Magnetoelectric coupling effect in transition metal modifiedpolycrystalline BiFeO3 thin films

Venkata Sreenivas Puli a,b,n, Dhiren Kumar Pradhan b, Sreenivasulu Gollapudi c,Indrani Coondoo d, Neeraj Panwar e, Shiva Adireddy a, Douglas B. Chrisey a, Ram S. Katiyar b

a Department of Physics and Engineering Physics, Tulane University, New Orleans, LA 70118, USAb Department of Physics and Institute of Functional Nanomaterials, University of Puerto Rico, San Juan, PR 00936, USAc Department of Physics, Oakland University, Rochester, MI 48309-4401, USAd Department of Materials and Ceramic & CICECO, University of Aveiro, 3810-193 Aveiro, Portugale Department of Physics, Central University of Rajasthan, Bandar Sindri, Kishangarh 305801, Rajasthan, India

a r t i c l e i n f o

Article history:Received 22 January 2014Received in revised form11 May 2014Available online 12 June 2014

Keywords:MultiferroicMagnetoelectric couplingDielectric propertiesPLDTransition metalBiFeO3

a b s t r a c t

Rare-earth (Sm) and transition metal (Co) modified polycrystalline BiFeO3 (BFO) thin films have beendeposited on Pt/TiO2/SiO2/Si substrate successfully through pulsed laser deposition (PLD) technique.Piezoelectric, leakage current and temperature dependent dielectric and magnetic behaviour wereinvestigated for the films. Typical “butterfly-shaped” loop were observed in BSFCO films with aneffective piezoelectric constant (d33) �94 pm/V at 0.6 MV/cm. High dielectric constant �900 and lowdielectric loss �0.25 were observed at room temperature. M–H loops have shown relatively highsaturation magnetization �35 emu/cm3 at a maximum field of H �20 kOe. Enhanced magnetoelectriccoupling response is observed under applied magnetic field. The multiferroic, piezoelectric, leakagecurrent behaviours were explored. Such studies should be helpful in designing multiferroic materialsbased on BSFCO films.

& 2014 Elsevier B.V. All rights reserved.

1. Introduction

Multiferroic magnetoelectric materials with the coexistence ofat least two ferroic orders (ferroelectric, ferromagnetic and ferro-elastic, ferrotoroidic) and exhibiting coupling between them havedrawn ever-increasing attention in the last decade due to theirpotential for applications in multifunctional devices. Such materi-als are rare in nature since they require simultaneously showferroelectric (empty d-orbitals) and ferromagnetic (filled transi-tion metal d-orbitals) properties which are mutually exclusive [1].BiFeO3 (BFO) is a room temperature, natural single phase perovs-kite multiferroic material, exhibiting ferroelectricity and antiferro-magnetism with, ferroelectric Curie temperature (TC) �1100 K,and Neel temperature (TN) �643 K [1]. It has a rhombohedraldistorted ABO3 type perovskite structure with R3c symmetry atroom temperature. With such specific features that both TC and TNin BFO are above room temperature and can be tailored orconverted to ferromagnetic state with proper substitution, BFOfinds great technological applications e.g. four-state memorydevices [1]. Nevertheless, its spontaneous polarization and

saturation magnetization are comparatively lower than manystandard ferroelectrics and ferromagnets. Various researchershave worked to investigate the effects of doping/substitution onthe physical properties of BiFeO3. For example, Bernardo et al. [2]synthesized Ti-doped BiFeO3 ceramics by a mixed-oxide route andobserved an enhancement in the dc resistivity and magneticbehaviour with Ti-substitution. Pradhan et al. prepared polycrys-talline samples of Bi0.9La0.1Fe1�xMnxO3 by a conventional solid-state reaction technique [3].

The dielectric constant, loss tangent and magnetization valuesincrease with increasing of Mn concentration. Panwar et al.reported thin films of Bi1�xPrxFe1�yCoyO3 via chemical solutiondeposition method, wherein the co-substituted film exhibited thelowest leakage current density and enhanced magnetic (M–Hcurves) behaviour [1]. This is attributed to the superimpositionof a spiral spin structure on BFO’s antiferromagnetic order. More-over, its ferroelectric and transport properties are degraded byhigh leakage currents making it difficult to pole the sample athigher electric fields.

