Room-temperature multiferroic properties and magnetoelectriccoupling in Bi42xSmxTi32xCoxO122d ceramics

Joginder Paul • Sumit Bhardwaj •

Kuldeep Kumar Sharma • Ravinder Kumar Kotnala •

Ravi Kumar

Received: 18 March 2014 / Accepted: 15 May 2014 / Published online: 31 May 2014

� Springer Science+Business Media New York 2014

Abstract We present the structural, dielectric, ferroelec-

tric, magnetic and magnetoelectric studies of lead free;

single phase Bi4-xSmxTi3-xCoxO12-d (0 B x B 0.07)

ceramics, synthesized using a standard solid-state reaction

technique. Raman spectroscopy analysis reveals the

relaxation of distortion in TiO6 octahedron. Field emission

scanning electron microscopy confirmed the growth of

plate-like grains. It is observed that with the substitution of

Sm3? and Co3? ions the dielectric constant, loss tangent

and ferroelectric transition temperature decreases. Electri-

cal dc resistivity, remnant polarization and magnetization

increases with increasing Sm3? and Co3? contents. The

magnetoelectric coupling co-efficient, a = 0.65 mV cm-1

Oe-1 is realized for Bi4-xSmxTi3-xCoxO12-d (x = 0.07)

ceramic sample. Our results clearly demonstrate the lead

free, multiferroic nature of Sm/Co-substituted Bi4Ti3O12,

which may find useful application in designing cost-

effective electromagnetic devices.

Introduction

Materials possessing two or more ferroic orders such as

ferroelectric, ferromagnetic or ferroelastic simultaneously

in the same phase, and also allows the coupling between

these ferroic orderings is known as multiferroic materials

[1–3]. These materials have attracted much attention dur-

ing the past decade. The search of these materials is driven

by the prospect of controlling charges by applying mag-

netic field and spins by electric field. The new effect, such

as magnetoelectric (ME) effect could be produced by the

coupling between different order parameters. For obser-

vation of ME effect, the co-existence of electric and

magnetic dipoles is the basic requirement. This is the most

fascinating characteristic of the multiferroic materials, as

interesting physics is associated with it. Due to the co-

existence of magnetism and ferroelectricity at room tem-

perature, these materials can be used in multiple state

memories, sensors, transducers and data storage devices

[4, 5]. The coupling between two order parameters exists, if

electric polarization is caused by applying either an electric

or a magnetic field. Due to coupling between two order

parameters makes it possible to write data bit with an

electric field and read it with magnetic field, and vice versa.

This provides an extra degree of freedom in device

designing [6, 7]. Much of the early work on multiferroic

was directed towards bringing these two order parameters

in one material [8]. However, in actual, there is shortage of

the materials exhibiting magnetoelectric behaviour at room

temperature possibly due to the fact that these two order

parameters make mutually exclusive groups [9, 10]. Since

Ferroelectricty requires empty d-orbitals, while magnetism

needs partially or half-filled d-orbitals, and the presently

known single-phase multiferroic compounds shows a weak

magnetoelectric (ME) coupling at room temperature. From

J. Paul (&) � S. Bhardwaj � R. Kumar

Centre for Materials Science and Engineering, National Institute

of Technology, Hamirpur 177 005, H.P., India

e-mail: jp_phy@yahoo.in

S. Bhardwaj

e-mail: sumit.bhardwaj4@gmail.com

R. Kumar

e-mail: ranade65@gmail.com

K. K. Sharma

Department of Physics, National Institute of Technology,

Hamirpur 177 005, H.P., India

e-mail: kknitham@gmail.com

R. K. Kotnala

National Physical Laboratory, New Delhi 110 012, India

e-mail: rkkotnala@nplindia.org

123

J Mater Sci (2014) 49:6056–6066

DOI 10.1007/s10853-014-8328-7

the technological application point of view, we require

those types of materials having large ME coupling

responses at room temperature [11]. Although the gigantic

ME effect in the composite structure has been well dem-

onstrated, the single-phase compounds are still greatly

concerned, partially because the multiferroic compounds

allow us to tune the ME effect in the quantum level. Hence

the search for new single-phase multiferroic materials

having large magnetoelectric effect at or above room

temperature still continues.

