Download Effect of Barium Substitution on Ferroelectric and Magnetic

Effect of Barium Substitution on Ferroelectric and
Magnetic Properties of Bismuth Ferrite
Rajasree Das and Kalyan Mandal
S. N. Bose National Centre for Basic Sciences, Kolkata 700 098, India
We successfully synthesized Bi
Ba FeO
replacing Bi by Ba using a chemical synthesis route. The phase
and crystal structure of the samples were studied by X-ray diffraction (XRD). Peaks in the XRD shifted toward lower value with
the increase in Ba concentration due to the increase in the unit cell size. Detail thermal behavior was performed by differential scanning calorimetry and differential thermal analysis (DTA) to investigate the change in magnetic and ferroelectric transition temperature,
respectively, due to Ba substitution. Magnetic, dielectric, and magnetoelectric properties of the samples were also studied in detail.
Antiferromagnetic bismuth ferrite was converted to ferromagnetic materials at room temperature on incorporating Ba in the crystal
structure. Polarization versus electric field (P-E loop) and dielectric constant were also found to change significantly in the presence of
a magnetic field in the aforementioned samples. Appreciable magnetodielectric effect
also indicated effective
magnetoelectric coupling within the materials.
Index Terms—Fatigue resistance, ferroelectric transition, magnetoelectric coupling, multiferroics.
I. INTRODUCTION
B
ISMUTH ferrite (BFO) is one of the most interesting
members [1] of multiferroic family, which shows large
magnetoelectric coupling in single phase at room temperature
(RT). It has been studied by many groups worldwide due to its
importance in fundamental research, as well as in commercial
applications.
BFO has a rhombohedrally distorted perovskite structure that
belongs to the R3c space group [2]. Compared to other multiferroics, BFO exhibits a higher ferroelectric Curie temperature
(
C) and a high G-type antiferromagnetic (AFM) ordering (
C) temperature [3]. At RT, pure bismuth ferrite shows an AFM nature because of its complicated cycloidal
spin structure with a wavelength of about 62 nm along the
axis at RT. Though BFO is considered as a promising
candidate for magnetic storage or in the applications of spintronics devices [4], leakage current due to oxygen vacancies or
impurities is the major problem in BFO. It has been observed
that doping [5]–[7] with ions at A-sites and B-sites reduce the
leakage current in BFO and enhance the multiferroic properties.
Though Ba-doped BFO shows promising results, few reports are available on this composition [8]–[10]. In this study,
Ba-doped BFO are prepared using a chemical route different
from the paper of Wang et al. [10]. The temperature dependence
of magnetic and dielectric behaviour, fatigue resistance, and
magnetoelectric properties are investigated in details, which
show some interesting results.
II. EXPERIMENTAL DETAILS
Bi
Ba FeO ceramics (
and 0.25) are
prepared by chemical synthesis route in ambient atmosphere.
Starting chemicals, such as Ba(N0 ) , Bi(N0 ) , and Fe(N0 ) ,
are weighed according to the stoichiometric compositions, all
supplied by Sigma-Aldrich, St. Louis, MO, with a purity of
99.9%. Solutions of these chemicals are mixed and dried up
at 80 C. Homogeneous powders are calcined in two steps: 1)
at 500 C for 4 hr and 2) at appropriate temperatures between
865 C and 885 C for 2 hr to get single-phase ceramics. For
the measurement of electrical properties, pellets with diameter
of 5 mm and thickness of 1 mm are prepared and sintered at
850 C. Both sides of the pellet are pasted with silver paste to
connect electric wires for the measurements of dielectric and
magnetoelectric properties.
Phase and structure of the samples are determined by X-ray
diffraction (XRD) using CuK radiation (PAnalytical). The
magnetic properties of BFO are measured using a vibrating
sample magnetometer (VSM; Lake Shore Cryotronics, Westerville, OH) up to a maximum field of 16 kOe at RT. Impedance
analyzer (Agilent 4294 A) is used to measure the change in
dielectric constant of the samples within the frequency range
40 Hz to 1 MHz. Ferroelectric hysteresis loops are studied by
a polarization versus electric field (P-E) loop tracer (Marine
India).
