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Materials Science and Engineering B 163 (2009) 48–56

Contents lists available at ScienceDirect

Materials Science and Engineering B

journa l homepage: www.e lsev ier .com/ locate /mseb

tructural, electrical and magnetic properties of Bi-substituted Co2MnO4

.E. Rajeevana, Ravi Kumarb,∗, D.K. Shuklac, P.P. Pradyumnana, S.K. Arorad, I.V. Shvetsd

Department of Physics, University of Calicut, Kerala 673635, IndiaMaterials Science Division, IUAC, New Delhi 110067, IndiaDepartment of Physics, Aligarh Muslim University, Aligarh 202002, IndiaCRANN, School of Physics, Trinity College Dublin, Dublin 2, Ireland

r t i c l e i n f o

rticle history:eceived 24 January 2009eceived in revised form 31 March 2009ccepted 2 May 2009

ACS:1.10.Nz2.80.Ga7.22.−d7.22.Gm7.80.−e7.80.Bh

eywords:

a b s t r a c t

Structural, electrical and magnetic properties of single phase BixCo2−xMnO4 (0 ≤ x ≤ 0.3) spinel materi-als synthesized by solid state reactions were studied. All the samples exhibit single phase with cubicspinel structure (space group Fd3m) and the lattice parameter increases with the Bi-substitution. Thegrain size was also observed to increase with the Bi-substitution. All the samples exhibit the semicon-ducting behaviour and overall resistivity decreases with the increase in the Bi-substitution. The dc as wellas ac conductivity data were analyzed in the light of various conductivity models. The dc conductivitydata is explained using variable range hopping (VRH) model. All the samples show diffused ferroelectric(FE) transition and follow the Debye-type relaxation, whereas ferroelectric transition temperature (TC)increases with the Bi-substitution. The ac conductivity calculated from the dielectric data as a functionof temperature and frequency demonstrate the cross-over from small polaron tunneling (SPT) to corre-lated barrier hopping (CBH) type conduction in these materials. The influence of cation composition onthe magnetic properties of BixCo2−xMnO4 (0 ≤ x ≤ 0.3) mixed cubic spinel system has been studied by

pinel oxideerroelectricelaxor behaviourc conductivityc conductivityerrimagnetism and multiferroic

dc magnetization. Soft magnetic type behaviour was observed in the Bi-substituted samples, showingmagnetic dilution and the increased value of saturation magnetization suggests the presence of cantedspin structure due to the incorporation of Mn and Bi. Nevertheless, ferrimagnetic (FM) nature of theCo2MnO4 is preserved in the Bi-substituted samples. The coexistence of ferroelectricity and ferrimag-netism in these materials is attributed to the off centering of cations that result in non-centrosymmetricarrangement and canted spin structure. These materials are promising candidates for multiferroic

applications.

. Introduction

Mixed transition metal oxides with general formulae AB2O4A = Zn, Fe, Co, Ni, Mn, Mg, Cd, etc.; B = Co, Fe, Cr, Al, Ga, Mn, etc.)here A and B represent divalent and trivalent cations respectively,

re time honored systems owing to their interesting magnetic andlectric properties, which can be tuned accurately by changinghe chemical composition through cation redistribution [1–5]. Therystal structure of a cubic spinel AB2O4 belongs to Fd3m (site sym-etry Oh

7) space group. Each unit cell contains 8[AB2O4] formulanits and therefore 32 O2− ions. This close packing contains 64etrahedral interstices and 32 octahedral interstices coordinated

ith O2− ions. The charge distribution of the normal spinel is

epresented by [A2+]8a[B23+]16d[O4

2−]32e, in which Wyckoff posi-ions 8a denote the tetrahedral sites and 16d the octahedral sitesurrounded by O2− ions at 32e sites. Empty interstitial space is

∗ Corresponding author. Tel.: +91 11 26893955; fax: +91 11 26893666.E-mail address: ranade@iuac.ernet.in (R. Kumar).

921-5107/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.mseb.2009.05.003

© 2009 Elsevier B.V. All rights reserved.

comprised of 16 octahedral sites (16c) and 56 tetrahedral sites (8band 48f). All spinel-like structures are characterized by an oxygenparameter u having a value around 0.375. For u = 0.375, the arrange-ment of O2− ions is exactly a cubic close packing (FCC structure).In actual spinel lattice, there is deviation from the ideal pattern,with u > 0.375, resulting in a deformation of oxygen tetrahedronsand octahedrons [6,7].

