Structural and magnetic properties of Ce1−xCoxO2 (0  x  0.1) nanocrystalline powders

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Phys. Scr. 86 (2012) 015605 (7pp) doi:10.1088/0031-8949/86/01/015605

Structural and magnetic properties ofCe1−xCoxO2 (0 6 x 6 0.1) nanocrystallinepowdersMayora Varshney1, Aditya Sharma2, K D Verma1 and Ravi Kumar3

1 Material Science Research Laboratory, Department of Physics, S V College, Aligarh-202001, UP, India2 Department of Applied Sciences and Humanities, Krishna Institute of Engineering and Technology,Ghaziabad-201206, UP, India3 Centre for Material Science and Engineering, NIT-Hamirpur, HP, India

E-mail: adityaiuac@gmail.com

Received 3 April 2012Accepted for publication 13 June 2012Published 4 July 2012Online at stacks.iop.org/PhysScr/86/015605

AbstractIn this work, we report detailed investigations on the structural and magnetic properties ofpure and Co-doped CeO2 nanoparticles. All the samples, with different cobalt concentrations,were prepared by the co-precipitation method. To investigate the structural properties andphases present in the samples, systematic, x-ray diffraction measurements were performed inthe θ–2θ mode. Phonon modes, from the pure and Co-doped samples, were studied by Ramanscattering measurements. Surface morphology of individual nanoparticles was studied by fieldemission scanning electron microscopy. Interestingly, room temperature ferromagnetism isobserved in, both, un-doped and Co-doped samples. The mechanisms of variation in theRaman active mode (F2g) and magnetization, by increasing the cobalt concentration, arebriefly discussed.

PACS numbers: 61.46.Bc, 63.22.Kn

(Some figures may appear in color only in the online journal)

1. Introduction

Diluted magnetic semiconductors (DMSs) are promisingcandidates for many applications in various fields such aslogic, storage, communications, quantum computation andmulti-functionality on the same chip, etc. Therefore, a numberof semiconductors doped with different transition metal (TM)ions have been synthesized [1–5]. However, some of them(e.g. GaAs:Mn and InAs:Mn) were disqualified because oftheir low Curie temperatures, low magnetic moments andmulti-phase nature [1, 2]. Hence, there is large incentiveto develop promising DMS materials that illustrate all therequired features of the ferromagnetic semiconductor. Inthis context, Dietl et al [6] and Sato Katayama-Yoshida[7] theoretically proposed oxide-based systems as interestingcandidates for room-temperature ferromagnetism (FM) andhigh magnetic moments. In addition, the discovery ofroom-temperature ferromagnetism in the Co-doped TiO2,ZnO and SnO2 systems [8–10] has also generated muchinterest in the wide bandgap, transparent semiconductor

and high k dielectric oxide materials. Although some ofthese works reported the observation of FM above roomtemperature, the origin of FM in these systems is not yetwell understood. The main unresolved question is whetherthe observed FM originates from uniformly distributed TMions in the host matrix or it is due to the precipitation ofsecondary phases such as metallic clusters/oxides [11]. Ifa DMS material contains TM ions below their equilibriumsolubility limit no secondary phases are expected. In this case,since the strength of magnetism is proportional to the numberof TM ions substituted on the cation cites in a DMS, theoccurrence of high magnetic moments is difficult. On the otherhand, at higher TM concentrations the doped atoms start toform unwanted metallic clusters.

