1.4907220

Multiferroic properties of stretchable BiFeO3 nano-composite filmJ. S. Hwang, J. Y. Cho, S. Y. Park, Y. J. Yoo, P. S. Yoo, B. W. Lee, and Y. P. Lee

Citation: Applied Physics Letters 106, 062902 (2015); doi: 10.1063/1.4907220 View online: http://dx.doi.org/10.1063/1.4907220 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/106/6?ver=pdfcov Published by the AIP Publishing

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Multiferroic properties of stretchable BiFeO3 nano-composite film

J. S. Hwang,1 J. Y. Cho,2 S. Y. Park,2 Y. J. Yoo,1 P. S. Yoo,3 B. W. Lee,3 and Y. P. Lee1,a)1Department of Physics, Hanyang University, Seoul 133-791, South Korea2Nano-Bio Convergence Research Center, Advanced Institutes of Convergence Technology,Seoul National University, Suwon 443-270, South Korea3Department of Electronic Physics, Hankuk University of Foreign Studies, Yongin 449-791, South Korea

(Received 9 December 2014; accepted 15 January 2015; published online 10 February 2015)

We present a simple drop-casting method for preparing multiferroic nano-composite film where

BiFeO3 (BFO) nanoparticles (NPs) were evenly dispersed into polyvinyl alcohol (PVA) polymer.

BFO NPs used in this work were synthesized by the conventional sol-gel method, having diameter

of tens of nm and being in good crystallinity. The BFO NPs were loaded into a highly insulating

PVA polymer solution as filler. The multiferroic properties of the film reveal ferromagnetic order-

ing due to the uncompensated spiral ordering and saturated ferroelectric curves due to the cut-off

of current leakage. Moreover, the prepared films show high flexibility and their multiferroicities

are preserved well even in a high curved condition, reflecting the possibility for fabricating weara-

ble devices based on multiferroic materials.VC 2015 AIP Publishing LLC.[http://dx.doi.org/10.1063/1.4907220]

Magnetoelectric multiferroics is well-known to have fer-

romagnetism and ferroelectricity simultaneously.1,2 Because

the magnetic and the electric properties could be controlled by

electric and magnetic fields, respectively, it could be applica-

ble potentially to magnetic memory device and spintronics.35

Over the past decade, various ternary oxides such as BaTi2O4,

YMnO3, BiMnO3, LuFe2O4, and BiFeO3 (BFO) have been

mentioned as candidates:611 YMnO3 being antiferromagnetic

[Neel temperature (TN) 70130K] and ferroelectric [Curie

temperature (Tc) 570990K], and BiMnO3 having ferromag-

netic (TN 100K) and ferroelectric (Tc 450K) characteris-tics. Among these candidates, BFO attracts the most attention

as a multiferroic material because it has simultaneously

ferroelectric and antiferromagnetic properties even at room

temperature. It exhibits rhombohedrally distorted perovskite

(a b c 5.63 A, a b c 89.4) corresponding to R3c,and its antiferromagnetism (G-type) and ferroelectricity occur

at TN 643K and TC 1103K, respectively.12,13 Particularly,it should be noted that BFO has a weak magnetism due to

canted spin ordering.14 Additionally, BFO film is found to

show a high remnant polarization (90lC/cm2) and 1.0Bohr magneton per unit cell, which are much greater than

those of the bulk. As a result, BFO is emerged as a leading

candidate multiferroic.15

However, BFO still has crucial problems for the applica-

tions. One of them is a high leakage current due to high volatile

Bi ions, which plays an important role in lowing the remnant

polarization.1619 Another is that BFO has weak magnetization.