Depending on the species and stoichiometry of cations occupy-ing A and B sites, some of the spinel phase may show multiferroicproperty. In case of magnetic spinel chromites, appearance of ferro-electricity has been explained based on the conical spin modulation[8]. Spin related ferroelectricity was also observed in many dis-torted perovskite type magnetic systems [9,10]. Bismuth basedmagnetic perovskites were investigated by many research groupsand their ferroelectric and ferromagnetic behaviour was attributed

to the covalent bonding between bismuth and oxygen atoms [11,12].For the mixed spinel oxide, LiFeTiO4, the dispersion behaviourin dielectric property and conductivity are thermally activatedquantities and are attributed to the processes controlled by anactivation energy related to the hopping of mobile cations [13,14].

nce and Engineering B 163 (2009) 48–56 49

Kaacmw[rt[a

ile(taIBciossctase[e2mIn

2

t(pAlwRstaeqycsLwfwaats((w

lattice constant increases almost linearly with Bi content up to 0.2,obeying Vegard’s law [23], as shown in the inset of Fig. 1. However,for x = 0.3, a small increase in ‘a’ is noticed (did not follow the linearbehaviour), which suggests that the system has approached to the

Table 1Lattice constant, average grain size, ε′ & tan ı (at 100 kHz and 300 K), and ferroelectrictransition temperature (at 100 kHz) for BixCo2−xMnO4.

Composition Lattice constant, Grain size, ε′ tan ı FE, TC (K)

N.E. Rajeevan et al. / Materials Scie

oops [15] explained the dispersion of dielectric properties usingphenomenological theory in which the dielectric spinel oxide is

ssumed to consist of well-conducting grains separated by poorlyonducting layers, with current flowing along the parallel align-ents of grains. Further the dielectric behaviour of MgAl2O4 spinelas explained by Maxwell–Wagner interfacial type polarization

16], which was in agreement with Koop’s theorem. The geomet-ical frustration of the lattice degrees of freedom is considered ashe origin of dispersive dielectric behaviour of cubic sulpho-spinels17]. There exist various spinels, which exhibit variety of electricalnd magnetic properties.

In the present study, our basic aim is to design a cubic spineln which the magnetic and ferroelectric properties can coexist. Inight of this Co3O4 is chosen, a very interesting material, whichxhibits an antiferromagnetic ordering at very low temperatureTN ∼ 20 K) and two non-equivalent sites of Co. In this material,he octahedrally coordinated Co3+ exists in low spin state and thentiferromagnetic ordering is due to weak Co2+–Co2+ interactions.n the above milieu, it will be interesting to study the effect ofi-substitution on cobalt based manganites because their electri-al and magnetic properties are influenced strongly by the changen cation distributions induced by the competition towards thectahedral preference between the pair of Co2+/Co3+ vs. the newubstituted cation [18,19]. The semiconducting properties of thesepinels can be described as in the case of manganites, where theonductivity reaches a maximum value due to the hopping of elec-rons when the numbers of cations with different valence statesre equal for the octahedral/tetrahedral sites. More precisely in thepinel compounds, the difference in valence state at two differ-nt sites by a unit charge is found to favour electrical conduction20,21]. Recently, we have reported [22] on the strong magneto-lectric coupling in Bi-substituted Co2MnO4, which persists up to00 K. Coexistence of ferroelectric and magnetic properties in theseaterials suggests a strong potential for multiferroic applications.

n this work, we present in detail the structural, electrical and mag-etic properties of Bi-substituted Co2MnO4.

. Experimental

The samples were synthesized by standard solid state reactionechnique from the pre-calcined mixture containing bismuth oxideBi2O3), cobalt oxide (Co3O4) and manganese oxide (MnO2) withurity >99.99%. The compositions were calcined at 820 ◦C for 24 h.ll the calcined compositions were uniaxially dye-pressed into pel-

ets of size 10 mm in diameter and 2 mm in thickness. Sinteringas performed at 1000 ◦C for 24 h, with intermediate grinding.oom temperature powder X-ray diffraction (XRD) studies of theamples were performed using a Bruker D8 X-ray diffractome-er with Cu K˛ radiation in 2� range of 15–75◦. Morphologicalnd microstructural features were investigated with a scanninglectron microscope, SEM Hitachi (Japan) model No.: S3400N anduantitative analysis was made with an electron beam microanal-ser equipped with an energy dispersive X-ray analysis (EDAX)apability. Temperature and frequency dependent dielectric mea-urements of the pellets were performed with a HP4192 precisionCR meter. For the dielectric measurement, surfaces of the pelletsere polished and silver coated, which were kept at 200 ◦C for 2 h