If these complications were not enough, in theTM-doped systems, things are further complicated bythe defected and/or nano-sized oxide materials. Thedefected and/or nano-dimensional oxides show unexpectedroom-temperature FM and introduce a special class of d0

0031-8949/12/015605+07$33.00 Printed in the UK & the USA 1 © 2012 The Royal Swedish Academy of Sciences

Phys. Scr. 86 (2012) 015605 M Varshney et al

oxide materials [12–15]. Various groups claim different rolesplayed by the defects. One common view on the roleplayed by the defects is that they help in generating morecharge carriers and hence stabilizing the FM in the systemand consequently the claim is that the observed FM ischarge-carriers mediated. Several reports on the observationof FM in un-doped ZnO, SnO2, CeO2 and ion-irradiatedTiO2 [12, 16] cast doubt as to whether TM doping isrequired in such systems or if some links are missing. Insuch a scenario, it is highly desirable to select a potentialmaterial and maneuver a growth method that not onlyoffers synthesis of, uniformly, TM ion-doped high-qualitymaterials of common size/shape with fair reproducibility, butis also financially viable. In order to understand the magneticproperties of TM-doped and un-doped oxide materials,we have employed a pH-controlled wet chemical method(co-precipitation) by which CeO2 nanoparticles, with differentcobalt concentrations, are synthesized at low temperaturewithout requiring any surfactant and/or capping agent. Thismethod has proven its efficiency in controlled concentrationdoping, uniform distribution of TM ions into the host latticevia chemical/ionic reaction and formation of mono-disperse(doped and un-doped) nanoparticles [17–20]. Our choiceof CeO2 as the test material was based on its very lowsolubility in the solvent used during synthesis, particularlyat lower temperatures. Besides this, CeO2 is one of the mostextensively investigated rare-earth oxides due to its interestingstructural, optical, di-electric and magnetic properties and richfunctionalities. It presents unique face-centered cubic (FCC)crystal structure, n-type carrier density and wide band gap(∼3.2 eV) and therefore offers its accessibility in the futuristicsmart and multifunctional devices.

2. Experimental details

2.1. Synthesis

All the reagents used were of analytical grade withoutfurther purification. 0.01 M solutions of CeCl4 · 5H2O andCo(CH3COO)2 · 5H2O were prepared with a molar ratio ofx = Co/(Ce + Co) by dissolving them into de-ionized water.Ammonium hydroxide (NH4OH) was added into the solution(drop by drop) by stirring to form white precipitates ofCeO2. The pH value during the reaction was kept constant,pH = 11, for all the compositions. The resultant precipitateswere washed several times with de-ionized water to removeother organic impurities. The precipitates were heated in airat 40 ◦C for 20 h followed by natural cooling up to roomtemperature and, finally, the light yellow colored powderswere carefully collected. From the nature of synthesisand experimental findings, the formation of Ce1−x Cox O2

nanoparticles in colloidal medium may occur via thefollowing reaction.

(1 − x) (CeCl4) + xCo (CH3COO)2 + mH2O → (1 − x) Ce+4

+ xCo+2 + mOH− + nHCl + 2xCH3COO−

(1 − x) Ce+4 + xCo+2 + mNH4OH → (1 − x) Ce (OH)4

+ xCo (OH)2 + mNH+4

(1 − x) Ce (OH)4 + xCo (OH)2

40 ◦C−−−→ Ce1−x Cox O2 + nH2O

In the reaction, cerium chloride, cobalt acetate and ammoniumhydroxide undergo dissociation into ionic reaction and formCe+4, Co+2, OH− and NH+

4 ions. The intermediate complexesof Ce(OH)4 and Co(OH)2 are expected to combine and formCe1−x Cox O2 granules in the colloidal medium. Meanwhile,pH-controlled growth and low-temperature drying is expectedto control the size and morphology of the so-formed particles.

2.2. Characterization

The phase and crystal structure of the as-synthesized sampleswere identified by powder x-ray diffraction (XRD) withBruker D8 advanced diffractometer using Cu Kα radiation(λ = 1.540 Å). The morphology was investigated by usinga field emission scanning electron microscope (FESEM).Raman scattering measurements were performed usingan In-Via (Ranishow) Raman microscope. Excitation wasprovided by an argon-ion laser of wavelength 514 nm and alow incident power to avoid thermal effects. dc magnetizationmeasurements were carried out at room temperature usingcommercial quantum design physical properties measurementsystem (PPMS).

