Influence of Mn doping on structural, electrical and magnetic properties of...

1 23

Journal of Materials Science:Materials in Electronics ISSN 0957-4522 J Mater Sci: Mater ElectronDOI 10.1007/s10854-014-1859-5

Influence of Mn doping on structural,electrical and magnetic properties of(0.90)BiFeO3–(0.10)BaTiO3 composite

Mohit Sahni, Naresh Kumar, SushantSingh, Aashish Jha, S. Chaubey, ManojKumar & M. K. Sharma

1 23

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Influence of Mn doping on structural, electrical and magneticproperties of (0.90)BiFeO3–(0.10)BaTiO3 composite

Mohit Sahni • Naresh Kumar • Sushant Singh •

Aashish Jha • S. Chaubey • Manoj Kumar •

M. K. Sharma

Received: 19 January 2014 / Accepted: 1 March 2014

� Springer Science+Business Media New York 2014

Abstract Structural, electrical and magnetic properties of

chemically synthesized polycrystalline Mn doped (0.90)Bi-

FeO3–(0.10)BaTiO3 composites [(0.90)BiFe1-xMnxO3–

(0.10)BaTiO3 (x = 0.0, 0.03, 0.05 and 0.10)] were studied.

The dielectric constant was observed to decrease when fre-

quency was increased from 20 Hz to 1 MHz and increased

with the increase in temperature from 313 to 773 K. An

interesting correlation between the antiferromagnetic Neel

temperature (TN) of bismuth ferrite and temperature

dependent dielectric constant was observed. The calculated

values of activation energies were in the order of

0.25–0.74 eV (\1.0 eV) and decreases with an increase of

Mn concentration. The variation of a.c. conductivity obeyed

the Jonscher’s power law (rac � xs). The observed value of

exponent‘s’ were in the range 0.09 \ ‘s’ \ 0.78 (\1.0) for

all the sample at temperature ranging from 473 to 598 K.

There was a systematic increase in the value of spontaneous

magnetization on increasing Mn concentration.

1 Introduction

Multiferroics, the materials which show simultaneous

existence of two or more ferroic properties such as ferro-

magnetic, ferroelectric and/or ferroelastic, are of growing

interest because of their applications in multiple state

memory elements, electronic field controlled ferromagnetic

resonance devices and transducers [1–7]. Most magnetic

ferroelectrics materials tend to have low magnetic ordering

temperatures and are often antiferromagnets, in which the

magnetoelectric effect is intrinsically small.

The perovskite like BiFeO3 compound, in which the ste-

reochemical activity of the Bi lone electron pair gives rise to

ferroelectric polarization, while the partially filled 3d orbi-

tals of the Fe3? ions cause G-type antiferromagnetic order,

seems to be one of the most suitable objects for multiferroic

research in view of its high magnetic and ferroelectric

ordering temperatures (TN * 640 K, Tc * 1100 K). Bi-

FeO3 has rhombohedrally distorted perovskite structure

(R3c space group) with lattice parameters: a = 3.958 A, and

a = 89.30� [8]. G-type antiferromagnetic spin ordering in

addition to a spiral spin structure with a wavelength of

62 nm, cancels the macroscopic magnetization in bulk Bi-

FeO3 [9]. In nanoparticles and thin films, the magnetization

value is high mainly due to the limitation of size, which

destroys the long range spin structure [10–13].

Recently, several groups reported on the synthesis and

characterization of multiferroic perovskite systems such as

[BiFeO3–BaTiO3] and [PrFeO3–PbTiO3] systems [14, 15].

These solid solutions of BiFeO3–ABO3 have attracted great

attention as a means to increase structural stability,

M. Sahni (&) � N. Kumar � S. Singh � A. Jha � S. Chaubey

Department of Physics, Motilal Nehru National Institute of

Technology Allahabad, Allahabad, U.P., India

e-mail: [email protected]

N. Kumar

e-mail: [email protected]; [email protected]

M. Sahni

Department of Physics, Sharda University, Greater Noida, U.P.,

India

M. Kumar

Department of Physics and Materials Science and Engineering,

Jaypee Institute of InformationTechnology, Noida 201307, India

M. K. Sharma

Department of Applied Sciences, Amity University, Noida, U.P.,

India

123

J Mater Sci: Mater Electron

DOI 10.1007/s10854-014-1859-5

Author's personal copy

sinterability, electric and magnetic properties. In order to

improve the electrical properties while preserving the

magnetic ordering the strategy of doping BiFeO3 with

different ?3 valence ions on A, B or both A and B sites has

been adopted in recent past [16–18]. It has been observed

that such doping has resulted in the reduction of the leak-

age current density and in the improvement of the ferro-

electric properties to some extent. These doping were also

expected to shift the transition temperature towards the

room temperature [19]. According to the Shannon et al.

