Composition-dependent structural, electrical, magnetic and ...
Transcript of Composition-dependent structural, electrical, magnetic and ...
Composition-dependent structural, electrical, magneticand magnetoelectric properties of (1 2 x)BaTiO32xCoFe2O4
particulate composites
D PADMAPRIYA1, D DHAYANITHI1, A RASHID1, M T RAHUL2,NANDAKUMAR KALARIKKAL3, M MUNEESWARAN4 and N V GIRIDHARAN1,*1Advanced Functional Material Laboratory, National Institute of Technology, Tiruchirappalli 620015, India2Department of Physics, The Cochin College, Kochi 682002, India3 International and Inter University Centre for Nano Science and Nanotechnology, Mahatma Gandhi University,
Kottayam 686560, India4Advanced Materials Laboratory, Department of Mechanical Engineering, University of Chile, Santiago 8370448, Chile
*Author for correspondence ([email protected])
MS received 7 January 2020; accepted 17 June 2020
Abstract. Multiferroic composite with the general formula (1 - x)BaTiO3-xCoFe2O4 (x = 0.05, 0.15, 0.25, 0.35 and
0.45) has been synthesized by a standard solid-state reaction route. Powder X-ray diffraction analysis confirms the
existence of ferrite (spinel CoFe2O4) and ferroelectric (tetragonal BaTiO3) biphase without any impurity phases in the
sintered composites. Microstructure of the composite displays two different grain sizes and shapes studied from SEM
analysis. The composites show both ferroelectric and ferromagnetic ordering: the saturation magnetization (Ms) and
retentivity (Mr) of the composite are improved with the increase in ferrite phase, while leakage current, ferroelectric and
dielectric properties of the composites show a drop. Existence of coupling between ferroelectric and ferromagnetic
ordering measured through magnetodielectric (MD) and magnetoelectric (ME) studies reveal an increase in % MD and
ME coefficients with an increase in ferrite content. An enhanced ME coupling coefficient of 17 mV cm-1 Oe-1 has been
realized at a dc magnetic field of 5 kOe with a ac frequency of 50 Hz in (1 - x)BaTiO3-xCoFe2O4 (x = 0.45) composite.
Keywords. Barium titanate; cobalt ferrite; composites; multiferroic; magnetodielectric; magnetoelectric.
1. Introduction
Research on multiferroic materials which displays simul-
taneous ferroelectric and ferromagnetic orderings have
drawn increasing interest due to their attractive physical
properties and potential application in spintronics, infor-
mation storage and sensors [1–4]. The coupling between
magnetic and electrical orderings can be studied through
magnetoelectric (ME) [5,6] and/or magnetodielectric (MD)
effects [7]. In ME effect, magnetization can be controlled
by applied electric field and vice versa, while in MD effect,
dielectric permittivity/capacitance can be modified by the
applied magnetic field. On the basis of constituent materi-
als, multiferroic materials are divided into two categories
i.e., single and composite phases. Single phase multiferroic
materials are rare and usually exhibit a weak ME effect near
or below room temperature, which limits their potential
applications [8–10]. Similarly, perceiving a significant MD
effect in these single phase multiferroic materials requires
an external magnetic field of several teslas [11]. To enhance
the ME coupling for practical applications, composite
structure (particulate, laminate, hybrid, core and shell, etc.)
consisting of various ferroelectric (BaTiO3, Sr0.5Ba0.5Nb2O6,
Pb(Zr,Ti)O3, BiFeO3, etc.) and magnetic phases (CoFe2O4,
NiFe2O4, MnFe2O4, Co, etc.) have been extensively studied
[2, 12–18]. The ME effect in these composites is a result of
the magnetostrictive effect in the magnetic phase and
piezoelectric effect in the ferroelectric phase [19]. BaTiO3–
CoFe2O4 is a widely investigated multiferroic composite
having ME coefficient in the range from 0.19 to 180 mV
cm-1 Oe-1 depending on the synthesis method, composi-
tional variation of piezoelectric/magnetostrictive phase and
their microstructure [20–22]. Compared to various syn-
thesis methods, such as unidirectional solidification of
Fe–Co–Ti–Ba–O system, sol–gel, electrospinning, etc.,
conventional solid state reaction affords a large yield
without leading to serious environmental problems.
