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Ceramics International 40 (2014) 6413–6419www.elsevier.com/locate/ceramint

Investigation of Tb-doping on structural transition and multiferroicproperties of BiFeO3 thin films

Guohua Dong, Guoqiang Tann, Yangyang Luo, Wenlong Liu, Huijun Ren, Ao Xia

School of Materials Science and Engineering, Shaanxi University of Science & Technology, Xi'an, Shaanxi 710021, China

Received 17 October 2013; received in revised form 18 November 2013; accepted 18 November 2013Available online 25 November 2013

Abstract

Pure BiFeO3 (BFO) and Bi1�xTbxFeO3 (BTFO) thin films were successfully prepared on FTO (fluorine doped tin oxide) substrates by thesol–gel spin-coating method. The effects of Tb-doping on the structural transition, leakage current, and dielectric and multiferroic properties ofthe BTFO thin films have been investigated systematically. XRD, Rietveld refinement and Raman spectroscopy results clearly reveal that astructural transition occurs from the rhombohedral (R3c:H) to the biphasic structure (R3c:HþR-3m:R) with Tb-doping. The leakage currentdensity of BTFOx¼0.10 thin film is two orders lower than that of the pure BFO, i.e. 5.1� 10�7 A/cm2 at 100 kV/cm. Furthermore, the electricalconduction mechanism of the BTFO thin films is dominated by space-charge-limited conduction. The two-phase coexistence of BTFOx¼0.10

gives rise to the superior ferroelectric (2Pr¼135.1 μC/cm2) and the enhanced ferromagnetic properties (Ms¼6.3 emu/cm3). The optimalperformance of the BTFO thin films is mainly attributed to the biphasic structure and the distorted deformation of FeO6 octahedra.& 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: BiFeO3 thin films; Tb-doping; Structural transition; Multiferroic properties

1. Introduction

Multiferroic materials exhibit the simultaneous existence ofelectric and magnetic orderings in a certain range of temperaturesand their potential applications are in multifunctional devices, sothey have drawn much attention in recent years [1–3]. Amongthese materials, BiFeO3 (BFO) has a distorted perovskitestructure, belonging to R3c space group. Owing to a high Curietemperature (1043 K) and Neel temperature (647 K), BFO hasbeen widely used in the high-density nonvolatile memories [4–7].One drawback of limiting its applications is the low electricalresistivity at room temperature. The origin of the leakage currentis often believed from the Fe2þ /Fe3þ valence fluctuation, theoxygen vacancies and the poor microstructure.

In terms of the reducing leakage current, it has been reportedthat defect engineering is an effective way. In particular, manystudies have been attempted on the doping at A-sites (Ln=La, Ce,Sm, Gd, Tb, Eu, etc.) [8–15] and the reducing leakage current hasbeen observed in Ce-doped BFO films. Meanwhile, 2Pr could bereached up to 92.3 μC/cm2 with better fatigue resistance [12].

e front matter & 2013 Elsevier Ltd and Techna Group S.r.l. All ri10.1016/j.ceramint.2013.11.089

g author. Tel.: þ86 13759878391.ss: tan3114@163.com (G. Tan).

Furthermore, some results showed that the superior ferroelectricproperties were closely related to the structural transition bydoping rare earth ions. Gd-doped BFO thin films showed thephenomenon that the rhombohedral structure was transformedinto a pseudo-tetragonal phase [13]. Moreover, Nd-doped BFOthin films exhibited the large and stable piezoresponse [14] andSm-doped BFO thin films also showed a phase transformationfrom the rhombohedra to the pseudo-cubic structure [15].As discussed above, many researches have focused on the

multiferroic and piezoelectric properties of the BFO throughadjusting Ln-doping concentration, but few researches haveinvestigated the structural transition in detail. In this paper, Tb-doped BFO thin films were prepared on FTO substrates by the sol–gel method. The effects of Tb-doping on the structural transition,surface morphology, leakage current density, and dielectric andmultiferroic properties of BFO thin films were investigatedsystematically.

