We have reported the structural, thermal, microscopic, magnetization, polarization, and dielectric properties of BiFeO3 ceramics synthesized by a rapid liquid-phase sintering technique. Optimum conditions for the synthesis of single-phase BiFeO3 ceramics were obtained. Temperature-dependent magnetization and hysteresis loops indicate antiferromagnetic behavior in BiFeO3 at room temperature. Although saturated ferroelectric hysteresis loops were observed in single-phase BiFeO3 ceramic synthesized at 880 °C, the reduced polarization is found to be due to the high loss and low dielectric permittivity of the ceramic, which is caused by higher leakage current

Bhattacharya, P.

Burger, A.

Cui, Y.

Dadson, J.B.

Hunter, D.

Katiyar, R.

Loiutts, G.B.

Pradhan, Aswini K.

Roy, U.N.

Sellmyer, David J.

Zhang, Jun

Zhang, Kai

English

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University of Nebraska - Lincoln 
DigitalCommons@University of Nebraska - Lincoln 
David Sellmyer Publications Research Papers in Physics and Astronomy 
5-1-2005 
Magnetic and electrical properties of single-phase multiferroic 
BiFeO3 
Aswini K. Pradhan 
Center for Materials Research, Norfolk State University, apradhan@nsu.edu 
Kai Zhang 
Center for Materials Research, Norfolk State University, Norfolk, Virginia 
D. Hunter 
Center for Materials Research, Norfolk State University, Norfolk, Virginia 
J.B. Dadson 
Center for Materials Research, Norfolk State University, Norfolk, Virginia 
G.B. Loiutts 
Center for Materials Research, Norfolk State University, Norfolk, Virginia 
See next page for additional authors 
Follow this and additional works at: https://digitalcommons.unl.edu/physicssellmyer 
 Part of the Physics Commons 
Pradhan, Aswini K.; Zhang, Kai; Hunter, D.; Dadson, J.B.; Loiutts, G.B.; Bhattacharya, P.; Katiyar, R.; Zhang, 
Jun; Sellmyer, David J.; Roy, U.N.; Cui, Y.; and Burger, A., "Magnetic and electrical properties of single-
phase multiferroic BiFeO3" (2005). David Sellmyer Publications. 21. 
https://digitalcommons.unl.edu/physicssellmyer/21 
This Article is brought to you for free and open access by the Research Papers in Physics and Astronomy at 
DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in David Sellmyer Publications 
by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. 
Authors 
Aswini K. Pradhan, Kai Zhang, D. Hunter, J.B. Dadson, G.B. Loiutts, P. Bhattacharya, R. Katiyar, Jun Zhang, 
David J. Sellmyer, U.N. Roy, Y. Cui, and A. Burger 
This article is available at DigitalCommons@University of Nebraska - Lincoln: https://digitalcommons.unl.edu/
physicssellmyer/21 
Magnetic and electrical properties of single-phase multiferroic BiFeO 3
A. K. Pradhan,a! Kai Zhang, D. Hunter, J. B. Dadson, and G. B. Loutts
Center for Materials Research, Norfolk State University, 700 Park Avenue, Norfolk, Virginia 23504
P. Bhattacharya and R. Katiyar
Department of Physics, University of Puerto Rico, San Juan, P.O. Box 23343, Puerto Rico 00931
Jun Zhang and D. J. Sellmyer
Department of Physics and Astronomy and Center for Materials Research and Analysis,
University of Nebraska, Lincoln, Nebraska 68588-0113
U. N. Roy, Y. Cui, and A. Burger
Department of Physics, Fisk University, 1000, 17 Avenue North, Nashville, Tennessee 37208
sReceived 21 June 2004; accepted 3 February 2005; published online 18 April 2005d
We have reported the structural, thermal, microscopic, magnetization, polarization, and dielectric
properties of BiFeO3 ceramics synthesized by a rapid liquid-phase sintering technique. Optimum
conditions for the synthesis of single-phase BiFeO3 ceramics were obtained. Temperature-dependent
magnetization and hysteresis loops indicate antiferromagnetic behavior in BiFeO3 at room
temperature. Although saturated ferroelectric hysteresis loops were observed in single-phase BiFeO3
ceramic synthesized at 880 °C, the reduced polarization is found to be due to the high loss and low
dielectric permittivity of the ceramic, which is caused by higher leakage current. ©2005 American
Institute of Physics. fDOI: 10.1063/1.1881775g
I. INTRODUCTION
Multiferroics are a class of materials with coupled elec-
tric, magnetic, and structural order parameters that yield si-
multaneous effects of ferroelectricity, ferromagnetism, and
ferroelasticity in the same material.1 These materials offer a
wide opportunity for potential applications in information
storage, such as spintronic devices and sensors, where both
electric and magnetic polarizations can be coupled, giving
enough opportunity for manipulating devices. The perovskite
BiFeO3 sBFOd belongs to this class of multiferroic materials
with ferroelectric transition temperatureTc,1103 K and an-
tiferromagnetic transitionTN,643 K. BFO belongs to other
magnetic ferroelectrics, such as YMnO3 and BiMnO3. Bulk
BFO, especially synthesized through solid solution, exhibits