Many multiferroic compounds such as YMnO3, Cr2O3,

TbMnO5 have been studied during the past decades [12–14].

The ME coupling responses of most of such known materials

at room temperature are too small to be used for technolog-

ical purposes. The demand of the materials with multifunc-

tional properties is increasing with development towards

device miniaturization. Initially the term multiferroic was

only referred to single-phase materials and was later on

expanded to include any material which exhibit two or more

type of long-range orderings. The single-phase multiferroics

exhibit the co-existing order parameters only at low tem-

perature, and they additionally have weak magnetic response

at room temperature. BiFeO3 (BF) and its solid solutions are

some of the widely studied ME systems in recent years.

World over, the aim of the researcher has been to improve the

ferroelectric and magnetic properties of BF along with

improved coupling. Although both ferroelectric and mag-

netic properties have been demonstrated in several of these

BF-based solid solutions. The ME effect in them is usually

too small to be used in applications. Recently, a few studies

on the Fe- and Ni-doped PbTiO3 have shown a convincing

magnetoelectric effect at room temperature [15, 16]. The

lead (Pb)-based compounds are highly toxic and non-eco

friendly, which restrict their uses from application point of

view. As an alternate to Pb-based compounds, bismuth

titanate Bi4Ti3O12 (BIT) has gained a lot of attention [17].

Layered structure BIT belong to the Aurivillius family of

compounds having general formula [Bi2O2]2? [An-1Bn-

O3n?1]2- with n = 3 has low processing temperature than

other bismuth layer-structured ferroelectric and a strong

anisotropy of the spontaneous polarization (Ps) along a-axis

and c-axis [18, 19]. Bismuth layer-structured materials are of

interest, since they have high Curie temperature, and exhibits

physical properties suitable for non-volatile memory devi-

ces. However, the high conductivity of these materials at

elevated temperatures often prevents the application of

higher electric fields during higher temperature poling. The

order of conductivity can be reduced in these materials by

proper choice of substitution with rare-earth ions at the bis-

muth site. To our knowledge, there are few reports on mul-

tiferroic behaviour of BIT, although Lu et al. have suggested

that the substitution of Ti by Fe in BIT will lead to ferro-

magnetism at room temperature [20]. Recently Chen et al.

reported the multiferroic nature of BIT by substituting Fe at

the Ti sites, but the system encountered a problem of high

loss due to conduction [21, 22]. The volatile nature of Bi3?

ions creates the Bi vacancies accompanied by oxygen

vacancies, which reduces the remnant polarization with high

dielectric losses of BIT. It has been reported that substitution

at the Bi-site by either La3? or Nd3? ions having smaller

ionic radius than Bi3? ion led to improved ferroelectric

properties [23, 24]. Sm3?ion substitution has been also

reported to improve the ferroelectric and magnetic properties

of such layered structure compounds by suppressing the

concentration of oxygen vacancies and canting of spins in the

respective sub-lattices [25, 26]. Hence, in the present study,

we have chosen rare-earth Sm3? ion for substitution at the

Bi-site in BIT, which have ionic radius smaller than Bi3? ion.

At present, single-phase multiferroic materials with large

magnetoelectric effect are under active experimental inves-

tigations. In order to search for the materials exhibiting both

the orders at room temperature with low value of dielectric

loss, we have partially substituted magnetic cobalt (Co3?)

ions at the Ti4? sites to introduce magnetic properties in the

purely ferroelectric compound, and at the same time rare-

earth (RE) samarium (Sm3?) ions at the Bi3?-sites to control

the high dielectric losses in addition to an improvement in the

ferroelectric properties of BIT.

In this paper, we will investigate the multiferroic behav-

iour of Bi4-xSmxTi3-xCoxO12-d (x = 0, 0.02, 0.05, 0.07)

ceramics. The major aim is to introduce the magnetic prop-

erties with controlled dielectric loss in purely ferroelectric

BIT and correlate it with the ME effect. Therefore, we uti-

lized the solid-state reaction method to prepare the bulk

ceramic samples for the structural, dielectric, electrical,

ferroelectric, magnetic and magnetoelectric measurements.