In BFO, ferroelectric and transport properties are also hinderedby leakage problems, which arise as a result of defects, nonstoi-chiometry, and low resistivity (high leakage current) [4]. Reduc-tion in leakage current and hence the ensuing increase in theelectrical resistivity and the ferroelectric behaviour can beachieved by proper substitution at either Bi/Fe-sites individually

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Journal of Magnetism and Magnetic Materials

http://dx.doi.org/10.1016/j.jmmm.2014.05.0500304-8853/& 2014 Elsevier B.V. All rights reserved.

n Corresponding author at: Department of Physics and Engineering Physics,Tulane University, New Orleans, LA 70118, USA. Tel.: þ1 239 537 4964;fax: þ1 504 862 8702.

E-mail addresses: pvsri123@gmail.com, vpuli@tulane.edu (V. Sreenivas Puli).

Journal of Magnetism and Magnetic Materials 369 (2014) 9–13

or at both the sites simultaneously that results in elimination ofimpurities phases and oxygen vacancies.

Improved multiferroic properties can be achieved to someextent by introducing suitable dopant ions in this material. A orB site substitution in BFO can lead to reduction in leakage currentand increase in resistivity by eliminating secondary impurities andoxygen vacancies, thereby improving its ferroelectric properties.

Yang et al. synthesized Bi1�xLaxFe1�yCoyO3 polycrystalline thinfilms on Pt/TiO2/SiO2/Si substrate via chemical solution depositionmethod [4]. The dielectric constant of the co-doped film was nearlytwo times higher than that of pristine BFO film while the dielectricloss was smaller than �0.07. They also reported the enhancedsaturation magnetization in the co-doped film. Zhu et al. grewMn-doped BiFeO3 (BFMO) thin films on SrTiO3 (0 0 1) substratewith a bottom electrode of SrRuO3 [5]. Hu et al. [6] fabricated Ndand Mo co-doped BNFM thin films on platinised silicon substrate bypulsed laser deposition (PLD) technique. The ferroelectric andferromagnetic properties were significantly enhanced by Nd andMo co-doping of the BFO film. Enhanced ferroelectric, piezoelectricproperties were reported for W6þ substituted at Fe3þ in BiFe1�x

WxO3 films [7]. Enhanced dielectric, ferroelectric and anti-fatigueproperties were observed by Yu et al., in La3þ and V5þ

co-substitued Ba0.85La0.15Fe1�xVxO3 (BLFV) ceramics [8]. Leakagecurrent density of BLFV ceramics was several orders of magnitudelower than individual substitution, La3þ at Bi3þ site or V5þ at Fe3þ

site in BFO lattice [8]. Low leakage current behavior was alsoobserved in La3þ and Ni2þ co-substituted BFO thin films [9].As well enhanced physical properties was also reported by Chenget al. in La3þ and Nb5þ co-substituted BFO films [10].

Low dielectric dissipation factor, leakage current density andimproved ferromagnetism with fatigue-free behaviour wasobserved in Nd3þ/Mo6þ co-substituted BFO samples [11], andCe3þ/Zr4þ co-substituted BFO samples [12]. Improved magneticbehavior with exchange bias effect was observed in La3þ , Zr4þ

co-substituted BFO films [13]. Suppressed leakage current densitybehaviour was observed in (Pr3þ , Mn4þ) co-substituted BFO thinfilms in the high electric field region [14]. Improved magnetizationand large piezoelectric response was observed in Co substitutedBFO films [15]. Enhanced piezoelectric coefficient (d33) wasobserved in (1�x)BiFeO3–xBiCoO3 solid solution [16]. Superiordielectric and ferroelectric properties were reported in Tb, Crco-substituted BFO chemical solution deposited films [17]. Besidesthe improvement in the electric and magnetic properties, therecan also occur structural transition with substation in BFOsamples. For example, Kan et al. reported a structural transitionfrom the rhombohedral to an orthorhombic phase accompaniedby a double hysteresis polarization loop in (Bi,Sm)(Fe,Sc)O3

co-doped thin films [18]. Similar behaviour was also observed forco-substituted Bi0.9Sm0.1Fe1�xMnxO3 system by Stefan et al. [19].Zhai et al. also reported structural transition in La3þ at Bi3þ site orNb5þ at Fe3þ site in BFO lattice [20].