[20], the Mn3? (ionic radius = 0.645 A) ions can occupy

the Fe3? sites in BiFeO3 materials, because both ions have

same valence state and similar ionic size to that of Fe3?

(0.645 A). Even if there are different oxidation states of

Mn (3? , 4?) in Mn-doped BiFeO3 materials, the ionic

radii of the different oxidation states of Mn are comparable

to that of Fe3?. Consequently, the Mn ions occupy Fe sites

only. Similar types of observations have been reported by

Chung et al. [21]. The reduction in leakage current and

increase in ferromagnetic properties is expected by the

substitution of Mn in BiFeO3- based compounds. An

enhancement in magnetization in Mn-doped BiFeO3 was

20 30 40 50 60

*

(a)

2θ (degree)

Inte

nsity

(A

.U)

*

(b)

* *

(c)*

(d)

(300

)

(122

)

(018

) (214

)

(116

)

(024

)

(006

)(012

) (104

)

(202

)

*

(e)(110

)

( *Bi2 Fe4 O9 )

Fig. 1 XRD pattern for a BiFeO3, b (0.90)BiFeO3–(0.10)BaTiO3

(BFO–BTO), c (0.90)BiFe0.97Mn0.03O3–(0.10)BaTiO3 (Mn-3),

d (0.90)BiFe0.95Mn0.05O3–(0.10)BaTiO3 (Mn-5), e (0.90)BiFe0.90-

Mn0.10O3–(0.10)BaTiO3 (Mn-10) (* represents Bi2Fe4O9 phase)

Fig. 2 a-c SEM images for a (0.90)BiFe0.97Mn0.03O3–(0.10)BaTiO3, b (0.90)BiFe0.95Mn0.05O3–(0.10)BaTiO3 and c (0.90)BiFe0.90Mn0.10O3–

(0.10)BaTiO3


123


also reported as result of the modification of the oxidation

state of Fe ion [22].

In the present work, the effect of Mn substitution on the

functional properties of the (0.90)BiFeO3–(0.10)BaTiO3

composites were investigated.

2 Experimental details

The BiFeO3 and Mn-doped (0.90)BiFe1-xMnxO3–

(0.10)BaTiO3 (x = 0.03, 0.05 and 0.10) nanocomposites

were synthesized by using chemical method. The precur-

sors used were 99.99 % pure (Merck) bismuth nitrate,

ferric nitrate, manganese nitrate and citric acid. Each pre-

cursor was dissolved in water separately in stoichiometric

ratio. Citric acid was used as complexing agent. The

solution of ferric nitrate and manganese nitrate were added

in bismuth nitrate solution slowly under optimized constant

stirring on a magnetic stirrer at 50 �C. After ensuring no

precipitate, the mixtures were put on a stirrer at 50 �C until

the clear solution converted to a floppy gel. The obtained

gel was placed in an oven for 8 h at 100 �C for drying. The

obtained powder was crushed and placed in furnace at

600 �C for 2 h to form a well-crystallized phase of BiFeO3.

10000 100000 10000000

50

100

150

200

250

300

350

Frequency (Hz)

BiFeO3 BFO-BTO

Mn-3 Mn-5 Mn-10

Die

letr

ic C

onst

ant

(ε)

(a)

10000 100000 10000000

1

2

3

4

5

6

tan

(δ)

Frequency (Hz)

BiFeO3 Mn-3

Mn-5 Mn-10(b)

