Magnetoelectric coupling and phase transition in BiFeO3 and (BiFeO3)0.95(BaTiO3)0.05
ceramics
T.-H. Wang, C.-S. Tu, H.-Y. Chen, Y. Ding, T. C. Lin, Y.-D. Yao, V. H. Schmidt, and K.-T. Wu 
 
Citation: Journal of Applied Physics 109, 044101 (2011); doi: 10.1063/1.3551578 
View online: http://dx.doi.org/10.1063/1.3551578 
View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/109/4?ver=pdfcov 
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Magnetoelectric coupling and phase transition in BiFeO3
and (BiFeO3)0.95(BaTiO3)0.05 ceramics
T.-H. Wang,1 C.-S. Tu,1,2,a) H.-Y. Chen,2 Y. Ding,1,3 T. C. Lin,2 Y.-D. Yao,1 V. H. Schmidt,4
and K.-T. Wu1,2
1Graduate Institute of Applied Science and Engineering, Fu Jen Catholic University, Taipei 242, Taiwan
2Department of Physics, Fu Jen Catholic University, Taipei 242, Taiwan
3Teaching Center of Natural Sciences, Minghsin University of Science and Technology, Hsinchu 304, Taiwan
4Department of Physics, Montana State University, Bozeman, Montana 59717, USA
(Received 17 June 2010; accepted 23 December 2010; published online 23 February 2011)
In situ high-resolution synchrotron x-ray diffraction reveals a local minimum in rhombohedral
distortion angle aR (associated with an inflection in the lattice constant aR) near 400 and 350 C in
BiFeO3 (BFO) and (BiFeO3)0.95(BaTiO3)0.05 (BFO–5%BT), respectively. It suggests a coupling
between ferroelectric and magnetic parameters near the antiferromagnetic–paramagnetic transition,
which is responsible for the broad frequency-dependent dielectric maxima. A rhombohedral
(R)–orthorhombic (O)–cubic (C) transition sequence takes place near 820 and 850 C in BFO upon
heating. BFO–5%BT exhibits a R–C transition near 830 C. The BaTiO3 substitution can enhance
dielectric and ferromagnetic responses and reduce electric leakage. The dielectric loss of
BFO–5%BT remains less than 0.04 below 150 C. VC 2011 American Institute of Physics.
[doi:10.1063/1.3551578]
I. INTRODUCTION
The multiferroic bismuth ferrite BiFeO3 (BFO) materi-
als have attracted much attention not only because they pos-
sess ferroelectric (FE) and ferromagnetic (FM) properties,
but also because the coupling between the electric and mag-
netic ordering leads to additional functionalities.1–3 BFO can
provide an alternate choice as a “green” FE/FM material
as compared with lead-containing compounds, such as
Pb(ZrxTi1x)O3 (PZT), Pb(Mg1/3Nb2/3)O3–PbTiO3, and
Pb(Zn1/3Nb2/3)O3–PbTiO3.
4 BFO has a high FE Curie tem-
perature (TC¼ 810–870 C) and antiferromagnetic Ne´el tem-
perature (TN¼ 352–397 C),5–13 which enable it to be used
reliably above room temperature.
