Effects of Ba0.7Sr0.3TiO3-based buffer layers andLa/Mn doping on the crystallization behavior andmultiferroic properties of BiFeO3 thin films

Bin Li,a Chunqing Wang,*ab Guangbin Dou*c and Zhenbin Wangd

A detailed study of the BiFeO3 (BFO)-based bilayer films with improved multiferroic properties is reported.

The BFO film without any impurity phase was obtained using Ba0.7Sr0.3TiO3 (BSTO) as a buffer layer. And the

leakage currents of the bilayer film were found to reduce effectively through La and Mn co-doping of the

BFO Layer. Moreover, magnetodielectric response and ferroelectric domains were observed in the

Bi0.9La0.1Fe0.95Mn0.05O3 (BLFMO)/BSTO film. We additionally investigated the effects of the

BSTO–Ni0.8Zn0.2Fe2O4 (NZFO) buffer layer on the multiferroic performances and surface morphology of

the BLFMO film. It was found that the BSTO–NZFO composite film could significantly enhance the

magnetic and magnetodielectric properties of the BLFMO film. In particular, the BLFMO/BSTO–NZFO

film possessed a higher coercivity value as compared to the BLFMO/BSTO film.

IntroductionMultiferroics have received considerable interest recently, asthey can simultaneously display ferroelectric and ferromagneticproperties.1,2 From an application perspective, multiferroicmaterials are promising candidates for many multifunctionaldevices such as memories and sensors.3,4 Among those mate-rials, BiFeO3 (BFO) has been most actively studied5–7 due to thecoexistence of ferroelectric and magnetic performances aboveroom temperature.8 Furthermore, the BFO combines a giantremanent polarization9 with high Curie and Neel tempera-tures.10 However, the high leakage current density,11 smallmagnetization12 and weak magnetoelectric coupling13 of BFOseriously impede its practical use. In addition, the BFO in lmform is apt to react with Pt bottom electrode during the thermaltreatment,14 and the formation of impurity phases can reduceits performances.

In order to enhance the multiferroic properties of BFO,considerable efforts have been devoted to the ionic substitu-tions onto its A or/and B sites in recent years.15–17 In particular,the La or/and Mn doping has been demonstrated to effectivelyimprove the leakage characteristics of BFO by many studies.18–21

However, its weak magnetic and magnetoelectric performances

remain a major challenge to use in multifunctional devices. It isworth mentioning that the dielectric and ferromagneticresponses of BFO ceramics can be improved by a ferroelectricBaTiO3 substitution.22 At the same time, a previous report hasproved that a signicant magnetoelectric coupling can beobserved in the BFO lm with embedded ferromagneticCoFe2O4 nanopillars.23 Additionally, the BFO thin lm withoutsecond phase and with improved leakage property was obtainedby using a SrRuO3 buffer layer.24 Therefore, these results mayopen the door to preparing a BFO-based lm with excellentproperties by using a ferroelectric and ferromagnetic compositelm as its buffer layer.

In this study, the ferroelectric Ba0.7Sr0.3TiO3 (BSTO) lm wasrstly used as a buffer layer to prevent the reaction between theBFO lm and Pt electrode. Then, the La/Mn doping was selectedto reduce the leakage currents of the BFO layer. And the changesof Fe3+ ion concentrations in the BFO/BSTO, BiFe0.95Mn0.05O3

(BFMO)/BSTO and Bi0.9La0.1Fe0.95Mn0.05O3 (BLFMO)/BSTOlms were investigated. To improve the magnetic and magne-todielectric (MD) performances, the BSTO–Ni0.8Zn0.2Fe2O4

(NZFO) ferroelectric–ferromagnetic lm was nally applied as abuffer layer of the BLFMO lm.