weak ferromagnetic properties at room temperature due to
residual moment from a canted spin structure,2 which is be-
lieved to be generated from the impurity phase related to Fe
phase separation. In addition, a major problem generally en-
countered in transport measurements is the high leakage cur-
rent due to the low resistivity of the sample, which creates
difficulties in the observation of the ferroelectric hysteresis
loops. The high leakage current in bulk is attributed to de-
fects and nonstoichiometry. Therefore, this material becomes
unsuitable for applications. However, there is tremendous
success in the fabrication of heteroepitaxially constrained
thin films of BFO which shows enhanced polarization with
saturated hysteresis loop and ferromagnetic behavior at room
temperature.3,4
There are several reports5,6 on the fabrication of BFO
dispersed in BaTiO3 matrix to restrict the second phase, leak-
age current, and low resistivity in the sample. Recently,
single-phase BFO ceramic samples have been synthesized by
rapid liquid-phase sintering technique.7 These ceramic
samples show relatively high resistivity and saturated ferro-
electric hysteresis loops at room temperature and could be
useful for potential applications. This is a dramatic improve-
ment over past techniques used for the fabrication of BFO
ceramics in which very often canted ferromagnetic spin
structures were observed due to the impurity phase. How-
ever, detailed studies on microscopic grains, thermal, mag-
netic, and electrical properties related to frequency-
dependent dielectric loss are still lacking in this potentially
important ceramic sample. In this paper, we report on the
development of various microstructures of liquid-phase-
sintered samples processed at various temperatures, thermal
properties, temperature-dependent magnetic behavior, and
electrical properties including ferroelectric loops and related
dielectric loss up tof =1 MHz. Our results suggest that pure
BFO samples exhibit antiferromagnetic behavior over the
whole temperature region, 5–300 K, and saturated ferroelec-
tric loops at room temperature. However, the ceramic sample
shows high dielectric loss, especially at higher frequency
range, which may be one of the major challenges for appli-
cations.
II. EXPERIMENT
BiFeO3 samples were synthesized by the solid-state re-
action method. Stoichiometric amounts of Bi2O3 s99.99%d
and Fe2O3 s99.99%d powders were thoroughly mixed using
isopropyl alcohol and ground. The mixtures were dried,
pressed, and sintered at different temperatures for 400–450 s,
as described in Ref. 7 for rapid liquid-phase sintering. Phase
identification was performed by the x-ray powder diffraction
adAuthor to whom correspondence should be addressed: electronic mail:
apradhan@nsu.edu
JOURNAL OF APPLIED PHYSICS97, 093903s2005d
0021-8979/2005/97~9!/093903/4/$22.50 © 2005 American Institute of Physics97, 093903-1
Downloaded 14 Nov 2006 to 129.93.16.206. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp
sXRDd method on a Rigaku x-ray diffractometer using
CuKa radiation. The magnetization was measured using
Quantum Design superconducting quantum interference de-
vice fSQUID magnetic property measurement system
sMPMSdg. RT6000 ferroelectric tester and Agilent dielectric
bridge measured the ferroelectric and dielectric properties of
the sintered ceramics at room temperature, respectively.
III. RESULTS AND DISCUSSION
Figure 1 shows the XRD patterns of the BFO samples
sintered at 850–880 °C, respectively, prepared by liquid-
phase sintering for,450 s. The sample sintered at 880 °C
shows a single phase, whereas the sample sintered at 850 °C
shows evidence for the presence of a second phase, probably
arising from Bi36Fe2O57 and related phases. Hence the rapid
sintering at 880 °C for a short time shows a completion of
reaction to form a single-phase BiFeO3. It is noted that the
sintering temperature higher than that of 880 °C, such as at
890 °C, forms many undesirable phases. In order to investi-
gate the thermal behavior related to phase changes and melt-
ing phenomena in BFO, differential scanning calorimetry
sDSCd results are presented in Fig. 2. A distinct endothermic
transition at 825 °C in heat flow versus temperature curve is
observed. This transition is very consistent with the change
in curvature in heat-flow curve, as shown in Fig. 2 as dashed
lines. The melting point of Bi2O3 is about 825 °C and this
transition can be certainly assigned due to the melting of
Bi2O3. Therefore, it remains very important at what tempera-
ture the liquid phase of Bi2O3 forms a single-phase material
with Fe2O3. It turns out that 880 °C becomes the exact tem-
perature for rapid sintering where single-phase BFO is
formed. On the other hand, there is a remarkable weight loss
after the second transition at 960 °C, indicating loss of Bi2O3
