Investigation of spin phonon coupling in BiFeO3 based system by Fourier transforminfrared spectroscopyV. M. Gaikwad and S. A. Acharya Citation: Journal of Applied Physics 114, 193901 (2013); doi: 10.1063/1.4831676 View online: http://dx.doi.org/10.1063/1.4831676 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/114/19?ver=pdfcov Published by the AIP Publishing

[This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

14.139.120.230 On: Sat, 16 Nov 2013 04:51:33

Investigation of spin phonon coupling in BiFeO3 based system by Fouriertransform infrared spectroscopy

V. M. Gaikwad and S. A. Acharyaa)

Department of Physics, Rashtrasant Tukadoji Maharaj Nagpur University, Nagpur, India

(Received 1 August 2013; accepted 1 November 2013; published online 15 November 2013)

In the present work, the low temperature infrared absorption spectra of BiFeO3 (BFO) are

measured to explore the spin-phonon coupling in this compound. At 303 K, 4 weak transverse optic

(TO) IR-active phonon modes E(TO6), E(TO7), E(TO8), and E(TO9) are observed. First two

modes are corresponded to the Fe3þ cations caused by the internal vibration of FeO6 octahedra,

E(TO8) is correlated to Fe-O bending vibration and E(TO9) is assigned to Fe-O stretching

vibrations, respectively. At 213 K, two new modes E(TO5) and A1(TO3) are emerging out. Both

are assigned to Fe3þ cations caused by the internal vibration of FeO6 octahedra. These modes get

stronger and stronger with lowering the temperature due to the lattice contraction. When the

temperatures decreases to T� 213 K, an additional phonon mode is start appearing at around

638 cm�1 suggesting local lattice distortion of FeO6 octahedra. The temperature is corresponding

with the FC and zero field cooled bifurcation temperature, which is related to the onset of spin

glass behaviour. The occurrence of this additional phonon mode at this particular temperature

suggests that there is strong spin-phonon coupling in BFO. This argument is further supported by

the temperature dependence of this additional phonon peak. It shows anomaly around 124 K, which

is related to spin reorientation of Fe3þ ions. This result clearly indicates that spin glass state and

spin reorientation of Fe3þ is accompanied with the local structure distortion of FeO6 octahedra,

providing evidence for the strong spin-phonon coupling in the BFO. VC 2013 AIP Publishing LLC.

[http://dx.doi.org/10.1063/1.4831676]

I. INTRODUCTION

Single phase BiFeO3 (BFO) has been attracted much

attention due to its multiferroic properties at room

temperature.1–4 BFO comes under the class of perovskite

type oxide having rhombohedrally distorted structure with

space group R3c.5–7 It displays ferroelectric and ferromag-

netic properties; interplay between these two orders would

give rise to potential application in spintronic devices.8,9

BFO in bulk form exhibits an antiferromagnet (G-type) and

ferroelectric ordering at room temperature with antiferro-

magnetic Neel temperature TN� 643 K, and ferroelectric

Curie temperature Tc� 1103 K. Microscopically, antiferro-

magnetic order of BFO is not homogeneous, it has incom-

mensurately modulated spin structure.10,11 Neutron

diffraction studies revealed a cycloid spin structure of BFO

with long period wavelength (k� 600 A�) due to the order-

ing of Fe3þ magnetic moments which remains same from

4 K up to Neel temperature, suggesting magnetic interaction

stable in this temperature range.11–13 By suppressing the spin

cycloid structure, the magnetic moment as well as ferroelec-

tric properties of BFO can be enhanced.

