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 (giri@nitt.edu)

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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