ExperimentalThe lms studied in this work were synthesized using a sol–gelspin-coating method. The preparation process of sols has beendescribed in detail elsewhere.25 The differences here were thatthe concentrations of all the single sols (BFO, BFMO, BLFMOand BSTO) were 0.3 M, and the composite sol (BSTO–NZFO witha molar ratio of 1 : 1) was formed by 0.4 M BSTO and 0.1 MNZFO precursors. Aer standing for at least one week, the BSTO

aState Key Laboratory of Advanced Welding and Joining, Harbin Institute ofTechnology, Harbin 150001, China. E-mail: wangcq@hit.edu.cnbKey Laboratory of Micro-systems and Micro-structures Manufacturing, Ministry ofEducation, Harbin Institute of Technology, Harbin 150080, ChinacElectrical and Electronic Engineering, Imperial College London, London SW7 2AZ, UK.E-mail: g.dou@imperial.ac.ukdDepartment of Materials Science and Engineering, University of Science andTechnology of China, Hefei 230026, China

Cite this: RSC Adv., 2014, 4, 55889

Received 23rd July 2014Accepted 20th October 2014

DOI: 10.1039/c4ra07492k

www.rsc.org/advances

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sol was rstly spin-coated onto a Pt/Ti/SiO2/Si substrate at 4000 rmin!1 for 40 s, and subsequently annealed at 700 "C for 5 minto form the buffer layer lm. Then, the BFO-based sols (BFO,BFMO and BLFMO) were deposited onto the BSTO buffer layersusing the same spin-coating process, followed by annealing at600 "C for 5 min. Finally, the BFO/BSTO, BFMO/BSTO andBLFMO/BSTO lms consisting of four BFO-based layers (380nm) and two BSTO layers (240 nm) were obtained, respectively.Additionally, the BSTO–NZFO buffer lm with the same thick-ness of 240 nm (ten layers) was fabricated by annealing at 700 "Cfor 5 min in the BLFMO/BSTO–NZFO lm synthesis process. Inorder to compare with the bilayer lms obtained above, the BFO(580 nm), BSTO (240 nm) and BSTO–NZFO (240 nm) single lmswere also synthesized. The heat treatment process of all lmshere was performed in air by a rapid thermal annealingtechnique.

The phase compositions of the thin lms were veried byX-ray diffraction (XRD) on a Philips X'Pert diffractometer withCuKa radiation. The Raman data were obtained using a JobinYvon HR 800 spectrometer, and an Ar+ laser was used as theexcitation source. The surface characterization of the bilayerlms was detected by X-ray photoelectron spectroscopy (XPS,PHI 5700 ESCA). For electrical and MD measurements, the Autop electrodes with 180 mm diameter were deposited on thelms through a shadow mask. The leakage currents andpolarization–electric eld (P–E) loops were measured by aferroelectric tester (Precision Premier II, Radiant Technologies,Inc.). The scanning electron microscopy (SEM) images wereinvestigated with an FEI Quanta 200 FEG instrument. Theatomic force microscopy (AFM) and piezoresponse forcemicroscopy (PFM) analyses were carried out by a BrukerDimension Icon system. The MD effects were observed bymeasuring the dielectric constant changes (using an AgilentE4980A precision impedance analyzer) in various magneticelds (using an electromagnet) at room temperature. And themagnetic elds from 0 to 6300 Oe were measured for the MDresponse. The magnetic hysteresis loops were examined by avibrating sample magnetometer (VSM, Lake Shore 7410). Thefocused ion beam (FIB, FEI Helios NanoLab 600i) technique wasused for the transmission electron microscopy (TEM) sample

preparation, then the high-resolution TEM (HRTEM) image andselected-area electron diffraction (SAED) pattern were charac-terized by a FEI Tecnai G2 F30 microscope equipped with anenergy dispersive spectroscopy (EDS) detector. The thermog-ravimetry (TG) and differential scanning calorimetry (DSC)curves were investigated by a Mettler Toledo TGA/DSC 1 system.

The rst-principles calculations of La and Mn doped BFOwere performed using Vienna ab initio simulation package(VASP) with a projector augmented wave (PAW) method.Exchange correlation effects were treated by employing thegeneralized gradient approximation (GGA) using Perdew–Burke–Emzerhof (PBE) functional. The energy cutoff for plane-wave basis set was 500 eV, and the energies were convergedwithin 1.0 # 10!5 eV. All structural relaxations were performeduntil the force on each atom reduced within 0.01 eV A!1. TheBrillouin zone integration based on the Monkhorst–Packmethod was carried out, and the number of k-points was chosento give a spacing of about 0.035 A!1. All calculations above werespin polarized.