2. Experimental

Pure BFO and Bi1�xTbxFeO3 (BTFO) thin films weredeposited on FTO/glass substrates by the sol–gel spin-coating

ghts reserved.

Fig. 1. (a) XRD patterns of the pure BFO and BTFO thin films deposited onFTO substrates; the inset shows the enlarged XRD patterns in the vicinities of2θ ¼32.01. (b) Rietveld refinement of the pure BFO and BTFOx¼0.10

thin films.

G. Dong et al. / Ceramics International 40 (2014) 6413–64196414

method. In order to prepare Bi1�xTbxFeO3 precursor solutions,Tb(NO3)3 � 6H2O, Fe(NO3)3 � 9H2O and Bi(NO3)3 � 5H2O wereused as raw materials. During the annealing process, 5 mol%excess Bi(NO3)3 � 5H2O was added to compensate for Bi loss.These solutions were mixed in 2-methoxyethanol. Aceticanhydride was used as the dehydrating agent. The volume ratioof 2-methoxyethanol and acetic anhydride was 3:1. The con-centration of cations was adjusted to 0.3 mol/L by adding2-methoxyethanol. The solutions were stirred for 2 h so as tobe dissolved to form the coating solution at room temperature.This solution was spread on FTO/glass substrates by spin-coating at 4000 rpm for 15 s prior to the sample dried on a hotplate of approximately 250 1C for 5 min to decompose theremaining organic compounds in the films. The dried films wereinserted into a furnace at 550 1C in the air. These procedureswere repeated for 15 times to obtain the desired film thickness.Au top electrode of 0.502 mm2 was sputtered over the surface ofthe thin films with a mask on the top. After getting annealed at300 1C for 20 min, the electrode could completely come incontact with the films. The capacitor was obtained for testingelectric properties.

The polycrystalline structures of the pure BFO and BTFOthin films were confirmed by a D/max-2200 X-ray diffract-ometer (Rigaku Company, Japan). With Cu target, the scan-ning step length was 0.021. The initial angle was 151 and theend angle was 701. Simulation of crystal structure based on themeasured XRD data was performed by a Maud program.Raman spectroscopy measurements were performed by aRenishaw inVia Raman microscope with argon laser(532 nm). The surface morphologies of the thin films wereobtained by a field emission scanning electron microscopy(FE-SEM, JSM-6700, JEOL, Japan). Agilent E4980A Preci-sion LCR Meter was used to measure the dielectric properties.Ferroelectric hysteresis loops and the leakage current densityof the pure BFO and BTFO thin films were measured by anaixACCT TF2000 Ferroelectric Analyzer.

3. Results and discussion

Fig. 1(a) shows XRD patterns of the pure BFO and BTFOthin films deposited on FTO substrates. As shown in Fig. 1(a),all the thin films were indexed for the randomly orientedpolycrystalline of the distorted rhombohedral structure onJCPDS Card (no. 85-1618). No impurity phases were detectedin the thin films. As shown in the inset of Fig. 1(a), theBTFOx¼0.10 thin film exhibits the merging of the (104) and(110) peaks. Meanwhile, the overlap of (214) and (300) peaksis also observed from the magnified XRD patterns of theBTFO thin films. All these changes indicate that Tb-dopinggives rise to the structural transition [16]. In the meantime, theshifts in the vicinity of both 32.01 and 39.51 are observed inthe BTFO thin films, showing that the interplanar spacing ofthe BTFO thin films could be changed dramatically. Moreover,the BTFO thin films show broad peaks around 32.01, indicat-ing that doping plays an important role in grain size reduction.

In order to further analyze the structural transition, Rietveldrefinement of XRD data has been performed by the Maud

program [17]. After the observation and calculation, the refinedXRD patterns of the two samples are shown in Fig. 1(b). TheXRD patterns show that the structural transition can be foundin BTFO thin films. The best fit to the measured data ofBTFOx¼0.10 is observed with using both R3c:H and R-3m:Rspace groups, indicating that BTFOx¼0.10 thin film keeps inthe state of two-phase coexistence. In the light of therefinement results, the schematics of the BFO crystal structureare shown in Fig. 2. The pure BFO thin film exhibits a singlephase of R3c:H (Fig. 2a) crystal structure, which is viewed as a(weak) rhombohedrally-distorted perovskite structure. How-ever, BTFOx¼0.10 thin film shows the state of two-phasecoexistence, which has both of R3c:H (Fig. 2a) and R-3m:R(Fig. 2b) space groups. The structural behavior is consistentwith Nd- and Co-doped polycrystalline BFO [18,19]. Therefined structural parameters of the Rietveld refinement of theXRD data are listed in Table 1.