Another crucial behaviour of BFO is their exotic state

such as spin-glass, which arise due to geometry frustrated

magnetic structure.14–16 The coupling between spins and lat-

tice degrees of freedom is important in geometry frustrated

magnetic materials. Infrared spectroscopy is very sensitive

to probe the spin-phonon coupling.17 Strong spin-phonon

interactions are expected to manifest themselves as either

anomalous frequency shifts or occurrence of new phonon

peaks with magnetic transition in infrared spectra.17–22

In this work, we measured the infrared absorption spectra

of BFO to explore the spin-phonon coupling in this com-

pound. At 303 K, we observed 4 weak transverse optic (TO)

IR-active phonon modes E(TO6), E(TO7), E(TO8), and

E(TO9), out of which first two modes are corresponded to the

Fe3þ cations caused by the internal vibration of FeO6 octahe-

dra, E(TO8) is correlated to Fe-O bending vibration and

E(TO9) is assigned to Fe-O stretching vibrations, respec-

tively.23,24 At 213 K, two new phonon modes E(TO5) and

A1(TO3) are observed. They became stronger below 213 K.

Both are assigned to Fe3þ cations caused by the internal vibra-

tion of FeO6 octahedra. The IR modes above 200 cm�1 are re-

sponsible for distortion and vibration of FeO6 octahedra.25

When the temperature decreases to T� 213 K, an additional

phonon mode is start appearing at around 638 cm�1, suggest-

ing local lattice distortion of FeO6 octahedra. With further

decrease of the temperature, intensity of this phonon mode

enhances. At 213 K, zero field cooled-Field cooled (ZFC-FC)

bifurcation is emerged out which commonly correlated with

onset of spin glass behavior.26,27 This result indicates that the

formation of spin glass state is accompanied with the local

structure distortion of FeO6 octahedra, providing clear evi-

dence for the strong spin-phonon coupling in the BFO.

II. EXPERIMENTAL DETAILS

In present report, BFO based system was synthesized by

microwave-assisted gel combustion route. Precursors for

a)Author to whom correspondence should be addressed. Electronic mail:

saha275@yahoo.com. Tel.: 91-09372078410. Fax: 91-0712-2041093.

0021-8979/2013/114(19)/193901/5/$30.00 VC 2013 AIP Publishing LLC114, 193901-1

JOURNAL OF APPLIED PHYSICS 114, 193901 (2013)

[This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

14.139.120.230 On: Sat, 16 Nov 2013 04:51:33

synthesis were procured from Sigma-Aldrich, USA and used

as raw materials without further purification. Analytical

grade Bismuth nitrate [Bi(NO3)3.5H2O, 99.9% pure], iron ni-

trate [Fe(NO3)3.9H2O, 99.9% pure], and citric acid [C6H8O7,

99.9% pure] were used as initial reagents. Bismuth nitrate

[Bi(NO3)3.5H2O] and iron nitrate [Fe(NO3)3.9H2O] in stoi-

chiometric proportions (1:1 mol ratio) were dissolved in

diluted nitric acid (HNO3) solution with citric acid (C6H8O7)

at a molar ratio of 6:1 with Bi(NO3)3.5H2O. The pH value of

the reaction was adjusted by adding liquor ammonia. The pH

values are varied from 4 to 8 by fine tuning in concentration

of liquor ammonia; best results are obtained for 5.5. During

the sol-gel process, citric acid was hydrolyzed in the solution

to induce (C6O7H5)3� as a complexing agent, which can

complex with Bi3þ and Fe3þ cations. Ragatech microwave

synthesis assembly was used to synthesize BFO based sys-

tem. It consists of microwave processor of size 360 mm

� 210 mm� 430 mm dimensions with a 2.45 GHz frequency

multimode source having maximum deliverable power out-

put as 700 W. In the microwave chamber, magnetic stirring

and refluxing arrangement are assembled to process the syn-

thesis reaction in presence of microwave. We prepared the

samples at different conditions of microwave power and irra-

diation time. After adjusting pH, the solution was mounted

in microwave refluxing system at different power and for dif-

ferent irradiation time. At the microwave power level 210 W

and for irradiation time 10 min the sample is found to change

its color as black from transparent yellow; the state is

observed near to combustion. To combust the solution, the

power level of microwave was finely tuned and the solution

was found to be spontaneously combusted at 300 W micro-

wave power resulting into dark brown color powder. The

as-synthesized powder is calcinated at 300 �C and used for

further characterization.

