, W. Walski, Phys. Status Solidi (a) 132 (1992)

51.[12] J. Smith, H.P.T. Wijin, Ferrites, Wiley, New York, 1959.[13] M.A. Amer, Phys. Status Solidi (a) 65 (1981) 479.[14] M.M. Girgis, A.M. El-Awad, Mater. Chem. Phys. 36 (1993)

48.[15] J.A. Dean, Lange’s Handbook of Chemistry, McGraw-Hill Book

Company, New York, 1979.[16] S.S. Ata-Allah, M.K. Fayek, H.S. Refai, M.F. Moustafa, J. Solid

State Chem. 149 (2000) 434.[17] Yu.P. Irkhin, E.A. Turov, Sov. Phys.-JETP 33 (1957) 673.[18] S.R. Mrrison, The Chemistry and Physics of Surface, Plenum Press,

New York, 1977.[19] C.C. Wu, S. Kumarkrishanan, T.O. Mason, J. Solid State Chem. 37

(1981) 144.[20] J.B. Goodenoug, Prog. Solid State Chem. 5 (1971) 146.