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ARTICLE IN PRESSG ModelASCER-116; No. of Pages 6

Journal of Asian Ceramic Societies xxx (2014) xxx–xxx

Contents lists available at ScienceDirect

Journal of Asian Ceramic Societies

HOSTED BY

j ourna l ho me page: www.elsev ier .com/ loca te / jascer

ombined structural, electrical, magnetic and optical characterizationf bismuth ferrite nanoparticles synthesized by auto-combustionoute

anjay Godaraa, Nidhi Sinhaa,b, Geeta Raya, Binay Kumara,∗

Crystal Lab, Department of Physics & Astrophysics, University of Delhi, Delhi 110007, IndiaDepartment of Electronics, SGTB Khalsa College, University of Delhi, Delhi 110007, India

r t i c l e i n f o

rticle history:eceived 16 July 2014eceived in revised form 28 August 2014ccepted 2 September 2014vailable online xxx

eywords:

a b s t r a c t

Phase-pure multiferroic bismuth ferrite (BFO) nanoparticles were synthesized by energy efficient, simpleand low temperature sol–gel followed by auto-combustion route. Highly crystalline and well-shapedBFO nanoparticles of size about 50 nm were observed in TEM. Thermal analysis was used to optimizethe calcination temperature as 500 ◦C. An endothermic peak at 834 ◦C has been detected in the DTAcurve, representing the Curie temperature. The dielectric anomaly around Neel temperature (TN) wasobserved signifying the magnetoelectric coupling. The BFO nanoparticles were found to be highly resistive

9 2

iFeO3

hemical synthesisanoparticlesielectric propertieserroelectricity

(� ∼ 3 × 10 �-cm) and had very low leakage current of the order of �A/cm , which resulted from phasepurity. A significantly enhanced weak ferromagnetism was observed due to smaller particles size andremnant magnetization and coercive field were 0.067 emu/g and 185 Oe, respectively. P–E loop confirmedthe ferroelectric behavior of BFO nanoparticles. The direct band gap energy was calculated to be 2.2 eVfrom UV–vis studies.

ic So

© 2014 The Ceram

. Introduction

Ferroic materials that exhibit spontaneous and switchable inter-al ordering have great importance due to a wide range of

unctional applications and their interesting fundamental physics1]. Multiferroic combines two or more primary ferroic orderingsut of ferroelectricity, ferromagnetism and ferroelasticity [1–4].he most interesting thing about these materials is their magneto-lectric coupling, as a result of which electric polarization can bewapped by magnetic field and vice versa. This coupling multiplieshe degrees of freedom, due to which multiferroics can be used inhe field of highly dense non-volatile memory devices, spintronics,elecommunication and sensors [4–8].

Mostly studied single-phase ABO3 type perovskite multiferroicsre BiFeO3 (BFO), BiMnO3 (BMO) and some rare earth manganitesike YMnO3 [9]. Out of these, BFO is the only promising can-idate that retains ferroelectric (TC ∼ 825 ◦C) along with G-typentiferromagnetic (Neel temperature, TN ∼ 360 ◦C) orderings fairlybove room temperature. It possesses rhombohedrally distorted

Please cite this article in press as: S. Godara, et al., Combined structuferrite nanoparticles synthesized by auto-combustion route, J. Asian C

∗ Corresponding author. Tel.: +91 9818168001; fax: +91 011 27667061.E-mail addresses: b3kumar69@yahoo.co.in, bkumar@physics.du.ac.in

B. Kumar).Peer review under responsibility of The Ceramic Society of Japan and the Korean

eramic Society.

187-0764 © 2014 The Ceramic Society of Japan and the Korean Ceramic Society. Producttp://dx.doi.org/10.1016/j.jascer.2014.09.001

ciety of Japan and the Korean Ceramic Society. Production and hosting byElsevier B.V. All rights reserved.

perovskite structure with space group R3c, which can also bedescribed by hexagonal system of basis with lattice parametersahex = 5.58 A and chex = 13.90 A [5]. It shows small residual magneti-zation as a result of canting of spins from their perfect antiparalleldirection, which is greatly enriched in nanoparticles of size less thanperiod of cycloid modulation, i.e. 62 nm [10]. Also, it has a smallband gap lying in visible region, so it finds potential application asa visible light photo-catalyst [11].