through sublimation process. Hence rapid heating rate and
short-time sintering of BiFeO3 at 880 °C remains a clue to
avoid loss of Bi, segregation of Fe2O3, and formation of
other undesired phases, as indicated by the XRD. Lower-
temperature rapid sintering is also unfavorable due to the
incomplete melting of Bi2O3 and the formation of interme-
diate phases.
Figure 3 shows the scanning electron microscopysSEMd
results of BFO synthesized by rapid thermal sintering at 850,
880, and 890 °C. The SEM micrographs elucidate the grain
growth phenomena in sintered BFO at various temperatures.
The grain size increases enormously with increasing tem-
perature. The average grain of 1–2µm increases to 5–10µm
in size when sintering temperature increases from 850 to
880 °C. However, the density and number of bright spots that
probably correspond to segregated Fe2O3 particles decreases
dramatically when sintering temperature increases from 850
to 880 °C. In fact, several red-colored spots embedded in a
black matrix were noticed when examined in an optical mi-
croscope. This suggests that the bright spots observed in
SEM are due to Fe2O3 particles. On the other hand, the SEM
micrograph of the sample sintered at 890 °C shows a uni-
form surface displaying very large grainss.50 mmd. How-
ever, this sample is multiphase due to partial loss of Bi2O3
from its melt, as discussed in x-ray and DSC studies. This
suggests that although large grains are produced by melting,
the grains contain many undesired phases.
In order to illustrate the magnetic ordering in BFO, the
temperature dependence of zero-field-cooledsZFCd and
field-cooledsFCd magnetization curves are presented in Fig.
4sad. In these measurements, the sample was cooled in either
ZFC to a desired temperature and then a magnetic field was
applied or FC. The measurements were done while warming
the sample in the magnetic field for both cases. ForH
=0.2 T, the FC and ZFC curves separate at around 160 K
and MFC−MZFC increases as temperature decreases. How-
ever, the FC and ZFC curves again merged at very low tem-
peratures. There is a distinct irreversibility at,160 K ac-
companied by a remarkable thermomagnetic hysteresissor
irreversibilityd with MZFCÞMFC at the irreversibility tem-
peratureTirr. This type of irreversibility in magnetization is
generally indicative of antiferromagneticsAFd ordering;8
however, for a simple AF no thermomagnetic hysteresis is
expected. This latter effect is traditionally accepted as a hall-
FIG. 1. X-ray diffraction pattern of BiFeO3 ceramics synthesized at 820 and
880 °C by liquid-phase sintering. The asterisks represent the undesired sec-
ond phase.
FIG. 2. Temperature dependence of weight loss and heat flow of BFO
measured by differential scanning calorimetry.
093903-2 Pradhan et al. J. Appl. Phys. 97, 093903 ~2005!
Downloaded 14 Nov 2006 to 129.93.16.206. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp
mark of a spin-glass system9 with a characteristic spin-glass
transition temperatureTg. The most interesting phenomenon
is the observation of an anomalous temperature dependence
of magnetization, such as a decrease in magnetization down
to 150 K accompanied by a small jump at about 265 K, and
then a sharp increase of magnetization, as the temperature is
further lowered. Although the latter temperature-dependent
magnetization behavior is understood in the framework of
antiferromagnetic correlation, the decrease of magnetization
with temperature in the temperature range of from 265 to
about 150 K remains puzzling and could not be explained.
One of the suspects for such anomalous magnetization
may be the oxygen vacancy concentration and also the exis-
tence of Fe2+ ions. It was proposed4 that the oxygen defi-
ciency may lead to valence fluctuation of Fe ions 3+ to 2+
statesd in BFO resulting in high conductivity. However, such
a possibility was recently ruled out in samples synthesized
by liquid-phase sintering7 where sintering time for the heat-
ing, sintering, and cooling times were limited to seconds
only. We argue that even though the sintering time is consid-
erably small, there may exist small concentrations of oxygen
vacancies resulting in Fe2+ and Fe3+ valence fluctuations
which may influence significantly both magnetic and electri-
cal properties. Hence, the observed anomalous magnetization
behavior may be related to such valence fluctuations, al-
though further experiments are necessary to reveal the com-
plete fact. However, the magnetic hysteresis loops, as shown
in Fig. 4sbd, illustrate a linear field dependence, indicating
that BFO shows antiferromagnetic behavior both at 5 and
300 K. On the other hand, the magnetization and magnetic
hysteresis results confirm the absence of canted ferromag-
netic behavior in this sample. This suggests the absence of