Considering these advantages, particulate composites of
(1 - x)BaTiO3- xCoFe2O4 (x = 0.00, 0.05, 0.15, 0.25,
0.35 and 0.45) have been prepared by solid-state reaction
route. A systematic investigation on the structural, elec-
trical, magnetic, MD and ME properties have been
Bull Mater Sci (2020) 43:276 � Indian Academy of Scienceshttps://doi.org/10.1007/s12034-020-02243-ySadhana(0123456789().,-volV)FT3](0123456789().,-volV)
carried out to understand the interaction between ferro-
electric/ferromagnetic phases.
2. Experimental
(1 - x)BaTiO3-xCoFe2O4 composite systems were made
by synthesizing individual phases by a standard solid-state
reaction route and later mixing them at different molar
ratios. High purity BaCO3 and TiO2 were mixed and
grounded by using agate motor for a few hours and then
sintered at 1100�C for 2 h to synthesize BaTiO3 (BTO).
Similarly high purity Co3O4 and Fe2O3 were selected,
grounded and sintered at 1000�C for 10 h to prepare
CoFe2O4 (CFO). The synthesized individual phases of BTO
and CFO were taken according to the formula
[(1 - x)BaTiO3-xCoFe2O4 with x = 0.05, 0.15, 0.25, 0.35
and 0.45] to have various compositions. Each of the com-
position was then rigorously mixed with an agate mortar for
an hour to have a homogeneous mixing of the synthesized
BTO and CFO phases. These composite samples were then
compressed into pellets (10 mm dia., 1–1.5 mm thickness)
by using uniaxial press with a load of 100 kg cm-2 and then
sintered at 1200�C for 24 h to carry out electrical and
magnetoelectrical measurements.
A Rigaku (D/Maxultima III) X-ray diffractometer with
CuKa1 radiation (1.5406 A) was used to investigate the
structural details of the ceramic composites. The
microstructure of the individual BTO, CFO phases and
composites were measured using scanning electron micro-
scope (SEM) (Zeiss EVO-18). Ferroelectric hysteresis and
leakage characteristics measurements were performed with
precision premier multiferroic system (Radiant Technology
PMF0713-334). Magnetic behaviour of the samples was
carried out using a vibrating sample magnetometer (Lake-
shore 7404) at applied field strength of ±15 kOe. The
temperature- and magnetic field-dependent dielectric prop-
erties were studied with the help of LCR meter (HIOKI
3532-50). Further, magneto-electric voltage coefficient of
all ceramic compositions was recorded using dynamic lock-
in amplifier setup.
3. Results and discussion
3.1 Phase and microstructural analyses
Figure 1 shows the X-ray diffraction (XRD) pattern of
pristine BTO (ferroelectric), CFO (ferromagnetic) and
the sintered composite systems [(1 - x)BaTiO3-xCoFe2O4
with x = 0.05, 0.15, 0.25, 0.35 and 0.45]. The room tem-
perature XRD patterns of pristine BTO and CFO exhibit
tetragonal perovskite structure and cubic inverse spinel
structure, respectively and they are in agreement with lit-
erature studies as well [23]. The composite system exhibits
both the parent phases, such as FE (BTO)/FM (CFO) phases
without any impurity within the detectable limit of X-ray
diffractometer. This indicates that no chemical reaction has
taken place between the ferroelectric and ferrite phases
during the sintering process. Further, the intensities of (311)
and (220) peaks of CFO increase with respect to its molar
fraction in the composite systems as shown in figure 1.