Experimental details

Polycrystalline samples of Bi4-xSmxTi3-xCoxO12-d (x = 0,

0.02, 0.05, 0.07, 0.1) ceramics were synthesized using standard

solid-state reaction technique. Raw materials of Bi2O3, TiO2,

Sm2O3 and Co3O4 with purity C99.95 % (Sigma Aldrich)

were weighed in stoichiometric amounts. The resultant powder

of each composition was ball milled for 5 h in the tungsten

carbide jars for better mixing with a 3 wt % excess Bi2O3 to

compensate for the bismuth (Bi) loss during post annealing.

The milled powders were calcined at 700 �C for 6 h. The post

calcined powders were then pressed into pellets of diameter

12 mm and thickness of 1–2 mm using hydraulic press

applying pressure of 120 MPa. The pellets were finally sin-

tered at optimized temperature of 850 �C for 4 h.

The structural analysis of the sintered samples was

carried out with the help of powder X-ray diffractometer

(X’Pert PRO PANalytical) using CuKa radiation

J Mater Sci (2014) 49:6056–6066 6057

123

(k = 1.54 A) over a range of angles (20� B 2h B 60�) at a

scanning rate of 2�/min at room temperature. In order to

analyse the surface morphology of the sintered pellets, field

emission scanning electron microscopy (FESEM) images

were taken with the help of FEI Quanta FEG-450 electron

microscope operating at an accelerating voltage of 20 kV.

The elemental composition of each sample was carried out

using energy dispersion X-ray spectroscope (EDX)

attached (Bruker-Nano-X-flash detector 5030), respec-

tively. Crystal structure distortions were studied using

Raman spectroscopy (Reinshaw invia Raman microscope)

equipped with Argon laser (k = 514.5 nm) operated at

20 mW. Dielectric measurements as a function of fre-

quency and temperature were carried out using LCR meter

(Wayne Kerr 6500B) with high temperature furnace

attached. The room temperature dc resistivity was mea-

sured by two probe methods (Keithley Electrometer 6221).

Ferroelectric hysteresis loop measurements were per-

formed using an automatic P-E loop tracer (Radiant

Technologies). Room temperature magnetic measurements

of powder samples were performed using a vibrating

sample magnetometer (Lake Shore Model No. 662).

Magnetoelectric effect was recorded using the laboratory

assembled dynamic magnetoelectric coupling set up.

Results and discussion

Structural and morphological studies

Figure 1 shows the X-ray diffraction patterns of Bi4-x-

SmxTi3-xCoxO12-d (x = 0, 0.02, 0.05, 0.07, 0.1) ceramics

at room temperature. For x B 0.07 samples all the dif-

fraction peaks correspond to the pure phase of Bi4Ti3O12

having orthorhombic structure (JCPDS card No. 89-7500).

At x [ 0.07, an impurity peak at around 2h = 27.89�(indicated by * in the Fig. 1) can be attributed to the for-

mation of secondary phase and is identified as bismuth

cobalt oxide (Bi25CoO40) matched with (JCPDS card No.

39-0871). The XRD pattern shows two closely spaced

(020) and (200) peaks around 2h = 33� in the ortho-

rhombic phase of BIT, and on increasing the Sm3? and

Co3? ions content in BIT, the relative 2h difference

between the peaks decreases with a slight shift towards

higher angle side. These observations correspond to the

decrease in the lattice parameters, as well as orthorhomb-

icity [27]. It is also confirmed from the calculated values of

lattice parameter, unit cell volume and orthorhombicity

(d = 2(a - b)/(a ? b)) for all compositions listed in the

Table 1. It can be clearly noticed that orthorhombicity (d)

decreases from 2.39 9 10-3 to 0.55 9 10-3 with the

increasing content of Sm3? and Co3? ions from x = 0 to

0.07 in BIT, implying a relaxation of the structural dis-

tortions in the system. The observed relaxation in structural

distortions may be attributed to different ionic radii of

substituting Sm3? (0.96 A) and Co3? (0.65 A) ions com-

pared to Bi3? (1.03 A) and Ti4? (0.61 A) ions of host sites.

[28]. Further, the observed slight decrease in c-parameter

with increasing Sm3? and Co3? content can be attributed to

the rotation of TiO6 octahedron [29].