In this work, Sm and Co co-substituted polycrystallineBi0.9Sm0.1Fe0.95Co0.05O3 (BSFCO) thin films have been grown byPLD technique and studied for their electrical, dielectric andmagnetoelectric properties. Other studies including x-ray diffrac-tion (XRD), atomic force microscopy (AFM), photovoltaic responseand piezoresponse force microscopy (PFM) on the same films haverecently been reported elsewhere [21].

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Fig. 1. (a) Scanning electron microscopy (SEM) (b) electron diffraction spectroscopy (EDAX) patterns and (c) elemental mapping of Bi0.9Sm0.1Fe0.95Co0.05O3 (BSFCO) thin films.

V. Sreenivas Puli et al. / Journal of Magnetism and Magnetic Materials 369 (2014) 9–1310

2. Experimental procedure

Ceramic target of Bi0.9Sm0.1Fe0.95Co0.05O3 (BSFCO) were synthe-sized by a sol–gel synthesis as described elsewhere [22]. Polycrys-talline multiferroic BSFCO thin films of thickness from 300 to360 nm were grown from these targets by pulsed laser deposition(PLD) on Pt/TiO2/SiO2/Si substrate in a temperature range of 700–750 1C, under an oxygen partial pressure of 100 mTorr, annealed for30 min and were subsequently cooled to room temperature in300 Torr oxygen partial pressure. An excimer laser (KrF, λ¼248 nm)with a laser energy density of 2–5 J/cm2, pulse repetition rate of10 Hz with a substrate target distance of 45 cm, was used for filmgrowth. Test structures were fabricated by sputtering Pt to createtop electrodes with 200 mm diameters at room temperature.Dielectric properties, ferroelectric hysteresis (P–V) loops wereobtained at 4 kHz and leakage current measurements were doneunder vacuum (10�4 Torr) with HP4294A impedance analyzer andRadiant Precision Multiferroic materials Analyzer respectively. Thetemperature control was achieved using a programmable tempera-ture controller (MMR Technologies, Inc.). The d33 measurement wascarried out with MTI-2100 Fotonic Sensor connected to radiantferroelectric tester. In order to characterize the magnetic propertiesof the films, a vibrating sample magnetometer (VSM, Lakeshore,Model 7300 series) was used.

3. Results and discussion

3.1. Surface morphology

The surface morphology of BSFCO thin film system wasobserved by SEM is displayed in Fig. 1(a). The SEM micrographs

of film have shown uniform distribution of grains throughout thesurface of the sample. The grains and grain boundaries are welldefined in the sample. Dense homogeneous microstructures withminimum number of voids were observed in SEM micrographs.Dense microstructure in the BSFCO system is attributed suitableannealing temperature of the PLD deposited films. The averagegrain size of BSFCO films is about 50 nm as shown in the figure.Grain growth in these films might be due to higher annealingtemperature �700 1C. Energy dispersive spectroscopy (EDX) mea-surement and atomic wt% of each element present in the filmswere tabled (Fig. 1(b)) and elementary mapping images of Bi, Fe,Sm, Co, and O, respectively, captured by SEM-EDX (Fig. 1(c)) showsall the elemental peaks of BSFCO along with peaks correspondingto Pt/TiO2/SiO2/Si substrate. From the SEM micrograph (Fig. 1(a)),BSFCO films did not show any obvious agglomeration.

3.2. Multiferroic and leakage current properties

The electric polarization vs. applied electric field (P–E loops) forBSFCO film is shown in Fig. 2(a). The BSFCO film has unsaturatedferroelectric characteristics: the remnant polarization (Pr), satura-tion polarization (Ps), and coercive field (Ec) at a maximumapplied field of �0.8 MV/cm were 16.06 mC/cm2, 22.91 mC/cm2,and 0.48 MV/cm, respectively. It should be noted that BSFCO filmshow leaky ferroelectric properties due to the comparatively highconductivity. In general pristine BFO films exhibit poor ferro-electric properties due to high leakage current behavior, howeverin contrast, Indrani et al. [15], reported that chemical solutiondeposited BiFe0.95Co0.05O3 films have shown low leakage currentwith improved polarization and is caused by Co3þ substitutionand suppression of Fe2þ ions at BiFeO3 lattice and they also

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Fig. 2. (a) Polarization-electric field hysteresis loops at 4 kHz frequency of Bi0.9Sm0.1Fe0.95Co0.05O3 (BSFCO) thin film; (b) leakage current density (J–E) behavior and(c) Electric field dependent piezoelectric coefficient (d33) of Bi0.9Sm0.1Fe0.95Co0.05O3 (BSFCO) thin films.