Results and discussionCrystallization and surface composition of the BFO/BSTO lm

Fig. 1a exhibits XRD analysis of the BFO, BSTO and BFO/BSTOlms deposited on Pt/Ti/SiO2/Si substrates. It can be seen thatthe impurity phase appears in the XRD pattern of the BFO lmsynthesized in this study. This is because the BFO lms caneasily react with Pt bottom electrode to form Bi–Pt alloy layerduring thermal annealing, as reported previously.14 It is worthto notice that there is no second phase detected in BFO lm byusing a BSTO buffer layer, and the BFO layer in the bilayer lmhas single phase and high quality. Meanwhile, the appearanceof all specic diffraction peaks reveals that the obtained BFOphase has a polycrystalline and tetragonal perovskite structure.In order to further conrm the effect of BSTO buffer layer on thecrystallization of BFO lm, we investigated the molecularstructure of the lms by Raman scattering (Fig. 1b). It is foundthat the characteristic peaks of the BFO/BSTO lm are strongerand sharper than those of the BFO lm. These observations

Fig. 1 (a) XRD patterns of the BFO/BSTO, BFO and BSTO thin films. (b) Raman spectra of the BFO/BSTO and BFO thin films (inset: Ramanspectrum of the BSTO thin film). The # in (a) denotes the impurity phase of Bi25FeO39.26 Two bands at 142 and 221 cm!1 shown in (b) are assignedto A1-1 and A1-3 modes of BFO, respectively.27

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indicate that the crystallization of BFO lm has been obviouslyimproved through the use of BSTO as a buffer layer.

The XPS spectrum in Fig. 2a shows that the BFO/BSTO lmsurface contains Bi, Fe, O and C elements. Apart from C, thisresult is in accordance with the chemical composition of BFOphase. The existence of C element could be from carbon

contamination of the sample surface. The strong Bi4f peak witha typical Gaussian distribution can be seen in Fig. 2b. Asreported in previous research,11 the BFO ceramics always exist alarge amount of Fe2+ ions, which is the main reason for theirhigh leakage current densities. Without exception, the Fe2+ ionshave been observed in the BFO/BSTO lm, and the concentra-tion of them is much higher than that of Fe3+ ions (Fig. 2c). Inorder to reduce the Fe2+ content in the BFO Layer, the effects ofLa and Mn doping on the BFO/BSTO lm are further studiedbelow.

Doping of the BFO layer with La and Mn elements

The tting spectra of Fe2p3/2 peaks for the BFMO/BSTO andBLFMO/BSTO lms are shown in Fig. 3a and b, respectively. TheFe3+ peak at binding energy near 712 eV and Fe2+ peak atbinding energy around 710 eV can be observed. It is found thatthe Fe3+ to Fe2+ ratio in the BFMO/BSTO lm or BLFMO/BSTOlm is much higher than that in the BFO/BSTO lm (Fig. 2c).In particular, the Fe3+ concentration of the BLFMO/BSTO lmhas increased to 40.57%, as presented in Fig. 3c. This obser-vation conrms that the La and Mn co-doping can considerablyreduce the Fe2+ content in the BFO layer studied here. In orderto better understand the effects of La/Mn doping, the leakagecurrents of three bilayer lms were also investigated (Fig. 3dand e). Compared to the BFMO/BSTO and BLFMO/BSTO lms(Fig. 3e), the BFO/BSTO lm in Fig. 3d has a jump in the leakagecurrent density near 200 kV cm!1. This is because the un-dopedBFO lm exhibits a low breakdown eld, which has been proved

Fig. 2 XPS analysis of the BFO/BSTO film surface. (a) Wide-scanspectrum. (b–d) High-resolution spectra.

Fig. 3 (a) Fitting spectra (Fe2p3/2 peaks) of the BFMO/BSTO thin film. (b) Fitting spectra (Fe2p3/2 peaks) of the BLFMO/BSTO thin film. (c)Concentrations of Fe3+ in the BFO/BSTO, BFMO/BSTO and BLFMO/BSTO thin films. (d) Leakage currents of the BFO/BSTO thin film (inset:schematic illustration of the test method for electrical and magnetodielectric properties in this study). (e) Leakage currents of the BFMO/BSTOand BLFMO/BSTO thin films.