A rhombohedral structure of R3c space group
(aR¼ 5.616 A˚ and aR¼ 59.35) was reported for bulk BFO
at room temperature,14 but can also be indexed based on the
pseudocubic lattice (apc¼ 3.96 A˚).15 The dielectric results of
BFO–PZT solid solutions predicted a FE Curie temperature
near 850 C for the pure BFO.13 From Raman scattering and
differential thermal analysis, a rhombohedral–orthorhombic–
cubic phase sequence was observed near 820 and 925 C
upon heating in bulk BFO.16,17 However, a monoclinic–
orthorhombic–cubic was proposed in (001) BFO thin film.16
The recent powder neutron diffraction result of BFO
revealed a phase sequence of rhombohedral-b orthorhombic-c
orthorhombic.18
The spontaneous polarization of bulk BFO was expected
to be 90–110 lC/cm2, predicted by ab initio calculations19,20
because of large atomic displacement. Spontaneous polariza-
tions of 60 and 40 lC/cm2 were reported in BFO crystal and
ceramic, respectively.21,22 However, most reported BFO
ceramics exhibit small electric polarization and nonsaturated
hysteresis loops,9,23,24 which were attributed to electric leak-
age. It has been a challenge to synthesize high-density BFO
ceramics, because a second phase or impurity can easily
result. To reduce electric leakage, many studies have focused
on doped BFO ceramics with ion substitutions to enhance
magnetic and electric properties.25–29 The structures of
(1–x)BiFeO3–xBaTiO3 solid solutions are rhombohedral,
cubic, and tetragonal for, respectively, 0 x 0.33,
0.33 x 0.925, and x> 0.925 in the low temperatures.30,31
Although BFO has been studied extensively in recent
years, its phase transition, dielectric properties, and magneto-
electric coupling still lack consistency and are not fully
understood. The main focus of this work is to study phase
transitions and magnetoelectric coupling of BFO and
(BiFeO3)0.95(BaTiO3)0.05 (BFO–5%BT) ceramics by in situ
high-resolution synchrotron x-ray diffraction (XRD).
Another task is to examine the BaTiO3-substitution effect on
dielectric and magnetic properties.
II. EXPERIMENTAL PROCEDURES
The BFO ceramic was prepared by the solid state reac-
tion method. The dried starting powders of Bi2O3 and Fe2O3
(purity 99.0%) were weighed in a 1.1:1 ratio to compen-
sate the low melting point of Bi, and then mixed in an agate
mortar for more than 24 h using alcohol as a medium. The
mixture was dried and mixed with polyvinyl acetate as a
binder for granulation. The ground mixture was pressed into
a 1.0-cm-diameter disk. The pressed BFO disk was sintered
in the region of 850–870 C for 1–3 h. The optimal sintering
condition for BFO ceramic is 860 C for 2 h and the density
is 90% of the theoretical density. For synthesis of BFO–
5%BT ceramic, the dried starting powders of BFO and BT
(purity 99.0%) were weighed in a 0.95:0.05 molar ratio
a)Author to whom correspondence should be addressed. Electronic mail:
039611@mail.fju.edu.tw.
0021-8979/2011/109(4)/044101/4/$30.00 VC 2011 American Institute of Physics109, 044101-1
JOURNAL OF APPLIED PHYSICS 109, 044101 (2011)
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and then a similar process was followed as for the BFO ce-
ramic. The optimal sintering condition for BFO–5%BT ce-
ramic is 960 C for 3 h and the density is 93% of the
theoretical density.
In situ high-resolution synchrotron XRD was performed
at the National Synchrotron Radiation Research Center (in
Taiwan) with a photon energy of 8.0 keV (k¼ 1.550 A˚). A
Wayne–Kerr Analyzer PMA3260A was used to obtain the
real (e0) and imaginary (e00) parts of dielectric permittivity.
Hysteresis loops of polarization versus E field were taken by
using a Sawyer-Tower circuit at f¼ 46 Hz.
III. RESULTS AND DISCUSSION
Figure 1 shows XRD spectra of BFO and BFO–5%BT
ceramics at room temperature. A two-peak splitting occurs
in the (110), (111), (210), and (220) reflections, suggesting a
rhombohedral structure.32 Some minor second phases (impu-
rity) of possible Bi25FeO39 (Ref. 21) and Bi2Fe4O9 (Ref. 33)
were observed as indicated by the symbols “*” and “#,”
respectively. BFO–5%BT has less impurity phases. The
XRD peaks of BFO–5%BT occur at lower 2h than those in
BFO, mainly due to the larger atomic radius of the Ba2þ ion
(1.35 A˚) compared with the Bi3þ ion (1.03 A˚) on the perov-
skite A site. The Fe3þ ion (0.64 A˚) and Ti4þ ion (0.605 A˚)
have similar atomic radii on the perovskite B site.