G. Dong et al. / Ceramics International 40 (2014) 6413–6419 6415

From Table 1, it is clear that the lattice parameters (a, b,and c) decreased with Tb-doping, resulting in the volumedecrease. Since the ionic radius of Tb3þ (0.923 Å) is smallerthan that of Bi3þ (1.03 Å), the compression stress generatedfrom the small ionic substitution makes the BTFO latticeshrink. At the same time, this structural transition may inducethe distorted deformation of FeO6 octahedra and the asymme-try of the structure. Under the circumstances, the largerpolarization is reasonably desired in the present work.

Fig. 2. Schematics of BFO crystal structure (a) R3c:H space group and(b) R-3m:R space group.

Table 1

Simples Crystal structure Space group

BFORhombohedralstructure

R3c:H

BTFOx¼0.10Rhombohedralstructure

R3c:H (23.26%)

R-3m:R (76.74%)

Weights of the phases in the thin films: BFO¼86.92% and FTO¼13.08%, BTFO

Effects of Tb-doping on the structural transition are furtherreflected in the Raman spectra, as shown in Fig. 3. Accordingto the group theory, as the high rhombohedrally distortedperovskite with R3c space group, 13 (ΓRa¼4A1þ9E) activeRaman modes are predicted [20]. The measured frequencyrange of the Raman scattering spectra is 100–700 cm�1 atroom temperature. All the Raman modes observed in theBTFO thin films are identical to those of the rhombohedrallydistorted perovskite structure [21]. The larger shift of A1

modes of BTFO to the higher frequency shows that Tb-dopingcan produce the significant structural transition on Bi-sites.The electronegativity of Tb is lower than those of O and Bi.Therefore, the electronegativity of Tb–O is higher than that ofBi–O, which strengthens Bi(RE)–O covalent bond. Theenhancement of the bond can induce more obvious off-centerdisplacement in BFO [10,20]. In the meantime, withTb-doping, some E modes become broader and disappeargradually. It is known that E-1 mode is associated with Fe ionsand A1-1 mode is related to Bi ions [22]. All these changes arethe major changes in the structure. The XRD, Rietveldrefinement and Raman analysis results clearly reveal thestructural transition in BTFO thin films, which just gives riseto such electronic property changes as the ferroelectriccharacteristics, leakage current density and dielectric proper-ties. All these results will be discussed below.Fig. 4 shows the surface morphologies of the pure BFO and

BTFO thin films. The significant changes in surface morphol-ogy have been observed clearly for the BTFO thin filmscompared with the pure BFO thin film. It can be found that thegrain size of the films is decreased gradually with Tb-doping.The pure BFO thin film exhibits the homogeneous grains witha large average size of about 80–100 nm. Compared with otherfilms, the surface morphology of BTFOx¼0.10 thin film ischanged to be dense and uniform and the grain size ofBTFOx¼0.10 thin film is about 50 nm in Fig. 4(c). Tb-dopingtends to suppress the grain growth, which is consistent with theXRD results: the decrease in the peak sharpness and theintensity (see Fig. 1a). The observed well-distributed finegrains and the film texture will undoubtedly be the contributedparameters toward the decrease in leakage current and theenhancement in ferroelectric behavior.The dielectric properties of the pure BFO and BTFO thin

films are measured at room temperature in the frequency range

Lattice parameters R-factors (%)

a¼5.57606 Å α¼90° Rw¼12.62c¼13.85842 Å γ¼120° Rwnb¼10.88

Volume¼373.16 Å3 Rb¼9.24a¼5.54000Å α¼90° Rw¼11.46c¼13.82123 Å γ¼120° Rwnb¼9.16

Volume¼ 367.36 Å3

a¼3.96604Å Rb¼8.42α¼90.44837°

Volume¼62.38 Å3

x¼0.10¼76.71% and FTO¼23.29%.

