Ferroelectric and magnetic properties of Fe-doped BaTiO3 thin films grownby the pulsed laser depositionE. Venkata Ramana, S. M. Yang, Ranju Jung, M. H. Jung, B. W. Lee et al. Citation: J. Appl. Phys. 113, 187219 (2013); doi: 10.1063/1.4801965 View online: http://dx.doi.org/10.1063/1.4801965 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v113/i18 Published by the American Institute of Physics. Additional information on J. Appl. Phys.Journal Homepage: http://jap.aip.org/ Journal Information: http://jap.aip.org/about/about_the_journal Top downloads: http://jap.aip.org/features/most_downloaded Information for Authors: http://jap.aip.org/authors

Ferroelectric and magnetic properties of Fe-doped BaTiO3

thin films grown by the pulsed laser deposition

E. Venkata Ramana,1,a) S. M. Yang,2 Ranju Jung,3 M. H. Jung,4 B. W. Lee,1 and C. U. Jung1,b)

1Department of Physics, Hankuk University of Foreign Studies, Yongin, Gyeonggi-do 449-791, South Korea2ReCFI, Department of Physics and Astronomy, Seoul National University, Seoul 151-747, South Korea3Department of Electrophysics, Kwangwoon University, Seoul 139-701, South Korea4Department of Physics, Sogang University, Seoul 121-742, South Korea

(Received 30 September 2012; accepted 5 January 2013; published online 8 May 2013)

Fe-doped BaTiO3 thin films were grown on (001) oriented SrTiO3 substrates using pulsed-laser

deposition technique. These films had a single-phase character and good epitaxial relationship with

the substrate. Polarization-electric field (P-E) hysteresis revealed saturated polarization with

remnant polarization (Pr) of 13.5 lC/cm2 for 10 mol. % Fe-doped BaTiO3 films. Further increase

of composition resulted in the large leakage currents and reduction of polarization. The

piezoelectric domain switching in the films was confirmed by local hysteresis using piezoelectric

force microscopy measurements. The Fe-doped BaTiO3 thin films exhibited room temperature

ferromagnetism, and the magnetization value increased with increasing Fe concentration. Our

results demonstrate that the addition of Fe �10 mol. % in BaTiO3 induces the ferromagnetism and

a switchable ferroelectric state. VC 2013 AIP Publishing LLC [http://dx.doi.org/10.1063/1.4801965]

I. INTRODUCTION

Multiferroics are functional materials that exhibit more

than one functionality. In particular, ferroelectricity and ferro-

magnetism have attracted considerable interest in the last few

years due to their interesting physical properties and wide

range of potential applications in magnetic data storage, sen-

sors, non-volatile memories, actuators, etc.1,2 Ferroelectricity

requires a non-centrosymmetric structural distortion, which is

usually not compatible with the presence of partially filled d-

level ions required for magnetic ordering.3 BiFeO3 is the only

known single-phase multiferroic material that exhibits both fer-

roelectricity and ferromagnetism, with coupling between the

two at room temperature.4 BiFeO3 is an interesting alternative

in this respect because it is lead free. Recently, researchers

have demonstrated that ferromagnetism can be achieved in

semiconducting and insulating oxides by the introduction of

transition metal (TM) ions. Cobalt ions implanted in polycrys-

talline layers of LiNbO3 ferroelectrics exhibited ferromagnet-

ism.5 Based on ab initio total energy calculations, Yoshida

et al.6 suggested that Cr, Mn, Fe, and Co are the most promis-

ing candidates in ferromagnetism for TM-doped BaTiO3

(TM¼Sc, V, Cr, Mn, Fe, Co, Ni, and Cu). Similarly, Co-

doped Ba(Sr,Ti)O3 thin films exhibited ferromagnetism.7 The

objective of the present work was to observe both ferroelectric-

ity and ferromagnetism in TM-doped BaTiO3 thin films.

