Solid State Communications 152 (2012) 525–529

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Solid State Communications

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Multiferroic, magnetoelectric and optical properties of Mn dopedBiFeO3 nanoparticlesSunil Chauhan a, Manoj Kumar a,∗, Sandeep Chhoker a, S.C. Katyal a, Hemant Singh b, Mukesh Jewariya c,K.L. Yadav b

a Department of Physics & Material Science & Engineering, Jaypee Institute of Information Technology, Noida-201307, Indiab Department of Physics, Indian Institute of Technology, Roorkee-247667, Indiac Department of Mechanical Engineering and Biomedical Sciences, Graduate School of Engineering Science, Osaka University, Osaka-560-8531, Japan

a r t i c l e i n f o

Article history:Received 24 October 2011Received in revised form20 December 2011Accepted 22 December 2011by J.F. SadocAvailable online 30 December 2011

Keywords:A. Bismuth ferriteC. Structural characterizationD. Magnetic propertiesD. Optical properties

a b s t r a c t

Mn doped BiFeO3 (5, 10 and 15 mol%) nanoparticles were synthesized using sol–gel technique. Theinfluence of Mn doping on structural, dielectric, magnetic, magnetoelectric and optical properties ofBiFeO3 was studied. Rietveld refinement of XRD patterns showed rhombohedral to orthorhombic phasetransition for 15 mol% Mn doped BiFeO3 sample. Magnetic measurements revealed the enhancementof ferromagnetic property with increasing Mn doping in BiFeO3. The characteristic dielectric anomaly,expected in the vicinity of antiferromagnetic transition temperature TN (Neel temperature) was foundin all Mn doped BiFeO3 samples. The magnetoelectric coupling was evidenced by the change incapacitance with the change in the applied magnetic field. On increasing Mn concentration from 5 to15 mol% in BiFeO3, a change in magnetocapacitance from 1.46% to 2.6% showed the improvement ofmultiferroic properties. In order to explore the optical properties of Mn doped BiFeO3 nanoparticles, theirphotoluminescent properties were also investigated.

© 2011 Elsevier Ltd. All rights reserved.

1. Introduction

In recent years, multiferroics, showing the coupling of at leasttwo of the possible three orders viz., ferromagnetic (or antiferro-magnetic), ferroelectric (or antiferroelectric) and ferroelastic, haveattracted a great deal of attention due to the fascinating funda-mental physics and potential applications in data storage me-dia, spintronics and multi-state memories [1–6]. The co-existenceof ferroelectricity and ferromagnetism and their coupling withelasticity provide an extra degree of freedom in the design of mul-tistage devices. As one of representative single phase multiferroic,BiFeO3 (BFO) has rhombohedrally distorted perovskite structureexhibiting simultaneously ferroelectric (Tc ∼ 830 °C) and G-type antiferromagnetic (TN ∼ 370 °C) nature. However, both itsspontaneous polarization and saturation magnetization are disap-pointingly low as compared to many standard ferroelectrics andferromagnets. This is due to the superimposition of a spiral spinstructure on BFO antiferromagnetic order. In this spiral spin struc-ture, the antiferromagnetic axis rotate through the crystal withan incommensurate long wavelength period of 62 nm, which can-cels the macroscopic magnetization and also inhibits the observa-tion of the linear magnetoelectric effect in bulk BiFeO3. Hence, for

∗ Corresponding author. Tel.: +91 01202594386; fax: +91 0120 2400986.E-mail address:manoj.chauhan@jiit.ac.in (M. Kumar).

0038-1098/$ – see front matter© 2011 Elsevier Ltd. All rights reserved.doi:10.1016/j.ssc.2011.12.037

novel electronic applications of BFO, its magnetic and electronicproperties must be enhanced. It is widely expected that the strate-gies based on nanotechnology and aliovalent ion doping will havea great impact on their magnetic, electrical and magnetoelectricproperties as a result of the suppression of the helical order andquantum confinement effect [7,8].