Although BFO is mostly investigated multiferroic, its practi-cal applications are limited on account of high leakage current,which is caused by defects and compositional instability [12].It is synthesized in various forms like single crystals, ceramics,nanoparticles/crystals and thin films [13–15]. However, stoichio-metric phase-pure synthesis is a challenging task. Conventionally,it is synthesized by solid-state reaction of Bi2O3 and Fe2O3 at hightemperature, but reaction kinematics suggests that the tempera-ture range for stability is very small and deviation from this rangeleads to impurity phases like Bi2Fe4O9 and Bi25FeO39 [16]. Further-more, bulk form of BFO offers insignificant electric and magneticmoments. So, it is beneficial to synthesize in the form of BFOnanoparticles and thin films.

Various low temperature wet chemical methods, like hydro-

ral, electrical, magnetic and optical characterization of bismutheram. Soc. (2014), http://dx.doi.org/10.1016/j.jascer.2014.09.001

thermal, co-precipitation, micro-emulsion technique, modifiedPechini’s method, mechanochemical, sol–gel and solution combus-tion route, have been reported for the synthesis of BFO nanopar-ticles/ceramics [17–22]. Besides, several literature describe

tion and hosting by Elsevier B.V. All rights reserved.

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2 Ceramic Societies xxx (2014) xxx–xxx

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ARTICLEASCER-116; No. of Pages 6

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uto-combustion route but it is rare to find collective studiesor structural, electrical, magnetic and optical properties of BFOanoparticles [23–25]. It is known that the phase purity is needed

or obtaining better magneto-electric properties of BFO. Therefore,he objective of this work is to synthesize stoichiometric phase-ure BFO nanoparticles at low temperature via auto-combustionoute which is a simple and energy efficient method, and to inves-igate structural, electrical, magnetic and optical properties while

aintaining nanoparticle nature of BFO intact. In the process,e have got enhanced properties than previous reports, particu-

arly in case of temperature-dependent dielectric study in whichn anomaly nearby TN signifying the magnetoelectric coupling isbserved.

. Experimental

.1. Sample synthesis

Bismuth ferrite (BFO) nanoparticles were synthesized byow temperature sol–gel followed by auto-combustion route,

hich was based on modified Pechini’s method. In this method,i(NO3)3·6H2O and Fe(NO3)3·9H2O (Sigma Aldrich, purity >99%)ere used as starting materials according to the stoichiometry.

irst, bismuth nitrate was dissolved in 1 N nitric acid at 60 ◦C, andhen iron nitrate was added to it while maintaining the heatingnd stirring to make a homogeneous solution. Citric acid as a che-ating agent was added in 2:1 molar ratio with metal ions and theolution was stirred vigorously for another 2 h. As-obtained lightrownish solution was kept on hot plate till it became viscous resin.hile continuous heating, dried resin auto-ignited and released

arge amounts of gases and became a dark brownish powder, whichas further ground and calcined at 500 ◦C for 3 h to get pure phase

f BFO.

.2. Characterization

The dried resin was subjected to TG-DTA analysis with Netzsch-09STA apparatus at a heating rate of 10 ◦C/min under nitrogen flowo obtain an optimum temperature range for the calcination. Struc-ure and phase study of BFO nanoparticles were carried out by XRDith PANanalytical Diffractometer (Model: X’Pert Pro; Cu-K�1,