F -related clusters or impurities in the sample.
The ferroelectric polarization hysteresis loops are pre-
sented in Fig. 5 for the BFO sample sintered at 880 °C. Ac-
cording to previous reports,2,10 the spontaneous polarization
in BFO ceramics and single crystals is significantly lower
than the expected value for other ferroelectric materials hav-
ing comparableTc, such as PbTiO3 with Tc,763 K, and
spontaneous polarization of 80–100mC/cm2. The spontane-
ous polarizationsPsd, remnant polarizationsRsd, and the co-
ercive fieldsEcd are about 3.5mC/cm2, 2.5 mC/cm2, and 40
kV/cm, respectively. Although these values are very compa-
rable to the values reported in the single crystal,10 they are
almost half of the reported values for BFO ceramics synthe-
sized using the same technique. However, a saturated polar-
FIG. 3. Scanning electron microscopic images of BFO ceramics synthesized
at sad 820, sbd 880, andscd 890 °C by rapid liquid-phase sintering. The
indicated scale in all images is 5µm.
FIG. 4. sad The temperature-dependent FC and ZFC magnetizations of BFO
ceramic synthesized by liquid-phase sintering at 880 °C.sbd The magneti-
zation hysteresis loops of BFO at 5 and 300 K.
093903-3 Pradhan et al. J. Appl. Phys. 97, 093903 ~2005!
Downloaded 14 Nov 2006 to 129.93.16.206. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp
ization hysteresis loop was observed at room temperature
under an applied field of 95 kV/cm. In order to understand
the origin of such low values forPs, Rs, andEc, the dielectric
constant and the loss as a function of frequency are presented
in Fig. 6. It is clear from Fig. 6 that the dielectric constant is
a strong function of frequency and it decreases significantly
with increasing frequency. However, a reverse trend is ob-
tained for the dielectric loss factor, tand. The high loss in
this material generally originated from the higher conductiv-
ity causing higher leakage current. This is one of the major
reasons for obtaining lower values forPs, Rs, andEc in BFO
pure materials. Therefore, it is necessary to synthesize BFO
in other dielectric matrix to enhance its dielectric and loss
properties in order to realize its practical applications.
IV. CONCLUSION
We have reported the x-ray, thermal, microscopic, mag-
netization, polarization, and dielectric properties of BiFeO3
ceramics synthesized by a rapid liquid-phase sintering tech-
nique. Temperature-dependent magnetization and hysteresis
loops indicate antiferromagnetic behavior in BiFeO3 at room
temperature. Anomalous temperature-dependent magnetiza-
tion behavior predicts the presence of a small concentration
of oxygen-related defects due to Fe valence fluctuations. Al-
though saturated ferroelectric hysteresis loops were observed
in single-phase BiFeO3 ceramic synthesized at 880 °C, the
reduced polarization is found to be due to the high loss and
low dielectric permittivity of the ceramic. In the present pa-
per, we have exclusively demonstrated the multiphase–
single-phase–multiphase transition in BiFeO3 ceramics with
increasing sintering temperature and their structural, thermal,
and microscopic evolutions. The persisting leakage current
problem in optimally sintered ceramics needs to be solved
for better applications.
ACKNOWLEDGMENTS
This work is supported by the National Aeronautics and
Space AdministrationsNASAd and National Science Foun-
dation sNSFd for Center for Research Excellence in Science
and TechnologysCRESTd Grant No. HRD-9805059. Re-
search at the University of Nebraska is supported by NSF-
MRSEC, ONR, and CMRA.
1E. K. H. Salje,Phase Transitions in Ferroelectrics, Ferroelastic and Co-
elastic CrystalssCambridge University Press, Cambridge, 1990d.
2G. A. Smolenski, Sov. Phys. Usp.25, 475 s1982d.
3J. Wanget al., Science299, 1719s2003d.
4V. R. Palkar, J. John, and R. Pinto, Appl. Phys. Lett.80, 1628s2002d.
5K. Ueda, H. Tabata, and T. Kawai, Appl. Phys. Lett.75, 555 s1999d.
6M. Mahesh Kumar, A. Srinivas, and S. V. Suryanarayan, J. Appl. Phys.
87, 855 s2000d.
7Y. P. Wang, L. Zhou, M. F. Zhang, X. Y. Chen, J.-M. Liu, and Z. G. Liu,
Appl. Phys. Lett.84, 1731s2004d.
8N. Hill and N. A. Spaldin,Magnetic MaterialssCambridge University
Press, Cambridge, 2003d.
9K. H. Fischer, J. A. Hertz, and D. Edwards,Spin GlassessCambridge
University Press, Cambridge, 1991d.
10J. R. Teague, R. Carson, and W. J. James, Solid State Commun.8, 1073
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FIG. 5. Polarization hysteresis loops of BFO ceramic synthesized by liquid-
phase sintering at 880 °C for different field cyclings.
FIG. 6. Frequency dependence of the dielectric constant and loss of the
BFO ceramic synthesized by liquid-phase sintering at 880 °C.
093903-4 Pradhan et al. J. Appl. Phys. 97, 093903 ~2005!
Downloaded 14 Nov 2006 to 129.93.16.206. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp