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in the previous report.10 We can see that the partial substitutionof La and Mn ions can effectively reduce the leakage currents ofthe bilayer lms, and thus improve their breakdown charac-teristics. The leakage current densities of the BFO/BSTO lm,BFMO/BSTO lm and BLFMO/BSTO lm are 1.8 # 10!5 A cm!2,9.7 # 10!9 A cm!2 and 5.2 # 10!9 A cm!2 at 300 kV cm!1,respectively. Moreover, the leakage current value of the BLFMO/BSTO lm is much lower than that of La/Mn co-doped BFOlm.28 Thus the improved leakage characteristics may be due tothe combined action of La/Mn co-doping and BSTO buffer layer.

The calculated results of crystal structures for the La and Mndoped BFO are shown in Fig. 4a–c. It can be seen that all theBFO, BFMO and BLFMO have perovskite structures. And therelative positions of the atoms do not change aer La/Mndoping. BFO was reported to exhibit a G-type antiferromag-netic ordering at room temperature.29 As observed in Fig. 4d, thedensities of states for two spin directions in BFO are mirror-image symmetry, conrming its weak magnetization nature.Aer doping of Mn, the electronic states at energy range from0 to 1 eV have been observed owing to the 3d orbit of Mn(Fig. 4e). This result indicates that the Mn doping can enhancethe magnetic properties of BFO, which is in accordance with ourprevious study.21

The effects of La and Mn doping on the molecular structuresare displayed in Fig. 5a. Compared to the BFO/BSTO lm(Fig. 1b), both the BFMO/BSTO lm and BLFMO/BSTO lmexhibit much stronger bands around 619 cm!1 (E-9 mode27).This can be explained by the distortion of [(Mn, Fe)3+O6] octa-hedra,30 and a similar result has been reported for the La and Coco-doped BFO ceramics.31 As presented in the inset of Fig. 5a,the BLFMO/BSTO lm without any impurity phase was alsoobtained aer using a buffer layer. From the BLFMO/BSTO lmcross-section shown in Fig. 5b, it can be seen that there are clearinterfaces between individual layers. Also, both the BLFMO andBSTO layers can be identied. As seen in Fig. 5c–e, the surface

morphologies reveal that the BLFMO/BSTO lm possesses adense and ne-grained structure. These indicate that theBLFMO/BSTO lm obtained is well-crystallized.

It is well known that multiferroics present magnetoelectricor MD effects.32 However, BFO-based compounds exhibit a weakcoupling between the ferroelectric and magnetic order param-eters.33 It is worth mentioning that the MD response of theBLFMO/BSTO lm has been observed here (Fig. 6a). However,MD properties of the BLFMO lm had not been observed in ourstudy. The MD coefficient is dened as:

MD ¼ 30ðHÞ ! 30ð0Þ30ð0Þ # 100 (1)

where 3'(H) and 3'(0) are the dielectric constants at appliedmagnetic eld and zero eld measured at same frequency,respectively. Fig. 6a shows that the MD values increase as themagnetic eld increases. Considering the eqn (1), it is indicatedthat the dielectric constant also increases with the increase ofthe magnetic eld. This may be because of the magneticallyinduced electric polarization. Compared to the epitaxiallygrown La-doped BFO thin lm,34 the BLFMO/BSTO thin lmexhibits a much stronger MD response. Additionally, no furtherincrease in the MD coefficient can be seen above 5000 Oe. Theobservation is probably due to the fact that the magnetizationtends to saturate at that magnetic eld, as presented inFig. 7a.

Fig. 6b shows a detailed polarization–electric eld charac-teristic of the BLFMO/BSTO lm. The ferroelectric hysteresisloop measured at 80 V is well-dened and clear because of thelow leakage densities. In order to further investigate electricalproperties of the BLFMO/BSTO lm, the out-of-plane piezores-ponse signal was extracted as shown in the inset of Fig. 6b. Thepolarization switching is conrmed by the change in contrastbetween the 5# 5 mm2 and 3# 3 mm2 areas. This observation inthe PFM image can be attributed to the presence of ferroelectric

Fig. 4 Crystal structures of the (a) BFO, (b) BFMO, and (c) BLFMO. Density of states of the (d) BFO, (e) BFMO, and (f) BLFMO.

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domains in the lm, and a similar result has been evidenced byother researchers.12,35

The magnetic properties of the BLFMO/BSTO lm werestudied by a VSM with the magnetic elds parallel (in-plane)and perpendicular (out-of-plane) to the lm surface (Fig. 7a).It is found that the in-plane and out-of-plane magnetic hyster-esis loops possess similar shapes. It shows the magneticanisotropy in the lm between the [100] and [001] directions isnot obvious owing to its polycrystalline structure. The coercivity

(Hc) values of the in-plane and out-of-plane loops are 63 and 72Oe, respectively.