Fig. 3 a Dielectric constant (e) as a function of frequency range from

20 Hz to 1 MHz at room temperature (315 K) for BiFeO3, (0.90)BiFeO3–

(0.10)BaTiO3, (0.90)BiFe0.97Mn0.03O3–(0.10)BaTiO3, (0.90)BiFe0.95

Mn0.05O3–(0.10)BaTiO3 and (0.90)BiFe0.90Mn0.10O3–(0.10)BaTiO3

b Dielectric loss (tand) as a function of frequency range from 20 Hz to

1 MHz at room temperature (313 K) for BiFeO3, (0.90)BiFe0.97Mn0.03-

O3–(0.10)BaTiO3 (Mn-3), (0.90)BiFe0.95Mn0.05O3–(0.10)BaTiO3(Mn-

5), (0.90)BiFe0.90Mn0.10O3–(0.10)BaTiO3 (Mn-10)

300 400 500 600 700 800

100

150

200

250

300

350

400

Die

letr

ic c

onst

ant

(ε)

Temeperature(K)

100KHz 75 KHz 50KHz 25 KHz(a)

300 400 500 600 700 800100

150

200

250

300

350

400

450

Die

letr

ic C

onst

ant

(ε)

Temperature(K)

100KHz 75KHz 50KHz 25KHz(b)

300 400 500 600 700 800

100

150

200

250

300

350

400

450

Die

letr

ic C

onst

ant

(ε)

Temperature (K)

100KHz 75KHz 50KHz 25KHz(c)

Fig. 4 Dielectric constant as a function of temperature range from

313 to 773 K at frequencies of 25, 50, 75, 100 kHz for

a (0.90)BiFe0.97Mn0.03O3–(0.10)BaTiO3 (Mn-3) b (0.90)BiFe0.95-

Mn0.05O3–(0.10)BaTiO3 (Mn-5) c (0.90)BiFe0.90Mn0.10O3–(0.10)

BaTiO3 (Mn-10)


123


The BaTiO3 has been added in pure and Mn-doped BiFeO3

in stoichiometric ratio and ball-milled for 5 h. The mixture

was kept in furnace at 600 �C for 1 h in air. After calci-

nations, the pellets were prepared with organic binder

polyvinyl acetate and sintered at 700 �C for 1 h.

The structural properties of thus synthesized composites

were studied using X-ray diffraction (XRD). The XRD

patterns of the compounds were recorded at room tem-

perature using X-ray powder diffractometer with CuKaradiation (k = 1.5405 A). A ZEISS scanning electron

microscope (SEM) was used to analyze the ceramics

microstructure. For dielectric measurements the prepared

pellets were polished and their flat surfaces were coated

with silver paste and then dried at 100 �C for 30 min for

electrical measurements. The dielectric measurements of

the samples were performed by using Wayne Kerr LCR

meter (model 4300) in a wide frequency range of 20 Hz to

1 MHz and temperature range from 313 to 773 K, also the

dielectric measurements were carried out as a function of

temperature at frequencies 10, 25, 50, 75, 100 kHz and

1 MHz for all the samples. The a.c. electrical conductivity

measurements were carried out as a function of frequency

and temperature using Wayne Kerr LCR metre (model

4300). The magnetic measurements were performed by

using a magnetic property measurement system Quantum

Design SQUID Magnetometer MPMSXL-7.

3 Results and discussions

3.1 Structural analysis

BiFeO3 belongs to the family of pervovskite ABO3 type

structures. BiFeO3 has rhombohedrally distorted perovskite

structure described by the space group R3c [a = 3.95 A

and a = 89.41]. Figure 1 shows the XRD patterns of Bi-

FeO3 and Mn-doped (0.90)BiFe1-xMnxO3–(0.10)BaTiO3

(x = 0.03, 0.05 and 0.10) nanocomposites. All the peaks

were indexed by Joint Committee on Powder Diffraction

Standards card No. 861518. In case of BiFeO3, it is difficult

to synthesize in a single phase because of the low structural

stability of perovskite BiFeO3 due to volatile nature of

Bismuth [23]. In our case, we also observed weak traces of

10000 100000 10000000

50000

100000

150000

200000

-Z''(

ohm

s)

Frequency (Hz)

323K 373K 423K 473K 523K 573 K

(b)

10000 100000 10000000

50000

100000

150000

200000

-Z''(

ohm

s)

Frequency(Hz)

323K 373K 423K 473K 523K 573K

(d)