In addition to the potential magnetoelectric applications,BiFeO3 might find application as photocatalytic material due toits small band gap [9]. It allows carrier excitation in BiFeO3 withcommercially available femtosecond laser pulses, hence enablesthe researchers to develop ferroelectric ultrafast optoelectronicdevices already been demonstrated in semiconductors [10].However, there are very few reports in the literature on opticalproperties of BiFeO3. In the present communication, we reportthe influence of Mn doping on structural, magnetic, dielectric,magnetoelectric and optical properties of BFO nanoparticles.

2. Experimental details

Sol–gel technique was used to synthesize 5, 10 and 15 mol%Mn doped BiFeO3 (BM-5, BM-10 and BM-15, respectively)nanoparticles [11,12]. The dried powders were calcined at 600 °Cfor 2 h to obtain doped BFO nanoparticles. Subsequently, thesematerials were structurally characterized by XRD (Brucker D-8 Advanced). Field effect scanning electron microscopy (FESEM)

526 S. Chauhan et al. / Solid State Communications 152 (2012) 525–529

Fig. 1. XRD pattern of 5, 10 and 15 mol% Mn doped BFO nanoparticles calcined at600 °C.

Fig. 2. Rietveld refined XRD patterns of 5, 10 and 15 mol% Mn doped BFOnanoparticles.

was used to determine grain size and surface uniformity ofthe sample matrix. The dielectric measurements were carriedout on silver coated pellets using HIOKI 3522-50 LCR Hi-Tester. Magnetoelectric coupling measurements were carriedout by measuring capacitance in the presence of magneticfield. Photoluminescence (PL) spectroscopy (Perkin Elemer, LS-55)measurements were performed with the excitation wavelength of380 nm at room temperature and photoluminescence excitation(PLE) spectroscopy were monitored at 570 nm for BM-10 and BM-15 samples.

3. Results and discussion

Fig. 1 Shows the XRD patterns of 5, 10 and 15 mol% Mn dopedBFO samples calcined in air at 600 °C. XRD patterns confirmedsingle phase formation of 10 and 15 mol% Mn doped BiFeO3nanoparticles without any trace of impurity phase. However, fewimpurity peaks of Bi25FeO40 phase are observed in BM-5 sample.It is very difficult to obtain phase pure BFO nanoparticles dueto kinetics of formation. Reflections (104) and (110) are clearlyseparated in the BM-5 and BM-10 samples as shown in Fig. 1.On increasing Mn content from 5% to 15%, peak position of (104)reflection shifted toward higher angle, while peak position of(110) reflection remained almost same. On increasing the Mncontent from 10 to 15 mol%, doublet (104) and (110) merged toa single (110) peak and (113) and (006) peaks are disappeared

Fig. 3. FESEM image of 5, 10 and 15 mol% Mn doped BFO samples (from top tobottom) sintered at 600 °C.

in 15 mol% doped Mn doped BFO sample. These results suggestlattice distortion in rhombohedral structure of BFO occurringwith increasing Mn content which lead to rhombohedral toorthorhombic phase transition in 15 mol% Mn doped BFO sample.Similar type of behavior is also reported in La and Dy doped BFOwhich correspond to a lattice distortion or phase change fromrhombohedral to orthorhombic structure [13,14].

To determine the structural features of Mn doped BFO samples,Rietveld refinement of XRD patterns have been performed usingthe FullProf program. The observed, calculated and the differencerefined XRD patterns of all Mn doped BFO powders are shownin Fig. 2. The Rietveld refinement of BM-5 and BM-10 samplesare carried out by considering the rhombohedrally distortedperovskite structurewith R3c space groupwhile for BM-15 samplePbnm space group is considered. Two phase refinement is used tofind out the amount of BFO phase and impurity Bi25FeO40 phasein BM-5 sample. The increasing Mn substitution in BFO causes adecrease in lattice parameters of rhombohedral phase which inturn leads to decrease in unit cell volume from 372.893 Å3 for

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Table 1Structural parameters for Mn doped BFO at room temperature obtained after Rietveld analysis.