V. Sreenivas Puli et al. / Journal of Magnetism and Magnetic Materials 369 (2014) 9–13 11

concluded that cobalt substitution improved the ferroelectricproperties of BFO sample. In the present case also, it seems thatSm and Co co-substitution in BFO lowered the electrical conduc-tivity which in turn lowered leakage current behavior of the films.

To investigate the leakage current behavior of the films, currentdensity was measured for the BSFCO films and Fig. 2(b) displaysthe results in the form of J–E curves. From Fig. 2(b), it can benoticed that the leakage current density increases with increasingelectric field, the J–E curves have shown asymmetry undernegative electric fields. The values of leakage current density areon the order of �5.8�10�4 A/cm2 and 5.3�10�6 A/cm2 wererespectively under positive and negative electric field due toasymmetry in J–E curves. This sharp increase in the leakagecurrent density might be the reason for loss ferroelectric loopsand is the major constraint for measuring saturated polarizationhysteresis loops. The co-substituted films exhibit lower leakagecurrent as compared to pure BFO and Bi0.95Pr0.5Fe0.95Co0.05O3 films[1]. Therefore, it is clear that the co-substitution of Sm, Co forBiFeO3 can effectively reduce the leakage current density of theBSFCO films. The substitution of Sm for Bi can stabilize theperovskite structure firstly due to higher bond strength of Sm–Obond (619713 kJ/mol) than that of Bi–O bond (34376 kJ/mol)and will decrease the volatilization of Bi, lower the concentrationof oxygen vacancies and significantly decrease the leakage currentof density of the film. Second, Bi3þ replacement by Sm3þ willsuppress the Bi3þ volatilization, because of the higher meltingpoint of Sm2O3 (1077 1C) than Bi2O3 [23].

Fig. 2(c) shows the variation of the piezoelectric coefficients(d33) as a function of electric field obtained at room temperature(300 K). Symmetric nature of butterfly d33-electric field loopssuggests the piezoelectric nature of the BSFCO films. Typical“butterfly-shaped” loop of a sample with preset polarization canbe observed in BFO films: after an initial low value, the d33 thenmakes an increase until the electric field of around 0.6 MV/cm, andthen decreases again to a remanent value as the field is released;the same behavior is exhibited for negative polarity fields. Thisbehavior is commonly connected to switching and movement ofdomain walls by an applied electric field [30]. The measurementsyield an effective piezoelectric constant (d33) �94 pm/V.

In order to characterize the magnetic properties of the films, avibrating sample magnetometer (VSM, Lakeshore, and Model7407) was used. We measured the field dependent magnetizationsin a plane magnetic field at room temperature of the BSFCO film.The M–H hysteresis loop got slimmer at room temperature asshown in Fig. 3(a). The room temperature M–H loops confirms theantiferromagnetic order with weak ferromagnetic behavior inBSFCO films.

M–H loops show that the present films have relatively highsaturation magnetization �35 emu/cm3 at a maximum field ofH �20 kOe, the magnetization is greatly enhanced withco-substitution, when compared to Pure BFO (Ms �1.05 emu/cm3,H �5 kOe), BiFe0.95Co0.05O3 (Ms �8.34 emu/cm3, H �5 kOe),Bi0.95Pr0.5Fe0.95Co0.05O3 (Ms �7.86 emu/cm3, H �5 kOe) [1,15].The enhanced magnetization in BSFCO films is due to the sub-stitution of rare-earth element (Sm) and transition metal (Co) isfound to suppress the modulated spin structure of BiFeO3 [24–26].On other hand, the substitution of rare-earth (RE), alkaline-earthelements and transition elements is found to suppress the spa-tially modulated spin structure of BFO that gives rise to enhancedmagnetic properties in these doped BFO compounds.

