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Journal of Superconductivity andNovel MagnetismIncorporating Novel Magnetism ISSN 1557-1939Volume 26Number 3 J Supercond Nov Magn (2013)26:657-661DOI 10.1007/s10948-012-1775-y

Structural, Dielectric, Ferroelectric andMagnetic Properties of Bi0.80A0.20FeO3(A=Pr,Y) Multiferroics

Vikash Singh, Subhash Sharma,R. K. Dwivedi, Manoj Kumar,R. K. Kotnala, N. C. Mehra &R. P. Tandon

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J Supercond Nov Magn (2013) 26:657–661DOI 10.1007/s10948-012-1775-y

O R I G I NA L PA P E R

Structural, Dielectric, Ferroelectric and Magnetic Propertiesof Bi0.80A0.20FeO3 (A = Pr,Y) Multiferroics

Vikash Singh · Subhash Sharma · R.K. Dwivedi ·Manoj Kumar · R.K. Kotnala · N.C. Mehra ·R.P. Tandon

Received: 5 September 2012 / Accepted: 28 September 2012 / Published online: 1 November 2012© Springer Science+Business Media, LLC 2012

Abstract Here we studied the effect of homovalent Pr3+and Y3+ substitution on the crystal structure, dielectric,electronic polarization and magnetic properties of theBiFeO3 multiferroic ceramic. The samples were synthesizedby the conventional solid-state reaction method. Pure phaseformation of Pr doped BiFeO3 (BFO) has been obtained,while Y3+ doped BFO has shown a few impurity peaks. Ithas shown that the crystal structure of the compounds is de-scribed within the space group R3c. Pr3+ modified BFO hasshown an anomaly in the εr vs. T plot around and a Néeltemperature ‘T N’ ∼ 370 ◦C. P –E hysteresis loops haveshown higher value of remnant polarization for Pr3+ mod-ified BFO. Magnetic properties of ceramics are determinedby the ionic radius of the substituting element. Experimentalresults propose that the increase in the radius of A-site ionleads to effective suppression of the spiral spin structure ofBiFeO3, resulting in the appearance of net magnetization.

Keywords Dielectric properties · Ferroelectric properties ·Magnetic properties · Multiferroics materials and bismuthferrites

V. Singh (�) · S. Sharma · R.K. Dwivedi · M. KumarDepartment of Physics and Materials Science and Engineering,Jaypee Institute of Information Technology, Noida 201307, Indiae-mail: vikas21jiit.in@gmail.com

R.K. KotnalaNational Physical Laboratory (CSIR), K.S. Krishna Marge, Pusa,New Delhi 110012, India

N.C. Mehra · R.P. TandonDepartment of Physics & Astrophysics, University of Delhi,Delhi 110007, India

1 Introduction

Presently the multiferroic materials have received a great at-tention due to exciting physics and potential applications inthe sensor, data storage and spintronics. Multiferroic mate-rials are those materials which have more than one ferroicproperties like ferroelectric, ferromagnetic and/or ferroelas-tic in the same materials. These materials are very rare be-cause localized ‘d’ electrons of transition metal, responsiblefor magnetism are not compatible with the requirement ofempty d orbitals for ferroelectricity [1–3]. BiFeO3 (BFO) isone of the well known multiferroic materials which showG-type antiferromagnetic behavior below Néel Temperature(T N) ∼ 370 ◦C and ferroelectric behavior below Curie tem-perature (T C) ∼ 830 ◦C [4]. In terms of practical appli-cations this property is very good because of coexistenceof ferroelectricity and magnetism simultaneously. However,problems of secondary phase due to bismuth volatilizationand low resistivity have fixed the practical application ofBFO ceramics to electronic devices. Ferroelectricity in BFOcomes from the long-range ordering of dipolar moments onBi-site with the existence of Bi lone pair and hybridiza-tion between Bi 6s and O 2p orbitals [1]. In last few years,various processing techniques have been used to synthesizeBFO. Bi and/or Fe sites substitution by rare-earth elementsin BFO has come out as an approach to suppress the for-mation of secondary phases and improved ferroelectric andmagnetic properties by destroying the spatial modulated spi-ral spin structures [5–11]. Zhang et al. [6] and Das et al. [7]recommended that La3+ doping for Bi3+ reduces impurityphases and destroys the spiral spin structure. The effect ofrare-earth dopants in BiFeO3 has been suggested for the en-hancement of ferroelectric and magnetic properties and alsodecreasing the Néel temperature T N from 370 ◦C [12].