Figure 2 shows frequency-dependent dielectric permit-
tivity (e0) and dielectric loss (tan d¼ e00/e0) upon heating. The
temperature (Tm) corresponding to the maximum in e0 exhib-
its a broad frequency dispersion. For BFO, Tm shifts from
420 C at 50 kHz to 490 C at 1 MHz. BFO–5%BT does
not exhibit obvious frequency dispersion in e0 below 200 C
and its Tm shifts to lower temperatures. This dielectric
response is likely activated by the antiferromagnetic–para-
magnetic transition, which takes place at the Ne´el tempera-
ture (TN).
5–13 A similar but rather pronounced frequency
dispersion in dielectric maxima was observed in BFO–
10%BT ceramic, associated with a local minimum in rhom-
bohedral distortion angle aR near TN.
29 It was attributed to
the changes in relative positions of Bi3þ and Fe3þ ions in the
perovskite structure as temperature approaches TN. In addi-
tion, the neutron scattering result of BFO also revealed
changes of distortion and strain in oxygen octahedra (FeO6)
at TN caused by the magnetoelectric and/or magnetoelastic
couplings,8 which can change electric polarization.
The dielectric loss (tan d) of BFO and BFO–5%BT are
respectively about 0.2–0.4 and 0.02–0.04 at room tempera-
ture, indicating that 5 mol % BaTiO3 substitution can effi-
ciently reduce electric conductivity. The tan d of BFO–
5%BT remains less than 0.04 below 150 C, which is close
to the tan d% 0.02 of the soft PZT-5 ceramic.34 The dielec-
tric losses of BFO and BFO–5%BT exhibit an exponential
upturn above 300 C with magnitude proportional to 1/f,
indicating conductivity activated by thermal energy.
Figure 3 shows temperature-dependent (110) synchro-
tron XRD spectra of BFO and BFO–5%BT upon heating.
The insets are the XRD spectra near structural transitions.
The (110) XRD spectrum of BFO–5%BT is much broader
than in BFO, likely caused by random distributions of Bi and
Ba ions on the A site, and Fe and Ti ions on the B site. The
random displacements can cause different 2h reflections and
result in a broadening effect in XRD. Upon heating, two
(110) R-phase reflections of BFO exhibit a triple splitting
near 820 C and then merge into a single peak near 850 C,
indicating a transition sequence of rhombohedral (R)–
FIG. 1. (Color online) XRD spectra of (a) BFO and (b) BFO–5%BT at
room temperature. “*” and “#” correspond to second phases of possible
Bi25FeO39 and Bi2Fe4O9. The dashed lines indicate 2h shifts between BFO
and BFO–5%BT.
FIG. 2. (Color online) Dielectric permittivity e0 and loss (tan d) of (a) BFO
and (b) BFO–5%BT upon heating. (Insets) Enlargements of loss below
200 C.
044101-2 Wang et al. J. Appl. Phys. 109, 044101 (2011)
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18:57:14
orthorhombic (O)–cubic (C) upon heating. A R–O–C phase
sequence was also revealed in bulk BFO near 820 and
925 C from Raman and thermal studies.16 Our result is con-
sistent with the dielectric result of BFO–PZT solid solutions,
which predicted a ferroelectric Curie temperature near
850 C for BFO.13 Additional XRD peaks appear above
860 C in BFO, implying possible decompositions of Fe2O3
and Bi2Fe4O9 (Ref. 33) due to the loss of Bi in high-tempera-
ture region.
For BFO–5%BT, two (110) R-phase reflections merge
into a broad single peak near 830 C, indicating a rhombohe-
dral (R)–cubic (C) transition upon heating. This result may
imply that the orthorhombic phase is not favored by replac-
ing BaTiO3 in the perovskite structure. Decomposition does
not occur in BFO–5%BT below 900 C, implying that
5%BT substitution can stabilize the perovskite structure.