Using the BSTO–NZFO composite lm as buffer layer

To enhance the magnetic properties of the bilayer lm, theBLFMO lm using a BSTO–NZFO buffer layer was furtherstudied. Fig. 7b exhibits a well-dened magnetic hysteresis loopof the BLFMO/BSTO–NZFO bilayer lm. Compared with the out-of-plane loop of the BLFMO/BSTO lm (Fig. 7a), a much largersaturation magnetization (Ms) has been observed in Fig. 7b. Itcan be seen that the Hc value of the BLFMO/BSTO–NZFO lm isup to 143 Oe, which is also much higher than that of theBSTO–NZFO lm (lower right inset of Fig. 7b). This can beresulted from the exchange coupling existing between theantiferromagnetic BLFMO lm and ferromagnetic BSTO–NZFOlayer. This coupling leads to a signicant increase of theHc, andthus make an improved magnetic property in the BLFMO/BSTO–NZFO lm.

The MD effects of the BLFMO/BSTO–NZFO lm are depictedin Fig. 8a. Compared to the BLFMO/BSTO lm (Fig. 6a), theBLFMO/BSTO–NZFO lm presents a signicantly larger MDcoupling, which is caused by the BSTO–NZFO buffer layer. Asreported previously,36 using a material with high dielectric andmagnetic susceptibilities is an important way to enhance theMD response. Thus, combining the ferroelectric BSTO andferromagnetic NZFO together can result in a large MD coeffi-cient for the BSTO–NZFO composite buffer layer. And a strongMD coupling has been proved in the BSTO–NZFO nanobers.37

Fig. 8b and c show the P–E hysteresis loop and leakage currentsof the BLFMO/BSTO–NZFO lm, respectively. It is found thatcompared with the BLFMO/BSTO lm, the BLFMO/BSTO–NZFOlm has lower polarizations and higher current densities due tothe existence of conductive ferromagnetic NZFO. In theBSTO–NZFO composite buffer layer with molar ratio of 1/1, theNZFO phase is liable to connect together. This may lead to aweakened interaction in the BSTO phase, and result in a lowresistivity of the BSTO–NZFO lm. However, both the magneticandMDproperties are obviously improved by using a BSTO–NZFO

Fig. 5 (a) Raman analysis of the BFMO/BSTO and BLFMO/BSTO thinfilms (inset: XRD pattern of the BLFMO/BSTO thin film). (b) SEM imageof the BLFMO/BSTO film cross-section. (c and d) SEM images, and (e)AFM image of the BLFMO/BSTO film surface.

Fig. 6 Magnetodielectric and electrical properties of the BLFMO/BSTO thin film. (a) Magnetodielectric property. (b) P–E hysteresis loop(inset: PFM image after being poled by applying +10 V within 5# 5 mm2

area and then applying !10 V within 3 # 3 mm2 area).

Fig. 7 (a) In-plane and out-of-plane magnetic hysteresis loops of the BLFMO/BSTO thin film (inset: enlarged sections of the same loops). (b)Out-of-plane magnetic hysteresis loop of the BLFMO/BSTO–NZFO thin film (upper left inset: enlarged section of the same loop, lower rightinset: out-of-plane magnetic hysteresis loop of the BSTO–NZFO thin film).

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buffer layer, thereby obtaining the BLFMO/BSTO–NZFO lm withan enhanced comprehensive performance.

Additionally, it is interesting to note that the BLFMO/BSTO–NZFO lm has a rough surface (Fig. 9a), and the reason will bediscussed in the section of growth mechanism below. Fig. 9aand b exhibit the surface morphologies of the BLFMO/BSTO–NZFO and BSTO–NZFO lms, respectively. We can see that theBSTO–NZFO lm possesses a much rougher surface than theBLFMO/BSTO–NZFO lm. Then combined with the BLFMO/BSTO lm surface (Fig. 5c–e), the special morphology of theBLFMO/BSTO–NZFO lm is believed to be from theBSTO–NZFO buffer layer. The interplanar spacings of the BSTO(110) and NZFO (220) planes exhibited in HRTEM image

(Fig. 9c) are conrmed by the reports for BaTiO3 lms38 andNiFe2O4 nanobres,39 respectively. Meanwhile, the SAEDpattern in Fig. 9d reveals a polycrystalline structure of theBSTO–NZFO lm. And the co-existing of BSTO and NZFO phasescan also be determined by the EDS spectrum in Fig. 9e. Theresults described above demonstrate that the BSTO and NZFOceramics are formed aer annealing at 700 "C.