Ion substitution effects on the magnetic and ferroelectricproperties of BFO is not clearly understood so far and hence

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research work is still required to understand the multifunc-tional properties for Bi-site substitution in this system. Itis therefore worthwhile to study Y3+ and Pr3+ doped BFOsystem completely. Primarily, in order to get pure phase for-mation, Y3+ and Pr3+ doped sample with x = 0.20 havebeen thought to be studied. For this reason in this report,we are reporting our studies on the structure, dielectric, fer-roelectric and magnetic properties of typical compositionBi0.80Y0.20FeO3 (BYFO) and Bi0.80Pr0.20FeO3 (BPFO). Tothe best of our knowledge, Pr3+ and Y3+ doped BFO forcomposition x = 0.20 has not been studied in detail so far.

2 Experimental Details

In this study Bi0.80Y0.20FeO3 (BYFO) and Bi0.80Pr0.20FeO3

(BPFO) ceramics were synthesized by conventional solid-state reaction method using high purity powder of bismuthoxide (Bi2O3 ∼ 99.99 %, Aldrich), iron oxide (Fe2O3 ∼99.99 %, Aldrich), praseodymium oxide (Pr2O3) and yt-trium oxide (Y2O3 ∼ 99.9 %, Aldrich) as starting raw ma-terials. After appropriate weighing, grinding, mixing in ace-tone medium and drying, the mixed powders were calcinedat 760 ◦C for 2 h. The obtained powders were again groundfor 1/2 hour using 0.2 wt% PVA binders and the mixtureswere pressed uniaxially into small cylindrical pellets with adiameter of 10 mm. The samples in the form of cylindricaldisc were put into air atmosphere in a furnace at temperature830 ◦C for sintering.

The X-ray diffraction patterns of the calcined powderswere taken at room temperature using an X-ray PowderDiffractometer (Bruker D8 Advance) with Cu Kα radiation(λ ∼ 1.5418 Å) in the range of 2θ form 20◦–60◦. Tempera-ture dependent dielectric measurements from room temper-ature to 550 ◦C were performed at few selected frequenciesusing an automated LCR Meter (HIOKI 3532-50 Hi Tester).The electric field controlled polarization (P –E hysteresisloops) was measured at room temperature by the modifiedSawyer–Tower circuit (Automatic P –E loop tracer system,Marine India Pvt. Ltd). Temperature dependent magnetiza-tion was measured from 30 ◦C to 450 ◦C and the M–H loopmeasurements for these samples were done at room tem-perature using vibrating sample magnetometer (model 7305,Lakeshore).

3 Results and Discussion

The substitution effect of different ionic size elements Y3+(1.01 Å) and Pr3+ (1.12 Å) on the crystallization of BiFeO3

(BFO) samples were identified by X-Ray Diffraction. Fig-ure 1 shows the room temperature X-Ray Diffraction pat-terns for the composition with x = 0.20 for Y3+and Prdoped BiFeO3 samples abbreviated as BYFO and BPFO,respectively. XRD pattern of BFO and BYFO show few im-purity phases of Bi2Fe4O9 (marked as ∗ in Fig. 1) whichwas also observed by Yuan et al. [8], whereas BPFO sam-ples show characteristics peaks with no signature of impu-rity phase. The occurrence of impurity peaks in BFO andBYFO samples may be according to the following reaction:

Bi2O3 + Fe2O3 → (1/2)Bi2Fe4O9 + (1/2)Bi2O3

Y3+ and Pr3+ are smaller, as compared to Bi3+ (∼1.17 Å)therefore, we have observed the decrease in lattice parame-ters and hence unit cell volumes for both the ions, but cellparameter for Pr3+ doped BFO are found to be more as com-pared to Y3+ doped BFO, which can be explained becauseof smaller Y3+ as compared to Pr3+ [13] (see in Table 1).

Figure 2 shows the temperature dependence of the dielec-tric constant and dielectric loss for BYFO and BPFO sam-ples at the frequencies of 50 kHz and 100 kHz. It was clearly

Fig. 1 X-ray diffraction patters for the BiFeO3, Bi0.80Y0.20FeO3 andBi0.80Pr0.20FeO3

Table 1 Lattice parameters,volume, remanent polarizationand remanent magnetization ofBiFeO3 (BFO),Bi0.80Y0.20FeO3 (BYFO) andBi0.80Pr0.20FeO3 (BPFO)samples

Sample a = b (Å) c (Å) V (Å3) Remanentpolarization (P r)

μC/cm2

Remanentmagnetization (M r)