Magnetic and electrical properties of single-phase multiferroic BiFeO\u3csub\u3e3\u3c/sub\u3e

DigitalCommons@University of Nebraska

University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln David Sellmyer Publications Research Papers in Physics and Astronomy 5-1-2005 Magnetic and electrical properties of single-phase multiferroic BiFeO3 Aswini K. Pradhan Center for Materials Research, Norfolk State University, apradhan@nsu.edu Kai Zhang Center for Materials Research, Norfolk State University, Norfolk, Virginia D. Hunter Center for Materials Research, Norfolk State University, Norfolk, Virginia J.B. Dadson Center for Materials Research, Norfolk State University, Norfolk, Virginia G.B. Loiutts Center for Materials Research, Norfolk State University, Norfolk, Virginia See next page for additional authors Follow this and additional works at: https://digitalcommons.unl.edu/physicssellmyer  Part of the Physics Commons Pradhan, Aswini K.; Zhang, Kai; Hunter, D.; Dadson, J.B.; Loiutts, G.B.; Bhattacharya, P.; Katiyar, R.; Zhang, Jun; Sellmyer, David J.; Roy, U.N.; Cui, Y.; and Burger, A., "Magnetic and electrical properties of single-phase multiferroic BiFeO3" (2005). David Sellmyer Publications. 21. https://digitalcommons.unl.edu/physicssellmyer/21 This Article is brought to you for free and open access by the Research Papers in Physics and Astronomy at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in David Sellmyer Publications by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. Authors Aswini K. Pradhan, Kai Zhang, D. Hunter, J.B. Dadson, G.B. Loiutts, P. Bhattacharya, R. Katiyar, Jun Zhang, David J. Sellmyer, U.N. Roy, Y. Cui, and A. Burger This article is available at DigitalCommons@University of Nebraska - Lincoln: https://digitalcommons.unl.edu/physicssellmyer/21 Magnetic and electrical properties of single-phase multiferroic BiFeO 3A. K. Pradhan,a! Kai Zhang, D. Hunter, J. B. Dadson, and G. B. LouttsCenter for Materials Research, Norfolk State University, 700 Park Avenue, Norfolk, Virginia 23504P. Bhattacharya and R. KatiyarDepartment of Physics, University of Puerto Rico, San Juan, P.O. Box 23343, Puerto Rico 00931Jun Zhang and D. J. SellmyerDepartment of Physics and Astronomy and Center for Materials Research and Analysis,University of Nebraska, Lincoln, Nebraska 68588-0113U. N. Roy, Y. Cui, and A. BurgerDepartment of Physics, Fisk University, 1000, 17 Avenue North, Nashville, Tennessee 37208sReceived 21 June 2004; accepted 3 February 2005; published online 18 April 2005dWe have reported the structural, thermal, microscopic, magnetization, polarization, and dielectricproperties of BiFeO3 ceramics synthesized by a rapid liquid-phase sintering technique. Optimumconditions for the synthesis of single-phase BiFeO3 ceramics were obtained. Temperature-dependentmagnetization and hysteresis loops indicate antiferromagnetic behavior in BiFeO3 at roomtemperature. Although saturated ferroelectric hysteresis loops were observed in single-phase BiFeO3ceramic synthesized at 880 °C, the reduced polarization is found to be due to the high loss and lowdielectric permittivity of the ceramic, which is caused by higher leakage current. ©2005 AmericanInstitute of Physics. fDOI: 10.1063/1.1881775gI. INTRODUCTIONMultiferroics are a class of materials with coupled elec-tric, magnetic, and structural order parameters that yield si-multaneous effects of ferroelectricity, ferromagnetism, andferroelasticity in the same material.1 These materials offer awide opportunity for potential applications in informationstorage, such as spintronic devices and sensors, where bothelectric and magnetic polarizations can be coupled, givingenough opportunity for manipulating devices. The perovskiteBiFeO3 sBFOd belongs to this class of multiferroic materialswith ferroelectric transition temperatureTc,1103 K and an-tiferromagnetic transitionTN,643 K. BFO belongs to othermagnetic ferroelectrics, such as YMnO3 and BiMnO3. BulkBFO, especially synthesized through solid solution, exhibitsweak ferromagnetic properties at room temperature due toresidual moment from a canted spin structure,2 which is be-lieved to be generated from the impurity phase related to Fephase separation. In addition, a major problem generally en-countered in transport measurements is the high leakage cur-rent due to the low resistivity of the sample, which createsdifficulties in the observation of the ferroelectric hysteresisloops. The high leakage current in bulk is attributed to de-fects and nonstoichiometry. Therefore, this material becomesunsuitable for applications. However, there is