or stabilization of electrodes. The dc conductivity measurementsere performed by standard four-probe technique in the temper-

ture range 150–400 K. Temperature was controlled with ±50 mKccuracy using a Lakeshore temperature controller. The magnetiza-

ion measurements were carried out using the VSM option of PPMSet up within a temperature range, 5–300 K in zero field cooledZFC) and field cooled (FC) modes under a constant magnetic fieldH = 1 kOe). Field dependent magnetization measurements (M–H)ere performed at various fixed temperatures.

Fig. 1. X-ray diffraction pattern for the samples BixCo2−xMnO4 (x = 0, 0.1, 0.2 and 0.3)measured at 300 K. Inset shows the variation of ‘a’ with x.

3. Results and discussion

3.1. Structural and morphological characterizations

The X-ray diffraction (XRD) patterns obtained at room temper-ature for BixCo2−xMnO4, with x = 0.0, 0.1, 0.2 and 0.3 are shownin Fig. 1. Indexing and refinement of XRD patterns indicate thepresence of a single-phase polycrystalline structure for the syn-thesized materials. The X-ray patterns confirm the existence ofspinel structure with the reflection arising from 1 1 1, 2 2 0, 3 1 1,2 2 2, 4 0 0, 4 2 2, 3 3 3, 4 4 0 and 5 3 3 planes. All the samples havegood crystallinity and can be indexed with the Joint Committeeon Powder Diffraction Standards (JCPDS) file 80-1544, in face cen-tered cubic (FCC) structure. The lattice parameters ‘a’ obtained fromthe analysis of XRD pattern for the samples with cubic unit cell fordifferent compositions are given in Table 1. From Fig. 1, the shiftof planes towards lower 2� with the increase in Bi-substitution isclearly observed, indicating the lattice expansion due to the substi-tution of Co3+ by Bi3+. This could be attributed to the fact that ionicradii of Bi3+ (1.17 Å) are greater than that of Co3+ (0.65 Å) and Mn3+

(0.785 Å). From the XRD results we infer that BixCo2−xMnO4 is hav-ing single phase cubic spinel structure (space group Fd3m) and the

a (Å) SEM (�m)

x = 0.0 8.27268 1.75 541 2.596 –x = 0.1 8.31045 3.82 186 1.593 366x = 0.2 8.32425 5.90 944 2.085 412x = 0.3 8.32796 7.42 620 2.707 420

50 N.E. Rajeevan et al. / Materials Science and Engineering B 163 (2009) 48–56

Fig. 2. SEM micrographs of BixCo2−xMnO4, for x = 0.0, 0.1, 0.2 and 0.3. Insets showthe respective histograms of grain sizes.

Table 2The compositional analysis of BixCo2−xMnO4, extracted from EDAX.

Nominal composition Bi Co Mn Actual composition

Bi0.0Co2.0MnO4 0.00 2.0 1.0 Co2MnO4

Bi Co MnO 0.09 1.9 1.0 Bi Co MnO

0.1 1.9 4 0.09 1.9 4

Bi0.2Co1.8MnO4 0.18 1.8 1.0 Bi0.18Co1.8MnO4

Bi0.3Co1.7MnO4 0.26 1.7 1.0 Bi0.26Co1.7MnO4

solubility limit of Bi in the system. In the above circumstances, itis not unfair to say that only a small amount of Bi3+ can be addedconsidering its solubility in the spinel system.

Fig. 2 shows the SEM micrographs of the BixCo2−xMnO4(0.0 ≤ x ≤ 0.3) samples taken at room temperature. The SEMimages show a uniformly developed grain morphology anddense microstructure. The micrographs of the samples with x > 0(Bi-substituted) showed relatively greater homogeneity in themicrostructure along with increased grain size. Histograms shownin the inset of each SEM micrographs quantitatively represent theincreased grain size (∼1–8 �m) due to Bi-substitution. This suggeststhat Bi-substituted compositions have higher fraction of biggercrystalline grains. The stoichiometry of the above compositions wascalculated by means of an electron beam microanalyser (EDAX)attached to SEM and is shown in Table 2. It is important to men-tion that, for higher Bi concentration, stoichiometry of the sinteredcomposition is found to deviate slightly from that of the nominalcomposition.