Samples Lattice parameters Atoms Positions x Y z Bonds Length Bonds Angle R-factors

BM-5 a = 5.575 (Å) Bi 6a 0.0 0.0 0.0 Bi–O 2.52 Fe–O–Fe 156.64 Rwp = 19.9Space group c = 13.8523 (Å) Fe/Mn 6a 0.0 0.0 0.2218 Fe–O 1.86 O–Fe–O 168.27 RF = 4.94R3c V = 372.893 (Å3

) O 18b 0.4355 −0.0026 0.9531 Fe–O 2.18 RB = 6.29BFO Bi25FeO40Phase = 93.7% Phase = 6.3%

BM-10 a = 5.5738 (Å) Bi 6a 0.0 0.0 0.0 Bi–O 2.458 Fe–O–Fe 155.73 Rwp = 25.7Space group c = 13.846 (Å) Fe/Mn 6a 0.0 0.0 0.2276 Fe–O 1.871 O–Fe–O 170.22 Rf = 4.87R3c V = 372.590 (Å3

) O 18b 0.4274 −0.0059 0.9612 Fe–O 2.178 RBragg = 6.29

BM-15 a = 5.5757 (Å) Bi 4c 0.9976 0.0232 0.25 Fe–O1–Fe 136.64 Rwp = 23.3Space group b = 5.6174 (Å) Fe/Mn 4a 0.50 0.0 0.0 Fe–O 2.010 Fe–O2–Fe 146.53 RF = 5.26Pbnm c = 7.8954 (Å) O1 4c 0.0571 0.629 0.25 Fe–O 2.124 O–Fe–O 180 RBragg = 6.41

V = 247.2914 (Å3) O2 8d 0.6971 0.3126 0.0360

BM-5 to 372.59 Å3 for BM-10 sample. This decrease in unit cellvolume on increasing Mn substitution is expected as ionic radii ofMn+2 is slightly less than that of Fe+3. In Table 1, we summarizedthe structural parameters, atomic positions, bond lengths, bondangles and reliability parameters. These results suggested that therhombohedral BFO structure undergoes distortion on doping ofMnin BFO samples, which involves Fe or Bi ion displacement relativeto the parent structure due to the ionic radii difference between Feand Mn ions.

Fig. 3 shows the FESEM images of BM-5 sample sintered at600 °C. It shows the dense and uniform distribution of particleshaving grain size in the range 50–100 nm. Themagnetic hysteresis(M–H) loops of Mn doped BFO nanoparticles, measured at roomtemperature, are shown in Fig. 4 which indicate soft ferromag-netic behavior in the samples. The magnetization increases withincreasing Mn concentration in BFO samples. Remnant magneti-zation (2Mr ) of 0.08, 0.25 and 0.51 emu/g are observed for 5, 10and 15 mol% Mn doped BFO samples, respectively. The coercivefield (2Hc) and saturation magnetization (Ms) are shown in Ta-ble 2. Compared to earlier reports on pure and doped BFO, Mndoped BFO samples exhibit large magnetization at room temper-ature [15,16]. The origin of spontaneous magnetization is due tonano range crystallite size. The helical order in nanocrystals canbe well suppressed through decreasing the particle size. It has al-ready been theoretically calculated that, suppression of helical or-der, with a periodicity about 62 nm on canted antiferromagneticorder between successive (111) ferromagnetic planes, might haveresulted in higher magnetization [17]. However, the increase inmagnetization with increasing Mn content may be due to distor-tion created in the rhombohedral stricture. BFO with the rhombo-hedral distorted perovskite structure allows a weak ferromagneticordering due to canting of spins. With Mn doping the structure ofBFO is distorted, that is confirm from the XRD pattern (Figs. 1 and2) of Mn doped BFO samples. However, adding of Mn+2 ions to theBFO requires charge compensation, which may be achieved by theformation of Fe+4 or oxygen vacancies. If Fe+4 exists, the statisticaldistribution of Fe+3 and Fe+4 ions in octahedralmay also lead to thenetmagnetization and ferromagnetism [18]. The increasedmagne-tization in BM-15 samplemay be attributed to the structural phasetransition from rhombohedral to orthorhombic phase resulting indestruction of spiral spin structure which in turn leads to releaseof latent magnetization. The helical order in nanocrystals can bewell suppressed through decreasing the particle size which leadsto the higher magnetization in the BiFeO3 nanoparticles. As we in-creased the sintering temperature of BM-10 sample, the grain sizeincreased (not shown here) with sintering temperature. The effectof increase in grain size on magnetization is shown in the inset ofFig. 4.With increasing sintering temperature, increase in grain sizehappens because of which magnetization reduces.