Although the magnetic moments of Fe are in ferromagnetic

ordering in the [111] plane, there is the canted antiferromag-

netic ordering between the neighboring plane. This results in

the cancelation of the net magnetic moment in bulk.20 In order

to address these issues, early studies have reported that, by

replacing atoms at A and B sites in perovskite structure, the

magnetic and electric properties could be tuned. Substitution of

Bi by rare-earth atom such as Dy and Ho at A sites makes it

possible to reduce the leakage current owing to suppression of

the volatility of Bi ions.21,22 Similarly, ferromagnetic phase

transition was reportedly realized by replacing Fe atom with

other ferromagnetic transition metal.2325 This substitution

method definitely has a theoretical limit for the coexistence of

ferromagnetism and ferroelectricity. This is due to the fact that

d-orbitals of the ions at B site should be occupied with an odd

number for ferromagnetic ordering while the vacancy for

ferroelectric ordering. In other words, the theoretical limit is

that the characteristics of d-orbitals for multiferroic phase are

contradictory to each other. Therefore, a new approach for the

coexistence is needed. Recently, it has been reported that anti-

ferromagnetic ordering could be transformed into a ferromag-

netic one by reducing the particle size to a nano-scale,

subsequently by improving the magnetic properties.2628

In this study, we report the dielectric and the magnetic

properties improved in flexible BFO film. BFO nanoparticles

(NPs) were prepared by the sol-gel method and mixed with

polyvinyl alcohol (PVA) solution as binder and/or matrix of

BFO NPs, because it has a strong thermal resistance, a good

processability, and a high elasticity.29,30 The BFO NP solu-

tion was made into a nano-composite film using drop-casting

method. The BFO/PVA nano-composite turned out to greatly

improve the dielectric properties and the magnetic proper-

ties, compared with the bulk. Moreover, it was found that it

retains multiferroic properties well even in a highly curved

condition. Thus, this result shows the possibility of flexible

multiferroic device for the application to electric devices.

BFO NPs were prepared by the sol-gel method. High-purity

bismuth nitrate [Bi(NO3)35H2O], iron nitrate [Fe(NO3)39H2O]powders, nitric acid, citric acid, and PVA were used as starting

materials. Bi(NO3)35H2O and Fe(NO3)39H2O were dissolvedinto 2-methoxyethanol (C3H8O2) at a ratio of 1/1. To control the

reaction rate depending on pH concentration, the precursor solu-

tion was prepared at 1/1 molar ratio of citric acid to nitric acid

and metal nitrate. The precursor solution was stirred by magnetic

bar on a hot plate, and dried at 50 C for 20min to obtain the sol.The temperature was maintained at 80 C until the dried gela)Email: [email protected]. Fax: 82-2-2281-5573

0003-6951/2015/106(6)/062902/5/$30.00 VC 2015 AIP Publishing LLC106, 062902-1

APPLIED PHYSICS LETTERS 106, 062902 (2015)


powders were fabricated. Finally, it was calcined under

atmospheric condition at 300 C for 30min. To fabricateBFO/PVA nano-composite film, as a filler, BFO nanoparticle

was added into insulating PVA polymer within weight per-

cent ranged from 0.04 to 0.5. To prepare the PVA solution,

PVA powders were placed into a hot-water bath at 90 C for1 h. BFO NPs were mixed with the PVA solution and ultra-

sonicated to disperse the NPs. 1ml of nano-composite solu-

tion was dropped onto a glass slide and dried at 70 C for 9 hand at atmospheric condition. Consequently, the final sample

was obtained by detaching the BFO nano-composite film

from the glass slide. The crystallinity and the morphology of

samples were analyzed by x-ray diffraction (XRD) (Rigaku

Miniflex-1) and scanning electron microscopy (SEM). The

magnetic and the dielectric properties were characterized with

vibrating sample magnetometer [VSM: Lakeshore, VSM-

7404] and standard ferroelectric tester (Radiant-RT66B),

respectively.

The crystallinity of BFO bulk powders, BFO NPs, PVA

film, and BFO/PVA nano-composite film was investigated by

XRD, as in Fig. 1(a). It is clearly seen that the prepared BFO

bulk powders and BFO NPs are in a high-quality perovskite

rhombohedral phase, since the peak position and the relative

intensity of resultant diffraction peaks are in accordance with

the diffraction pattern of BiFeO3 in JCPDS. Unidentified

small peaks at 2h 27.91 and 32.10 (marked by asterisk)are also observed, which might stem from Bi24Fe2O39. Many

previous studies showed that the secondary phase like non-

magnetic Bi24Fe2O39 coexists in BFO nano-powders prepared

by the sol-gel method because low calcining temperature of

300 C makes it possible to crystallize not only perovskitephase but also various impurity phases such as Bi36Fe2O57,

Bi2Fe4O9, Bi25FeO40, and Bi12(Bi0.5Fe0.5)O19.5.3134 The

XRD patterns of all the BFO/PVA nano-composite films turn

FIG. 1. (a) XRD patterns of BFO powders, BFO NPs, PVA film, and BFO/

PVA nano-composite film. (b) Tensile strength and strain of PVA film and

BFO/PVA nano-composite films at room temperature.

FIG. 2. FESEM images of BFO NPs

and BFO/PVA nano-composite film. (a)

BFO NPs, (b) BFO/PVA 1/1, (c) BFO/

PVA 1/8, and (d) BFO/PVA 1/16.