The microstructures of the composite systems
(1 - x)BaTiO3-xCoFe2O4 (x = 0.25 and 0.45) along with
individual BTO and CFO (as reference) are shown in fig-
ure 2a–d. The calculated average grain size of BTO and
CFO measured by using image J software on the SEM
image yielded a value of about 0.31 and 1.1 lm, respec-
tively. The composite samples (1 - x)BaTiO3-xCoFe2O4
with (x = 0.25 and 0.45) show the presence of grains with
two different morphologies and sizes. The ratio of these two
different grains changes with respect to the ratio of BTO
and CFO phases as evidenced from figure 2c–d.
3.2 Leakage current studies
Figure 3 shows the leakage current characteristics of pris-
tine BTO, CFO and (1 - x)BaTiO3-xCoFe2O4 composites
as a function of applied electric field. The leakage current
Figure 1. XRD pattern of pristine BTO, CFO and composite
systems [(1 - x)BaTiO3-xCoFe2O4 with x = 0.05, 0.15, 0.25,
0.35 and 0.45] measured at room temperature.
276 Page 2 of 8 Bull Mater Sci (2020) 43:276
density is found to increase with the increase in applied field
and CFO content of the composite. The measured leakage
current density in pure BTO and CFO are in the order of
10-6 and 10-4 A cm-2 at 8 kV cm-1, respectively. The
measured leakage current densities in the case of compos-
ites are found to be between 10-5 and 10-4 A cm-2,
attributed to the increase of lower resistive CFO content in
the higher resistive BTO phase.
Figure 2. SEM micrographs of (a) BTO, (b) CFO, (c) (1 - x)BaTiO3-xCoFe2O4 (x = 0.25) and
(d) (1 - x)BaTiO3-xCoFe2O4 (x = 0.45).
Figure 3. Leakage current density vs. electric field for BTO,
CFO and (1 – x)BaTiO3–xCoFe2O4 (x = 0.05, 0.15, 0.25, 0.35 and
0.45).
Figure 4. Variations of dielectric constant as a function of
frequency for (1 - x)BaTiO3-xCoFe2O4 (x = 0.05, 0.15, 0.25,
0.35 and 0.45). Inset figure represents pure BTO and CFO.
Bull Mater Sci (2020) 43:276 Page 3 of 8 276
3.3 Dielectric and ferroelectric studies
The frequency-dependent dielectric constant measured at
room temperature in the frequency range between 1 kHz
and 1 MHz for pure BTO, CFO and (1 - x)BaTiO3-xCoFe2O4 composites are shown in figure 4. At 1 kHz, fer-
roelectric BTO has the highest dielectric constant value,
while the ferrite (CFO) has the lowest value. The composite
systems have dielectric constant values between BTO and
CFO at 1 kHz as x is varied from 0.05 to 0.45. A gradual fall
of dielectric constant value with increase in ferromagnetic
CFO content and ferroelectric BTO is observed at 1 kHz.
Further, it has been noticed from the plot that as the fre-
quency increases from 1 to 100 kHz, the dielectric constant
decreases gradually and then become almost constant up to
1 MHz for all the samples. The higher dielectric constant at
lower frequencies could be attributed to the Maxwell–
Wagner type interfacial polarization explained as the
accumulation of charges at the interface, such as grain
boundaries/electrodes/materials having different conduc-
tivities. At higher frequencies, the charge carriers cannot
follow the rapid change in the electric field directions
leading to the fall of dielectric constant values [24]. The
possible compositional inhomogenities in the composites
can lead to easier movements of charge carriers and hence,
decrease in activation energy to change their dielectric
properties [25–27].
Figure 5. Temperature vs. dielectric constant for pristine BTO,
CFO and (1 – x)BaTiO3–xCoFe2O4 (x = 0.0, 0.05, 0.15, 0.25, 0.35
and 0.45) measured at 1 kHz.
