Materials Research Bulletin 47 (2012) 4240–4245

Nanoscale piezoresponse and magnetic studies of multiferroic Co andPr co-substituted BFO thin films

Neeraj Panwar a, Indrani Coondoo a,*, Amit Tomar b, A.L. Kholkin a, Venkata S. Puli c, Ram S. Katiyar c

a Department of Materials and Ceramics Engineering & CICECO, University of Aveiro, Aveiro 3810-193, Portugalb N. A. S. College, C.C. S. University, Meerut, Indiac Department of Physics, University of Puerto Rico, San Juan, PR 00931, USA

A R T I C L E I N F O

Article history:

Received 25 June 2012

Received in revised form 23 August 2012

Accepted 5 September 2012

Available online 16 September 2012

Keywords:

A. Thin films

B. Chemical synthesis

C. Atomic force microscopy

D. Magnetic properties

A B S T R A C T

Piezoresponse Force Microscopy (PFM) technique has been employed to acquire out-of-plane (OPP)

piezoresponse images and local piezoelectric hysteresis loop in rhombohedrally distorted Bi1�xPrx-

Fe1�yCoyO3 [x = 0, 0.05; y = 0.05] polycrystalline thin films fabricated via chemical solution deposition

method. PFM images revealed that piezoelectric contrast is dependent upon the film composition.

Furthermore, negative self-polarization effect was observed in the cobalt substituted BFO film. Well

saturated local piezo-hysteresis loops were monitored and an increase was noticed in the piezoelectric

coefficient (d33) value with cobalt doping (25.1 pm/V) whereas with Pr co-substitution in BFCO film, the

piezoelectric behavior was almost suppressed. Pr and cobalt co-substituted film exhibited the lowest

leakage current density. Magnetic behavior (M–H curves) exhibited nearly eight times enhancement in

the saturation magnetization values in the Co- and Co–Pr substituted films. The present study provides

the different elements’ substitution effect on the local piezoelectric and magnetic properties of BiFeO3

multiferroic thin film.

� 2012 Elsevier Ltd. All rights reserved.

Contents lists available at SciVerse ScienceDirect

Materials Research Bulletin

jo u rn al h om ep age: ww w.els evier .c o m/lo c ate /mat res b u

1. Introduction

With an ever-increasing demand for data storage, transducers,and microelectromechanical (MEMS) systems applications, mate-rials with superior ferroelectric and piezoelectric responses are ofgreat interest. A number of multiferroics have been identified;however, their low transition temperatures hinder the potentialapplications. BiFeO3 (BFO) with its high ferro-paraelectric transi-tion temperature (TC) � 1100 K and high antiferromagnetic toparamagnetic Neel temperature (TN) � 643 K, stands out among allothers [1,2]. Ever since, BFO has been widely studied in bulk andthin film forms [3–10]. It has been shown that the addition ofdopants, affects the electrical and magnetic properties of BFOsamples through structural modifications and the control over theconcentration of oxygen vacancies. Consequently, several attemptshave been employed to tailor the properties of BFO films by A- or B-site substitutions [2,11–15]. In order to further improve theelectric and magnetic properties, co-substitutions have beenattempted [16–22]. For example, Yu et al. reported enhanceddielectric, ferroelectric and anti-fatigue properties in La3+ and V5+

co-substituted Bi0.85La0.15Fe1�xVxO3 (BLFV, x = 0–0.1) ceramics

* Corresponding author. Tel.: +351 910446473; fax: +351 234 401470.

E-mail address: indrani_coondoo@yahoo.com (I. Coondoo).

0025-5408/$ – see front matter � 2012 Elsevier Ltd. All rights reserved.

http://dx.doi.org/10.1016/j.materresbull.2012.09.026

[16]. The leakage current density of BLFV ceramic was only2.1 � 10�6 A cm�2, several orders of magnitude lower than that forsingle element substituted BLF and BFV ceramics. Singh et al.studied La3+ and Ni2+ co-substituted BFO films and observed threeorders of magnitude lower leakage current density than pure BFO[17]. This was even much lower than that for single ion La3+ or Ni2+

substituted films. The lower leakage current density helped in theobservation of well saturated ferroelectric hysteresis loop in theco-substituted film. Zhai et al. prepared Bi0.9La0.1Fe0.98Nb0.02O3