Emu/g m

BFO 5.580 13.876 374.15 0.121 0.0021

BYFO 5.5652 13.705 367.59 0.133 0.038

BPFO 5.584 13.848 373.93 0.25 0.057

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Fig. 2 Dielectric constant andloss versus temperature plot(a) Bi0.80Y0.20FeO3 and(b) Bi0.80Pr0.20FeO3

Fig. 3 SEM microstructure ofsamples (a) Bi0.80Y0.20FeO3and (b) Bi0.80Pr0.20FeO3

observed from the εr vs. T plot of BYFO that a typical fre-quency dependent dielectric anomaly occurs near 310 ◦Cand dielectric loss was very low at the peak position (nearly0.6). The dielectric constant vs. temperature plot of BYFOsample exhibits a small anomaly at around 310 ◦C (whichis far below the Néel temperature), which is consistent withearlier reports [14]. This anomaly attributed to a transient in-teraction between oxygen ion vacancies and the Fe3+/Fe2+redox couple. Replacement of Bi3+ by Y3+ modifies the di-electric characteristics of BFO, resulting in vanishing of theanomaly and substantial reduction of tan δ. Pr3+ modifiedBFO has shown a dielectric anomaly near the Néel tem-perature (T N) ∼ 370 ◦C and at this temperature dielectricloss is quite large as compared to BYFO sample. Dielec-tric anomaly observed around magnetic transition tempera-ture, indicating magnetoelectric coupling in these samples.This type of dielectric anomaly is predicted by the Landau–Devonshire theory of phase transition in magnetoelectricallyordered systems as an influence of vanishing magnetic orderon the electric order [15]. In both samples, a strong increaseof the dielectric characteristics is observed with increasingtemperature or decreasing frequency. The behavior can beexplained in the following way [16, 17]: the defect–relateddipoles are able to follow the alternating field at low fre-quencies, providing high values of εr. The increase of the di-electric constant and loss factor with increasing temperature

is related to thermally induced enhancement of the hoppingconduction. The replacement of some volatile Bi3+ withnon-volatile Pr3+ and Y3+ may prevent oxygen ion vacan-cies causing stabilization of the Fe3+/Fe2+ couple–oxygenvacancy interaction [18–20]. Due to Bi evaporation duringsintering in BFO as a result of Bi3+ loss, it is difficult tocontrol oxygen loss. In order to maintain charge neutralityover all in the sample, this oxygen loss is leading to gener-ation of oxygen vacancies. This occurs as per the followingreactions:

Bi2O3 → 2Bi3+ + 3O2−

2Bi3+ + 3O2− → 2Bi (gas) + 3/2O2 (gas) + 2V3−Bi + 3V2+

O

4Fe3+ + O2− → 3Fe2+ + 3O2 (gas) + 6V2+O

Thus, the defects may be responsible for high dielectric loss.Figure 3 shows the Scanning electron micrographs of

BYFO and BPFO ceramics. SEM micrograph of BYFOshows spherical grain growth. The average grain size of thissample is in the range of 2–3 μm. SEM of BPFO sam-ple shows agglomeration. It appears that nuclei aggregateinto clusters. The higher dielectric constant in BYFO sam-ple may be attributed to the better quality microstructureand small grain size. The poor quality of microstructure forPr3+ doped BFO could be responsible for the lower dielec-tric constant.

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Figure 4 shows the variation of dielectric polariza-tion (P ) with applied electric field (E) for BYFO and BPFOsystems at room temperature. The samples are highly con-ductive at room temperature and only partial reversal ofthe polarization takes place quite similar to that observedby Pradhan et al. [21]. The relatively high conductivity ofBiFeO3 is known to be attributed to the variable oxidationstates of Fe ions (Fe3+/Fe2+) which require oxygen vacan-cies for charge compensation. Also, during synthesis theslow heating rate and long sintering time will enable theequilibrium concentration of the oxygen vacancies at hightemperature to be reached and will result in the high oxygenvacancy concentration in the synthesized product. So thepresence of Fe2+ ions and oxygen deficiency leads to highconductivity. No saturated polarization hysteresis loop hasbeen observed. For BYFO sample the value of remnant po-larization is 0.036 μC/cm2. Remnant polarization of BPFOsample is 0.44 μC/cm2. The value of remnant polarization(P r) is higher for BPFO sample. The low value of P r inpure and Y3+ doped BFO samples may be attributed to thepresence of minor impurity phases. The unsaturated state ofpolarization in P –E curve may be due to leakage currentsdevelop because of oxygen vacancies and other possible de-fects.