III. RESULTS AND DISCUSSIONS
A. Structural Analysis
Fig. 1 shows the XRD pattern of Bi
Ba FeO
. Except the first one, all other samples show single-phase perovskite structure, which implies that
the doping does not lower the stabilization of BiFeO . BiFeO
shows additional low intensity peaks of Bi O and Bi Fe O at
, and 30.52 . As Bi is more volatile than Fe,
iron-rich phases, such as Bi Fe O , are usually formed while
preparing BFO. But those impurities disappear in Ba-doped
BFO as Ba(NO ) in the solid solution could accelerate the
formation of BiFeO perovskite structure. Peak positions shift
toward left with the increase of doping concentration, indicating
increment in the lattice parameter. Thus, Ba substitution leads
to an increase in the unit cell volume since the ionic radius of
Ba
is larger than that of Bi
. Separation
Fig. 3. (a) Magnetization M as a function of applied field. (b) Magnetization
Ba FeO .
versus temperature plot of Bi
with increase in cell volume. In BaTiO , the various transition
temperatures shift down on compression [12].
Fig. 1. XRD patterns for Bi
Ba FeO samples. The symbols “*" and “ "
indicate the presence of the impurity phase in the pure BFO sample.
Fig. 2. (a) DSC heating curve indicating magnetic transition temperature
. (b) DTA curve indicating ferroelectric transition temperature
for
Ba FeO
.
Bi
between (0 1 2) and (1 1 0) diffraction peaks (see the inset of
Fig. 1) is reduced with Ba substitution, which implies that
the rhombohedral structure may be distorted to a monoclinic
or a tetragonal structure. This change might be important for
the ferroelectric properties of the compounds. However, an
Mn-doped BFO thin film showed similar kind of changes [11].
B. DSC and DTA Analysis
Thermal analysis of the samples has been carried out with
DSC (30 C to 500 C) and DTA (200 C to 900 C) to study
the magnetic and ferroelectric transition temperatures, respectively. DSC results for Bi
Ba FeO are shown in Fig. 2(a).
As can be seen in the figure, the heat flow data show a kink-like
anomaly at 369.2 C for
(heating curve).
Magnetic transition shifts toward higher temperature on Ba
doping, which is 386.2 C for
. DTA results are shown
in Fig. 2(b). For the parent compound, a peak obtained at 831 C
is attributed to the ferroelectric transition
in this compound. The peak is shifted toward higher temperature in the
Ba-substituted compounds and reaches 868 C for
exhibiting ferroelectric characteristics over a wide temperature
range. This increase in
can be due to decrease of pressure
C. Magnetic Analysis
The magnetic hysteresis loops, as shown in Fig. 3(a), illustrate the linear-field dependence of magnetization M for pure
BFO indicating its AFM nature. However, Ba-doped samples
are ferromagnetic at RT and the characteristics of the loop depend on doping concentrations. Remanent magnetization and
coercivity are maximum in the case of
. Although the
reason behind the occurrence of FM in Ba-doped BFO samples
is not clearly understood, this can be attributed to the change in
interaction between Fe and Fe breaking down the balance
between the antiparallel sublattice magnetization [13]. Change
in canting angle [14] or spiral spin modulation [15] may also be
responsible for this phenomenon.
Fig. 3(b) shows the temperature dependence (250 C to
700 C) M of Bi
Ba FeO in the presence of a magnetic
field,
Oe. For all Ba-doped samples, an AFM transition is observed, with a ferro/ferrimagnetic nature above
C
. However, above
C
also, these
samples have another magnetic transition. In the case of pure
BFO, a sharp increase in M takes place at
C
. On
attaining a maximum at 500 C, M starts decreasing and reaches
the paramagnetic state at
C
. Other samples also
show the similar results. M is the maximum for
beyond which it decreases.
To understand the magnetic behavior, hysteresis loops are
measured at temperatures below
, above
and within
them, and they are shown in Fig. 4(a)–(c) for the pure BFO.
In between
and
, pure BFO shows a hysteresis loop
with very small coercivity
Oe. This magnetic behavior
above the Neel temperature can be due to the existence of the
impurity phase of Bi Fe O because Bi Fe O is paramagnetic
at RT [16]. Linear-field dependence of M is observed at temperatures higher than
.
To confirm the magnetic state of the pure BFO at different
temperature, Arrott’s plots (
versus H) have been plotted in
Fig. 4(d). It shows a positive intercept on the y-axis indicating
spontaneous magnetization in between
and
only [17].