The structure and phase purity of the sample was

checked by X-ray powder diffraction (XRD) using Bruker

AXS D8 Advance X-ray diffractometer equipped with

copper target (k1, CuKa1¼ 1.5405 A�). The data were col-

lected with a step size of 0.067� and step time of 10 s.

Magnetization measurements were performed with Vibrating

Sample Magnetometer (Model 14 T-VSM) at temperature

(2–350 K). Fourier Transform Infrared Spectroscopy (FT-IR)

measurements were carried out on Fourier Transform

Infrared spectrometer [Model: MAGNA 550 (USA),

range—4000 cm�1 to 50 cm�1].

III. RESULT AND DISCUSSION

The XRD pattern of BFO system is shown in Fig. 1.

Rietveld refinement of the XRD pattern confirms that the

sample is single phase rhombohedrally distorted perovskite

structure with space group R3c and goodness of fit was

found to be v� 2.57. The lattice parameters are determined

as: a¼ b¼ 5.57A� and c¼ 13.85 A� yielded satisfactory fits

by using Rietveld refinement which is good agreement with

standard JCPDS card No. 20-0619.

DC magnetization versus temperature curve for 2 kOe

in the FC and ZFC is shown in Fig. 2. The sample exhibits

an irreversible thermomagnetization process below 213 K;

there exists an obvious difference between the ZFC and FC

curve. The splitting and separation of ZFC and FC curves is

due to the spin-glass type behaviour. A broad anomaly

observed near 124 K is related to spin reorientation transition

of Fe3þ ions. However, for bulk BFO single crystal, the spin

reorientation has been reported near 50 K.18,28–32 This devia-

tion in BFO can be attributed to particle size effect, which is

responsible for induce strain, coordination distortion and lat-

tice disorder. Below 124 K, decrease in magnetization is

observed for ZFC curve; however for FC, there is no any no-

ticeable change in magnetization is detected with decrease of

temperature. The origin of spin reorientation transition

observed in the present work is due to the orientation of

Fe3þ spins which are generated by the breaking of antiferro-

magnetic spiral ordering.

For BFO system, magnetic ordering is quite complicated

because Dzyaloshinkii-Moriya interaction which results in a

canted AFM ordering of Fe3þ spins in the system. The AFM

ordering resulting in rotation of spins, the order parameter of

this helical ordering is 62 nm. The particles having size less

than 62 nm, breaking of the helical ordering of the spins

along AFM is observed. It suppresses the modulated spin

FIG. 1. Powder XRD patterns of BFO system with Rietveld fitting.

FIG. 2. Temperature dependence magnetization of BFO system at H¼ 2kOe

field.

193901-2 V. M. Gaikwad and S. A. Acharya J. Appl. Phys. 114, 193901 (2013)

[This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

14.139.120.230 On: Sat, 16 Nov 2013 04:51:33

structure and improves magnetization. The field dependence

magnetization isothermal curve of BFO at various tempera-

tures confirms ferromagnetic nature (Fig. 3 displays hystere-

sis curve at 300 K). Slim hysteresis loop of BFO based

system saturates at about 5 kOe magnetic field reaching a

saturation magnetization (Ms� 5.16 emu/g) and has coercive

field (Hc) of 216 Oe with remanence (Mr) 0.68 emu/g. It

clearly exhibits that cycloid spin structure of BFO almost get

completely destroyed at about 5 kOe magnetic field gives

rise to high saturation magnetization (Ms� 5.16 emu/g) with

the high alignment of ferromagnetic Fe3þ ions.6,21,33 This

behaviour attributes to nanosize effect of microwave-assisted

synthesized BFO sample.