tremendoussuccess in the fabrication of heteroepitaxially constrainedthin films of BFO which shows enhanced polarization withsaturated hysteresis loop and ferromagnetic behavior at roomtemperature.3,4There are several reports5,6 on the fabrication of BFOdispersed in BaTiO3 matrix to restrict the second phase, leak-age current, and low resistivity in the sample. Recently,single-phase BFO ceramic samples have been synthesized byrapid liquid-phase sintering technique.7 These ceramicsamples show relatively high resistivity and saturated ferro-electric hysteresis loops at room temperature and could beuseful for potential applications. This is a dramatic improve-ment over past techniques used for the fabrication of BFOceramics in which very often canted ferromagnetic spinstructures were observed due to the impurity phase. How-ever, detailed studies on microscopic grains, thermal, mag-netic, and electrical properties related to frequency-dependent dielectric loss are still lacking in this potentiallyimportant ceramic sample. In this paper, we report on thedevelopment of various microstructures of liquid-phase-sintered samples processed at various temperatures, thermalproperties, temperature-dependent magnetic behavior, andelectrical properties including ferroelectric loops and relateddielectric loss up tof =1 MHz. Our results suggest that pureBFO samples exhibit antiferromagnetic behavior over thewhole temperature region, 5–300 K, and saturated ferroelec-tric loops at room temperature. However, the ceramic sampleshows high dielectric loss, especially at higher frequencyrange, which may be one of the major challenges for appli-cations.II. EXPERIMENTBiFeO3 samples were synthesized by the solid-state re-action method. Stoichiometric amounts of Bi2O3 s99.99%dand Fe2O3 s99.99%d powders were thoroughly mixed usingisopropyl alcohol and ground. The mixtures were dried,pressed, and sintered at different temperatures for 400–450 s,as described in Ref. 7 for rapid liquid-phase sintering. Phaseidentification was performed by the x-ray powder diffractionadAuthor to whom correspondence should be addressed: electronic mail:apradhan@nsu.eduJOURNAL OF APPLIED PHYSICS97, 093903s2005d0021-8979/2005/97~9!/093903/4/$22.50 © 2005 American Institute of Physics97, 093903-1Downloaded 14 Nov 2006 to 129.93.16.206. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jspsXRDd method on a Rigaku x-ray diffractometer usingCuKa radiation. The magnetization was measured usingQuantum Design superconducting quantum interference de-vice fSQUID magnetic property measurement systemsMPMSdg. RT6000 ferroelectric tester and Agilent dielectricbridge measured the ferroelectric and dielectric properties ofthe sintered ceramics at room temperature, respectively.III. RESULTS AND DISCUSSIONFigure 1 shows the XRD patterns of the BFO samplessintered at 850–880 °C, respectively, prepared by liquid-phase sintering for,450 s. The sample sintered at 880 °Cshows a single phase, whereas the sample sintered at 850 °Cshows evidence for the presence of a second phase, probablyarising from Bi36Fe2O57 and related phases. Hence the rapidsintering at 880 °C for a short time shows a completion ofreaction to form a single-phase BiFeO3. It is noted that thesintering temperature higher than that of 880 °C, such as at890 °C, forms many undesirable phases. In order to investi-gate the thermal behavior related to phase changes and melt-ing phenomena in BFO, differential scanning calorimetrysDSCd results are presented in Fig. 2. A distinct endothermictransition at 825 °C in heat flow versus temperature curve isobserved. This transition is very consistent with the changein curvature in heat-flow curve, as shown in Fig. 2 as dashedlines. The melting point of Bi2O3 is about 825 °C and thistransition can be certainly assigned due to the melting ofBi2O3. Therefore, it remains very important at what tempera-ture the liquid phase of Bi2O3 forms a single-phase materialwith Fe2O3. It turns out that 880 °C becomes the exact tem-perature for rapid sintering where single-phase BFO isformed. On the other hand, there is a remarkable weight lossafter the second transition at 960 °C, indicating loss of Bi2O3through sublimation process. Hence rapid heating rate andshort-time sintering of BiFeO3 at 880 °C remains a clue toavoid loss of Bi, segregation of Fe2O3, and formation ofother undesired phases, as indicated by the XRD. Lower-temperature rapid sintering is also unfavorable due to theincomplete melting of Bi2O3 and the formation of interme-diate phases.Figure 3 shows the scanning electron microscopysSEMdresults of BFO synthesized