D. Ferroelectric Properties
It is very difficult to get good ferroelectric properties in
BFO ceramics due to the presence of oxygen vacancies. How-
Fig. 4. (a)–(c) Magnetic hysteresis loops at different temperatures. (d)
versus H plots of BiFeO .
Fig. 6. Dielectric constant
versus frequency (50 Hz to 1 kHz) plots of
Ba FeO ceramics at RT.
Bi
Fig. 5. Ferroelectric hysteresis loop of Bi
kV cm and
Hz.
Ba FeO
ceramics at
Fig. 7. Dielectric constant versus frequency (
Bi Ba FeO sample at different applied fields.
ever, with Ba substitution, better ferroelectric properties are
observed. This can be explained by the distortion of oxygen
octahedral by the relative displacement of the equatorial and
apical oxygens in Ba-doped samples as observed in A-site
ion-substituted Bi Ti O [18]. Enhanced ferroelectricity observed in Bi
La Ti O is due to rotation of TiO octahedra
in the – plane accompanied with a shift of the octahedron
along the axis.
RT P-E loops of Bi
Ba FeO
samples
are shown in Fig. 5 at the frequency of 50 Hz and a maximum
field of
kV cm . Saturation can be obtained with the application of higher electric field or by poling the samples with
high voltage. The Ba
BFO sample shows the maximum
(
C/cm ) among the Ba-doped BFO ceramics.
Electrical switching causes the trapping of electronic charge
at the domain wall. Fatigue endurance of the samples is also
an important factor for practical applications. Fatigue occurs in
BFO mainly because of the change in the positions of oxygen
vacancies. Doping of Ba reduces movable charge density due
to which fatigue resistance changes with an appropriate doping
concentration. Fatigue characteristics of all the samples are carried out at an applied field
kV cm and frequency 50 Hz.
Hz–2 kHz) plots of the
Inset a of Fig. 5 shows that the
value remain almost the same
up to
read and/or write switching cycles for sample
.
TheP-E loop shows the same nature before and after the fatigue
measurement (see inset b of Fig. 5). Though Ba
BFO shows
a higher
value initially than the other Ba-doped samples,
the fatigue resistance of Ba
BFO is very much less than the
sample. The value of Ba
BFO decreases continuously from
C/cm to a lower value with the switching
cycles. Compared to others, the sample with the
concentration shows the maximum fatigue resistance with
value
about
C/cm throughout the measurement cycles.
E. Dielectric Measurements
Fig. 6 shows the variation of dielectric constant of the samples measured from 50 Hz to 1 kHz at RT. changes noticeably
with the Ba concentration. Ba BFO shows the maximum at
RT. The trend observed here is closely similar to that of ferroelectric measurements.
The magnetoelectric coupling of the sample Ba
BFO
(which shows the maximum fatigue resistance) is clearly
shown in Fig. 7. The dielectric constant of the sample increases
ACKNOWLEDGMENT
The work of R. Das was supported by Council of Scientific
and Industrial Research, India, for a fellowship. This work was
supported by S. N. Bose National Center for Basic Sciences
under the Project SNB/KM/10-11/57.
REFERENCES
Fig. 8. MD effect versus field plots of the Bi
frequency.
Ba
FeO sample at a different
with the applied magnetic field. Temperature dependence of
of the sample Ba BFO from RT to 460 C is shown in the
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due to the limitation of sample holder. It is observed that of
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Fig. 8 shows a field dependence of the magnetodielectric
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at a different
frequency . A large change in the MD value is observed while
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IV. CONCLUSION
Magnetic and ferroelectric properties of BFO are found to
change significantly with Ba substitution in place of Bi. Above
the Neel temperature of the pure BFO, a large enhancement in
magnetization is observed in Ba-doped samples. Transition temperatures (magnetic and ferroelectric) are shifted toward higher
temperature with the increase in the Ba concentration. Canting
of spins and displacement of oxygen atoms from its original position could be the reasons for enhanced magnetic and ferroelectric properties, respectively. Magnetoelectric coupling also
increases due to the Ba substitution. Measurements of electric
properties illustrate a remarkable difference in the dielectric
constant above the magnetic transition temperature of the parent
compound.
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