10000 100000 10000000

50000

100000

150000

200000 323K 373K 423K 473K 523K 573K

-Z''(

ohm

s)Z(ohms)

(a)

10000 100000 10000000

50000

100000

150000

200000 323K 373K 423K 473K 523K 573K

-Z''(

ohm

s)

Z(ohms)

(c)

Fig. 5 Variation of

Z0 0(imaginary component of

impedance) as function of

frequency range from 20 Hz to

1 MHz and at different

temperature (323, 373, 423,

473, 523, 573 K) for a BiFeO3,

b (0.90)BiFe0.97Mn0.03O3–

(0.10)BaTiO3, c (0.90)

BiFe0.95Mn0.05O3–

(0.10)BaTiO3,

d (0.90)BiFe0.90Mn0.10O3–

(0.10)BaTiO3

0 2000000 40000000

200000

400000

600000

800000

1000000

1200000

-Z''(

ohm

s)

Z'(ohms)

(c) (b) (a)

Fig. 6 Nyquist plot of impedance measured at room temperature

(313 K) for a (0.90) BiFe0.97Mn0.03O3–(0.10)BaTiO3, b (0.90)BiFe0.95-

Mn0.05O3–(0.10)BaTiO3, c (0.90)BiFe0.90Mn0.10O3–(0.10)BaTiO3


123


impurities of Bi2Fe4O9 [marked by * in XRD plot in Fig. 1]

in data of pure BiFeO3. The diffraction lines of this

impurity phase disappeared with 10 % doping of BaTiO3 in

BiFeO3. Lin et al. [19] have also been reported similar

traces of impurities in pure BiFeO3 and this impurity peak

disappears with the 15 % doping of La in BiFeO3. This

means that doping of BaTiO3 and some other rare earths

hinders the formation of second phase, as reported by

several authors [24]. The doping of Mn (x = 0.03, 0.05 and

0.10) in (0.90)BiFeO3–(0.10)BaTiO3 results in reappearing

of impurity phase. Zhenga et al. [25] have also been

reported similar impurity peaks with the doping of Mn and

Co in BiFeO3. The observed dhkl values of all diffraction

lines of BiFeO3 and Mn-doped (0.90)BiFeO3–(0.10)Ba-

TiO3 (x = 0.03, 0.05 and 0.10) suggest that the structure is

maintained to be rhombohedral and in space group R3c.

Figure 2a–c shows SEM images of (0.90)BiFe(1-x)-

MnxO3–(0.10)BaTiO3 (x = 0.03, 0.05 and 0.10) compos-

ites. The grain sizes of all the composites are nonuniform

but with the increase in Mn concentration, the grain size

decreases and the samples become denser.

3.2 Dielectric relaxation

Figure 3a and b illustrate the frequency dependence of

dielectric constant (e) and dielectric loss (tand) respectively

for BiFeO3 and (0.90)BiFe1-xMnxO3–(0.10)BaTiO3

(x = 0.0,0.03,0.05 and 0.10) composites at 313 K. All

samples display decreasing trend in dielectric constant (e)and dielectric loss (tand) with increasing frequency from

20 Hz to 1 kHz. The observation has been explained by the

phenomenon of dipole relaxation wherein at low

0 10000 20000 30000 40000 500000

50000

100000

150000

200000

-Z''(

ohm

s)

Z'(ohms)

323K 373K 398K 423K 448K 473K 498 523K

(a)

0 20000 40000 600000

50000

100000

150000

200000

-Z''(

ohm

s)

Z'(ohms)

323K 373K 398K 423K 448K 473K 498K 523K

(b)

0 20000 40000 60000 800000

50000

100000

150000-Z

''(oh

ms)

Z'(ohms)

323K 373K 398K 423K 448K 473K 498K 523K

(c)

Fig. 7 a–c Nyquist plot of impedance measured at different temperature (323, 373, 398, 423, 448 , 473, 498, 523 K) for a (0.90)BiFe0.97-

Mn0.03O3–(0.10)BaTiO3, b (0.90)BiFe0.95Mn0.05O3–(0.10)BaTiO3, c (0.90)BiFe0.90Mn0.10O3–(0.10)BaTiO3