by rapid thermal sintering at 850,880, and 890 °C. The SEM micrographs elucidate the graingrowth phenomena in sintered BFO at various temperatures.The grain size increases enormously with increasing tem-perature. The average grain of 1–2µm increases to 5–10µmin size when sintering temperature increases from 850 to880 °C. However, the density and number of bright spots thatprobably correspond to segregated Fe2O3 particles decreasesdramatically when sintering temperature increases from 850to 880 °C. In fact, several red-colored spots embedded in ablack matrix were noticed when examined in an optical mi-croscope. This suggests that the bright spots observed inSEM are due to Fe2O3 particles. On the other hand, the SEMmicrograph of the sample sintered at 890 °C shows a uni-form surface displaying very large grainss.50 mmd. How-ever, this sample is multiphase due to partial loss of Bi2O3from its melt, as discussed in x-ray and DSC studies. Thissuggests that although large grains are produced by melting,the grains contain many undesired phases.In order to illustrate the magnetic ordering in BFO, thetemperature dependence of zero-field-cooledsZFCd andfield-cooledsFCd magnetization curves are presented in Fig.4sad. In these measurements, the sample was cooled in eitherZFC to a desired temperature and then a magnetic field wasapplied or FC. The measurements were done while warmingthe sample in the magnetic field for both cases. ForH=0.2 T, the FC and ZFC curves separate at around 160 Kand MFC−MZFC increases as temperature decreases. How-ever, the FC and ZFC curves again merged at very low tem-peratures. There is a distinct irreversibility at,160 K ac-companied by a remarkable thermomagnetic hysteresissorirreversibilityd with MZFCÞMFC at the irreversibility tem-peratureTirr. This type of irreversibility in magnetization isgenerally indicative of antiferromagneticsAFd ordering;8however, for a simple AF no thermomagnetic hysteresis isexpected. This latter effect is traditionally accepted as a hall-FIG. 1. X-ray diffraction pattern of BiFeO3 ceramics synthesized at 820 and880 °C by liquid-phase sintering. The asterisks represent the undesired sec-ond phase.FIG. 2. Temperature dependence of weight loss and heat flow of BFOmeasured by differential scanning calorimetry.093903-2 Pradhan et al. J. Appl. Phys. 97, 093903 ~2005!Downloaded 14 Nov 2006 to 129.93.16.206. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jspmark of a spin-glass system9 with a characteristic spin-glasstransition temperatureTg. The most interesting phenomenonis the observation of an anomalous temperature dependenceof magnetization, such as a decrease in magnetization downto 150 K accompanied by a small jump at about 265 K, andthen a sharp increase of magnetization, as the temperature isfurther lowered. Although the latter temperature-dependentmagnetization behavior is understood in the framework ofantiferromagnetic correlation, the decrease of magnetizationwith temperature in the temperature range of from 265 toabout 150 K remains puzzling and could not be explained.One of the suspects for such anomalous magnetizationmay be the oxygen vacancy concentration and also the exis-tence of Fe2+ ions. It was proposed4 that the oxygen defi-ciency may lead to valence fluctuation of Fe ions 3+ to 2+statesd in BFO resulting in high conductivity. However, sucha possibility was recently ruled out in samples synthesizedby liquid-phase sintering7 where sintering time for the heat-ing, sintering, and cooling times were limited to secondsonly. We argue that even though the sintering time is consid-erably small, there may exist small concentrations of oxygenvacancies resulting in Fe2+ and Fe3+ valence fluctuationswhich may influence significantly both magnetic and electri-cal properties. Hence, the observed anomalous magnetizationbehavior may be related to such valence fluctuations, al-though further experiments are necessary to reveal the com-plete fact. However, the magnetic hysteresis loops, as shownin Fig. 4sbd, illustrate a linear field dependence, indicatingthat BFO shows antiferromagnetic behavior both at 5 and300 K. On the other hand, the magnetization and magnetichysteresis results confirm the absence of canted ferromag-netic behavior in this sample. This suggests the absence ofF -related clusters or impurities in the sample.The ferroelectric polarization hysteresis loops are pre-sented in Fig. 5 for the BFO sample sintered at 880 °C. Ac-cording to previous reports,2,10 the spontaneous polarizationin BFO ceramics and single crystals is significantly lowerthan the expected value for