Fig. 4 shows the temperature dependence of inverse of

susceptibility. It can be clearly seen that there is a region,

which follow the Curie-Weiss law. Curie-Weiss fitting for

the susceptibility yields a Curie-Weiss temperature at 90 K

by extrapolating the graph indicating the paramagnetic

region of graph. The degree of frustration (h/Tg) is obtained

�0.72. Curie constant (C) is calculated from the inverse of

slope of the graph. The curie constant is found

C¼ 6.15 emu K Oe�1 g�1. By using the value of suscepti-

bility, [v¼C/(T� h)], effective magnetic moment is calcu-

lated in Bohr magneton/ion (lB/ion) unit. The effective

magnetic moment is determined leff¼ 0.86 lB/Fe which is

much smaller than the spin only value for S¼ 5/2

(l¼ 5.93 lB/Fe).17,30,31 It concludes that in the present

study BFO system has shown ferromagnetic nature with

very less magnetic moment per Fe3þ ion.

To probe the spin-phonon coupling in this compound,

FTIR spectra of BFO based system were studied at various

temperatures (98 K to 313 K). The results are shown in

Fig. 5. There are only 4 weak Transverse Optic (TO) IR-

active absorption phonon modes observed at 303 K (was not

appeared for temperature> 303 K). The modes are detected

at 351, 391, 437, and 552 cm�1 assigned to symmetry

E(TO6), E(TO7), E(TO8), and E(TO9), respectively. First

two modes are corresponded to the Fe3þ cations caused by

the internal vibration of FeO6 octahedra, E(TO8) is corre-

lated to Fe-O bending vibration and E(TO9) is assigned to

Fe-O stretching vibrations, respectively.21,24 At 213 K, two

new absorption modes are emerging out at 307 cm�1 and

331 cm�1 corresponding to E(TO5) and A1(TO3), respec-

tively. They became stronger below 213 K. Both are

FIG. 3. M-H Hysteresis loop for BFO system at room temperature (300 K).

FIG. 4. Temperature dependence of inverse susceptibility. Blue line repre-

sents the Curie-Weiss fitting for susceptibility in linear region.

FIG. 5. The infrared absorption spectra

measured at various temperatures for

BFO system, plotted as normalized ab-

sorbance versus the wave no. The

arrow marks the additional phonon

mode start appearing from T� 213 K.

193901-3 V. M. Gaikwad and S. A. Acharya J. Appl. Phys. 114, 193901 (2013)

[This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

14.139.120.230 On: Sat, 16 Nov 2013 04:51:33

assigned to Fe3þ cations caused by the internal vibration of

FeO6. The modes above 200 cm�1 are responsible for distor-

tion and vibration of FeO6 octahedra.22 E(TO8) and E(TO9)

are also related to Bi-O groups vibrations corresponding to

BiO6 octahedral structural unit and found to be overlapped

with Fe-O.24,25 All these modes get stronger and stronger

with lowering the temperature due to the shrinkage in lattice.

Fig. 5 reveals an additional phonon mode, which started

appearing at around 638 cm�1 from 213 K onwards. It can be

clearly observed in magnified view in Fig. 6(a). This addi-

tional phonon mode of vibration can be assigned to new

stretching vibration mode of FeO6 octahedron. This tempera-

ture is corresponding with the FC and ZFC bifurcation tem-

perature in M versus T study (Fig. 2). Commonly, FC-ZFC

bifurcation temperature is related to the onset of spin glass

behaviour.17 The occurrence of this additional phonon mode

at this particular temperature suggests that there is strong

spin-phonon coupling in BFO. This argument is further sup-

ported by the temperature dependence of this additional pho-

non peak. It can be seen that the intensity of this phonon

mode enhances upon cooling [Fig. 6(b)], hinting that the

strength of the local lattice distortion get increased. Both

these results clearly demonstrate the local distortion in FeO6

octahedra. The anomaly around 124 K observed in tempera-

ture versus normalized peak intensity of FTIR curves

[Fig. 6(b)] is corresponding with anomaly related to spin

reorientation transition of Fe3þ ions in M-T curves (Fig. 2).

It gives strong evidence for spin-phonon coupling in BFO

system.

Here, we assigned the observed vibrational IR bands on

the basis of previous first principle calculations and infra-red

reflectance results.24,34,35 The data is tabulated in Table I.