BaTiO3 (BTO) is a classical ferroelectric material with a

relatively low Curie temperature, (TC)¼ 120 �C, in bulk. It can

be grown epitaxially on a (001)-oriented perovskite SrTiO3

(STO) substrate. Because its in-plane lattice constant,

a¼ 3.992 A, is larger than that of STO, (a¼ 3.905 A), a large

(2.2%) lattice mismatch exists between BTO and the cubic

STO substrate. Thus, BTO thin films grown on the STO sub-

strates usually have misfit dislocations near the hetero interface

to relax misfit strains. Such misfit dislocations and the strain

fields around the defects often have considerable effects on the

physical properties of the thin films.8 Recently, Mn-doped BTO

films grown on STO (001) substrates exhibited ferroelectricity

and ferromagnetism for doping up to 2 mol. %.9 Rajamani

et al.10 found ferromagnetic behavior and Faraday rotation in

Fe-doped BTO thin films grown on MgO (100) substrates using

a pulsed-laser deposition method for typical compositions of

Fe� 15 mol. %.10 In a similar study, Maier et al.11 estimated

the magnetic Curie temperature to be greater than 450 �C for

BaTi0.5Fe0.5O3 thin films grown on STO (001) substrates.

In this study, we grew Fe-doped BTO thin films using

pulsed-laser deposition to investigate their phase, micro-

structure, ferroelectricity, and ferromagnetic behavior at

room temperature. We used Fe-doping up to 20 mol. %; our

results indicate that the films with 10 mol. % Fe doping

exhibited the best multiferroic physical properties.

II. EXPERIMENTAL

Polycrystalline ceramic targets of Ba(Ti1�xFex)O3

(BTFO) with x¼ 0.0 (BTO), 0.1 (BTFO10), and 0.2

(BTFO20) were synthesized according to the regular ceramic

route. High purity (99.9%) BaCO3, TiO2, and Fe2O3 were

used as starting materials. Stoichiometric amounts were cal-

cined at 1200 �C for 10 h and sintered at 1300 �C for 4 h as

ceramic targets. Thin films of BTFO were grown on (001)-

oriented single-crystalline STO substrates using pulsed-laser

deposition.12,13 The conductive SrRuO3 (SRO) layer was

first grown on STO substrate, which serves as the bottom

electrode for electrical measurements. Polycrystalline BTFO

and SRO targets were ablated by a KrF excimer laser

(k¼ 248 nm) with an energy density of 2 J/cm2 at a repetition

rate of 2–4 Hz. The target-to-substrate distance was kept at

5 cm. A SRO layer (�50 nm) was grown on the STO with a

a)Presently at I3N-Aveiro, Department of Physics, University of Aveiro,

Aveiro-3810-193, Portugal.b)Author to whom correspondence should be addressed. Electronic mail:

cu-jung@hufs.ac.kr.

0021-8979/2013/113(18)/187219/5/$30.00 VC 2013 AIP Publishing LLC113, 187219-1

JOURNAL OF APPLIED PHYSICS 113, 187219 (2013)

substrate temperature (TS) of 750 �C and an oxygen partial

pressure (PO2) of 100 mTorr. BTFO films were grown on

SRO/STO and STO substrates at TS¼ 750 �C and

PO2¼ 20 mTorr. After deposition, the films were post-

annealed at 500 �C for 1 h in ambient oxygen (�0.6 atm).

The thickness of the films was estimated using a high-

resolution transmission electron microscope (HRTEM,

Tecnai 20F). All BTFO films were 15–20 nm thick.

Structural analysis was performed using a four-circle high-

resolution X-ray diffractometer (D8, Bruker). The surface

morphology of the films was examined by atomic force mi-

croscopy (AFM, PSIA). To measure the electrical properties

of the BTFO/SRO/STO (001) heterostructures, we fabricated

square-shaped Pt capacitors with areas of 40� 40 lm using

an electron-beam evaporation process. Polarization was

measured as a function of the electric field (P–E loops) at

2 kHz using a TF analyzer 2000 equipped with FE-module

(HV) (aixACCT). The switching of the local ferroelectric

polarization was studied using piezoresponse force micros-

copy (PFM, XE-100, Park Systems) and Au–Cr-coated Si

tips with a spring constant (k) �0.65 N=m. The amplitude

(R) and phase (h) of the piezoelectric signals were measured

using a lock-in amplifier (SR830). The magnetic properties

were determined using a superconducting quantum interfer-

ence device (MPMS SQUID VSM).

III. RESULTS AND DISCUSSION

Fig. 1(a) shows the X-ray h-2h scan patterns of the

BTFO films grown on SRO-buffered STO (001) substrates.