The magnetic hysteresis (M–H ) loops of BFO, BYFOand BPFO samples have been measured using VSM at room

Fig. 4 Ferroelectric hysteresis loop of BiFeO3, Bi0.80Y0.20FeO3 andBi0.80Pr0.20FeO3

temperature (see Fig. 5a). All the samples show non linearmagnetization loops representing weak ferromagnetic be-havior. The ferromagnetic nature is more visible for BPFOsamples and lesser for BFO and BYFO samples. BFO is re-ported to be a G-type antiferromagnetic at room tempera-ture [4]. The crystal structure of BFO allows the appearanceof weak ferromagnetic arising from the canting of the anti-ferromagnetic sublattices [4]. The appearance of remanentmagnetization of BPFO and BYFO are attributed to the sup-pression of the spiral spin structure by Pr3+ and Y3+ dopingat Bi-site [6]. But the spiral spin structure of BFO is notcompletely destroyed by doping. The relatively higher valueof remnant magnetization (M r) for the BPFO sample hasbeen obtained as compared to BYFO sample. This relativehigh value of M r for BPFO sample may occur due to smallerbond angle of Fe–O–Fe bond in the lattice and hence sig-nificant improvement in the value of magnetization [22, 23].Fe–O–Fe angle and Fe–O distances are changed by Pr3+ andY3+ doping. As the super exchange interaction is responsiveto bond angles and bond distances, the spiral spin structuremight be destroyed by doping, which leads to the release oflatent magnetization and the enhancement of remanent mag-netization. The data are tabulated in Table 1. However, thisvalue of M r is very low for both samples may be attributed tothe rhombohedrally distorted perovskite structure with spacegroup R3c, in which both ferroelectricity due to lone pair ofBi ion and weak ferromagnetic ordering due to the cantingof spin moments [22].

Plots of M vs. T for BYFO and BPFO above room tem-perature are shown in Fig. 5b and inset of 5b. The tempera-ture dependence of magnetization of the samples was mea-sured at a magnetic field of 500 Oe from room temperatureto 450 ◦C to determine the magnetic ordering. The magneti-zation of both samples shows a sharp decrease at around of383 ◦C for Pr3+ and Y3+ (Y3+ see in inset), which is consid-ered to be the magnetic transition temperature of both sam-ples. This transition temperature is close to the spin orderingtemperature of BFO (370 ◦C [4]). This result indicates thatthe addition of Pr3+ and Y3+ does not affect the magnetictransition temperature too effectively. The antiferromagnetic

Fig. 5 (a) Room temperatureM–H loops (b) M–T curves forBi0.80Y0.20FeO3 andBi0.80Pr0.20FeO3

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interaction seems enhanced in BPFO since the increase ofmagnetization with temperature is faster than BYFO. Pr3+and Y3+ content, the lattice volume decreases and the over-lapping Fe-orbitals in Fe–O–Fe bond increases. Eventually,this leads to the enhancement of the antiferromagnetic su-perexchange [24]. The occurrence of the low value of mag-netization in BYFO sample may be attributed to d0 characterof yttrium [2].

4 Conclusion

A typical composition with x = 0.20 in Bi1−xYxFeO3

(BYFO) and Bi1−xPrxFeO3 (BPFO) has been successfullysynthesized by solid-state ceramic method. BPFO samplehas shown complete single phase formation, whereas BYFOsample has shown single phase formation with minor pres-ence of impurity phase and their crystal structure is de-scribed by R3c space group. Low value of lattice constantsin BYFO sample may be attributed to small ionic radii ofY3+ as compared to Pr3+. Spherical grains of size 2–3 μmhave been observed in the microstructure of BYFO. The di-electric constant anomaly observed at temperature 370 ◦Cin BPFO sample may be due to the magnetic transition tem-perature. However, this anomaly clearly appears for BYFOsample at reduced at around temperature (310 ◦C). Thissample has shown typical frequency dependence behavior,which is not reported so far. BPFO sample has shown highvalue of remnant polarization (P r) as well as remnant mag-netization (M r) as compared to BYFO sample. Magnetiza-tion measurements showed that the magnetic state of thesample is determined by the ionic radius of the substitutingelements, and dopants with the biggest ionic radius effec-tively suppress the spiral spin structure of BFO. The M–H

hysteresis loops exhibit a weak ferromagnetic behavior ofthe samples.

Acknowledgements We are thankful to Defense Research and De-velopment Organization (DRDO), Government of India, New Delhifor financial support to this work through research project (No.:ERIP/ER/0803744/M/01/1246). One of the authors, Vikash Singh isalso thankful to Jaypee Institute of Information Technology for teach-ing assistance ship.

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