Figure 6. Polarization vs. electric field (P–E) hysteresis loops of(1 - x)BaTiO3-xCoFe2O4 (x = 0.05, 0.15, 0.25, 0.35 and 0.45).
Inset figure shows P–E loop of pristine BTO.
Table 1. Data on ferroelectric and magnetic parameters of BTO,
CFO and (1 - x)BaTiO3-xCoFe2O4 composites.
(1 - x)BaTiO3
-xCoFe2O4
Ferroelectric
parameters Magnetic parameters
Ps
(lCcm-2)
Pr
(lCcm-2)
Ec
(kV
cm-1)
Mr
(emu
g-1)
Ms
(emu
g-1)
Hc
(Oe)
BTO (x = 0) 6.84 2.06 7.34 — — —
x = 0.05 2.62 1.24 7.38 0.69 2.68 384.46
x = 0.15 1.78 1.00 8.57 2.99 11.28 431.75
x = 0.25 1.74 0.85 8.37 4.70 18.22 470.69
x = 0.35 1.05 0.70 9.94 5.53 24.09 504.22
x = 0.45 0.90 0.65 10.05 6.89 29.13 571.40
CFO (x = 1) — — — 15.33 68.08 909.36
Figure 7. Magnetization vs. magnetic field (M–H) loops of
(1 - x)BaTiO3-xCoFe2O4 (x = 0.05, 0.15, 0.25, 0.35 and 0.45).
Inset figure shows M–H loop of pristine CFO.
276 Page 4 of 8 Bull Mater Sci (2020) 43:276
Figure 5 shows the variation of dielectric constant as a
function of temperature measured at 1 kHz for the pure
and composite samples. A clear dielectric anomaly is
noticed at the vicinity of 420 K for (1 - x)BaTiO3-xCoFe2O4 (x = 0.00, 0.05) samples and becomes diffusive
for all other compositions. This dielectric anomaly is the
signature of ferroelectric to paraelectric transition tem-
perature (Tc) of BTO. It is interesting to note that the
dielectric constant for x = 0.25, 0.35 and 0.45 in the
paraelectric region (above 420 K) increases with increase
in the temperature and also with increase in the concen-
tration of CFO. This can be due to the existence of
interfacial polarization at the boundaries between BTO
and CFO with different resistivities. Such interfacial
polarization is predominant in low frequency region and
hence, the dielectric constant increases with increase in
the temperature [16,28].
The room temperature polarization vs. electric field (P–E) hysteresis loops of (1 - x)BaTiO3-xCoFe2O4 compos-
ites are shown in figure 6. The inset figure presents the
P–E loop of pure BTO. The P–E loops of all the composite
samples have unsaturated characteristics due to the leakage
of the polarized charges through ferrite content CFO having
higher leakage current density (about 10-4 A cm-2)
[13,29,30]. Hence, introduction of ferrite content reduces
the ferroelectric properties of the composite samples, which
are observed by the reduction in saturation polarization (Ps),
remnant polarization (Pr) and enhancement in coercive field
(Ec) values [29–31]. The values of Ps, Pr and Ec of com-
posites are listed in table 1.
Figure 8. (a–e) Room temperature variations of % MDC for the composites (1 - x)BaTiO3-xCoFe2O4 samples measured at
different frequencies under varying magnetic fields.
Bull Mater Sci (2020) 43:276 Page 5 of 8 276
3.4 Magnetic studies
The magnetic hysteresis (M–H) loops of all samples have
been recorded at room temperature in the field range of
±1.5 T and shown in figure 7. The inset figure presents the
M–H loop of pure CFO exhibiting well-saturated ferro-
magnetic loop with saturation magnetization (Ms), magnetic
retentivity (Mr) and coercive field (Hc) values of 68, 15 emu
g-1and 909 Oe, respectively. The unbalanced anti-parallel
spins between Fe3? ions at tetrahedral sites and Co2? ions at
octahedral sites could have led to this behaviour [23,32].