3.2. dc conductivity studies

The dc conductivity measured in the temperature range150–400 K exhibits semiconducting behaviour, with resistivityreducing from 600 �-cm to 300 �-cm at room temperature, on Bi-substitution (Fig. 3). For the sample with x = 0, the resistivity is ofthe order of k�-cm. The decrease in resistivity can be attributedto the Bi-induced stabilization of the solid solution instead of lessconducting pure phase and an increase of grain size that reducesthe number of grain boundaries. Another consideration that can bemade to support the increased dc conductivity in Bi substitutedsystem is slight oxygen deficiency (see Table 2). The basic con-

duction mechanism is taken as the hopping of electrons betweennon-equivalent valence states existing between octahedral andtetrahedral sites. The dc conductivity behaviour can be explained

Fig. 3. The plot of dc resistivity as a function of temperature for BixCo2−xMnO4 (x = 0,0.1, 0.2 and 0.3) with x = 0.0, 0.1, 0.2 and 0.3.

N.E. Rajeevan et al. / Materials Science and Engineering B 163 (2009) 48–56 51

Fl

u

�

wmktccslfptf

TAB

C

xxxx

ig. 4. log �dcT vs. 1000/T plots for BixCo2−xMnO4 (x = 0, 0.1, 0.2 and 0.3). The dashedine fit to the data obtained using Eq. (1).

sing the Arrhenius relation [24]:

dcT = �0 exp(

− Eact

kBT

)(1)

here �0 is the pre-exponential factor and is a characteristics of theaterial depending on lattice parameter and phonon frequency,

B is Boltzmann’s constant, T is absolute temperature and Eact ishe activation energy associated with the hopping process. Eact cal-ulated from the plots of log �dcT vs. 1000/T (Fig. 4) for all theompositions is given in Table 3, and is found to decrease with Bi-ubstitution in the higher temperature region. The deviation from

inearity in the log �dcT vs. 1000/T plots at low temperatures, 290 Kor x = 0.0 and 212 K for x = 0.3 (see Fig. 4), indicates that the smallolaron hopping model is valid only at relatively higher tempera-ures for lower Bi-substitution and over a wider temperature rangeor higher Bi-substitution.

able 3ctivation energy values calculated from dc conductivity, relaxation loss, forixCo2−xMnO4.

omposition dc activationenergy (eV)

Loss relaxation activation energy(eV) (100–1100 kHz)

= 0.0 0.1813 –= 0.1 0.1651 0.1511= 0.2 0.1628 0.1492= 0.3 0.1591 0.1493

Fig. 5. log �dc vs. 1/T1/4 plots for BixCo2−xMnO4 (x = 0, 0.1, 0.2 and 0.3).

In the low temperature region, the conduction may follow thevariable range hopping (VRH) model [25–27]. According to VRHModel, the dc conductivity (�dc) is proportional to exp (−T0/T)1/4,T0 being the characteristic temperature. Fig. 5 shows the variationof log �dc vs. T−1/4 which fits linearly rather well for x = 0, indicat-ing the validity of variable range hopping mechanism. Accordingto VRH theory [25], kBT0 = 18˛3/N(E), where N(E) is the density ofstates and 1/˛ corresponds to the localization length. From Fig. 5,we have found that the value of T0 increases with Bi-substitution(T0

1/4 varying from 100 for x = 0.0–118 for x = 0.3), which suggeststhe modification of effective localization length and the density ofstates.

3.3. Dielectric constant and dielectric loss

Fig. 6(a)–(d) shows the dielectric constant (ε′) as a functionof temperature (T) at various frequencies (100 kHz–1.1 MHz), forBixCo2−xMnO4 (0.0 ≤ x ≤ 0.3) samples. It is clearly evident fromFig. 6(a) that for x = 0 composition, the dielectric constant remainsalmost constant up to 204 K at 100 kHz, then increased sharplywith the increase in temperature. Within the investigated temper-

′

ature range, no peak is observed in ε vs. T. With the Bi-substitution(x = 0.1) the overall dielectric constant values are increased and thetemperature up to which the dielectric constant remains constantis also increased to T = 230 K (for 100 kHz). However, a well definedbroad peak-like structure (FE transition) appears at a temperature

52 N.E. Rajeevan et al. / Materials Science and Engineering B 163 (2009) 48–56

FBd

TtiaaiaitbwsxdrtNsaTtiaadubs

ig. 6. The plot of dielectric constant, ε′ as a function of temperature forixCo2−xMnO4 (x = 0.0, 0.1, 0.2 and 0.3). The top inset shows the composition depen-ence of ε′ .