3. Results and discussion

Figure 1 shows the XRD patterns of Ce1−x Cox O2 (x = 0.00to 0.1) nanoparticles. The diffraction peaks in the figure areindexed to the florite-type structure of CeO2 (JCPDF no.750390) with an FCC unit cell. No trace of cobalt metal,oxides or other binary cerium–cobalt phases could be detectedup to the cobalt concentration of x = 0.07. At the highest Coconcentration (x = 0.1), a few extra peaks were detected andmarked by an asterisk in figure 1. These peaks are assignedto the oxide of cobalt (i.e. Co2O3). This indicates that thesolubility limit of Co into CeO2 (by using co-precipitationmethod) is only up to x = 0.07. The average size of thenanoparticles was calculated using the Scherrer relation D =

0.9λ/(β cos θ), where β is the full-width at half-maximumof the peaks, expressed in radian. Thus calculated particlesizes fall in the range of ∼6.2–6.7 nm for all the samples.The calculated lattice parameters of nanocrystalline CeO2

are listed in table 1. It is clear from the table that thelattice parameter of nanocrystalline Ce1−x Cox O2 is higherthan the corresponding bulk lattice parameter values (JCPDFno. 750390). This is in agreement with earlier reports thatthe lattice expands in CeO2 nanoparticles due to the decreasein electrostatic forces caused by the valence reduction of Ceions (i.e. formation of O-ion vacancies in the material) [21].Interestingly, the position of the XRD peak (i.e. (220) peak)shows systematic variation with cobalt doping. The peaksshift toward the higher 2θ values as x increases up to 0.07(see figure 1(b)). Further increase in the Co concentrations(i.e. x = 0.1) leads to the (220) XRD peak shifting towardthe lower 2θ values. The peak shifting causes interestingvariations in the lattice parameters and, hence, unit cellvolume. Table 1 summarizes evolution in the cell parametersand unit cell volume. It can be seen from table 1 that thelattice parameters decrease, and hence the volume of the unitcell decreases, with increase in cobalt concentration up tox = 0.07. Such a contraction of lattice parameters can be

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Phys. Scr. 86 (2012) 015605 M Varshney et al

2θ (Degree) 2θ (Degree)

Figure 1. (a) XRD patterns of Ce1−x Cox O2 (x = 0.00–0.1) nanoparticles. (b) Evolution of the (220) peak.

Table 1. Particle size, lattice parameter and unit cell volume ofdifferent samples.

Sample Particle Lattice Cell volumename size (nm) parameters (Å) (Å)3

CeO2 6.2 5.452 162.056Ce0.97Co0.03O2 6.6 5.412 158.516Ce0.95Co0.05O2 6.3 5.406 157.989Ce0.93Co0.07O2 6.7 5.401 157.551Ce0.9Co0.1O2 6.5 5.435 160.545

understood, qualitatively, by considering the size of the ions.Here, substitution of bigger Ce4+ ions (ionic radius = 0.97 Å)by the smaller Co2+ ions (ionic radius = 0.79 Å) reduces theinter-atomic spacing significantly and, hence, contraction inthe lattice parameters is observed with Co-doping. Similarcontraction in the lattice parameters has also been observedin the case of Co-doped SnO2 nanoparticles [19]. At theconcentration of x = 0.1, the lattice parameters and unit cellvolume expand, which could be due to the secondary phasepresent in the material (marked by an asterisk). Thus, theXRD measurements suggested that the doped Co ions aresubstituting the Ce in the host matrix and there are no Cometallic clusters or other impurity phases in the Ce1−x Cox O2

nanoparticles, up to the x = 0.07.