(BLFNO) polycrystalline ceramic and noticed a rhombohedral tomonoclinic structural transition with Nb5+-substitution in BLFO.Further, BLFNO sample showed a large coercive field and remnantmagnetization � 1.35 T and 0.23 A m2 kg�1, respectively at roomtemperature [18]. The enhanced magnetic behavior was attributedto several factors including collapse of cycloid spin structure due tostructural transition, destruction of magnetic balance between theantiparallel sub-lattices of Fe3+ ions and drastic grain size decreasewith Nb5+ co-substitution in BLFO compound. Enhancement inthese physical properties was also obtained by Cheng et al. in La3+

and Nb5+ co-substituted BFO films [19]. Hu et al. prepared pureBFO, Nd-substituted BFO (BNF), Nd and Mo co-substituted BFO(BNFM) thin films and reported minimum dielectric loss andleakage current density, well-saturated ferroelectric hysteresisloop, fatigue-free behavior and improved ferromagnetism with asaturation magnetization of �15 emu/cm3 in BNFM film [20]. Lan

Fig. 1. (a) X-ray diffraction patterns of BFO, BFCO and BPFCO thin films; (b)

magnified view of the (1 0 4) and (1 1 0) peaks (the vertical dotted lines are

guidelines to the eyes).

N. Panwar et al. / Materials Research Bulletin 47 (2012) 4240–4245 4241

et al. investigated La3+ and Zr4+ co-substitution effect in BFOceramics (BLFZ) and observed improved magnetic behavior andexchange bias effect [8]. The enhanced effects in BLFZ ceramicswere ascribed to grain size reduction and antiferromagnetic–ferromagnetic core–shell structure formation. Liu et al. concludedthat among various A-site Ce and B-site Zr co-substitutedBi1�xCexFe1�yZryO3 (BCFZ) thin films, Bi0.97Ce0.03Fe0.97Zr0.03O3 filmshowed the lowest dielectric loss and leakage current density, awell-squared P-E loop and fatigue-free characteristics as well asthe strong magnetization [22]. Kawae et al. suppressed the leakagecurrent density in the high electric field regions with co-substitution of Pr3+ and Mn4+ in BFO thin films [21]. As far aspraseodymium (Pr) is concerned, it has been found more effectivein enhancing the ferroelectric and magnetic properties than otherrare earth ions substituted BFO films [4]. Similarly, cobalt (Co) hasalso been observed a suitable substituent for improving ferroelec-tric and magnetic properties of BFO [13]. Therefore, in this paper,we planned to investigate the simultaneous doping effect of Pr andCo on the electrical and magnetic properties of BiFeO3 film. Theelectrical properties, e.g. local piezoelectricity, were studied byPiezoresponse Force Microscopy (PFM) technique. This techniquehelps to characterize the samples with higher conductivity whichhampers macroscopic polarization measurement. The piezoelec-tric and magnetic measurements on Co- and Co–Pr-doped BFOfilms are discussed.

2. Experimental details

BiFeO3 (BFO), BiFe0.95Co0.05O3 (BFCO) and Bi0.95Pr0.05Fe0.95-

Co0.05O3 (BPFCO) films were deposited on Pt(1 1 1)/Ti/SiO2/Sisubstrates by chemical solution deposition technique. Theprecursor solutions were prepared by dissolving bismuth nitratepentahydrate, iron nitrate nonahydrate, cobalt nitrate hexahy-drate, praseodymium nitrate hexahydrate in acetic acid, and 2-methoxyethanol according to the metal ion ratios. Molarity of theresultant solutions was 0.5 M. The films were deposited onto thesubstrates by spin coating and pyrolysed layer by layer at 300 8Cfor 3 min. The process of coating and drying was repeated 15 timesresulting in film thickness � 300 nm. Finally the films wereannealed at 550 8C for 1 h in N2 atmosphere to avoid secondaryphase formation. Phase analysis of the films and investigation oftheir crystal structure were performed by X-ray diffraction (XRD)technique using Siemens diffractometer with Cu Ka radiation overan angular range 208 � 2u � 608. Local ferroelectric/piezoelectricproperties of the films were investigated with PFM (simultaneous-ly acquired with topography) using a commercial setup multimodenanoscope IIIA (Veeco) equipped with a lock-in amplifier (SR-830A, Stanford Research) and a function generator (FG-120,Yokagawa). Platinum coated silicon cantilever (force constant42 N/m and resonance frequency 204–497 kHz) was used for thisstudy. PFM images were obtained by applying ac voltage (5 V,peak-to-peak) with frequency of 50 kHz (out-of-plane) betweenthe grounded tip and the bottom electrode. A standard ferroelectrictester (Radiant Technology) was used to measure the leakagecurrent. The magnetization hysteresis loop (M–H loop) measure-ment was carried out using a vibrating sample magnetometer(VSM) (Lakeshore 7407).