C = 366 K. This peak temperature is found to shift towards a higheremperature with the increase in frequency. With a further increasen the Bi-substitution (x = 0.2), the dielectric behaviour is modifiednd exhibits two transitions, first transition is observed at 307 Knd the second one at 412 K, for 100 kHz. The peak-like structuresn temperature dependent ε′ and the shift in transition temper-ture (corresponds to ε′

m) towards higher temperature with thencrease in frequency indicate that these materials exhibit relaxorype ferroelectric behaviour. The first transition has strong relaxorehaviour, whereas the second one does not change appreciablyith the frequency. It has been observed that the dielectric con-

tant falls slightly for higher Bi-substituted compound (x = 0.2 and= 0.3). Also, for x = 0.3, low temperature FE transition is found toisappear and high temperature transition became almost non-elaxor type. This may be due to the polarization associated withhe 6s2 lone pair electrons of Bi, and stabilization of grain growth.otable broadening at FE transition indicates a diffusive phase tran-

ition. Inset of Fig. 6 shows the value of the dielectric constants a function of Bi-substitution measured at 300 K and 100 kHz.he temperature dependence of the dielectric loss, tan ı, for allhe samples at selected frequencies (100 kHz–1.1 MHz) is shownn Fig. 7. At low temperature (below 200 K), tan ı is found to belmost independent of frequency for all the samples. However for

′

given composition and temperature, ε and tan ı were found toecrease with the increase in the frequency. The overall higher val-es of tan ı are observed in all Bi-substituted samples, which maye due to an increase in dc conductivity of the materials with the Bi-ubstitution. The temperature, at which the peak in tan ı appears,

Fig. 7. The plot of tan ı as a function of temperature for BixCo2−xMnO4 (x = 0, 0.1, 0.2and 0.3). The top inset shows the composition dependence of tan ı and lower insetsare depicting the loss relaxation.

shifts with frequency. Insets of Fig. 7 for the doped samples show theplots between frequency vs. 1000/T i.e. Arrhenius relation, whichagain confirms the typical relaxor type behaviour. Loss relaxationenergy obtained from these Arrhenius plots is given in Table 3. Thestrong frequency dependent dielectric properties are attributed tothe interaction among the free carriers (electrons or holes) withpotential barriers at grain boundaries, resulting in the enhancementof conductivity [28].

Among the various contributions to polarization such aselectronic, ionic, dipolar and space charge polarizations, the con-tribution of space charge depends upon the purity of the system. Inthe present case, Bi ions are likely to occupy octahedral positionsand create bonding defects. Capacitance measurements on thesesamples have been carried out in the high frequency range startingfrom 100 kHz to 1.1 MHz, greater than the frequencies correspond-ing to the time constants (RC) suggested by the Catalan [29], to ruleout the possibility contribution from the Maxwell–Wagner inter-facial polarization towards capacitance which is dominant at lowfrequencies. At high frequencies most of the systems exhibits theintrinsic capacitance and need not be an artifact effect. In a paperby Wang et al. [30], two kind of capacitive contribution above 100 Kin TbMnO3 system by grains (dipolar effect i.e. hopping of charge

carriers within the grain) and grain boundary effect (i.e. internallayer barrier capacitor, IBLC) was observed and the system followsthe universal dielectric response (UDR). In UDR model, localizedcharge carriers hopping between spatially fluctuating lattice poten-tials not only produce the conductivity but also give rise to dipolar

N.E. Rajeevan et al. / Materials Science an

Ft

ebFswtToBiitetb

sTmtdtbCs[

rit�oSamsa

xpvm0ot

�ac is found to increase for the BixCo2−xMnO4 (0.0 ≤ x ≤ 0.3) sam-ples with a greater Bi content especially in the high frequency region(Fig. 9). This indicates that the movement or hopping of the chargecarriers is influenced by the neighborhood, i.e. enhanced by pres-ence of Bi3+ ions. At higher frequencies (∼2 MHz), �ac is decreased

ig. 8. Plot of log (fε′) vs. log f for the BixCo2−xMnO4 (x = 0, 0.1, 0.2 and 0.3) at roomemperature.