To further examine the morphology of the so-formednanoparticles FESEM measurements were performed andare presented in figure 2. Figures 2(a)–(c) represent FESEMmicrographs of CeO2, Ce0.97Co0.03O2, and Ce0.93Co0.07O2

nanoparticles, respectively. It is clear from the images thatthe nanoparticles are of spherical shape. However, someaggregation of the nanoparticles has been observed in allthe samples. This aggregation in wet chemically synthesizednanoparticles is expected due to the presence of substantialOH− ions in samples [22–23]. The size of individualnanoparticles, evaluated from the FESEM images, is about20 nm. The difference in the calculated size from XRD andFESEM may arise because of the fact that the XRD measuresonly the crystalline part (in which long-range ordering ofatoms exist) of the particle, whereas the FESEM gives themorphology of overall particle in which a non-crystallinepart may exist. The non-crystalline part in such chemicallysynthesized samples may exist due to the presence of, slight,organic matters, such as C, H, Cl, etc., present in the parentchemicals, used to prepare the samples, and aggregated withwater molecules.

It is evidenced that the doped systems are often difficultto analyze by XRD because of the high dispersion and lowconcentration of the doped constituents. Raman analysesare usually easier to perform than the XRD, and they can

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Phys. Scr. 86 (2012) 015605 M Varshney et al

Figure 2. FESEM images of (a) CeO2, (b) Ce0.97Co0.03O2 and (c) Ce0.93Co0.07O2 nanoparticles.

often yield useful information. Indeed, Raman scattering isable to provide information on different phenomena suchas determination of the different phases of same material(e.g. anatase and rutile phases of TiO2) [24], identificationof different systems in parent host, allowing the investigationof amorphous-to-crystalline phase transitions, oxygen defects,stress states and the size effects. A detailed analysis ofRaman spectral features leads to the derivation of bandparameters, such as line-width, asymmetrical shape, relativeintensity ratio etc, and allows gain of important informationto understand the functional behavior of the probed material.Fluorite structured metal dioxides have only a single allowedRaman mode, F2g, and appear as a symmetric breathing modeof the O atoms around each Ce cation [25]. Since only the Oatoms move the mode frequency should be nearly independentof the cation mass. In CeO2 this frequency is 465 cm−1 [25],whereas in ThO2, where the cation mass is 65% larger, thefrequency is only about 1 cm−1 higher [26]. Therefore, thepeak shifting (of F2g mode) is treated as the consequenceof variation in the O ion concentrations in the material [27].Figure 3 shows the room temperature Raman spectra ofun-doped and Co-doped CeO2 nanoparticles. It is clearfrom figure 3 (the main panel) that the Ce1−x Cox O2 (x =

0.00, 0.03, 0.05 and 0.07) nanoparticles show an intenseRaman active mode, appearing at 458.5 cm−1. This mode(at 458.5 cm−1) is present in all the samples and assignedto the Raman active mode (F2g) of CeO2. However,the peak position of this mode is, slightly, toward thelower wave number side in comparison to the reportedvalue of F2g mode (465 cm−1) of the bulk CeO2. Thiscould be due to two different facts (i) crystalline sizeeffect and (ii) variation in the oxygen ion concentration.

Figure 3. Raman spectra of Ce1−x Cox O2 (x = 0.00–0.07)nanoparticles.

In case of a perfect crystal only the phonons near thecenter of the Brillouin zone (q0 ∼ 0) contribute to thescattering of incident radiation due to the momentum

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Phys. Scr. 86 (2012) 015605 M Varshney et al

conservation rule between phonons and incident light. Asthe size of the crystal is reduced, the vibration is limited tothe size of the crystal, which gives rise to the breakdownof the phonon momentum selection rule (q0 ∼ 0), allowingphonons with q 6= 0 to contribute to the Raman spectrumand leads to asymmetric broadening and even peak positionchanges in the Raman active modes [28]. The peak positionshifting and asymmetric peak broadening is best describedusing the spatial correlation model (also known as the phononconfinement model) [28]. In the present case, the peakposition of F2g mode (458.5 cm−1) does not match with theF2g mode position (465 cm−1) of bulk CeO2; however, itmatches well with the reported F2g mode of nanocrystallinesamples of the size of 4–10 nm [29] and, hence, strengthenedthe formation of nano-crystalline CeO2 samples.