= 1.5405 A) at room temperature. Morphology of nano-sized par-icles was studied by TEM (Tecnai 300 kV Ultra twin; FEI Company).–E characteristic for leakage current analysis was obtained bypplying a DC field and using a computer controlled Keithley Sourceeter (Series 2400). Ferroelectric hysteresis loop was traced by an

ndigenously built Sawyer–Tower circuit (Marine India) driven by lock-in amplifier. Dielectric measurement was carried out by theour-point probe system on Agilent E4980A LCR meter in the tem-eratures range of 30–460 ◦C and at frequencies ranging from 20 Hzo 2 MHz. These all characterizations were performed on 0.5 mmhick pellet of BFO nanoparticles, annealed at 500 ◦C for 2 h onhich high-grade silver paste was coated uniformly to serve as

lectrodes. The magnetic hysteresis loops of the powder samplesithin a field range of ±2.2 T were measured at room temperaturesing a vibrating sample magnetometer (JDAW-2000D VSM). TheV–vis absorption spectra of the BFO nanoparticles were measuredn Varion Cary-100 UV–vis spectrophotometer.

. Results and discussion

Please cite this article in press as: S. Godara, et al., Combined structuferrite nanoparticles synthesized by auto-combustion route, J. Asian C

.1. Thermal analysis

Thermal analysis (TG-DTA) was used to get appropriate tem-erature range of calcination for obtaining pure phase of bismuth

Fig. 1. TG-DTA curves of (a) as-obtained precursor and (b) BFO nanoparticles cal-cined at 500 ◦C.

ferrite. For this, as-obtained precursor was heated up to a tempera-ture of 900 ◦C with heating rate of 10 ◦C/min in a continuous flow ofN2 gas. TG curve (Fig. 1a) showing weight of dried resin decreasedin a step-like manner up to temperature of 450 ◦C. There is a suddendip at temperature of 146 ◦C, accompanied with a large exothermicpeak in DTA, which indicates the starting of combustion of metalcitrate complex that formed during reaction and other organicmaterials. So, large amount of energy and gases like CO2, NO2, etc.were released. Then, there is a plateau region from 450 ◦C to 650 ◦C,which is the most suitable range of temperature for calcination.This auto-combustion route of BFO synthesis is based on principlesof propellant chemistry [25], where a high-energy redox reactiontakes place. As formation of oxide requires enormous energy, a hightemperature is required for the reaction to begin. However, in thismethod, energy is supplied at molecule level by the combustionof complex carbonized ions, which get used for formation of oxidewithout attaining a high temperature.

Fig. 1b shows the TG-DTA curve of BFO nanoparticles calcined at500 ◦C, where straight TG curve (no weight loss) confirms the quitestability on heating of BFO nanoparticles throughout the tempera-ture range. An endothermic peak at ∼834 ◦C in DTA curve may bedue to change in the crystal structure of BFO which clearly indicatesthe Curie temperature (TC).

3.2. Structural analysis

ral, electrical, magnetic and optical characterization of bismutheram. Soc. (2014), http://dx.doi.org/10.1016/j.jascer.2014.09.001

The room temperature powder X-ray diffraction pattern of BFOnanoparticles calcined at 500 ◦C is shown in Fig. 2. All the diffractionpeaks are indexed with various (h k l) planes according to hexago-nal system of basis. The position of each peak was found to be in

ARTICLE ING ModelJASCER-116; No. of Pages 6

S. Godara et al. / Journal of Asian Ceram

Far

gFtsBsinnt

p

ig. 2. XRD pattern of BFO nanoparticles calcined at 500 ◦C; peaks were indexedccording to space group R3c (* and #: some traces of secondary phases) and Reitveldefined pattern with refinement parameters (inset).

ood agreement with the perovskite BiFeO3 [JCPDS Card 86-1518].urther, hexagonal crystal structure of the BFO was confirmed byhe clear splitting of peaks (1 0 4) and (1 1 0). Sharp and high inten-ity XRD peaks support the good crystallinity of the as-synthesizedFO nanoparticles. Traces of some very low intensity peaks corre-ponding to secondary phases like Bi2Fe4O9, Bi25FeO39, etc. weredentified [22]. But, overall there had been a good control over theon-perovskite phases which confirms the phase purity of BFO

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anoparticles. Thus, loss of bismuth was minimized due to lowemperature synthesis in this method.