3. Results and discussion

Fig. 1a shows the XRD patterns of the single phase BFO, BFCOand BPFCO films with rhombohedrally distorted BiFeO3 perovskitestructure. The lattice constants of both the substituted films (BFCOand BPFCO) are smaller (a = 3.961 A and 3.956 A) as compared tothose of pristine BFO film (a = 3.977 A) due to smaller ionic sizes ofthe substituents, viz. Pr3+ (112.6 pm) and Co3+ (54.5 pm) than

those for Bi3+ (117 pm) and Fe2+ (61 pm)/Fe3+ (55 pm) ions,respectively [23]. Due to this effect, an increase in the diffractionangles of the peaks (1 0 4) and (1 1 0) of the substituted films canbe noticed in Fig. 1b which represents the magnified XRD patternsfrom 318 to 338 of Fig. 1a. The rhombohedral distortion of thepristine BFO film is reduced toward the orthorhombic or tetragonalstructure with Co/Pr substitution, as can be deduced from thebroadening of (1 0 4) diffraction peak in Fig. 1b. Such effect hasalso been earlier reported in smaller ions substituted BFO samples[11,12].

Fig. 2a–c shows the surface topographies of the films acquiredusing standard AFM technique. The films present granularmicrostructure with root-mean-square (rms) roughness being4.4 nm (BFO) and 3.9 nm (BFCO) and 4.8 nm (BPFCO). For the BFOand BFCO films, the average grain size is almost similar (�200 nm)whereas for the co-substituted BPFCO film, it is about 125 nm. InBPFCO film, clustering of grains can also be seen. The decrease inthe grain size in the co-substituted film can be understood in termsof the decrease in the oxygen vacancies which are mobile anddiffuse easily. With Pr at Bi-site in BFO, the bond strength withoxygen ion increases, since the bond dissociation energy of Pr–Obond (753 � 17 kJ mol�1) being higher than that of Bi–O bond(343 � 6 kJ mol�1), it firmly holds oxygen [4,24]. This also affirms thatBi-site substitution is more effective in oxygen vacancies suppressionthan Fe-site substituted BFO. Fig. 2d–f compares out-of-plane PFMimages of the co-substituted films with that of pure BFO film. PFM

Fig. 2. (a–c) Surface topography of BFO, BFCO and BPFCO thin films; (d–f) out-of-plane PFM (OPP-PFM) image of BFO, BFCO and BPFCO thin films, (g) piezo-histogram of BFO,

BFCO and BPFCO thin films.

N. Panwar et al. / Materials Research Bulletin 47 (2012) 4240–42454242

measurements reveal a clear piezoelectric contrast associated withthe direction of the polarization, with bright and dark contrastscorresponding to domains having polarization components directednormal to the free surface of the films and to their bulk, i.e. along[0 0 1] and ½0 0 1� directions, respectively. Moreover, comparison ofthe PFM images with topographic features reveals that most of thedomains are limited by the grain boundary. From the PFM images, itcan also be noticed that pure BFO film consists of domains withpolarization directions either up (white contrast) or down (darkcontrast) whereas BFCO film comprises majority of dark domains andonly a small portion of domains exhibits polarization orientedupward. In case of BPFCO film, very few domains are there withpolarization directions up or down, predicting this film has a largerpiezoelectricity property corresponding to the 718 and 1098ferroelastic domain types [11,12]. From this interpretation, weascertain that pristine BFO film consists of mostly antiparalleldomains and hence it should be the most conducting film (1808domain walls are the most conducting ones). It is generally acceptedthat the characteristics of the domains and domain walls directlyimpact the ferroelectric switching behavior [25,26]. We will discussthis aspect later. In order to acquire more information from the PFMimages, we carried out the statistical analysis of ferroelectric domaindistributions of the films (shown in Fig. 2g). These curves representstandard histograms of the piezoresponse signal, i.e. the number ofpixels with a given piezoresponse value. The distribution maximumcorresponds to the most probable domain configurations and its