Temperature dependent dielectric constant at different fre-quencies for Mn doped BFO samples are shown in Fig. 5. Dielec-tric constant shows anomaly in the vicinity of antiferromagnetic(AFM) Neel temperature TN depicting coupling between ferroelec-tric andmagnetic order parameters. This type of dielectric anomalyis predicted by the Landau–Devonshire theory of phase transitionin magnetoelectrically ordered systems as an influence of van-ishing magnetic order on the electric order [6]. The pronouncedanomaly in the dielectric constant (ϵ/) near themagnetic transitiontemperature for 15 mol% Mn doped BFO sample is shown in Fig. 6.In this case we observed that the temperature (TN ) correspondingto the peak in ϵ/(T ) showed considerable frequency dispersion. Itshifts from 265 °C at 10 kHz to 365 °C at 100 kHz with increas-ing the measuring frequency, with a concomitant decrease in thepeak value of the dielectric constant at TN , mimicking a ferroelec-tric relaxor behavior [19,20]. In order to demonstrate the couplingbetween electric andmagnetic polarization inMn doped BFO sam-ples, the variation of capacitance at 10 kHz with applied magneticfield is measured at room temperature (Inset of Fig. 6). The mag-netocapacitance is defined as

Magnetocapacitance =CP(H) − CP(0)

CP(0)× 100

where CP(0) is the capacitance in zero magnetic field, CP(H) isthe capacitance in the presence of magnetic field. We observethat with increasing magnetic field, the capacitance decreasesand the negative values of magneto capacitance depend upon thedoping concentration of Mn in BFO. At room temperature and∆H = ±9 kOe, the values of magnetocapacitance are foundto be ∼1.46%, ∼2.2% and ∼2.6% for BM-5, BM-10 and BM-15samples, respectively. In multiferroic, when a magnetic field isapplied, thematerial will be strained. Due to the coupling betweenmagnetic and ferroelectric domain, the strainwill induce stress andthen generate an electric field on the ferroelectric domain, thusmodifying the dielectric behavior of the samples.

The room temperature optical properties of BFO samples areunderstood with photoluminescence (PL) spectra. Fig. 7(a) showsthe PL spectra of Mn doped BFO samples at room temperatureexcited with the wavelength of 380 nm (3.26 eV). It is reportedthat PL spectra of pure BFO sample shows only one blue emissionaround 454 nm (2.73 eV) which could be assigned to self-activated(SA) center [21]. In Mn doped BFO samples, two emission bandsare observed in the PL spectra (Fig. 7(a)). The peak correspondingto 484 nm (2.56 eV) is due to blue emission band and dominantyellow–orange band is found at 570 nm (2.17 eV). The blue bandemission around 484 nm in Mn doped BFO samples could beassigned to self activated center. The shift in blue emission bandtoward higher wavelength in our Mn doped samples as compared

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Table 2Shows the magnetic parameters for Mn doped BFO samples.

Sample Coercive field 2Hc (Oe) Remnant magnetization 2Mr (emu/g) Spontaneous magnetizationMs (emu/g)

BM-5 1161 0.08 0.046BM-10 1172 0.25 0.16BM-15 1191 0.51 0.33

Fig. 4. Magnetization hysteresis (M–H) loops of Mn doped BFO samples. Inset showingM–H loops of BM-10 sample sintered at different temperatures.

