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Physica B 383 (2006) 31–32

www.elsevier.com/locate/physb

Piezoelectric coefficient of BiFe1�xMnxO3 thin films measuredby piezoresponse force microscopy

S.-H. Leea,�, C.-H. Yanga, Y.H. Jeonga, N.O. Birgeb

aDepartment of Physics & Electron Spin Science Center, Pohang University of Science and Technology, Pohang, KoreabDepartment of Physics and Astronomy, Michigan State University, E. Lansing, MI 48824, USA

Abstract

BiFe1�xMnxO3 thin films were grown by pulsed laser deposition on Nb-doped SrTiO3 substrates. Piezoresponse force microscopy was

utilized to investigate the piezoelectric properties of these films. The piezoelectric hysteric behavior of the films was confirmed at room

temperature. It is further shown that the piezoelectric coefficient of BiFe1�xMnxO3 decreases rapidly as x increases initially, but it

becomes saturated above x�0:4.r 2006 Elsevier B.V. All rights reserved.

Keywords: PFM; BiFe1�xMnxO3; Piezoelectric coefficient

A prospect of devices using both charge and spin ofelectrons continuously gives impetus to the activities in thefield of spintronics. In particular, recent years have seen arenewal of interest in multiferroic materials showingsimultaneously magnetic and ferroelectric orders due tomultifunctional application possibilities. BiFeO3 with theperovskite structure is such a material with ferroelectricand antiferromagnetic transition temperatures TFE�1103Kand TAF�643K, respectively [1]. It possesses a large ferro-electric polarization P�70mC=cm2 [2]. Another distortedperovskite BiMnO3 is also multiferroic with ferromagneticand ferroelectric transition temperatures TFM�105K andTFE�770K, respectively [3,4]. While it is known that bothBiFeO3 and BiMnO3 are ferroelectric, the related propertiesof their mixture BiFe1�xMnxO3 remain unexplored. Pre-viously we have established the synthesis method of BiFe1�x

MnxO3 thin films and studied their magnetic properties [5].Here we wish to report on the piezoelectric properties ofBiFe1�xMnxO3 thin films as investigated with a piezo-response force microscope (PFM).

BiFe1�xMnxO3 (x ¼ 0:1; 0:2; 0:308; 0:438; 0:5) films weredeposited by the combinatorial pulsed laser depositiontechnique using two stoichiometric targets of BiMnO3 and

e front matter r 2006 Elsevier B.V. All rights reserved.

ysb.2006.03.043

ng author. Fax: +8154 279 8056.

ss: ysh8270@postech.ac.kr (S.-H. Lee).

BiFeO3 on Nb-doped SrTiO3(100) substrates [5]. Thedeposition temperature and oxygen pressure were 823Kand 3� 10�3 Torr, respectively. The manganese contents x

represents nominal values determined from the depositiontime. We only briefly mention our PFM measurements,and the details are referred to Ref. [6]. The contact-modeatomic force microscopy (XE-100, PSIA) with a Cr-Ptcoated silicon cantilever (BS-ElectriMulti75, Budgetsen-sors) was used to investigate surface morphology of a film.To perform PFM measurements simultaneously, thecantilever was modulated at frequency 17 kHz with a smallAC voltage, which then induces a piezoelectric response inthe film. The induced piezoelectric response was picked upfrom the associated cantilever deflections with a lock-inamplifier. For hysteresis measurements the DC bias voltagewas also applied in series with the AC voltage.In Fig. 1 displayed are the hysteresis loops, amplitude as

well as phase signals at room temperature, for aBiFe0:5Mn0:5O3 film obtained by sweeping the bias voltage.The clear presence of piezoelectric hysteresis provides theproof of ferroelectricity of the film. The amplitude signal ofFig. 1(a) contains information on the magnitude of thepiezoelectric coefficient along the normal direction, whilethe phase signal of Fig. 1(b) represents the direction ofspontaneous polarization. Similar hysteresis loops werealso obtained for other films of different composition.

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-6 -4 -2 0 2 4 6Voltage (V)

-6 -4 -2 0 2 4 6Voltage (V)

Pha

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Am

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Fig. 1. Hysteresis loops of the piezoelectric signal from a BiFe0:5Mn0:5O3 thin film: (a) the amplitude represents the piezoelectric coefficient along the

normal direction and (b) the phase indicates the polarization direction.

12

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6

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d zz

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10 20 30 40 50

Mn (%)

Fig. 2. Local piezoelectric coefficient of BiFe1�xMnxO3 thin films as a

function of manganese contents.

S.-H. Lee et al. / Physica B 383 (2006) 31–3232

Having proved the ferroelectric nature of the films, weturn to the variation of the piezoelectric coefficient ofBiFe1�xMnxO3 films as a function of x. The local piezo-electric coefficient was obtained by sweeping the amplitudeof the AC voltage, that is, the coefficient was determinedfrom the linear dependence of the signal on the ACamplitude as follows:

ho ¼ SAFMdzzAo, (1)

where ho is the amplitude of the piezoresponse signal, Ao isthe amplitude of the input AC voltage and SAFM represents

the sensitivity of the optical detector, i.e., the conversionfactor between the mechanical displacement of thecantilever and the electric deflection signal [7]. Fig. 2displays the local piezoelectric coefficient of BiFe1�x

MnxO3 films as a function of x. From the figure it is seenthat the local piezoelectric coefficient for a BiFe0:9Mn0:1O3

thin film (dzz � 9:7 pm=V) is appreciably larger than that ofBiFe0:8Mn0:2O3 (dzz � 2:7 pm=V). It is also seen that whilethe piezoelectric coefficient continues to decrease withx, the decreasing rate reduces severely above x�0:4and the coefficient becomes saturated at a small value(dzz � 0:456 pm=V).

In conclusion, we successfully synthesized BiFe1�xMnxO3

thin films and characterized their piezoelectric propertieswith PFM. More detailed investigation of various piezo-electric properties of this important multiferroic system is inprogress.

References

[1] J. Wang, et al., Science 299 (2003) 1719.

[2] K.Y. Yun, et al., Appl. Phys. Lett. 83 (2003) 3981.

[3] H. Chiba, et al., J. Solid State Chem. 132 (1997) 139.

[4] A. Moreira dos Santos, et al., Phys. Rev. B 66 (2002) 064425.

[5] C.-H. Yang, et al., Phys. Status Solidi (B) 241 (2004) 1453.

[6] A. Gruverman, et al., Annu. Rev. Matter. Sci. 28 (1998) 101.

[7] C. Harnagea, et al., in: M. Alexe, et al. (Eds.), Nanoscale Characteri-

zation of Ferroelectric Materials: Challenges in the Analysis of the

Local Piezoelectric Response, Springer, Berlin, 2004, pp. 51–55.