Raman scattering study provides valuable information

about local structures in the materials. The Raman spectra

of Bi4-xSmxTi3-xCoxO12-d (x = 0, 0.02, 0.05, 0.07)

ceramic samples at room temperature are shown in the

Fig. 1 X-ray diffraction

patterns of sintered

Bi4-xSmxTi3-xFexO12-d

samples with x = 0, 0.02,

0.05, 0.07 and 0.1

6058 J Mater Sci (2014) 49:6056–6066

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Fig. 2. Theoretically, there are 24 Raman active modes for

orthorhombic BIT [30, 31]. In the present system, the

Raman modes are observed at around 116, 144, 224, 268,

328, 357, 535, 564, 610 and 850 cm-1. The modes above

200 cm-1 have been assigned as the internal modes of

TiO6 octahedra of BIT [30]. In addition, the formation of

BIT with orthorhombic structure is identified by splitting

modes at 193 and 224 cm-1, 535 and 564 cm-1. Although

the mode at 224 cm-1 is Raman inactive according to the

Oh symmetry of TiO6, it is often observed because of the

distortion of octahedron. The suppression of the mode at

224 cm-1 with increasing substitution can be attributed to

the decrease in distortion of TiO6 octahedra and hence the

decrease of orthorhombicity [32]. The two bands in the

frequency range between 500 cm-1 and 600 cm-1 and the

other two bands at 328 cm-1 and 361 cm-1 tend to merge

into one another with increase in Sm3? and Co3? content,

which correspond to the vibrations of the O–Ti–O bending

and Ti–O torsional modes [33]. The phonon modes at 116

and 144 cm-1 reflect the vibration of A-site Bi3? ions in

layer-structured pervoskite. The appearance of Raman

mode at about 716 cm-1 becomes more prominent with

increasing Co content and its frequency is in the high

frequency phonon band of the octahedron with A1g sym-

metry [34]. Hence, it is reasonable to believe that the new

Raman mode resulting from the vibration of Co–O due to

the substitution of Ti by Co in TiO6 octahedron. Moreover,

the gradual increase in the intensity of the new Raman

Table 1 Structural, electrical,

ferroelectric, magnetic and

magnetoelectric parameters of

Bi4-xSmxTi3-xCoxO12-d

(x = 0.0, 0.02, 0.05, 0.07)

Parameters x = 0 x = 0.02 x = 0.05 x = 0.07

Lattice parameters (A) a = 5.430 a = 5.425 a = 5.417 a = 5.412

b = 5.417 b = 5.416 b = 5.411 b = 5.409

c = 32.731 c = 32.728 c = 32.720 c = 32.719

Unit cell volume (A3) 962.75 961.60 959.06 957.79

Orthorhombicity (d) 2.39 9 10-3 1.66 9 10-3 1.10 9 10-3 0.55 9 10-3

q (X cm) at RT 2.2 9 107 1.24 9 108 2.31 9 108 3.54 9 108

Tc (�C) 674 662 655 646

2Pr (lC/cm2) 8.66 11.84 14.02 15.82

2Ec (kV/cm) 24 26 29 31

2Mr (10-4 emu/g) Diamagnetic 10.52 13.46 19.86

amax (mV/cm Oe) – 0.29 0.39 0.65

Fig. 2 Raman spectra of

Bi4-xSmxTi3-xCoxO12-d

(x = 0, 0.02, 0.05, 0.07)

at room temperature

J Mater Sci (2014) 49:6056–6066 6059

123

mode also confirms the cobalt substitution at the B-site in

Bi4-xSmxTi3-xCoxO12-d ceramic sample. Hence, the

results obtained from Raman spectra correlate well with the

XRD patterns.

The FESEM micrographs of Bi4-xSmxTi3-xCoxO12-d

(0 B x B 0.7) are shown in Fig. 3. From the surface

morphology, it is clearly observed that randomly oriented

plate-like grains appear in the samples, and the grain size

increases gradually with increasing Sm and Co content in

BIT. Figure 4 shows the elemental composition of all the

considered samples, which indicates that Sm3? and Co3?

ions are well incorporated into the system. The weight and

atomic percentage of each sample are listed in Table 2.