Since the lower valence state of the doped ions (e.g. Co+2

or Co+3) leads to generation of oxygen ion vacancies in theCeO2 (where Ce is in the +4 valence state, i.e. Ce+4) to makecharge neutrality in the system. Therefore, the F2g mode isexpected to show a little shifting in the peak position [27] withlower valence ion doping. However, in the present case thepeak position of the F2g mode is not affected by increasingthe Co concentration; an indicator of unlikely formationof oxygen ion vacancies in the Co-doped CeO2 and maybe intrinsic to the low temperature and pH controlled wetchemical synthesis process. Therefore, the present Ramanspectra precisely validates the formation of nanocrystallinepowder samples, using the co-precipitation method, and theoxygen ion vacancy concentration is not varied in the sampleswith Co concentrations. One can also note from figure 3(main panel and insets) that the Raman spectra consists oftwo other peaks located at 302.5 and 608.2 cm−1, respectively.The fluorite structure of CeO2 (space group Fm-3m) doesnot allow other modes to appear in the Raman spectra,except the F2g mode. Therefore, the presence of these Ramanactive modes in the present study may be the consequenceof structural constrains in the as-synthesized nanoparticles.There are reports available on the appearance of such Ramanactive modes in the narrow sized CeO2 nanoparticles [29].Feng et al [30] have reported the relationship between theratio of the number of superficial atoms to the total numberof atoms and crystal size. According to their results, the ratioof superficial atoms to the total atoms decreases, swiftly, withan increase in crystal size. The surface atoms have differentatomic arrangement/orientation than that of interior atoms andgive a surface mode in the Raman spectra. Therefore, the peakappearing at 458.5 cm−1, F2g mode, can be attributed to theinterior phonon modes of the CeO2 crystal, whereas the peaksappearing at 302.5 and 608.2 cm−1 are phonon modes relatedto the surface of nanoparticles. However, the peak intensityof the Raman modes from the surface was found to decreaseby increasing the crystalline size [29]. However, our XRDand FESEM measurements confirm the formation of, almost,equal-sized nanoparticles for all the samples. Therefore, theobserved decrease in the intensity (see figure 3; the main paneland insets) of Raman modes may not be due to the increase inparticle size, with increasing Co concentration. The observeddecrease in the Raman modes is probably related to thestructural properties of the Ce1−x Cox O2 system. Similarobservations, on the reducing Raman intensity with increasing

Figure 4. Room-temperature hysteresis curves of Ce1−x Cox O2

(x = 0.00, 0.03, 0.05 and 0.07) nanoparticles.

doping concentrations, have also been reported in the Nd, Tband Pr-doped CeO2 samples [27]. The authors have concludedthat the increased concentrations of doped atoms lead toshrinkage in unit cell volume and increase optical absorption,thus results in reducing the intensity of all the Ramanpeaks. Therefore, the observed decrease in the intensity ofRaman active mode is the consequence of Co-doping inducedstructural modification in CeO2 nanoparticles and not dueto variation in the particle size of the samples. Furthermore,we have not observed any Raman active modes of cobaltoxides (i.e. CoO and Co2O3, etc) in this study, up to theCo doping of x = 0.07. Hence, the Raman measurementsstrengthened the XRD data and help us to conclude that:(i) all the samples, prepared by co-precipitation method, arein nanocrystalline form; (ii) the oxygen ion vacancies maybepresent in the as-synthesized samples but their concentrationis not varied with Co doping; and (iii) no secondary phasescould be detected, up to the Co concentration of x = 0.07,indicative of substitutional doping of Co.