The peaks corresponding to (00l) reflections, but without

secondary phases in the sensitivity range of HRXRD, indi-

cate highly textured epitaxial growth. The position of the

(002) peak shifted systematically toward lower angles with

increasing Fe content in BTO, suggesting that the Fe was

doped at the B-site. The out-of-plane lattice parameters cal-

culated from the (002) reflections were 4.186 A, 4.213 A,

and 4.233 A for BTO, BTFO10, and BTFO20, respectively.

The c-axis lattice constant of the BTO film was larger than

that of bulk BTO (4.036 A) by �4%, which may have been

due to the compressive strain.14 The increase in the lattice

constant with increasing Fe content in the BTO was similar

to a previous study of BTFO films grown on MgO sub-

strates.10 Such an increase in unit cell parameters is expected

due to the volume expansion and the formation of oxygen

vacancies that occur when large Fe3þ(64.5 pm) ions replace

Ti4þ(60.5 pm) ions.15 From the earlier work on these com-

pounds with Fe� 15 mol. % (grown on MgO and STO sub-

strates),10,11 it can be understood that the expansion of lattice

is independent of lattice mismatch.

The epitaxial relation of the BTFO films was verified by

the X-ray reciprocal space mapping measured around the

(103) reflection of STO (001). Fig. 1(b) shows that the peaks

corresponding to the BTO, SRO, and STO lie on the same

horizontal line, indicating that the film was grown coherently

with the STO substrate and had the same in-plane lattice

parameter.

From the cross-sectional HRTEM image (Fig. 2), it can

be seen that the BTFO thin films have highly oriented growth

on the STO (001) substrate with a single crystalline quality.

A sharp interface between film and the substrate clearly con-

firms the Fe doping into the BTO lattice. Figure 3 shows the

surface morphology of the BTFO10/SRO/STO (001) thin

films. The step-terrace structure of the films following post-

annealing was seen in the atomic force microscopy image.

The BTFO and SRO had the similar surface morphology as

the STO substrate. The line profile confirmed that the steps

likely had a height of one unit-cell, and the terraces were

atomically smooth (RMS roughness of the BTFO films was

0.4–1 nm).

The ferroelectric polarization of the BTFO/SRO/STO

(001) heterostructures with Pt top electrode is shown in

Fig. 4. Here, it can be noted that the SRO buffer layer

(50 nm) on STO (001) serves as lattice-matched bottom elec-

trode. According to the Tagantsev and Stolichnov’s model,16

FIG. 1. (a) h-2h XRD scan of the BTFO thin films grown on the SRO/STO

(001) substrate for x¼ 0.0, 0.1, and 0.2. (b) X-ray reciprocal map for BTO/

SRO/STO(001) film measured around (103) Bragg reflection.

FIG. 2. (a) Cross-sectional bright field TEM image showing the thickness

and (b) HR-TEM image at the interface for the BTFO10 film grown on STO

(001).

187219-2 Venkata Ramana et al. J. Appl. Phys. 113, 187219 (2013)

SRO with good conductivity decreases the thickness of inter-

facial dielectric layer between the ferroelectric and electrode

layers, thereby improving ferroelectric limits. Fig. 4 depicts

that the BTO films had a typical ferroelectric nature, whereas

the BTFO10 exhibited switchable polarization, with a trend

of saturation within the applied voltage range. However, the

BTFO20 film exhibited leaky dielectric behavior with unsat-

urated polarization. The inset of Fig. 4 clearly shows the

peaks in the polarization current for BTFO10 confirming the

existence of a switchable ferroelectric state that was absent

in the BTFO20. The remnant polarizations (Pr) for the BTO

and BTFO10 films were �22 lC/cm2 and 13.5 lC/cm2,

respectively; decreased polarization as the Fe content

increased. The Pr for the BTO film was slightly smaller than

that of the bulk BTO (26 lC/cm2)17 and comparable to that

of the BTO/SRO/STO(001) films with NiFe electrodes

reported by Zhang et al.18 The most significant result in the

present study is that the Pr for 10 mol. % Fe-doped BTO

films was much larger (with lower leakage current) than

those of 2 mol. % Mn-doped BTO films (Pr¼ 3.7 lC/cm2).9

These results clearly indicate that the films (BTO and

BTFO10) were insulated in an electric field up to �5 MV/cm

without causing dielectric breakdown. To confirm the

ferroelectric behavior of the BTFO10 and BTFO20, the PFM

response was recorded. As shown in Fig. 5(a), the hysteresis

loops for phase vs. bias voltage indicate 180� phase reversal

for both films, confirming the existence of ferroelectric

switching. The piezoresponse hysteresis loops (Fig. 5(b))