The composite samples also show saturated M–H loops
attributed to the presence of the ordered magnetic structure
in all the composite systems. The cobalt ferrite is the only
carrier of magnetic properties in these composites. The
values of magnetic parameters (Ms, Mr and Hc) of all the
composite samples increase with increase in the ferrite
content, but they are lower than those of pure CFO due to
dilution of magnetic properties of CFO by the additional
ferroelectric BTO phase. The Ms, Mr, and Hc values of all
samples are listed in table 1.
3.5 MD studies
The MD parameter is used to explain the interaction
between the ferroelectric and ferromagnetic phases in the
composite system. Due to piezo-magnetic effect in the
ferrite phase, the dielectric constant varies under applied
magnetic field. The MD coefficient (MDC) is defined as the
ratio between the difference in dielectric constant with and
without magnetic field to the dielectric constant without
field, given in mathematical form as:
%MDC ¼ e Hð Þ � e 0ð Þ½ �e 0ð Þ � 100;
where e(H) is themeasured dielectric constant in the presence of
magnetic field and e(0) is the dielectric constant without mag-
netic field. ThisMD property could be explained on the basis of
two effects, such as Grindev’s effect and Catalon type effect.
According to these two effects, theMD response arises not only
due to multiferroicity, but also due to magnetostrictive beha-
viour of the piezo-magnetic system.The stress in the sample due
to the application of magnetic field is called magnetostriction
effect represented by magnetostriction coefficient (k), whichmayhavepositive or negativevalue dependingon the expansion
or contraction of the composite system. This expansion/con-
traction will cause changes in the dielectric constant values and
can therefore be measured [16,31,33].
Figure 8(a–e) represents variation of % MDC for the
composite (1 - x)BaTiO3-xCoFe2O4 samples measured at
different frequencies under varying magnetic fields. It has
been observed from the figure that %MDC increases with the
increase in ferrite content till x = 0.35 and a small decrease for
x = 0.45 except at 1 MHz. Further, for all the composite
systems, the lower is the applied frequency, the higher is the
% MDC. The variation of % MDC with frequency and
compositions can be understood as follows: under the appli-
cation of an external magnetic field to this composite system,
strain is induced in the magnetic component (as they tend to
align along the field direction), which is mechanically
transferred to the ferroelectric component, inducing electrical
polarization through piezoelectric effect attributing direct
ME effect [34,35]. Due to this strain-mediatedME effect, the
% MDC increases with the increase in ferrite content except
for x = 0.45. The lower value observed for x = 0.45 (at 0.1, 1,10 and 100 kHz) is likely due to the relatively higher dielectric
loss factor as high conducting phase (CFO) is introduced
leading to interdiffusion of ions from the phases and/or
increase in the porosity in that particular sample. The pores
provide an insulating path of the electrons leading to a
decrease in resistivity, which in turn results in a reduction in
the percentage of magnetodielectric coefficient (% MDC)
[36]. The % MDC (-12.5212, -7.58949, -2.93746,
-2.45776 at 100 Hz, 1, 10 and 100 kHz, respectively)
obtained for (1 - x)BaTiO3-xCoFe2O4: x = 0.35 at 7000
kOe is higher than the values reported by Shen et al [20] forBTO/CFO composites synthesized by two-step solid-state
reaction method, and that of Rani et al [37] for [0.5Ba(Zr0.2Ti0.8)O3 - 0.5(Ba0.7Ca0.3)TiO3] composites synthesized by
solid-state reaction method.
The ME susceptibility is the direct measure of degree of the
ME coupling, which is the crucial material parameter of the
MD multiferroics. It has been proposed by Jang et al [38] thatMD changes detected above 10 kHz can be used to determine
the intrinsic ME susceptibility. The ME coupling susceptibility
(vME) can be measured by using the MD constant values
Figure 9. Variation of MD loss for the composites
(1 - x)BaTiO3-xCoFe2O4 samples under varying magnetic fields
measured at 1 MHz.