To further understand the magnetic properties ofCe1−x Cox O2 (x = 0.00 to 0.07) nanoparticles, systematic,hysteresis loops were collected at room temperature andpresented in figure 4. It is clear from figure 4 that all thesamples show room temperature ferromagnetic behavior withcoercivity of ∼178 Oe. One can easily see from figure 4that un-doped CeO2 nanoparticles present larger saturationmagnetization compared to the Co-doped samples. Sincethere is no reason to attribute the introduction of FMto any dopant, and moreover, in this CeO2 case (Ce in+4 valence state) there are no d electrons involved, onecannot think of any interaction that may originate fromthat. Thus, we must reconsider the possibility that waspreviously assumed for the other ferromagnetic un-dopedoxides; FM due to oxygen vacancies and/or defects andconfinement effects [12, 31]. As for CeO2, it seems that boththe factors are equally important. A few groups reported thattheir bulk CeO2 samples show diamagnetic behavior with4f 0 electronic configuration of Ce+4. However, nanoparticles

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Phys. Scr. 86 (2012) 015605 M Varshney et al

and nanocubes of CeO2 were ferromagnetic in nature. [31].Sundersan et al [12] reported an increase in the magnetization(maximum ∼0.002 emu g−1) by annealing the samples at500 ◦C, which is due to the removal of organic matterfrom the surface of as-synthesized nanoparticles. However,in this case (samples are not annealed at higher temperature)the observed magnetization is almost three times larger (∼0.006 emu g−1) than that observed by Sundersan et al. Thisfurther validates the superiority of the synthesis procedure,employed in the present work, which helped us to synthesizeorganic matter free and high quality compounds. The previousexperimental and theoretical work suggested that the oxygenvacancies in pure CeO2 cause spin polarization of f electronsfor Ce ions surrounding oxygen vacancies, resulting innet magnetic moment for pure CeO2 samples [31]. Theoxygen vacancy-induced magnetic moment depends on thelocation of the vacancy; i.e., one oxygen vacancy at surfacecan induce more magnetic moments than those inducedby one oxygen vacancy in the interior of the sample. Ascerium can have variable valence states (Ce4+/Ce3+) andoxygen vacancies may form on the surface of the CeO2

nanoparticles [31], oxygen vacancies can create magneticmoments on the neighboring Ce-ions, as shown by x-rayspectroscopy studies [31, 32]. In our XRD results, significantlattice expansion is observed while the F2g mode was foundto be situated at lower wave-number side (in comparison toposition of F2g mode in bulk CeO2) in the Raman spectrum,which could be an indication of the formation of oxygenion vacancies in the as-synthesized samples. We thereforebelieve that oxygen vacancies are probably responsible forthe observed ferromagnetic behavior in the un-doped CeO2

nanoparticles. The net magnetization, in the samples, isexpected to increase by increasing the TM ion concentrationdue to the expected direct exchange interaction among Coatoms [33]. In contrast, we observed a significant reductionin magnetization by adding Co ions in the CeO2 nanoparticle.The magnetization decreases almost one-third times for theCe1−x Cox O2 (x = 0.03, 0.05 and 0.07) systems with respectto the value of un-doped CeO2. For some DMSs likeMn:GaAs, for example, the magnetic exchange is describedas the Ruderman–Kittel–Kasuya–Yosida (RKKY)–Zenerinteraction [34]. On the other hand, in some oxide-basedDMSs the observed magnetism suggests that some parts of theexchange are mediated by the bound magnetic polarons [35].In both of the circumstances, free charge carriers are, indeed,involved in the magnetic exchange interaction. In the purestoichiometric form CeO2 is considered as an insulatormaterial, with a large band gap, in which four valenceelectrons of Ce are communal with two neighbor oxygenatoms. However, in the usual CeO2 crystals naturally formedoxygen vacancies are present, since the required energy for thecreation of oxygen deficiency in this crystal is much less. Eachoxygen deficiency provides free charge carriers originatingfrom two shallow donor levels. In CeO2, cations of hostmaterial can be substituted either by Co+2 or by Co+3, sincethe ionic radius of both of the Co ions (0.79 Å for the low spinand 0.89 Å for the high spin state) is smaller than the ionicradius of Ce+4 (0.97 Å). Doping of Co ions manifests lowervalence states (i.e., Co+2 or Co+3) in the material comparedto the valence of host cations (i.e., Ce+4). In this scenario Co