show that the BTO and BTFO10 films had switchable pie-

zoresponse by the external bias, whereas that of BTFO20

film was not saturated within the applied voltage range. The

decrease in the piezoresponse with increasing Fe content

clearly indicates that the polarization decreased with increas-

ing Fe doping. The increase of leakage current due to the for-

mation of oxygen vacancies during the growth and the

existence of multiple Fe valences (Fe3þ$Fe4þ) may have

played a vital role in the decrease in polarization. In general,

the polarization and piezoelectric strength of the ferroelec-

trics is known to decrease with the addition of acceptor ions

to the host lattice. The oxygen vacancies created as part of

the charge compensation have a clamping effect on the

motion of domain walls.19 Such effects associated with the

appearance of oxygen vacancies could suppress the degree

of modulating spontaneous polarization, which is obvious

from the results of Figs. 4 and 5.

Fe 2p spectra of BTFO thin films measured by X-ray pho-

toelectron spectroscopy (XPS) are shown in Fig. 6. It can be

seen that the 2p3/2 and 2p1/2 spin-orbit doublet components

were located at about 710 eV and 724 eV, respectively, in both

BTFO10 and BTFO20 film. For BTFO20, a satellite structure

is observed at about 718 eV which is 8 eV away from the 2p3/2

peak. This satellite structure is similar to that of LaFe3þO3.20

It supports that the valence state of Fe in BTFO20 is þ3. In

the case of BTFO10, Fe 2p spectrum is very noisy due to

smaller amount of Fe in BTO. The satellite feature is very

weak or hardly discriminated. This weak satellite present

looks like that of SrFeO3.21 It reveals that the valence state of

Fe in BTFO10 is a mixture of þ3 andþ4, or þ4.

FIG. 3. Atomic force microscopy image for 20 nm thick BTFO10 thin film

grown on SRO/STO (001) substrate.

FIG. 4. Ferroelectric hysteresis loops of BTFO/SRO/STO (001) films meas-

ured at a frequency of 2 kHz. Inset: polarization current (IP) for the BTFO10

and BTFO20 films.

FIG. 5. Piezoforce response for the BTFO/SRO/STO (001) films: (a) phase

vs. bias voltage and (b) piezoresponse vs. voltage.

187219-3 Venkata Ramana et al. J. Appl. Phys. 113, 187219 (2013)

Fig. 7 shows the magnetic hysteresis loops at room tem-

perature for BTFO10/STO (001) and BTFO20/STO (001)

films after subtracting the diamagnetic signal (STO sub-

strate) from the total magnetization [we observed a diamag-

netic signal for the BTO/STO (001) thin film]. The

magnetization had a saturation character with the applied

magnetic field due to the ferromagnetism. The saturation

magnetization (MS) and coercive field (HC) were 4.75 emu/

cc (0.038 lB/Fe) and 50 Oe for the BTFO10 film and

10.3 emu/cc (0.1 lB/Fe) and 40 Oe for the BTFO20 film,

respectively. The value of MS for the BTFO20 film was com-

parable to that for film grown on a MgO substrate (12 emu/

cc).10 The ferromagnetic (FM) nature of Fe-doped BTO films

with Fe content �0.15 was previously reported.10 Present

results for samples with Fe> 10 mol. % confirm the earlier

observation. Small differences in the zero-field-cooled

(ZFC) and field-cooled (FC) magnetizations measured under

H¼ 500 Oe in the temperature range of 5–400 K suggest the

presence of FM behavior.

The observed magnetization in BTFO can be ascribed to

the contribution of Fe valence fluctuation or oxygen vacan-

cies and their interactions with Fe3þ (Fe3þ � Vo � Fe3þ, Vo

is the oxygen vacancy) rather than the impurities. We arrived

at this possibility based on the facts that (a) the HC of Fe2O3-

thin films exceeds 600 Oe at room temperature22 and (b) the

XRD patterns of the films do not show any traces of impur-

ities corresponding to BaFeO3/BaFe12O19 (thin films of

BaFeO3/STO(100) show parasitic ferromagnetism/antiferro-

magnetism, and BaFe12O19 thin films have much larger MS

and HC� 100 Oe),23,24 Fe2O3, or Fe3O4. In addition, the sat-

uration magnetization for Fe2O3 or Fe3O4 as impurities

exceeds 70 emu/cc, which is much higher than the observed

magnetization values of our BTFO films. If the magnetic sig-

nal came only from Fe2O3 or Fe3O4 (if any), the volume per-

cent of these impurities inside the film cannot be negligible,

which is not consistent with the XRD data without a impurity

peak and AFM data with the atomic flatness. Thus, the FM

must have been intrinsic and most likely due to defect com-

plexes from the substitution of Fe3þ for B-site Ti4þ.