Dielectric studies

Figure 5a shows the variation of dielectric constant (er) at

room temperature as a function of frequency over the range

1 kHz–1 MHz for all the considered Bi4-xSmxTi3-xCox-

O12-d

(0 B x B 0.7) ceramic samples. It is observed that

dielectric constant of BIT without Sm3? and Co3? content

decreases rapidly with frequency (up to 105 Hz) and

remains constant at higher frequencies, indicating strong

dielectric dispersion. The observed decrease in dispersion

in dielectric constant with increase in both the frequency

and Sm3? and Co3? content may be attributed to the

possible reduction in the space charge effect. Due to vol-

atile nature of Bi3? ions, the defects related to oxygen

vacancies are created during heat treatment of the material.

Addition of Sm3? improves dielectric properties of BIT as

it suppresses the volatility of Bi3? ions and reduces the

oxygen vacancies in the system. The Bi and oxygen

vacancies get trapped at sites like grain boundaries with the

creation of space charges. No doubt, the oxygen vacancies

should arise naturally to ensure charge neutrality when the

Co3? ions are substituted at the Ti4? sites of the sample.

The decrease in dielectric constant and loss tangent with

composition and frequency indicates that the process of

reduction in oxygen vacancies due to substitution of Sm3?

ions dominates over the process of their creation by the

Co3? ions in the system leading to overall decrease in the

oxygen vacancies and hence the space charge effect. It is

worth mentioning here that when the pure specimen is

substituted with Sm3? and Co3? ions, a significant reduc-

tion in dispersion of dielectric constant can be clearly

noticed at low frequencies (see Fig. 5a), such type of

behaviour has been reported in Fe-doped PbTiO3 system by

Verma et al. [35]. Hence substitution of Sm3? ions plays

Fig. 3 FESEM images of Bi4-xSmxTi3-xCoxO12-d samples for a x = 0, b x = 0.02, c x = 0.05 and d x = 0.07

6060 J Mater Sci (2014) 49:6056–6066

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the major role to reduce the defects and dielectric losses in

the BIT system. Similar inference can be drawn from the

frequency dependence loss tangent (tand) curves for these

samples as shown in Fig. 5b, where the pure sample

appears to be more conducting compared to the samples

with Sm and Co content. The decrease in the interfacial or

space charge polarization has been also supported by the

results obtained from the FESEM (see Fig. 3), which

reveals that the size of the grain increases with increasing

content of Sm and Co in BIT. Moreover, the growth in the

size of the grain reduces the volume fraction of the grain

boundaries, leading to decrease in the space charge

polarization. The dc resistivity (q) of the considered cera-

mic samples measured at room temperature (RT) also

increases upon increasing the concentration of Sm3? and

Co3? ions (see Table 1). All these results show that the

dielectric properties of BIT system improve with Sm3? and

Co3? ions substitution.

Figure 6a and b shows the variation of dielectric con-

stant and loss tangent as a function of temperature mea-

sured at 100 kHz. It is observed that the ferroelectric phase

transition temperature (Tc) decreases with increase in Sm3?

and Co3? ions content (see Fig. 6a; Table 1), which may

be attributed to the reduction of orthorhombic distortions in

Fig. 4 EDX of Bi4-xSmxTi3-xCoxO12-d samples for a x = 0, b x = 0.02, c x = 0.05 and d x = 0.07

Table 2 EDX analysis of Bi4-xSmxTi3-xCoxO12-d (x = 0.0, 0.02, 0.05, 0.07)

Elements x = 0 x = 0.02 x = 0.05 x = 0.07

wt % at % wt % at % wt % at % wt % at %

Bi L 74.13 23.76 74.02 23.54 74.81 24.60 74.54 24.40

Ti K 11.51 16.10 11.23 15.60 10.78 15.47 10.74 15.35

O K 14.36 60.14 14.24 59.90 13.87 59.57 13.98 59.76

Sm L – – 0.18 0.08 0.37 0.17 0.50 0.23

Co K – – 0.08 0.09 0.16 0.19 0.22 0.26

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the present system with the substitution. [36].The temper-

ature dependence of loss curves illustrates the decreasing

trend in the loss tangent with increasing content of Sm and

Co.