width reflects the number of available domain states [7]. It isapparent that in BFO and BPFCO films, domain distribution is almostsymmetric about zero piezoresponse whereas in case of BFCO film,the distribution is shifted to negative values. The asymmetry of thedistribution, as observed in BFCO film, is a manifestation of the localself-polarization effect. This indicates that the alignment of domainsoccurs locally and these domains are oriented towards the bottomelectrode. Such effect has already been discussed in the literature to agreat extent [27–33]. Whatever be the origin of this self-polarizationeffect in BFCO film, one thing is clear in present case that it occurs ondoping at Fe-site and disappears with co-doping at Bi-site. Thisimplies that co-doping at different sites has produced opposite effectas far as self-polarization is concerned. Further, to facilitate theseresults, square patterns were written on these films with bigger�10 mm � 10 mm area at �30 V and the central �5 mm � 5 mm areawith +30 V dc voltage. The images are shown in Fig. 3a–c. The imagesclearly show the oppositely written regions establishing that bothBFCO and BFO films support ferroelectricity while BPFCO does not. Itshould also be noted that in the case of pure BFO film, an almostuniform contrast is obtained, whereas the BFCO film clearly showssome dark regions inside the bright square, which might be attributedto the presence of some piezoelectric inactive regions inside the film.The poor contrast in case of BPFCO film may be attributed to theadditional deterioration of ferro-/piezoelectricity in BPFCO film. Thisobservation is further corroborated by the local piezoelectrichysteresis loop acquired on the films (Fig. 3d). Well saturated loops

Fig. 3. (a–c) Square patterns of different areas written on the surface of BFO, BFCO and BPFCO thin films with �30 V dc voltage, (d) local piezo hysteresis loop acquisition on thin

films.

604530150-15-30-45-60

10-9

10-8

10-7

10-6

10-5

10-4

10-3

10-2

J (

A/c

m2 )

E(kV/cm)

BiFeO BiFe Co O Bi Pr Fe Co O

Fig. 4. Leakage current density measurement of BFO, BFCO and BPFCO thin films.

N. Panwar et al. / Materials Research Bulletin 47 (2012) 4240–4245 4243

are observed for BFO and BFCO films, with vertical component of thepolarization amplitude fully reversible with bias voltage. A clearenhancement in the piezosignal can be noticed in case of BFCO film.The piezoelectric coefficient (d33) values for BFO, BFCO and BPFCOfilms are 17.4, 25.1 and 1 pm/V, respectively. It is worth mentioninghere that d33 value is equal to the product of PFM instrumentsensitivity (in this case �193 nm/V) and the lock-in amplitude atsaturation divided by a factor of 10 (since the deflection output wasamplified ten times compared to the internal deflection signal). Theenhancement in d33 values for BFCO film can be attributed to themechanism of polarization rotation in favorable direction with cobaltdoping [34]. These results are consistent with previous reports of d33

measurement of BFCO film [35,36], however, smaller values in thepresent case may be attributed to the difference in preparationmethods and the mismatch between the film and the substrate.Furthermore, the reduction in piezoelectric activity of BPFCO filmmay be explained in terms of origin of ferroelectricity in BFO. It isknown that the off-center ferroelectric distortion in BFO is mainly dueto the stereochemical activity of 6s2 lone electron pairs of Bi whichhybridize with an empty p orbital of oxygen ions [25]. Thesubstitution of Pr3+ at Bi3+ site might cause electron redistribution,owing to strong electronegativity of Pr3+ ions (1.13 on Pauling scale),which might be detrimental to ferroelectricity/piezoelectricity andresults in reduction of remnant polarization (Pr) in the BPFCO film.The other reason for the destruction of polarization in case of co-doped BPFCO film may be the presence of more 71 and 1098ferroelastic domain types, as seen in the PFM images. These resultsindicate that co-doping is always not helpful in enhancing themultiferroic properties of BFO sample as reported earlier andtherefore the selection of the co-dopants should be an importantcriterion for designing a multiferroic material with optimal proper-ties.

To investigate the leakage current behavior of the films,current density was measured for the films and Fig. 4 presents

the obtained results. The doped films exhibit lower leakagecurrent as compared to pure BFO film. It is well known that, inthe BFO, Fe ions often exist in a mixed-valence state of Fe2+/3+

due to the presence of oxygen vacancies. As a consequence ofthis, there may be electron transfer between these ions and theensuing higher conductivity of the sample. The other reason forhigher current density of pure BFO film could be the presence oflarge number of 1808 domain walls which are conducting innature. With Co3+ ion substitution at Fe-site in BFO, the Fe2+ ionsconcentration will reduce, resulting in the decrease of the filmconductivity. Further, Pr3+ co-substitution at Bi-site will lead tothe suppression in Bi-volatilization and the oxygen vacanciesdue to the effect discussed earlier in the paper. As aconsequence, the co-substituted film exhibits the lowest leakagecurrent density.