G. Dong et al. / Ceramics International 40 (2014) 6413–64196416

from 1 kHz to 1 MHz. As shown in Fig. 5(a), it is found thatthe dielectric constant is not increased monotonously with theincrease of Tb-doping. However, within the whole testingfrequency range, the dielectric constant of BTFOx¼0.10 thinfilm is much higher than that of the pure BFO thin films due tothe amount of Tb-doping playing an important role in varyingthe defect concentration, such as O2� and Bi3þ vacancies inthe films [23]. That is further confirmed by leakage currentproperties of BTFO films shown in Fig. 6. Inversely, thedielectric constant of BTFOx¼0.15 thin film is decreased (156–140). Synchronously, the dependence of BTFOx¼0.15 thin filmon the frequency is more serious, indicating that the excessdoping leads to the increase of the internal defects inBTFOx¼0.15 thin film.

Fig. 3. Raman spectra of the pure BFO and BTFO thin films at roomtemperature.

Fig. 4. FE-SEM images of surface micrographs of the pure BFO and B

Fig. 5(b) shows dielectric loss of Bi1�xTbxFeO3 thin films.At the low frequency range (o50 Hz), the dielectric loss ofthe thin films is less than 0.03. However, the dielectric loss ofthe thin films is rapidly increased with increasing frequency.This is due to the failure of the polarization mechanism of thedefective dipoles in the thin films and on the boundary [24].The higher dielectric loss of the thin films with x¼0.10 and0.15 could be related to the internal structure transition.Fig. 6(a) shows the leakage current density at the applied

electric field for BFO and BTFO thin films at positive biasmeasured at RT. The leakage current density of BTFOx¼0.10

thin film is 5.10� 10�7 A/cm2, which is about two orderslower than that of the pure BFO thin film (1.93� 10�5 A/cm2)at the applied electric field of 100 kV/cm. The large leakagecurrent mainly results from the oxygen vacancies and triplevalence fluctuations of iron, as well as from various defects,such as stoichiometry, pores and cracks [25]. The BTFOx¼0.10

thin film has the lowest leakage current density, which isattributed to two aspects: One is that Tb-doping remarkablyreduces the size of grains (from 100 to 50 nm) so as to increasethe density of grain boundary, which gives rise to increasingthe resistivity directly. The other is due to the reduction ofoxygen vacancies by Tb-doping, which has been confirmed bya lot of researches [14,26].In order to understand the origin of the leakage current, the

leakage current mechanisms of the pure BFO and BTFO thinfilms are investigated. The log(J)–log(E) plots are shown inFig. 6(b). It is observed that the slope (α) values of BTFO thinfilms are between 1 and 2 at the whole electric field region (0–220 kV/cm). Whereas the leakage current density curve of thepure BFO thin film shows a bigger slope value (α¼2.62). Thisleakage behavior indicates the power law relationship J1En,n41, which is the characteristic of space-charge-limited

TFO thin films (a) x¼0, (b) x¼0.05, (c) x¼0.10 and (d) x¼0.15.

Fig. 5. Dielectric behaviors of the pure BFO and BTFO thin films. Fig. 6. (a) Current densities of the pure BFO and BTFO thin films. (b) Currentdensities versus the electric field in a logarithmic scale of the pure BFO and BTFOthin films.

Fig. 7. Ferroelectric P–E hysteresis loops of the pure BFO and BTFOthin films.

G. Dong et al. / Ceramics International 40 (2014) 6413–6419 6417

conduction (SCLC) [27]. The BTFOx¼0.05 thin film shows theideal Ohmic conduction (J1En, n�1) within the whole testingelectric field range. According to Child's law (J1En, n�2)[28], the conduction behavior of BTFOx¼0.10, 0.15 thin filmdoes not show the ideal SCLC mechanism on the breakdownfield. Therefore, in present work, the conduction behavior ofBFO thin films is transformed by Tb-doping.