The Rietveld refinement was performed using Fullprof softwareackage with starting parameters for hexagonal system of basis

Fig. 3. TEM micrographs at various magnification scales an

PRESSic Societies xxx (2014) xxx–xxx 3

and space group R3c. The refined unit cell parameters ahex, chexand Vhex are 5.5820 A, 13.8758 A and 374.429 A3, respectively andhave resemblance with that reported previously [19]. There is alsogood agreement between observed and calculated XRD patterns[Fig. 2 (inset)]. In our case, the refinement goodness parameterslike �2 (2.28) and R-factors are very low compared with some pre-vious reports [19,23]. The obtained peaks of the powder XRD werefound to be broad which is due to the nano-crystallite size of BFO.The crystallite size of the BFO nanoparticles was calculated usingScherrer formula,

D = 0.9�

cos �(1)

The average crystallite size of the diffraction pattern was foundto be about 50 nm which is less than the period of spiral modulatedspin order, 62 nm [24].

3.3. Transmission electron microscopy (TEM) analysis

TEM micrographs (Fig. 3) show the typical morphology of thelow temperature synthesized BFO nanoparticles. For TEM samplepreparation, BFO powder was dispersed in methanol by sonica-tor and dropped on carbon coated copper grid. Uniformly shapednanoparticles can be observed in the TEM images. The diameterof the spherical BFO particles was measured to be about 50 nm,which is in good agreement with that calculated from powderXRD using Scherrer’s formula. The selected area electron diffrac-tion (SAED) shows bright diffraction spots, which confirms theformation of a high-quality well-crystalline structure of perovskite

ral, electrical, magnetic and optical characterization of bismutheram. Soc. (2014), http://dx.doi.org/10.1016/j.jascer.2014.09.001

BiFeO3 nanoparticles. X-ray energy dispersive spectroscopy (EDS)attached to TEM was used to find elemental composition of pre-pared sample, which fairly agreed with expected stoichiometry ofthe compound.

d SAED pattern of low temperature grown BFO NPs.

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Fpb

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ftctarslfrr

ig. 4. Log(J) versus log(E) curve of BFO nanoparticles at room temperature showingower law dependence, J ∝ En and (inset) symmetric behavior of J versus E plot foroth the polarities of applied field.

.4. J–E analysis

The conductivity analysis is one of the important tools fornspecting the feasibility of any material to be multiferroic. For thisurpose, leakage current density versus electric field (J–E) measure-ent of BFO nanoparticle was carried out at room temperature.

he leakage current was measured for both polarities of the biasoltage and no remarkable difference was obtained on reversinghe bias voltage, which can be clearly seen from the inset of Fig. 4.hus, metal-insulator (MI) contacts were ohmic in nature and con-uction process was mainly bulk limited. Fig. 4 shows relationshipetween log(J) and log(E) and indicates power law dependence,

∝ En where ‘n’ is indices. In the low field region, ‘n’ is close tonity and it exhibits ohmic nature of conduction, whereas, in higheld region, ‘n’ is nearly 2 and follows the space-charge limitedonduction (SCLC) mechanism [26]. In this mechanism, injectedharge carriers dominate over volume generated free charge carri-rs. This shows that defects mediated conductivity is relatively lessominated, so oxygen vacancies, which are caused by volatility ofismuth (Bi), may be suppressed due to low temperature synthesis.eakage current density was found to be in the range of �A/cm2 andesistivity (�) of sample was found to be about 3 × 109 �-cm, whichas decreased to about 2 × 109 �-cm at higher field. Conductionechanism of BFO is also explained on the basis of Poole–Frenkel

PF) emission [27], where field-assisted hopping of charge betweene2+ and Fe3+ occurs. But, this type of defects activated conductivityas not observed in our sample, which may be due to low appliedeld and less defect centers.