The observed Spin-glass behaviour is due to pinning of

incomplete cycloid spin structure of BFO at the nanoparticles

boundary and thus spins reorientation of Fe3þ ions. It leads to

distortion in FeO6 octahedra and induces the structural insta-

bility. The structural instability induces electric polarization

effect. Thus the behaviours can be manifested by spin phonon

coupling. The additional phonon mode observed in the present

work at the spin glass phase onset is related to stretching

vibration mode of FeO6 octahedra is the clear evidence of

magnetostructural transition and spin-phonon coupling.

According to our knowledge, this first reports on spin

phonon coupling at the spin glass phase onset of BFO will

help to understand physics of low temperature multiferroic

phases of BFO.

In summary, we have performed low temperature FTIR

study in BFO to investigate its spin-phonon coupling. Six

phonon modes have been identified to Fe3þ cations caused

by the internal vibration of FeO6 octahedra. We find that an

additional phonon mode appear at 638 cm�1 when the tem-

perature decreases to �213 K around which ZFC-FC bifurca-

tion begins spin glass phase, suggesting occurrence of local

distortion of FeO6 octahedra. With further decrease of the

temperature, its intensity enhances. Our results indicate that

the spin glass phase onset is accompanied with the occur-

rence of the local structure distortion of FeO6 octahedra. The

temperature dependence behaviour of the additional phonon

mode exhibits anomaly due to local distortion of FeO6 octa-

hedra exactly coincides with the temperature at which spin

reorientation of Fe3þ ions dominantly occurred (M-T curve).

Both these observations provide evidence for the strong

spin-phonon coupling in the BFO system.

ACKNOWLEDGMENTS

UGC-DAE CSR at Indore, India is acknowledged for

providing the Magnetic and low temperature FTIR character-

ization facility. Authors want to give special thank to

FIG. 6. (a) Enlarged view of new addi-

tional phonon mode, vertical dashed

line indicates the position of absorp-

tion peak. (b) Temperature dependence

of normalized absorption intensity of

this phonon mode.

TABLE I. Frequencies (cm�1) and assignments of Transverse Optic (TO)

phonon modes in BiFeO3 using the notation of Hermet et al.34,35

Mode Calculated (Ref. 34) Crystal (Ref. 35) Present work

E(TO5) 274 274 307

E(TO6) 335 340 351

E(TO7) 378 375 391

E(TO8) 409 433 437

E(TO9) 509 521 551

A1(TO3) 318 – 331

193901-4 V. M. Gaikwad and S. A. Acharya J. Appl. Phys. 114, 193901 (2013)

[This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

14.139.120.230 On: Sat, 16 Nov 2013 04:51:33

Dr. T. Shripathi and Mr. U. P. Deshpande, Scientist, UGC-

DAE CSR, Indore for their help in recording FTIR spectra

and valuable discussion.

1Z. Cheng, X. Wang, S. Dou, H. Kimura, and K. Ozawa, Phys. Rev. B 77,

092101 (2008).2J. Dho, X. Qi, H. Kim, J. L. MacManus-Driscoll, and M. G. Blamire, Adv.

Mater. 18, 1445 (2006).3Z. X. Cheng, X. L. Wang, C. V. Kannan, K. Ozawa, H. Kimura, T.

Nishida, S. J. Zhang, and T. R. Shrout, Appl. Phys. Lett. 88, 132909

(2006).4J. Wang, J. B. Neaton, H. Zheng, V. Nagarajan, S. B. Ogale, B. Liu, D.

Viehland, V. Vaithyanathan, D. G. Schlom, M. Wuttig, and R. Ramesh,

Science 299, 1719 (2003).5W. Eerenstein, N. D. Mathur, and J. F. Scott, Nature (London) 442, 759

(2006).6D. Lebeugle, D. Colson, A. Forget, M. Viret, P. Bonville, J. F. Marucco,

and S. Fusil, Phys. Rev. B 76, 024116 (2007).7C. Michel, J.-M. Moreau, G. D. Achenbach, R. Gerson, and W. J. James,

Solid State Commun. 7, 701 (1969).8F. Kubel and H. Schmid, Acta Crystallogr., Sect. B: Struct. Sci. 46, 698 (1990).9C. Ederer and N. A. Spaldin, Phys. Rev. B 71, 060401(R) (2005).