Figure 4 shows temperature-dependent lattice parame-
ters calculated from the (110) XRD peaks. A local minimum
in rhombohedral distortion angle aR (associated with a slight
inflection in lattice constant aR) occurs near 400 and 350
C
in BFO and BFO–5%BT, respectively. A similar minimum
of aR was reported near TN in BFO and BFO–10%BT.
29,35
This local minimum in aR suggests that the position shift
(or distortion) of Bi3þ cation gradually reaches a maximum
as temperature approaches TN. This confirms a coupling
between ferroelectric and magnetic order parameters near
TN, which causes the broad frequency-dependent dielectric
maxima as shown in Fig. 2.
Electric hysteresis loops at room temperature are given
in Fig. 5 and show attainable remanent polarizations of
0.01 and 0.1 lC/cm2 in BFO and BFO–5%BT, respec-
tively. However, the saturated polarization cannot be
achieved in either case. A similar hysteresis loop with an un-
saturated remanent polarization of 0.01–0.015 lC/cm2 was
observed from the BFO ceramic.9 As evidenced in Fig. 5,
BFO–5%BT can sustain a higher measuring field than BFO,
confirming that 5%BT substitution can reduce electric leak-
age and thus enhance polarization.
Figure 6 shows the magnetic hysteresis loops (magnet-
ization versus magnetic field) of BFO and BFO–5%BT at
FIG. 3. (Color online) The (110) synchrotron XRD spectra of (a) BFO and
(b) BFO–5%BT upon heating. (Insets) XRD spectra near structural transi-
tions. The dashed lines indicate the transition temperatures.
FIG. 4. (Color online) Temperature-dependent lattice parameters of (a)
BFO and (b) BFO–5%BT. aR, aR, and aC are lattice parameters of rhombo-
hedral (R) and cubic (C) structures and aO, bO, and cO are lattice constants
of orthorhombic (O) structure. The lattice parameters were calculated based
on the pseudocubic lattice.
FIG. 5. (Color online) Hysteresis loops of electric polarization vs E field of
BFO and BFO–5%BT at room temperature.
044101-3 Wang et al. J. Appl. Phys. 109, 044101 (2011)
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18:57:14
room temperature. The magnetization curve of BFO is linear
with the field, which is typical for an antiferromagnetic
arrangement of the Fe3þ magnetic moments. BFO–5%BT
exhibits a rather weak ferromagnetic nature, which is similar
to the magnetization curve observed in the BFO–10%BT ce-
ramic.29 These results suggest that BT substitution can
enhance the FM feature.
IV. CONCLUSIONS
A local minimum in rhombohedral distortion angle aR
(associated with an inflection in the lattice constant aR) was
revealed near 400 and 350 C in BFO and BFO–5%BT,
respectively. The local minimum in aR indicates that the
position of Bi3þ reaches the largest distortion near TN. This
suggests a magnetoelectric coupling near TN, which is re-
sponsible for the broad frequency dispersion in dielectric
maxima. This work suggests that the Ne´el temperatures of
BFO and BFO–5%BT are near 400 and 350 C, respectively.
A rhombohedral–orthorhombic–cubic transition sequence
takes place near 820 and 850 C in BFO upon heating. BFO–
5%BT exhibits a rhombohedral–cubic transition near
830 C. The dielectric loss of BFO–5%BT remains less than
0.04 below 150 C, which is close to the tan d% 0.02 of the
soft PZT-5 ceramics.34 The attainable polarization at room
temperature was improved from 0.01 lC/cm2 in BFO to
0.1 lC/cm2 in BFO–5%BT. The 5%BT substitution can
enhance dielectric and ferromagnetic responses.
ACKNOWLEDGMENTS
The authors would like to thank Dr. C.-S. Ku and Dr.
H.-Y. Lee for their assistance on the synchrotron experiment.
This work was supported by National Science Council of
Taiwan Grant No. 97-2112-M-030-003-MY3.
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FIG. 6. (Color online) Hysteresis loops of magnetization vs magnetic field
of BFO and BFO–5%BT at room temperature.
044101-4 Wang et al. J. Appl. Phys. 109, 044101 (2011)
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