Fig. 5. Dielectric constant vs temperature at different frequencies ofMn doped BFOsamples.

to reported value by Yu X et al. may be due to large crystallitesize of our samples which leads to decrease in band gap. The570 nm emission band was attributed to the 4T1( 4G) →

6A1(6S)

transition within the 3d shell of Mn+2 ions [22]. The differenceof few nm in the position of the yellow–orange emission bandswith increasingMn contentmay arise from slightly different defectconcentrations in the Mn doped BFO samples. With the increase inMn concentration in BFO, Mn+2 ions come closer and an exchangemechanism leads to the excited Mn+2 ion resonantly transferring

Fig. 6. Dielectric constant vs temperature at different frequencies for BM-15sample. Inset showingmagnetocapacitance vsmagnetic fieldmeasurement at roomtemperature for Mn doped BFO samples.

its energy to its neighbors, until a red center is encountered. Thesimultaneous appearance of both the 484 nm blue and 570 nmyellow–orange bands in the PL spectra of Mn doped BFO samplesproves that the Mn+2 ions were indeed incorporated with in theBFO lattice.

In order to understand the nature of the emission bands in Mndoped BFO samples, we studied PLE spectra at room temperature.The PLE spectra recorded at the 570 nm band at room temperaturefor 10 and 15 mol% Mn doped BFO samples are shown in Fig. 7(b).This PLE spectra contains 5 absorption bands peaked at 391 nm(3.17 eV), 434 nm (2.86 eV), 474 nm (2.62 eV), 504 nm (2.46 eV)

S. Chauhan et al. / Solid State Communications 152 (2012) 525–529 529

a

b

Fig. 7. PL spectrum of Mn doped BFO samples at room temperature (a) and PLEspectrum of BM-10 and BM-15 samples at room temperature (b).

and 522 nm (2.37 eV). These five absorption bands at 391, 434,474, 500 and 522 nm are attributed to the 6A1(

6S) →4E2(4D);

6A1(6S) →

4T2(4D); 6A1(6S) →

4E(4G); 6A1(6S) →

4T2(4G)and 6A1(

6S) →4T1(4G) transitions within Mn ion, respectively.

Additional peak at 415 nm (2.99 eV) is observedwhich is attributedto the oxygen vacancy defects [23]. The energy positions of the fivebands are in good agreementwith the energies of the excited statesof the Mn doped ZnS nanocrystals [24,25]. These luminescentemissions will open up many opportunities of Mn doped BFOnanoparticles for both further fundamental studies and nanoscaleoptical applications.

4. Conclusion

We have reported the synthesis, structural, multiferroic, mag-netoelectric and photoluminescence optical characterizations ofMn doped BiFeO3 nanoparticles. Sol–gel technique is used to syn-thesized Mn doped BFO samples. We observed a structural phasetransition in BM-15 sample due to the distortion in the rhombo-hedral structure with increasing Mn substitution. The grain size ofMn doped BFO samples are found to be in the range 50–200 nm.Magnetic measurements showed the enhancement in remnantmagnetization (2Mr ) from 0.08 emu/g for BM-5 to 0.51 emu/g forBM-15 with increasing Mn concentration in BFO samples due tothe destruction in the spiral spin structure. Magnetoelectric cou-

pling was evidenced by dielectric anomaly near Neel temperaturein all Mn doped BFO samples. However, considerable frequencydispersion near Neel temperature in BM-15 sample showed theferroelectric relaxor behavior. Magnetoelectric coupling increasesfrom1.46% for BM-5 to 2.6% for BM-15 sample indicating improvedmultiferroic properties of Mn doped BFO samples. The PL spectraof Mn doped BFO nanoparticles exhibited both the blue defect re-lated emission and the yellow–orange Mn+2 ion related emission.In addition to enhanced multiferroic and magnetoelectric proper-ties, interesting optical properties were observed inMn doped BFOnanoparticles which leads to potential applications of this materi-als as optoelectronic devices.

Acknowledgments

This work was financially supported by Department of Scienceand Technology (DST) India. Sunil Chauhan is thankful to JIIT Noidafor providing financial support.

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