10I. Sosnowska, T. Peterlin-Neumaier, and E. Steichele, J. Phys. C 15, 4835

(1982).11N. Wang, J. Cheng, A. Pyatakov, A. K. Zvezdin, J. F. Li, L. E. Cross, and

D. Viehland, Phys. Rev. B 72, 104434 (2005).12R. Przenioslo, A. Palewicz, M. Regulski, I. Sosnowska, R. M. Ibberson,

and K. S. Knight, J. Phys.: Condens. Matter 18, 2069 (2006).13S. Vijayanand, M. B. Mahajan, H. S. Potdar, and P. A. Joy, Phys. Rev. B

80, 064423 (2009).14R. Palai, H. Hutinen, J. F. Scott, and R. S. Katiyar, Phys. Rev. B 79,

104413 (2009).15J. E. Greedan, J. Mater. Chem. 11, 37–53 (2001).

16V. M. Gaikwad and S. A. Acharya, Adv. Mat. Lett. 4, DOI: 10.5185/

amlett.2013.fdm.82.17Z. Qu et al., J. Appl. Phys. 113, 17E137 (2013).18A. B. Kuz’menko et al., Phys. Rev. B 63, 094303 (2001).19A. B. Shushkov, O. Tchernyshyov, W. Ratcliff, S. W. Cheong, and H. D.

Drew, Phys. Rev. Lett. 94, 137202 (2005).20G. Caimi, L. Degiorgy, H. Berger, and L. Forro, Europhys. Lett. 75, 496

(2006).21J. Kim et al., Phys. Rev. B 74, 052406 (2006).22C. C. Homes et al., Phys. Rev. B 79, 125101 (2009).23M. Arora, P. C. Sati, S. Chauhan, S. Chhoker, A. K. Panwar, and M.

Kumar, J Supercond. Novel Magn. 26, 443–448 (2013).24P. Chen, X. Xu, C. Koenigsmann, A. C. Santulli, S. S. Wong, and J. L.

Musfeldt, Nano Lett. 10, 4526–4532 (2010).25G. Biasotto, A. Z. Sim~oes, C. R. Foschini, S. G. Antonio, M. A. Zaghete,

and J. A. Varela, Process. Appl. Ceram. 5(3), 171–179 (2011).26V. Tuboltsev, A. Savin, W. Sakamoto, A. Hieno, T. Yogo et al., J. Mater.

Chem. 21, 781–788 (2011).27M. K. Singh, W. Prellier, M. P. Singh, R. S. Katiyar, and J. F. Scott, Phys.

Rev. B 77, 144403 (2008).28A. Jaiswal et al., J. Phys. Chem. C 114, 2108–2115 (2010).29C. J. Cheng, C. Lu, Z. Chen, L. You, L. Chen et al., Appl. Phys. Lett. 98,

242502 (2011).30S. Goswami, D. Bhattacharya, and P. Choudhury, J. Appl. Phys. 109,

07D737 (2011).31A. K. Kundu, V. K. Jha, M. Seikh, R. Chatterjee, and R. Mahendiran,

J. Phys.: Condens. Matter 24, 255902 (2012).32M. K. Singh, R. S. Katiyar, W. Prellier, and J. F. Scott, J. Phys. Condens.

Matter 21, 042202 (2009).33T. J. Park, G. C. Papaefthymiou, A. J. Viescas, Y. Lee, H. Zhou, and S. S.

Wong, Phys. Rev. B 82, 024431 (2010).34R. P. S. M. Lobo, R. L. Moreira, D. Lebeugle, and D. Colson, Phys. Rev. B

76, 172105 (2007).35P. Hermet, M. Goffinet, J. Kreisel, and P. Ghosez, Phys. Rev. B 75,

220102(R) (2007).

193901-5 V. M. Gaikwad and S. A. Acharya J. Appl. Phys. 114, 193901 (2013)

[This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

14.139.120.230 On: Sat, 16 Nov 2013 04:51:33