Rajamani et al.10 and Maer et al.11 suggested that the magne-

tism in BTFO grown on MgO is due to the presence of Fe3þ

and Fe4þ. In addition the increase in leakage current (from

P-E and I-V) with increasing Fe content in our films indicates

that the doping induced the related defects. Lin et al.9

explained the existence of FM in these systems based on the

defects (cited above) related to the bound magnetic polaron

model proposed by Coey et al.25 for dilute magnetic semi-

conductors (DMS). According to this model, the magnetism

in DMS depends on the cation and the donor polaron concen-

tration, and the local magnetic structure is related to the dis-

tribution of the magnetic polarons. In a recent study, Yao

et al.26 demonstrated that Mn-doped BTO thin films grown

by the PLD under smaller oxygen partial pressure of

10 mTorr exhibited a FM nature while the films under higher

oxygen pressure have the paramagnetic order (100 mTorr)

and related the disparity to the presence of Vo concentration.

In our BTFO films, which are grown at lower oxygen partial

pressure (20 mTorr) oxygen vacancies are expected to form

in the vicinity of Fe ions to maintain charge neutrality, lead-

ing to the formation of bound polarons which overlap with

each other resulting in a long range ferromagnetic order. At

the same time, the Fe3þ ion with 3d5 has unoccupied spin

orbitals in which trappd electron will spin down and the two

neighbors will spin up. The superexchange interaction

between two Fe3þ ions results in antiferromagnetism while

the exchange interaction Fe3þ � Vo � Fe3þ will be a FM. As

the content of Fe in BTO increases, the resulting competition

between the FM and AFM interactions will lead to the

observed magnetic behavior shown in Fig. 7. In a study, by

means of ab initio calculations, Ray et al.27 related the FM

in BTFO single crystals to intrinsic magnetic inhomogene-

ities arising from positional disorder. Overall, the magnetic

behavior in BTFO can be considered to arise due to the

defect complexes such as oxygen vacancies and Fe valence

fluctuations. In BaTiO3, empty d orbitals are necessary to

stabilize the Ti off-centering in order to maintain the long-

range ferroelectric order. Substitution of Fe for some Ti ions

destroys the stability of the off-centering displacement to

FIG. 7. Magnetic hysteresis loops for the BTFO10 and BTFO20 thin films

on STO (001) at room temperature. Inset: ZFC-FC curves of the BTFO10

sample as a function of temperature under H¼ 500 Oe.

FIG. 6. XPS spectra of Fe 2p state of BTFO thin films on STO (001).

187219-4 Venkata Ramana et al. J. Appl. Phys. 113, 187219 (2013)

some extent, resulting in reduced polarization and room tem-

perature ferromagnetism. The present study demonstrated

that the BTFO films (BTFO10 in particular) exhibit FM

behavior with reasonably good ferroelectricity.

IV. CONCLUSIONS

Thin films of 10% and 20% Fe-doped BaTiO3 were

grown on SrRuO3-buffered SrTiO3(001) substrates.

Structural analysis showed that the films had an epitaxial

relationship with the SrTiO3 substrate and the SrRuO3 layer.

All the samples exhibited ferroelectric nature, as confirmed

from the polarization vs. applied voltage and PFM response.

All the films showed nearly 180� phase reversal with the

applied bias voltage, and both films exhibited ferromagnetic

nature. These results indicate that insulating BTFO10 film

with a switchable ferroelectric and ferromagnetic state is a

promising candidate for magnetoelectric applications.

ACKNOWLEDGMENTS

This work was supported by Basic Science Research

Program through the National Research Foundation of Korea

(NRF) funded by the Ministry of Education, Science and

Technology (2012R1A1A2008595 and 2012R1A1A2008845).

C. U. Jung is supported by Hankuk University of Foreign

Studies Research Fund of 2010. R. Jung is supported by

Kwangwoon University Research Fund (2011).

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