062902-2 Hwang et al. Appl. Phys. Lett. 106, 062902 (2015)


out to be the same as that of BFO NPs and do not vary as a

function of weight ratio of BFO to PVA, reflecting that the

addition of PVA does not affect the crystal structure of BFO.

Unlike BFO NPs, a single sharp peak at 2h 29.58 is seen,whose intensity increases linearly with the amount of PVA in

the film. This indicates that the peak is mainly due to the crys-

tallization of PVA. On the other hand, the average particle

size (D) of BFO NPs is calculated by the Debye-Scherrerequation. The resultant particle size in all the samples turns

out to be as small as 23.56 1.6 nm. Figure 1(b) includes themeasured tensile strength and strain of PVA film and BFO/

PVA nano-composite film at room temperature. The tensile

strength turns out to be 43, 40, 27, and 21MPa for PVA film,

BFO/PVA ratio of 1/16, 1/8, and 1/1, respectively. Our film

has large strain (6.6%, 6.5%, 5.6%, 6.7% for PVA film, BFO/

PVA ratio of 1/16, 1/8, and 1/1, respectively) before failure, in

agreement with the large tensile ductility. Though the tensile

strength was decreased, the strain was preserved according to

the BFO content. These results indicate that all the nano-

composite films are highly flexible and have stretchable

properties.

Figure 2 shows field-emission SEM (FESEM) images of

BFO/PVA film. BFO clusters with diameter of a few lm areevenly dispersed in PVA. The average size of BFO cluster is

found to be approximately 3.8 lm, indicating that a singlecluster is made of about 106 of irregularly shaped BFO NPs,

regardless of the concentration of BFO NPs. This is not only

because surfactant molecule is not used for mono-dispersion

of the NPs, but also because the use of PVA with relatively

hydrolyzed degree of 87% results in further aggregation of

BFO NPs.

To investigate the magnetic properties of BFO/PVA

nano-composite samples, we performed the magneticfield

vs. magnetization (M-H) hysteresis at room temperaturemeasurements using a VSM equipped with a 12 kOe electro-

magnet. The M-H loops were taken in a field sweep from10 kOe to 10 kOe at a rate of 25Oe/s. In Fig. 3(a), it isclearly observed that BFO bulk powders exhibit nearly linear

M-H loop with small values of the remnant magnetizationand the coercive field, suggesting antiferromagnetic behavior

with weak ferromagnetic. On the other hand, the magnetic

hysteresis loop for PVA film shows linear curve correspond-

ing to paramagnetic ordering. As shown in Fig. 3(b), the

M-H loops of all nano-composite samples exhibit non-linearcurve corresponding to ferromagnetic ordering. In contrast to

the BFO bulk powders, which exhibited linear M-H loopwith absence of remnant magnetization, our results for all

nano-composite samples indicate a clear nano-size effect on

the magnetic phase transition. As we mention earlier that the

average size of our BFO NPs was much smaller than the

minimum size for the spiral magnetic ordering, which results

in breaking of the spiral ordering to induce a small ferromag-

netism in the antiferromagnetic lattice. It should be noted

that saturated magnetization is linearly increased as a func-

tion of the concentration of BFO NPs in the film, which

means that the surrounding PVA do not affect the magnetic

properties of BFO NPs [see Fig. 3(c)]. It is also found that

the magnetic coercive field goes up slightly as the concentra-

tion of BFO NPs decreases. This implies that there are

remote magnetic interactions among BFO clusters, which act

as an obstacle against the flip of magnetic moments.35

Figure 4 presents the ferroelectric properties of BFO

bulk powders, BFO NPs, PVA film, and BFO/PVA nano-

composites. The ferroelectric hysteresis loops (P-E loops)were measured in electric field sweep from 0.8 to 0.8 kV/cm at 100Hz. BFO bulk powders and BFO NPs show unsat-

urated lossy loops, indicating a relatively high conductivity

due to the space charge defects.36 In the case of BFO NPs,

NPs have smaller particle size compared with the bulk pow-

der sample, the grain boundary is larger, the leakage current

is increased, and the dielectric property is degraded. In our

experiment, however, these disadvantages are overcome

using polymer composite. The PVA film reveals nearly satu-

rated loop with small double remnant polarization (2Pr) of0.5 102nC/cm2. Unlike BFO powders, BFO NPs, andPVA film, all the BFO/PVA nano-composites commonly

show that the polarization is saturated at 0.8 kV/cm, as

FIG. 3. (a) Magnetization versus magnetic field for BFO powders and PVA

film at room temperature. (b) Magnetization versus magnetic field for BFO

NPs and BFO/PVA composite film at room temperature. (c) Saturation mag-

netization and coercive field versus BFO wt.%.



shown in Figs. 4(d)4(f). The double remnant polarization

turns to be 2.84 102, 2.46 102, and 3.28 102nC/cm2for the BFO/PVA ratio of 1/1, 1/8, and 1/16, respectively.