We now focus on the formation mechanism of the BLFMO/BSTO–NZFO bilayer lm. As illustrated schematically inFig. 10a, the BSTO and BSTO–NZFO lms exhibit smooth andrough surfaces, respectively. This difference can be ascribed tothe various grain shapes and sizes of two phases in thecomposite lm as compared to that in the single lm. As a

Fig. 8 (a) Magnetodielectric properties, (b) P–E hysteresis loop, and (c) leakage currents of the BLFMO/BSTO–NZFO thin film.

Fig. 9 SEM, AFM and TEM analysis of the BLFMO/BSTO–NZFO and BSTO–NZFO thin films. (a) SEM image of the BLFMO/BSTO–NZFO filmsurface (inset: enlarged image of the same sample). (b) AFM image of the BSTO–NZFO film surface (insets: SEM images of the same sample). (c)HRTEM image, (d) corresponding SAED pattern, and (e) EDS spectrum of the BSTO–NZFO film.

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result, the grains of BSTO and NZFO phases become larger andcloser to each other as the annealing temperature increases,and an uneven surface (Fig. 9b) of the BSTO–NZFO compositelm is nally formed. In addition, the TG–DSC curves of theBSTO, BSTO–NZFO and BLFMO gels are plotted in Fig. 10b–d.The exothermic peaks observed in the DSC curves can beattributed to the decomposition of solvents used for sol prep-aration, and the burning of decomposed carbon. Compared theDSC curve in Fig. 10c to that in Fig. 10b, more exothermic peakscan be clearly seen during the thermal decomposition processof the BSTO–NZFO gel. This observation further demonstratesthat the BSTO–NZFO lm has amore complicated process of thegrain growth due to the co-existence of BSTO and NZFO phases.In the synthesis process of the BLFMO/BSTO–NZFO lm, theconvex and concave parts on the BSTO–NZFO layer surfacecould be partially lled with the BLFMO sol. Therefore, it isfound that the BLFMO/BSTO–NZFO lm possesses a relativelysmoother surface (Fig. 9a) than the BSTO–NZFO lm.

ConclusionsIn summary, the inuences of BSTO-based buffer layers andLa/Mn doping on multiferroic properties of the BFO lms wereinvestigated. The phase compositions indicated that the use ofBSTO buffer layer could prevent the reaction between the BFOlm and Pt electrode. The results of reducing leakage currentdensity by the La and Mn doping of the BFO layer were dis-cussed in detail. And the magnetodielectric, electrical andmagnetic properties of the BLFMO/BSTO lm were also detec-ted. In addition, it was found that BSTO–NZFO buffer layercould effectively improve the magnetic and magnetodielectric

performances of the BLFMO lm. In particular, a signicantincrease of the Hc value occurred due to the existence ofexchange coupling between the antiferromagnetic BLFMO lmand ferromagnetic BSTO–NZFO layer. Meanwhile, a roughsurface was observed in the BLFMO/BSTO–NZFO lm, and theformation mechanism of this special morphology had beenproposed. Further study of the magnetoelectric coupling anddielectric property will be carried out in our future research.

AcknowledgementsThe work was supported by State Key Laboratory of AdvancedWelding and Joining, Harbin Institute of Technology. Theauthors would like to thank Dr Mao Ye and Dr Qiu Sun for theirsupport on testing ferroelectric properties, as well as Mr Yang Lifor his help in the AFM and PFM measurements at HarbinInstitute of Technology.

Notes and references1 N. A. Spaldin and M. Fiebig, Science, 2005, 309, 391–392.2 A. Stroppa and S. Picozzi, Phys. Chem. Chem. Phys., 2010, 12,5405–5416.

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Fig. 10 Analysis of growth mechanism of the BLFMO/BSTO–NZFO thin film. (a) Schematic illustration of grain growth in the BSTO andBSTO–NZFO thin films. (b–d) TG–DSC curves of the BSTO, BSTO–NZFO and BLFMO gels.

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