.5. Dielectric analysis

The dielectric measurement of BFO nanoparticles was per-ormed over a wide range of frequency 20 Hz to 2 MHz andemperature 30–460 ◦C. Fig. 5a shows the variation of real dielectriconstant (ε′) and loss factor (tan ı) with log of frequency at roomemperature. The value of real dielectric constant was found to bebout 65 for 1 kHz and loss factor (tan ı) in the range of 0.8–0.02 atoom temperature. In lower frequency region, the dielectric con-tant abruptly decreased following a maxima (ωp) in dielectric

Please cite this article in press as: S. Godara, et al., Combined structuferrite nanoparticles synthesized by auto-combustion route, J. Asian C

oss and then attained nearly constant value (ε∝ ∼ 40) at higherrequencies. This dielectric behavior is expected from every fer-oelectric material and is known as low frequency space-chargeelaxation. At lower frequencies, various polarizations viz. space

Fig. 5. (a) Real dielectric constant and loss (tan ı) with log frequencies at roomtemperature and (b) real dielectric constant as a function of temperature at variousfrequencies.

charges, ionic defects, permanent dipoles, induced atomic and ionicdipoles, contribute toward dielectric resulting in high values. As fre-quency increases, different participating mechanisms for dielectricconstant decrease gradually and subsequently dielectric constantdecreases [28,29]. Also, each polarization mechanism is accompa-nied with relaxation frequency (fr) for which a maximum phaseshifts between polarization (P) and applied electric field (E), caus-ing maximum dielectric loss. The high value of dielectric constantmay also be attributed to smaller particle size [30].

Fig. 5b illustrates the variation of dielectric constant as a func-tion of temperature at various frequencies. The dielectric constantshows a fairly constant behavior up to 230 ◦C for all the frequencies.Above 230 ◦C dielectric constant gradually increases and a broadpeak is observed around Neel temperature (TN). This broad peaknear TN is frequency dependent and shifts to a higher temperatureand decreases in intensity with an increase in frequency, which isa typical characteristic of relaxor ferroelectrics [28,31]. This type ofdielectric anomaly may be a direct consequence of magnetoelectriccoupling and Huang et al. have also observed this for YMnO3 [32,33].As for a magnetic transition, only modifications in spin structuretake place but it is coupled with ferroelectric ordering, on accountof which, there is a little change in structural parameters, thus ananomaly in dielectric behavior may be observed.

3.6. P–E loop measurement

ral, electrical, magnetic and optical characterization of bismutheram. Soc. (2014), http://dx.doi.org/10.1016/j.jascer.2014.09.001

Ferroelectric hysteresis (P–E) loop for BFO nanoparticles wastraced using a standard Sawyer–Tower circuit. An external AC fieldof frequency 50 Hz and strength 85 kV/cm was applied to measure

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Fig. 6. P–E loop of BFO nanoparticles measured at room temperature.

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ig. 7. M–H hysteresis curve of BFO nanoparticles measured at room temperature.

–E loop. Fig. 6 shows the relationship between polarization andpplied field at room temperature. As sample was able to bear anlectric field as high as 85 kV/cm without breaking down, whichas because of minimized leakage current, it is observed that even

or such a high electric field, the P–E loop could not be fully satu-ated. The remnant polarization (Pr) and coercive field (Ec) of thisnsaturated hysteresis loop were obtained to be 0.4 �C/cm2 and5 kV/cm, respectively. However, in the case of thin films, a well-aturated P–E loop with very large remnant polarization has beeneported for BFO [13,34]. The high coercive field and leakage cur-ent may be responsible causes for unsaturated PE loop and lowalue of polarization in BFO [14].