doping is expected to reduce the number of potential bindingelectrons to the neighboring oxygen atoms. Meanwhile, freeelectron capture takes place by oxygen, into the localizedacceptor states, because of the high electronegative nature ofoxygen [36]. If the Co+2 substitute the Ce+4, the oxygen ionvacancy may be completely compensated. This mechanismof free charge carrier compensation is well documentedand helps to explain the decrease of magnetization in theCo-implanted SnO2 thin films [36]. In this case, compensationof charge carriers may take place by adding a littleconcentration (i.e., x = 0.01) of Co in the CeO2, which leadsto a decline in the saturation magnetization. Doping of higherCo concentrations is further expected to compensate thecharge carriers and hence decrease in saturation magnetizationis observed in the x = 0.03, 0.05 and 0.07 Co dopedsamples. However, we cannot exclude the possibility of othermagnetic interactions in the higher concentration Co-dopedsamples. The well-known super-exchange interaction orexchange via oxygen anions will probably happen whenthe magnetic ions have a very small mean distance (lessthan 10 nm) in the host material [37]. This interaction isantiferromagnetic in nature rather than ferromagnetic. In thepresent case the size of nanoparticles is very small (<7 nmcalculated by the XRD) in which Co ions are distributed,substitutionally. As the doping concentration increases pairsof much closer Co atoms are expected to form within thenanoparticle. Therefore, one may expect a large magnitudeof super-exchange interaction between neighbor Co ions,with increasing doping concentration. This in turn leads toa decrease in the net ferromagnetic character of Co-dopedCeO2 with an increase in its concentration. This is inaccordance with our previous results of Co-doped SnO2

nanoparticles [19]. Our experimental results suggest thatthe origin of FM in the un-doped and Co-doped CeO2

nanoparticles is intrinsic to the system, however, studies onthe defects and TM-doping induced FM may lead to furtherinsight into the relationship between the structure and theother properties of nanocrystalline oxide materials.

4. Conclusions

In summary, sufficiently crystalline, smaller sized (∼7 nm),Co-doped CeO2 nanoparticles were synthesized using theco-precipitation method at low temperature and constantpH. The (220) peak, in the XRD measurements, showssignificant shifting toward the higher angles side and leadsto contraction in the cell parameters, and hence in unitcell volume, up to the Co concentration of x = 0.07. Thisis an indication of substitutional doping of Co in CeO2.Secondary phases of cobalt oxide are detected in the x =

0.1 cobalt-doped sample, an indicator of the solubilitylimit of Co in CeO2 by the Co-precipitation method.Raman spectroscopy measurements strengthen the synthesisof narrow-sized, oxygen-deficient and Co-substituted CeO2

nanoparticles. The room temperature FM in un-doped CeO2

is probably related to the oxygen-ion vacancies. Hysteresiscurves of Co-doped CeO2 infer that the free charge carriersare supposed to be compensated, with Co doping, leading toan observed decrease in net magnetization. Further decreasein saturation magnetization, with increasing Co concentration,

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is the consequence of the combined effect of charge-carrierconcentration and super-exchange interaction between nearbyCo ions. Our experimental results indicate that the observedFM in the samples is intrinsic to the system and offers a fluentphysics to analyze.

Acknowledgments

The authors are grateful to Inter University AcceleratorCentre, New Delhi for providing financial assistance underthe research project (UFUP-46301) and to Dr R K Kotnala(NPL, Delhi) for helping in room temperature magneticmeasurements.

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