3.3. Magnetoelectric voltage coupling coefficient (MEVC)

The coexistence of the ferromagnetic and ferroelectric phasesin the present BSFCO thin films gives rise to a ME effect, which ischaracterized by the magnetoelectric voltage coefficient (αE). For

ME characterization, an ac field δH¼1 Oe at 1 kHz and a biasmagnetic field H were applied parallel to the sample plane and thevoltage δV across the BSFCO film was measured. The ME voltagecoefficient αE¼δE/δH¼δV/tδH, where E is the electric field, H isthe magnetic field and t is the total thickness of BSFCO. First wemeasured the αE of BSFCO; consider the data in the figure for theBSFCO film that shows increase in αE with H to a maximum of500 mV/cm Oe, followed by a sharp decrease to �142 mV/cm Oe.The small alternating magnetic field H (10 Oe) was generated by asolenoid that was superimposed onto a magnetic bias Hbias up to6 kOe. The electric field E induced by H and Hbias was measuredusing a lock-in amplifier (SRS Inc., SR830). The variations of αE

with Hbias are shown in Fig. 3(b). At Hbias¼0, the film exhibits aninitial high αE value (e.g., 220 mV/cm Oe at f¼1 kHz), much largerthan that of the bulk ferroelectric–ferromagnetic composite whichis nearly close to zero. The zero-bias ME effect and the decrease inMEVC upon reversal of H direction indicate the possible presenceof a uniaxial “built-in” magnetic field in the film.

The reason that causes the ME effect of the present BSFCO thinfilm will now be discussed. In the magnetostrictive-ferroelectriccomposite system (e.g., BSFCO) ME coupling mainly arises fromthe magnetic–mechanical–electrical transform through the stress-mediated transfer in the interface [22]. Similar to present resultsenhanced magnetoelectric coupling results were also observed inSm and Co doping Bi0.95Sm0.05Fe0.95Co0.05O3, and is two orders ofmagnitude higher than that of pure BiFeO3 samples in magnitude,showing that Sm and Co doping lead to enhanced magnetizationin the samples [27].

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Fig. 3. (a) a magnetization-magnetic field hysteresis loops at room temperature ofBi0.9Sm0.1Fe0.95Co0.05O3 (BSFCO) thin film and (b) MEVC vs H for Bi0.9Sm0.1Fe0.95-Co0.05O3 (BSFCO) thin films.

V. Sreenivas Puli et al. / Journal of Magnetism and Magnetic Materials 369 (2014) 9–1312

3.4. Temperature dependent dielectric properties

The dielectric properties as a function of temperature for BFSCOfilm is shown in Fig. 4 It is evident that the dielectric constantdecreases with increasing frequency indicating the signature ofpolar dielectrics. As the temperature increase dielectric peakbroadening is observed around 350–450 K up to 1 kHz. The BSFCOsamples show decreasing trends of their dielectric constant anddielectric loss values on increasing frequency, at higher frequencyregion dielectric constant value remains almost remains constantabove 100 Hz frequency. The observed dielectric behavior has beenexplained on the basis of dipole relaxation phenomenon whereinat low frequencies the dipoles are able to follow the frequency ofthe applied field relaxation wherein at low frequencies the dipolesare able to follow the frequency of the applied field [28]. Howeverdielectric loss is moderately low and there is no consistency in thedielectric loss behavior at different temperature measured. Thehigh dielectric constant at lower frequency might be attributed tothe oxygen vacancies, piling of interfacial dislocations, grainboundary effect and the decrease in dielectric constant withincreasing frequency is due to the difference in the contributionsof different polarization mechanisms [29,30]. As shown in Fig. 4,low dielectric loss values at high frequencies might be due todiploes with small effective mass (electrons and ferroelectricdomains mainly) [31]. High dielectric loss in high temperatureregion is attributed to space charge polarization [32].

4. Conclusions

In conclusion we have studied the effect of co-substitution onBiFeO3 films on its multiferroic, piezoelectric, leakage current

behavior, magnetoelectric coupling and dielectric properties.M–H loops show that the present films have relatively high saturationmagnetization �35 emu/cm3 at a maximum field of H �20 kOe.Enhance mangetoelectric coupling (αE with H to a maximum of500 mV/cm Oe) behavior was witnessed in the BSFCO films. A notablepiezoelectric constant of approximately �94 pm/V was found; this isclose to values obtained for PZT films.

Acknowledgements

This work was supported by the DOD Grant no. W911NF-11-1-0204. Acknowledgment is also due to DOE Grant no. FG02-08ER46526 for providing financial support to the student DhirenK. Pradhan and to NSF-EFRI RESTOR no. 1038272 Grant forsupporting Dr. Venkata S. Puli.

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Fig. 4. Temperature dependent dielectric properties for Bi0.9Sm0.1Fe0.95Co0.05O3

(BSCFO) thin films.

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