Fig. 7 shows the electric hysteresis loops of BTFO thin filmsat 1 kHz. It can be found that the testing results of theferroelectric property are confirmed to the dielectric frequencyspectra. The enhancement of the ferroelectric polarization canbe obtained by Tb-doping. As for BTFOx¼0, 0.05, due to thesmall dielectric constant and the large leakage current density,the electric hysteresis loop is not saturated and the remanentpolarization is lower (2Pr¼3.2 μC/cm2 and 6.1 μC/cm2,respectively). However, BTFOx¼0.10 thin film obtains thebetter electric hysteresis loop and the maximum remanentpolarization (2Pr¼135.1 μC/cm2) in surprise. On the onehand, the enhancement of the ferroelectric property is corre-sponding to XRD results. BTFOx¼0.10 thin film keeps inthe state of two-phase coexistence, which may appear on themorphotropic phase boundary [14]. On the other hand, the

Fig. 8. Magnetic hysteresis (M–H) loops of the pure BFO and BTFOx¼0.10

thin films; the inset is an enlarged section of the loops.

G. Dong et al. / Ceramics International 40 (2014) 6413–64196418

lower leakage current density can increase the effective electricfield for polarization reversal. As known to all, the high orderferroelectric switching and the spontaneous polarization ofBFO primarily result from the stereochemically active 6s2 lonepairs on Bi3þ ions [29]. Thus, the systematic substitution byTb3þ ions will not only change the spacing between Bi3þ /Tb3þ ions and FeO6 octahedra, but also alter the long-rangeferroelectric order.

The magnetic hysteresis (M–H) loops of the pure BFO andBTFOx¼0.10 thin films are shown in Fig. 8 and the inset showsthe enlarged section of the loops. The pure BFO shows theweak ferromagnetic behavior. The saturated magnetization(Ms) of pure BFO is about 0.7 emu/cm3, while BTFOx¼0.10

presents the higher saturation magnetization (Ms¼6.3 emu/cm3). This result arises from the destruction of the antiferro-magnetically ordered spins due to the structural transition. Inaddition, the enhanced ferromagnetic property is closelyrelated to the small grain size effect, which significantlymodifies the long-range spiral spin structure of BFO [30].Tb-doping also results in the different bond interactionsbetween 4f and 5d orbitals of Tb–O and 6p orbitals of Bi–Oleading to the ferromagnetic coupling. Simultaneously, withTb-doping, the structural transition releases the latent magne-tization locked within the spin cycloid [31]. These reasonscause the enhanced ferromagnetism.

4. Conclusion

In summary, Bi1�xTbxFeO3 thin films were prepared onFTO substrates by the sol–gel method. The effects of Tb-doping on the structural transition, leakage current, anddielectric and multiferroic properties of BTFO thin films wereinvestigated systematically. In BTFO thin films, the structuraltransformation from the rhombohedral (R3c:H) to the biphasic(R3c:HþR-3m:R) structure is confirmed through XRD, Riet-veld refinement and Raman analysis. The surface morphologyof BTFOx¼0.10 thin film obtains the dense and uniform

structure. The leakage current density of BTFOx¼0.10 thinfilm is two orders lower than that of the pure BFO thin film.Furthermore, the conduction mechanism of BTFO thin films isdominated by space-charge-limited conduction. Two-phasecoexistence of BTFOx¼0.10 gives rise to the superior ferro-electric property (2Pr¼135.1 μC/cm2 and 2Ec¼1000.7 kV/cm), which is mainly attributed to the lower leakage currentdensity, the state of two-phase coexistence and the distorteddeformation of FeO6 octahedra. The higher saturated magne-tization (Ms¼6.3 emu/cm3) of BTFOx¼0.10 is also bound upwith the structural transition.

Acknowledgments

This work was supported by the Project of the NationalNatural Science Foundation of China (Grant no. 51372145);Research and Special Projects of the Education Department ofShaanxi Province (Grant no. 12JK0445); the Graduate Innova-tion Fund of Shaanxi University of Science & Technology(SUST-A04).

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