20151050-5-10-15-20-12

-9

-6

-3

0

3

6

9

12M

agn

etiz

atio

n (

emu

/cm

3 )

Field (kOe)

BiFeO3

BiFe0.95

Co 0.05O

3

Bi0.95

Pr 0.5Fe 0.95

Co 0.05O

3

Fig. 5. Magnetization versus magnetic field measurement of BFO, BFCO and BPFCO

thin films.

N. Panwar et al. / Materials Research Bulletin 47 (2012) 4240–42454244

Fig. 5 shows the magnetization versus magnetic field curvesof BFO, BFCO and BPFCO thin films in an in-plane magnetic fieldmeasured at room temperature. As observed, pure BFO filmshows a weak ferromagnetism at room temperature whereas inBFCO film, the magnetization is greatly enhanced with Co-substitution. The saturated magnetic field, as observed from thehysteresis loops, is around 5 kOe for all the films. Pure BFO thinfilm exhibits weak ferromagnetic moment with small saturationmagnetization (Ms � 1.05 emu/cm3) because net magnetizationis cancelled by the spiral spin structure [37,38]. However, the Ms

is enhanced in the BFCO (Ms � 8.34 emu/cm3) and BPFCO(�7.86 emu/cm3) films. The enhancement of magnetization indoped BFO-based materials is usually attributed to either thedestroying of the spiral spin-modulated incommensuratestructure, or the increasing of the spin canting angle resultingin the net macroscopic magnetization [39,40]. A negligiblechange in Ms value is observed for the BPFCO film with respect toBFCO which indicates that 5 at.% Pr substitution at the A-site didnot affect the magnetization much. From this we ascertain theabsence of any magnetic interaction between Pr3+ and Fe3+ ionsin the present case. This is contrary to the recent report on Eu3+

and Co3+ co-doped BFO nanoparticles [41]. Eu3+ becomesmagnetically active (due to the special distribution of 4felectrons) and interacts with Fe3+ ions ferromagnetically. Therole of Eu3+ in enhancing magnetic behavior of BFO has also beenconfirmed by theoretical analysis using first-principles calcula-tions [42]. The superposition of two interactions between Eu3+/Fe3+ and Co3+/Fe3+ ions result in larger enhancement in magneticbehavior (almost 20 times as compared to pure BFO) of Eu and Coco-doped BFO than in Pr and Co co-doped BFO film (eight timesbetter than that for pristine BFO film). In the present case, theenhanced magnetic properties in doped films could be attributedonly to the strong coupling between Co3+ and Fe3+ ions throughd6 Co3+ and d4 Fe3+ (4 mB per Co3+ and 5 mB per Fe3+). This willresult in a net magnetization and suppression or breaking of thespiral spin structure.

4. Conclusions

Bi1�xPrxFe1�yCoyO3 polycrystalline thin films were prepared bychemical solution deposition method. Local piezoelectric activityof the films was investigated by Piezoresponse Force Microscopy(PFM) technique. A decrease in the grain size was observed in theCo and Pr co-doped BFO film presumably due to the suppression of

oxygen vacancies. Out-of-plane PFM (OPP-PFM) images revealed aclear piezoelectric contrast associated with the direction of thepolarization. While pristine BFO film possesses maximum numberof antiparallel domains, their density decreases in the doped filmsand the BPFCO film exhibited the presence of 718 and 1098ferroelastic domains. Further, BFCO film exhibited negative self-polarization effect. Well saturated local piezo-hysteresis loopswere monitored and an increase was noticed in the d33 values withCo-doping whereas with Pr co-doping in BFCO film, the piezoelec-tric behavior was almost suppressed. Pr and cobalt co-substitutedfilm exhibited the lowest leakage current density and corroboratedthe PFM results. Magnetic behavior (M–H curves) exhibited anenhancement in the saturation magnetization values both in Co-and Co–Pr substituted films. Saturated magnetic hysteresis loopswere observed in the thin films and the saturation magnetizationof BFO films was enhanced by Co doping and also with Pr–Co co-doping. The saturation magnetization (Ms) increased from1.05 emu/cm3 to �7.86 emu/cm3 and �8.34 emu/cm3 in BPFCOand BFCO, respectively, i.e. nearly eight times greater than that inpure BFO thin film.

Acknowledgements

The authors N.P. and I.C. would like to thank PortugueseFoundation for Science and Technology (FCT) for their Postdoctoralgrants through SFRH/BPD/71289/2010 and SFRH/BPD/81032/2011respectively. The authors from the University of Puerto Rico wouldlike to acknowledge DoD project grant W911NF-11-1-0204, USA.

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