276 Page 6 of 8 Bull Mater Sci (2020) 43:276
obtained at 1 MHz as the tan d of all the samples is\\0.1 and
is independent of magnetic field (H) as shown in figure 9. The
expression for ME coupling susceptibility is given by
e Hð Þ � eð0Þe 0ð Þ ¼ vME
E0
H;
where E0 is the applied excitation signal of 2 V, H is the
applied magnetic field in Oe and [e(H) - e(0)]/e(0) is the
MD changes. The calculated vME of (1 - x)BTO-xCFOcomposites are found to be 0.0349, 0.0656, 0.787, 0.606 and
3.49 mV cm-1 Oe-1 for x = 0.05, 0.15, 0.25, 0.35 and 0.45,
respectively, at 2500 Oe.
3.6 DME analysis
Direct ME (DME) effect is the electrical response of the
multiferroic composite to an applied magnetic field and is
represented by ME coefficient (aME). In DME measurement
technique, the sample has been subjected to an ac and dc
magnetic fields (Hac and Hdc) to obtain ME coefficient
(aME). Figure 10a represents the ME voltage vs. ac mag-
netic field plot of (1 - x)BaTiO3-xCoFe2O4 composite
samples that shows a typical linear behaviour under con-
stant dc magnetic field. Figure 10b–f depicts the variation
of aME with Hdc at a constant Hac. The value of aME of the
composites is strongly influenced by the ferrite content. The
aME value increases with an increasing ferrite content and
has an enhanced value of 17 mV cm-1 Oe-1 at the magnetic
field of *5.1 kOe for (1 - x)BaTiO3-xCoFe2O4): x =
0.45. The homogeneous distribution of grains of ferroelec-
tric/ferrite phases and a tight interfacial connection between
the two phases attribute to this enhanced ME coefficient
value. The observed value in the present case is larger than
the values reported for BTO/CFO particulate composites by
Mahajan et al [12] and Botello-Zubiate et al [40], as well as
Figure 10. (a) Room temperature ME voltage vs. ac magnetic field of (1 - x)BaTiO3-xCoFe2O4 composite samples under constant
dc magnetic field. (b–f) Depicts the variations of ME coefficient (aME) with Hdc at a constant Hac for the composites measured at room
temperature.
Bull Mater Sci (2020) 43:276 Page 7 of 8 276
for BTO/CFO core–shell structure [21], and BTO/Ni0.92Co0.03Mn0.05Fe2O4 [40]. But the observed values are
smaller than the BTO/CFO particulate nanocomposites
reported by Verma et al [23], and laminated [13] or spark
plasma-sintered BT/CFO composites [22].
4. Conclusion
ME (1 - x)BaTiO3-xCoFe2O4 particulate composites have
been successfully prepared by a standard solid-state reac-
tion method. The XRD analysis of the composites con-
firmed the cubic spinel and perovskite structure of CFO and
BTO, respectively, without any additional phase formation.
A gradual decrease of dielectric constant value and increase
in leakage current density have been perceived with
increase in ferrite content. An interfacial polarization at the
boundaries of BTO and CFO phases (attributed to the dif-
ferent resistivities of the two phases) led to an increase in
dielectric constant value with increase in the temperature in
the paraelectric region for the composites with x = 0.25,
0.35 and 0.45. Composites showed both ferroelectric and
ferromagnetic orderings. The P–E loops of all the com-
posite samples have unsaturated characteristics, while they
show saturated M–H loops from the magnetic measure-
ments ascribed to the presence of the ordered magnetic
structure in all the composite systems. The MD and ME
effects have also been observed in the composites and
among the studied compositions, the composite with x =
0.45 showed an enhanced ME coefficient (aME max) value of
17 mV cm-1 Oe-1 at a dc magnetic field of 5.1 kOe
demonstrates its suitability towards ME devices.
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