Ferroelectric studies

Figure 7 shows the room temperature polarization–electric

field (P–E) hysteresis loops of Bi4-xSmxTi3-xCoxO12-d

(0 B x B 0.7) ceramic samples observed at a fixed fre-

quency of 50 Hz. It can be noticed that the shape of fer-

roelectric loop improves on increasing content of Sm3? and

Co3? ions, indicating the reduction in losses related to

conduction. This observation also compliments the results

of our dc electrical resistivity measurements. On increasing

the substitution, a significant enhancement in the remnant

polarization (2Pr) and coercive field (2Ec) has been also

observed (see Table 1). This clearly shows that the ferro-

electric properties of BIT are improved significantly with

the addition of Sm and Co content. It is evident, since

Sm3? and Co3? ions substitution in BIT leads to a decrease

in the orthorhombicity, which in turn weaken the segre-

gation of vacancy related defects at domain walls and

speed up the movement of the domain walls [37].

Moreover, the grain size of Bi4-xSmxTi3-xCoxO12-d

ceramics increases gradually with increasing content of Sm

and Co in BIT which leads to increase the domain variants

[38]. Consequently, the volume fraction of grain boundaries

reduces, thereby decreasing the probability of trapping

space charge at the boundaries. Due to this, the pinning of

neighbouring domains gets reduced, which makes domain

reorientation easier [39]. This then leads to an increase in

the domain alignment, and hence the remnant polarization.

Magnetization studies

Figure 8 shows isothermal magnetization (M) versus

applied magnetic field (H) curves for all the samples

Fig. 5 Frequency dependence of a dielectric constant (er) and b loss tangent (tand) of Bi4-xSmxTi3-xCoxO12-d (x = 0, 0.02, 0.05, 0.07) at room

temperature

Fig. 6 Temperature dependence of a dielectric constant (er) and b loss tangent (tand) of Bi4-xSmxTi3-xCoxO12-d (x = 0, 0.02, 0.05, 0.07) at

100 kHz

6062 J Mater Sci (2014) 49:6056–6066

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measured at room temperature (300 K). An anti S-type

curve has been obtained for pure (x = 0) sample, demon-

strating the diamagnetic nature of BIT. Upon increasing the

content of Sm and Co, the M-H curve gradually changes

into symmetric S-type hysteresis loop (although non-satu-

rating), indicating the origin of weak ferromagnetism. It

might be speculated that the present magnetism originated

from the Sm3? and Co3? ions that have successfully

entered into the pseudo-pervoskite unit cell. The remnant

magnetization (2Mr) of the considered samples increases

with increasing content of magnetic ions (see Table 1),

which can be attributed to the decrease in interionic dis-

tances as revealed from XRD analysis and the enhance-

ment in the strength of magnetic interaction. The origin of

room temperature ferromagnetism in sample with x = 0.07

might be explained on the basis of following mechanisms;

(i) The oxygen vacancies arise naturally to ensure charge

neutrality, when Co3? is substituted at the Ti4?-site,

leading to Co3?–h–Co3? network in the structure, where

(–h– denotes oxygen vacancy). In these vacancies, an

electron gets trapped having a down spin, which form the

F-centre [22, 40]. Since Co3?, 3d6 also have unoccupied

minority spin orbitals, so it might be speculated that

F-centre exchange mechanism is also suitable for Co3?–

h–Co3? network [41]. The F-centre exchange mechanism

was employed earlier to explain the ferromagnetism in Fe-

doped PbTiO3 and Fe-doped BIT [22, 42]. The orbital of

the trapped electron will overlap the d-shells of both the

Co3? ions having spin-up. The exchange interaction

between the two neighbouring Co3? ions via F-centre gives

rise to direct ferromagnetic coupling. (ii) In another

mechanism Co3?–O–Co3? coupling might occur, which

would favour the formation of ferromagnetic state. (iii)

There is a possibility that cobalt may exist in the ?2

valence state. This may lead to the emergence of Co3?–O–

Co2? group in the structure. The double exchange inter-

action existing in this group would also lead to ferromag-

netic state [41]. To confirm the exact valence state of Co in

the system further studies are required. The presence of

Sm3? ions in the system also contributes to the magnetism,

because in pervoskite layered structure of bismuth titanate,

the spins of Sm3? ions in the sub-lattices are canted instead

of being parallel. This leads to the weak canted antiferro-

magnetic behaviour. The extent of canting of spin structure

increases with the substitution of Sm3? ions [26]. Hence

simultaneous substitution of Sm3? and Co3? ions leads to

the enhancement in magnetization of the Bi4-xSmxTi3-x-

CoxO12-d ceramic samples.