ffects. If it is true, at a given temperature a linear behaviour shoulde obtained in the plot of log (fε′) vs. log (f), where f is the frequency.ig. 8 represents the log (fε′) vs. log (f) plot for BixCo2−xMnO4amples at room temperature and in the range 100 kHz–1.1 MHz,here exact straight line behaviour is observed for undoped sys-

em i.e. Co2MnO4, whereas for doped samples it is not the case.his analysis straight away rules out the possibility of any kindf conductivity contribution in dielectric constant as per UDR ini-substituted system. Again it is well known that the exponential

ncreasing background in loss tangent implies that the backgrounds associated with the hopping conductivity. The loss tangent vs.emperature shown for Bi-substituted Co2MnO4 does not show thexponential increase up to a reasonably high temperature and fur-her support to the fact that these samples do not follow the UDRehaviour.

From the above mentioned results it is evident that the presentystem exhibit relaxor ferroelectric behaviour for all the samples.o understand the nature of relaxor behaviour, the well knownethod is to construct the Cole–Cole plot [31]. In this conven-

ional method ε′ is plotted as a function of ε′′ (imaginary part ofielectric constant, i.e. dielectric loss factor) to get information onhe distribution of relaxation times. For the Debye-type relaxorehaviour the data fits into a semicircular curve. However, theole–Cole plots for BixCo2−xMnO4 (0.0 ≤ x ≤ 0.3) samples are notemicircular (not shown here), owing to conductivity of the samples32].

From the plots representing the temperature dependence of theeal part of the dielectric constant, ε′ at different frequencies (Fig. 6),t is observed that ε′ varies very slowly with temperature up to aemperature �D/2, where �D is the Debye temperature [33]. AboveD/2, the values of ε′ for all the samples increase with increasef temperature and decreases with an increase in the frequency.uch a behaviour again indicates the Debye-type dielectric relax-tion process. Frequency dependent shift in ε′

m (dielectric constantaxima) towards the higher temperature was also exhibited by the

amples, up to x = 0.3, as expected in relaxor ferroelectric materi-ls.

The dielectric constant, ε′ of the BixCo2−xMnO4 samples with= 0, was about an order of magnitude smaller as compared to sam-les with x = 0.1 and 0.2. The FE transition was not visible from the ε′

s. T plots, within the high temperature limit (450 K) of our experi-

ental setup. In other samples (BixCo2−xMnO4 with x = 0.1, 0.2 and

.3), FE transition was observed within the high temperature limitf the setup, with increased diffuseness. Finally, the variation inhe dielectric constant and dielectric loss appears to be non-linear

d Engineering B 163 (2009) 48–56 53

with the composition (shown in the top insets of Figs. 6 and 7).It is due to the stabilization of structure with the increase in Bi-substitution up to x = 0.2. Also there is chance of the formation ofvacancies at the octahedral sites as the Bi content increases. In ourearlier report [22], the capacitance vs. voltage (C–V) measurementsof the BixCo2−xMnO4 with x = 0–0.3 (using an applied voltage of15 V) showed a typical butterfly-shaped loop in the C–V curves (verynarrow for x = 0), which demonstrates the ferroelectric nature of thesubstituted samples. Similar to the dielectric constant (ε′) measure-ment, the C–V curves show diffuseness that increases with x, andmay be attributed to the strain induced by Bi-substitution, owingto its larger ionic radius compared to the other cations.

3.4. ac conductivity

The ac conductivity (�ac) was studied over a frequency rangeof 100 kHz–5 MHz, for temperature varying from 150 K to 425 K.The �ac was calculated from the dielectric data, using followingequation:

�ac = ε0 ε′ ω tan ı (2)

Fig. 9. The plot of log �ac vs. log ω at 300 K, for BixCo2−xMnO4 (x = 0, 0.1, 0.2 and 0.3).