The electric resistivity at applied electric field of 0.8 kV/cm

is estimated to be 1.7 1013, 6.13 1012, and 7.76 1012,and 1.16 1013X cm for the PVA film and ratio of 1/1, 1/8,and 1/16, respectively, which is much greater than that of

BFO NP pellet (2.56 109X cm). As a result, the increase inresistivity is attributed to the enhancement of ferroelectric

properties, since the existence of PVA cuts off current fila-

ment and restrains spread of the current leakage throughout

the sample. Our results is quite similar to previous studies on

ferroelectric nano-composite material.37,38 Furthermore, the

dielectric permittivity of PVA film and BFO NPs are nearly

4 and 100 at room temperature and 100Hz, which were also

reported.39,40 While BFO NPs has large leakage current,

above-mentioned, the PVA film has good electric resistivity.

Thus, the ferroelectric property of the BFO/PVA nano-

composites is improved due to the fact that the leakage cur-

rent of the BFO/PVA nano-composites is reduced by the

dielectric property of the PVA film. The dielectric permittiv-

ity of BFO/PVA 1/16 sample was estimated by us to be

nearly 9, which is in good agreement with the composition

ratio.41 The permittivity of composite film is increased

because of the effect of high permittivity of filler (BFO

NPs), compared with the polymer matrix. Even though we

reduce the leakage current, it still remains. That is why the

electron tunneling phenomenon works in external field.42 In

order to investigate the possibility of the flexible multiferroic

device, we measured the P-E loop for the BFO/PVA nano-composite at a highly curved condition [shown in Figs.

4(d)4(f)]. Importantly, the P-E loops are found not to besignificantly changed by cylindrical bending strain (Pr andPs alter by only less than 20%). These results are differentfrom early study in which the cylindrical bending strain

induced energy dissipation increased, and contributed to

increase in the electric coercive field.43 Our results suggest

that the PVA plays an important role in relaxation of the

strain because it is highly deformable and flexible. The insets

of Figs. 4(c)4(f) show the P-E loops for PVA film andBFO/PVA composite films by applying a magnetic field of

140 Gauss. From the P-E results, both Ps and Pr in magneticfield are found to be greater than those in absence of mag-

netic field at room temperature, implying that there is the

magneto-electric effect in BFO-PVA composite.44

To investigate the phase transition versus temperature of

BFO and BFO NPs, the TGA pattern for BFO bulk powders

and BFO NPs was measured in a temperature range of

4001270K by thermogravimetric analysis (TGA), as in

Fig. 5. The weight was fluctuated by temperature, especially,

near magnetic TN and electric Tc points, the weight wasdecreased sharply. This indicates that the weight, the magnetic

permeability, and the dielectric permittivity are closely related,

from which we obtain that the Tc point of BFO powders is1239K and that of the BFO NPs is 863K. BFO NPs turn out

to have lower TN and Tc, compared with the BFO powders.26

FIG. 4. P-E loops of curved BFO/PVA

nano-composite film. (a) BFO pow-

ders, (b) BFO NPs, (c) PVA film, (d)

BFO/PVA 1/1, (e) BFO/PVA 1/8, and

(f) BFO/PVA 1/16. Insets in (c)(f)

show the P-E loops with (red) and

without (blue) magnetic field.

FIG. 5. TGA patterns of BFO powders and BFO NPs.



We report the simple method to fabricated BFO/PVA

nano-composite film. The film exhibits the improved multi-

ferroic properties: (i) ferromagnetic hysteresis loop due to

the uncompensated spiral ordering and (ii) saturated ferro-

electric curves due to the cut-off of current leakage. We also

emphasize that the prepared films have highly flexibility and

their multiferroicity is preserved well even at a highly curved

condition, reflecting the possibility for fabricating wearable

devices based on multiferroic materials.

This research was supported by the ICT R&D program

of the Ministry of Science, ICT and Future Planning (MSIP)/

IITP, Korea [KCA-2013-005-038-001], and by Basic

Science Research Program through the National Research

Foundation of Korea funded by the Ministry of Education,

Korea (2013R1A1A2011917).

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