.7. M–H loop measurement

The M–H hysteresis loop has been measured at room tempera-ure to study magnetic properties of BFO nanoparticles, which ishown in Fig. 7. It confirms that as-synthesized BFO nanoparti-les exhibit weak ferromagnetic ordering with residual magneticoment of 0.067 emu/g and coercive field of 185 Oe on applying aagnetic field of 22 kOe. The observed remnant magnetization is

Please cite this article in press as: S. Godara, et al., Combined structuferrite nanoparticles synthesized by auto-combustion route, J. Asian C

igher than many other previous reports [18], which may be due tomaller size and uniform shape of nanoparticles. The particle sizef our as-synthesized BFO nanoparticles is about 50 nm, which isess than the wavelength 62 nm of cycloid structure of canted spin.

Fig. 8. UV–vis optical absorbance spectrum and (inset) calculated band gap Eg.

Thus, residual magnetic moment may be a result of uncompensatedspins on the surface and broken order of cycloid spin structure [11].Other possibilities for this magnetization may be ruled out on thebasis of phase purity and high resistivity of the sample. Saturationin hysteresis loop was not attained, which may be expected fromG-type antiferromagnetic nature of BFO.

3.8. UV–vis spectra

The UV–vis absorption spectrum of as-synthesized BFOnanoparticles is shown in Fig. 8. This is seen from the spectrumthat BFO nanoparticles have strong absorption in UV–vis region300–550 nm and slight absorption in 550–700 nm. This absorp-tion characteristic can be used in the photo-catalytic activity indecomposition of organic compounds. The absorption curve givesan absorption edge at 510 nm. The inset of Fig. 8 shows (˛h)2–hplot for the direct band gap calculation. For any semiconductor,optical absorption near band edge obeys following equation,

(˛h) ∝ (h − Eg)n, (2)

where ˛, h, and Eg are the optical absorption coefficient, Plank’sconstant, light frequency and band gap energy, respectively. For thedirect band gap, the variable ‘n’ in the equation is equal to 1/2. AsBFO is a direct band gap semiconductor, the extrapolated line inter-cept at horizontal axis gives the value of direct energy band gap as2.2 eV. The band gap value is comparable with the earlier reportedvalues [12,17]. Also, the band gap of BFO at bulk level is reportedto be 2.6 eV, which decreases to 2.2 eV for nanoparticles due to thesize effect. Thus, BFO nanoparticles may be used for photo-catalysisin degradation applications.

4. Conclusion

In conclusion, phase-pure BFO nanoparticles have been synthe-sized using low temperature sol–gel followed by auto-combustionroute. Phase purity and crystallite size of the BFO nanoparticleswere confirmed by XRD and TEM studies. TG-DTA analysis revealedthe optimum temperature range of calcination and showed anendothermic peak at 834 ◦C corresponding to ferroelectric phasetransition (TC) of BFO. Temperature and frequency-dependentdielectric study confirmed relaxor behavior, whereas a dielec-tric anomaly near Neel temperature has provided a direct proofof magnetoelectric coupling. Particles size (∼50 nm) was foundto be smaller than the period of spin cycloid which resulted in

ral, electrical, magnetic and optical characterization of bismutheram. Soc. (2014), http://dx.doi.org/10.1016/j.jascer.2014.09.001

weak ferromagnetism and exhibited net magnetic moments ofMr = 0.067 emu/g. High resistivity of � = 3 × 109 �-cm and reducedleakage current have been achieved which is a direct conse-quence of suppression of oxygen vacancies. P–E loop confirmed the

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ARTICLEASCER-116; No. of Pages 6

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erroelectric behavior of BFO nanoparticles and observed valuesf Pr and Ec were 0.4 �C/cm2 and 25 kV/cm, respectively. Also,mall band gap of Eg = 2.2 eV makes it suitable candidate forhoto-catalysis applications. Hence, this method can be useful forynthesizing highly resistive and uniform-sized BFO nanoparticlesor various applications.

cknowledgements

We are thankful for the financial support received through UGCroject (Sanction No. 41-914/2012: SR) and DST project (Sanctiono. SR/S2/CMP-0068/2010). Mr. Sanjay Godara and Ms. Geeta Rayre thankful to UGC and CSIR (India) for Junior Research Fellow-hips.

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