5 nce and Engineering B 163 (2009) 48–56

fhTa

�

wsdab

�

wefda

soTF�(

F0

4 N.E. Rajeevan et al. / Materials Scie

or all compositions (not shown here), indicating the inability ofopping of charges to follow the high frequency of the applied field.he total conductivity of the material at a frequency can be writtens:

total(ω) = �dc + �ac(ω) (3)

here hopping conduction arising from Bi-substitution is respon-ible for the enhancement of both dc and ac conductivities. Thec conductivity is the ω → 0 limit of �total (ω). For semiconductorsnd disordered systems the ac conductivity follows the power lawehaviour [34]:

ac(ω) = A ωs (4)

here A is temperature dependent constant and s is the frequencyxponent (s ≤ 1). From the log–log plots drawn for �ac vs. ω (Fig. 9)or the different compositions at room temperature, the slopeirectly provides the value of dimensionless frequency exponent, s,nd it is clear that frequency exponent s varies with Bi content.

The ac conductivity also shows frequency dependent disper-ion similar to dielectric behaviour discussed earlier. In the case

f disordered solids, �ac is an increasing function of frequency.he temperature dependence of frequency exponent s is shown inig. 10. Frequency exponent s is a measure of correlation betweenac and frequency. For random hopping of carriers, s should be 0

frequency independent �ac) and tends to 1, as correlation between

ig. 10. Temperature dependence of frequency exponent, s for BixCo2−xMnO4 (x = 0,.1, 0.2 and 0.3).

Fig. 11. Zero field cooled (ZFC) and field cooled (FC) magnetization data plottedas a function of temperature of BixCo2−xMnO4 (x = 0, 0.1. and 0.3). Insets show therespective temperature dependence of 1/�M.

�ac and ω increases. Qualitatively small polaron tunneling (SPT),usually associated with an increase of s with the increase in tem-perature indicates the activated behaviour of polarons, which isindependent of intersite separation. On the other hand corre-lated barrier hopping (CBH) shows a decrease in s with increasingtemperature, indicating the thermally activated behaviour of elec-tron transfer over the barrier between two sites having their ownCoulombic potential wells. Temperature dependence arising fromthe correlation between barrier height and intersite separation arethe characteristics of CBH model [35]. From the temperature depen-dence plots of s (Fig. 10), it is clear that for pure Co2MnO4, thefrequency exponent, s increases up to 260 K and then decreases. Forthis sample, SPT model is suitable up to 260 K at which maximumcorrelation is exhibited between �ac and ω. Above 260 K, CBH modelis valid, where dc conductivity contribution is not negligible. Forthe Bi-substituted samples also, SPT and CBHs models seem to fit,which explain the conduction mechanism with better correlationat higher temperature ∼ 320 K.

3.5. Magnetization measurements

Temperature dependence of the field cooled (FC) and zerofield cooled (ZFC) magnetization of BixCo2−xMnO4, with x = 0.0,

0.1 and 0.3, in the presence of an applied magnetic field of 0.1 Tare shown in Fig. 11. It has been found that these samples obeythe Curie–Weiss law and exhibits ferrimagnetic transition in thetemperature range 180–186 K depending on Bi content with a diver-gence of FC and ZFC at lower temperatures (∼143–147 K). Below the

N.E. Rajeevan et al. / Materials Science an

F5

dZcioc(tThlAfmToucrbndspdvfaah

ig. 12. Isothermal magnetization hysteresis of BixCo2−xMnO4 (x = 0, 0.1 and 0.3) atK and 120 K.

ivergence point the FC magnetization increases linearly, whereasFC curve exhibit a wide maximum around 133 K. Inverse sus-eptibility (1/�M) obtained for these samples are plotted in thensets of Fig. 11, which revealed that the ferrimagnetic transitionccurred at slightly varied temperature depending on Bi con-entration. Isothermal magnetization hysteresis for BixCo2−xMnO40.0 ≤ x ≤ 0.3) samples have been performed at various tempera-ures below the transition temperature. In Fig. 12 the M–H plots at= 5 and 120 K for BixCo2−xMnO4 (0.0 ≤ x ≤ 0.3) samples are shown,ysteresis loops clearly depicting ferrimagnetic behaviour. Larger

oop area at lower temperature indicates increased magnetization.t all measured temperatures, saturation magnetization (MS) is

ound to be increasing with the increasing Bi content. Remnantagnetization (MR) observed in these compounds was not so high.