Fig. 7 Ferroelectric hysteresis loops of Bi4-xSmxTi3-xCoxO12-d (x = 0, 0.02, 0.05, 0.07) at room temperature

J Mater Sci (2014) 49:6056–6066 6063

123

Magnetoelectric studies

The ferroelectric and magnetization studies reveal that both

ferroic orders exist in Bi4-xSmxTi3-xCoxO12-d ceramic for

x = 0.02, 0.05 and 0.07 respectively. To observe the extent

of coupling between these two ferroic order in the single-

phase material, ME voltage across each sample under

applied dc magnetic field was measured using laboratory

assembled magnetoelectric coupling setup. All the mea-

surements were recorded under a constant ac magnetic field

(Hac = 3Oe) and at frequency f = 993 Hz. The ME cou-

pling co-efficient (a = dE/dH), is calculated from the

output voltage [43]. Figure 9 shows the variation of a as a

function of applied dc magnetic field (Hdc) at a fixed ac

magnetic frequency 993 Hz for Bi4-xSmxTi3-xCoxO12-d

(x = 0.02, 0.05 & 0.07), measured at room temperature.

The coupling co-efficient has shown a non-linear behaviour

with the applied dc magnetic field for all the samples and is

found to increase with Sm and Co substitution (see

Table 1). The maximum value of coupling co-efficient

a = 0.65 mV cm-1 Oe-1 has been recorded for x = 0.07.

These results clearly demonstrate the existence of a cou-

pling between the two order parameters. The optimized dc

magnetic field at which maximum coupling occurs between

the two ferroic order shifts towards higher field with

increase in Sm and Co content, which is a critical param-

eter for the operation of multiferroic-based devices to

maximize the energy conversion between electric and

magnetic fields. In single-phase multiferroic materials, the

ME coupling arises mainly from the interaction between

electric and magnetic sub-lattices through the stress or

Fig. 8 Isothermal magnetization hysteresis (M-H) of Bi4-xSmxTi3-xCoxO12-d (x = 0, 0.02, 0.05, 0.07) samples at room temperature

Fig. 9 Magnetoelectric coupling co-efficient (a) as a function of dc

magnetic field under Hac = 3Oe at 993 Hz

6064 J Mater Sci (2014) 49:6056–6066

123

strain transmitting from one sub-lattice to another [44]. In

the presence of magnetic field, the strain-induced magnetic

sub-lattice induces a stress on the electrical sub-lattice

which is realized as ME output.

Conclusion

In summary, we have synthesized the Lead free, single-

phase polycrystalline samples of Bi4-xSmxTi3-xCoxO12-d

(x = 0, 0.02, 0.05, 0.07) ceramics by standard solid-state

reaction technique. The introduction of Sm3? and Co3?

ions leads to the relaxation of the orthorhombic distortion

in the system. The dielectric constant and loss tangent are

found to decrease with substitution, which has been

explained in terms of reduction of space charge effect.

Substitution has significantly reduced the dispersion both in

dielectric constant and loss tangent. There is a composi-

tional decrease in ferroelectric phase transition temperature

(Tc), which may be due to reduction of orthorhombic dis-

tortions. It is observed that dc electrical resistivity, remnant

polarization (2Pr) and magnetization (2Mr) increases with

increasing Sm3? and Co3? contents. Magnetoelectric

coupling co-efficient (a) value of 0.65 mV cm-1 Oe-1 has

been found in Bi3.93Sm0.07Ti2.93Co0.07O12-d ceramic sam-

ple. The present study clearly demonstrates the multiferroic

behaviour of the Sm3? and Co3? ion-substituted BIT,

which may find useful application in designing cost-

effective electromagnetic devices such as multiple state

memories and data storage devices.

Acknowledgements We would like to acknowledge Dr. Mahavir

Singh and Dr. Nagesh Thakur, Department of Physics Himachal

Pradesh University, Shimla, India for magnetic, ferroelectric and

dielectric measurements. Joginder Paul is thankful to the department

of Higher Education Himachal Pradesh-India for providing study

leave to conduct this work.

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