other ferroelectric materials hav-ing comparableTc, such as PbTiO3 with Tc,763 K, andspontaneous polarization of 80–100mC/cm2. The spontane-ous polarizationsPsd, remnant polarizationsRsd, and the co-ercive fieldsEcd are about 3.5mC/cm2, 2.5 mC/cm2, and 40kV/cm, respectively. Although these values are very compa-rable to the values reported in the single crystal,10 they arealmost half of the reported values for BFO ceramics synthe-sized using the same technique. However, a saturated polar-FIG. 3. Scanning electron microscopic images of BFO ceramics synthesizedat sad 820, sbd 880, andscd 890 °C by rapid liquid-phase sintering. Theindicated scale in all images is 5µm.FIG. 4. sad The temperature-dependent FC and ZFC magnetizations of BFOceramic synthesized by liquid-phase sintering at 880 °C.sbd The magneti-zation hysteresis loops of BFO at 5 and 300 K.093903-3 Pradhan et al. J. Appl. Phys. 97, 093903 ~2005!Downloaded 14 Nov 2006 to 129.93.16.206. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jspization hysteresis loop was observed at room temperatureunder an applied field of 95 kV/cm. In order to understandthe origin of such low values forPs, Rs, andEc, the dielectricconstant and the loss as a function of frequency are presentedin Fig. 6. It is clear from Fig. 6 that the dielectric constant isa strong function of frequency and it decreases significantlywith increasing frequency. However, a reverse trend is ob-tained for the dielectric loss factor, tand. The high loss inthis material generally originated from the higher conductiv-ity causing higher leakage current. This is one of the majorreasons for obtaining lower values forPs, Rs, andEc in BFOpure materials. Therefore, it is necessary to synthesize BFOin other dielectric matrix to enhance its dielectric and lossproperties in order to realize its practical applications.IV. CONCLUSIONWe have reported the x-ray, thermal, microscopic, mag-netization, polarization, and dielectric properties of BiFeO3ceramics synthesized by a rapid liquid-phase sintering tech-nique. Temperature-dependent magnetization and hysteresisloops indicate antiferromagnetic behavior in BiFeO3 at roomtemperature. Anomalous temperature-dependent magnetiza-tion behavior predicts the presence of a small concentrationof oxygen-related defects due to Fe valence fluctuations. Al-though saturated ferroelectric hysteresis loops were observedin single-phase BiFeO3 ceramic synthesized at 880 °C, thereduced polarization is found to be due to the high loss andlow dielectric permittivity of the ceramic. In the present pa-per, we have exclusively demonstrated the multiphase–single-phase–multiphase transition in BiFeO3 ceramics withincreasing sintering temperature and their structural, thermal,and microscopic evolutions. The persisting leakage currentproblem in optimally sintered ceramics needs to be solvedfor better applications.ACKNOWLEDGMENTSThis work is supported by the National Aeronautics andSpace AdministrationsNASAd and National Science Foun-dation sNSFd for Center for Research Excellence in Scienceand TechnologysCRESTd Grant No. HRD-9805059. Re-search at the University of Nebraska is supported by NSF-MRSEC, ONR, and CMRA.1E. K. H. Salje,Phase Transitions in Ferroelectrics, Ferroelastic and Co-elastic CrystalssCambridge University Press, Cambridge, 1990d.2G. A. Smolenski, Sov. Phys. Usp.25, 475 s1982d.3J. Wanget al., Science299, 1719s2003d.4V. R. Palkar, J. John, and R. Pinto, Appl. Phys. Lett.80, 1628s2002d.5K. Ueda, H. Tabata, and T. Kawai, Appl. Phys. Lett.75, 555 s1999d.6M. Mahesh Kumar, A. Srinivas, and S. V. Suryanarayan, J. Appl. Phys.87, 855 s2000d.7Y. P. Wang, L. Zhou, M. F. Zhang, X. Y. Chen, J.-M. Liu, and Z. G. Liu,Appl. Phys. Lett.84, 1731s2004d.8N. Hill and N. A. Spaldin,Magnetic MaterialssCambridge UniversityPress, Cambridge, 2003d.9K. H. Fischer, J. A. Hertz, and D. Edwards,Spin GlassessCambridgeUniversity Press, Cambridge, 1991d.10J. R. Teague, R. Carson, and W. J. James, Solid State Commun.8, 1073s1970d.FIG. 5. Polarization hysteresis loops of BFO ceramic synthesized by liquid-phase sintering at 880 °C for different field cyclings.FIG. 6. Frequency dependence of the dielectric constant and loss of theBFO ceramic synthesized by liquid-phase sintering at 880 °C.093903-4 Pradhan et al. J. Appl. Phys. 97, 093903 ~2005!Downloaded 14 Nov 2006 to 129.93.16.206. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp

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Magnetic and electrical properties of single-phase multiferroic BiFeO\u3csub\u3e3\u3c/sub\u3e

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