he coercive field (HC) was found to decrease with Bi-substitutionwing to a larger grain size, which results in the reduction ofncompensated spins. The appearance of ferrimagnetism in thisompound may be attributed to either the canting of the antifer-omagnetically ordered spins by the structural distortion [36] or areakdown of the balance between the antiparallel sublattice mag-etization of Co2+ due to the substitution of Mn or Bi ions withifferent valence states [37]. Regarding the magnetic behaviour ofpinel oxides, there are mainly three types of magnetic interactionsossible between the ions at A-site and B-site through the interme-iate oxygen ions (O2−) via super exchange interaction. It has beenerified experimentally that these interaction energies are negative

avouring antiferromagnetism when the d orbitals of the metal ionsre half filled or more than half filled, while a positive interactionccompanied by ferrimagnetism appears when d orbital is less thanalf filled. So ferrimagnetism in the samples prepared is considered

d Engineering B 163 (2009) 48–56 55

to originate from Mn3+/Mn4+ ions occupying the octahedral sites.In general, the saturation magnetization Ms shows a monotonicincrease with increasing grain size. Bi-substituted samples are hav-ing large grain size resulting the increased Ms. Even though weak,super exchange interaction between A sites (Co2+–Co2+) throughO2− and Co3+ ions will have an effect on magnetic properties. Inthe present series, a sizeable portion of Co3+ is replaced by Mn3+

and Bi3+ ions. The exchange interaction is strongly influenced bythe greater ionic radius of Bi3+, that distorts the oxygen octahe-dron and causes A-site frustration, resulting in an incommensuratemagnetic ordering in the system. In other words the presence of Biand Mn at the cation sites modifies the magnetic behaviour of theparent compound Co3O4 because of the competition among intr-asite (JAA and JBB) and intersite (JAB) interactions that leads to themagnetic frustration. This disrupts the antiferromagnetic orderingin Co2+ sublattice and results in ferrimagnetism [37,38]. Analysisof temperature dependence of inverse susceptibility showed thatCurie–Weiss temperature, �CW is highly negative and found variedwith Bi-substitution, indicating the frustration of antiferromagneticinteractions in the compound, besides the ferrimagnetic character,which sets at TC. So the compounds show ferrimagnetism, con-trolled by Bi-substitution.

4. Conclusions

(1) Polycrystalline bulk samples of BixCo2−xMnO4 (0 ≤ x ≤ 0.3) havebeen successfully synthesized by standard solid state reactiontechnique.

(2) The XRD analysis of BixCo2−xMnO4 (0 ≤ x ≤ 0.3) indicates thatall samples exhibit a single phase nature. All the samples areindexed in cubic spinel structure and lattice parameter ‘a’ isfound to increase with Bi content. Analysis of XRD data andSEM micrographs shows that Bi content x > 0.3 is difficult toaccommodate, while retaining the cubic spinel structure.

(3) The temperature dependent dielectric data represents adiffused ferroelectric phase transition; with an increasing fer-roelectric TC as the Bi content increases. Bi-substituted sampleshave moderately high dielectric constant depicting better fer-roelectric characteristics along with Debye-type relaxationbehaviour.

(4) The dispersion of ac conductivity has been estimated in termsof frequency exponent s, which varies with temperature andis explained using small polaron tunneling (SPT) model andcorrelated barrier hopping (CBH) model.

(5) The dc conductivity analysis reveals the semiconducting natureof the spinel compounds. The hopping of charges betweencations with different valence states at the octahedral sites isconsidered to be the origin of electrical conductivity of the Bi-substituted Co2MnO4.

(6) The dc magnetization measurements show that all the samplesare ferrimagnetically ordered, with TC < 190 K and saturationmagnetization increases with increasing the concentration ofBi3+ ions due to the redistribution of cations at the octahedralsites and consequent frustration in magnetic ordering.

Bi-substitution in cobalt manganite spinel introduces struc-tural distortion and modifies magnetic exchange interaction, whicheffects both ferroelectric and ferrimagnetic transition. The coex-istence of ferroelectric and magnetic properties proposes thecandidature of this material for the future multiferroic application.

Acknowledgements

Authors are thankful to Dr. Amit Roy, Director IUAC, New Delhifor his keen interest and encouragement to this work, and DST,

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6 N.E. Rajeevan et al. / Materials Scie

ndia for the financial support under the project No. SR/S2/CMP-051/2007. One of the authors (N.E.R.) is thankful to IUAC, Newelhi, UGC, India, DCE and KSCSTE of State Govt. of Kerala and Z.G.ollege, Kerala, India for their support to carry out the researchork. SKA and IVS are grateful to the Science Foundation of Ireland

or the financial support under the project no. 06/IN.1/I91.

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