Fig. 8 equivalent circuits that represent the electrical properties of

grains and grain boundaries

Table 1 Various calculated parameters for the Pure BiFeO3 and

(0.90)BiFe1-xMnxO3–(0.10)BaTiO3 (x = 0.03, x = 0.05 and

x = 0.10) at 313 K

Sample Rg (KX cm) Cg (pF cm-1)

(0.90) BiFe0.97Mn0.03O3–(0.10)

BaTiO3

5,030 221.4

(0.90) BiFe0.95Mn0.05O3–(0.10)

BaTiO3

3,520 156.5

(0.90) BiFe0.90Mn0.10O3–(0.10)

BaTiO3

1,000 133.8


123


frequencies the dipoles are able to follow the frequency of

the applied field. In frequency region from 1 kHz to 1 MHz

the dielectric constant becomes independent of frequency,

resulting end of relaxation and is well explained by the

Maxwell–Wagner type relaxation [26, 27], often occurring

in the heterogeneous systems. The Koop’s phenomeno-

logical theory [28] postulates that the contributions of grain

and grain boundaries are effective at high and low fre-

quencies respectively. The polarization decreases with the

increase in frequency resulting decrease in dielectric con-

stant. At low frequency more energy is required for the

motion of charge carriers, thereby a higher resistance is

offered by the grain boundaries, causing higher value of

dielectric loss (tand). Similarly at high frequency low

resistance is offered by grains, resulted in lower value of

dielectric loss (tand). It has also been observed that the

dielectric loss in Mn doped (0.90)BiFeO3–(0.10)BaTiO3

samples in low frequency region is lower than BiFeO3

sample.

Figure 4a–c depict the temperature dependence of

dielectric constant at the frequencies of 25, 50, 75 and

100 kHz. It has been observed from the data that the value

of dielectric constant increases with the increase in tem-

perature for all the samples. One can also notice an

anomalous peak in dielectric constant around 600 K for all

the samples in Fig. 3a–c, which is very close to the

magnetic transition temperature (643 K) of BiFeO3. It has

also been observed this peak shift towards the lower tem-

perature side with the decrease in frequency. Singh et al.

[29] had also reported the similar kind of anomalies and

shift in (0.90)BiFeO3–(0.10)BaTiO3 composite and con-

cluded with the possibility of magnetoelectric coupling and

ferroelectric relaxor behaviour. The Landau-Devonshire

theory of phase transition predicts such types of dielectric

anomaly in magnetoelectrically ordered systems as an

influence of vanishing magnetic order on the electric order

[30]. Since there is a possibility of coupling in electric and

magnetic orderings in these materials, a change in mag-

netic ordering clearly affects the dielectric (e) constant

value.

3.3 Impedance spectroscopy

Impedance spectroscopy is a very well known technique to

study the electrical properties of polycrystalline compos-

ites. The electrical behavior of the system was studied over

a range of frequency from 10 to 1,000 kHz and temperature

from 323 to 573 K. Fig. 4a–d shows the variation of the

imaginary part of impedance -Z00 with frequency at tem-

peratures ranging from 323 to 573 K in BiFeO3 and Mn-

doped (0.90)BiFe1–MnxO3–(0.10)BaTiO3 (x = 0.03, 0.05

1.0 1.5 2.0 2.5 3.0 3.5

-10

-9

-8

-7

-6

-5

-4

-3

Ln(

σ dc)

(ohm

-1cm

-1)

10KHz 25KHz 50KHz 75KHz 100KHz 1000KHz

(b)

region I

region II

1.0 1.5 2.0 2.5 3.0 3.5-10

-9

-8

-7

-6

-5

-4

-3

Ln(

σ dc)

(ohm

-1cm

-1)

Ln(

σ dc)

(ohm

-1cm

-1)


(c)

region I

region II

1.0 1.5 2.0 2.5 3.0 3.5-8

-7

-6

-5

-4

-3

-2

Ln(

σ dc)

(ohm

-1cm

-1)

1000/T(K-1) 1000/T(K-1)

1000/T(K-1)1000/T(K-1)


(a)

region I

region II

1.0 1.5 2.0 2.5 3.0 3.5-10

-9

-8

-7

-6

-5

-4

-3 1000KHz 100KHz 75KHz 50KHz 25KHz 10kHz

region I

region II

(d)

Fig. 9 a–d ac conductivity

versus 1,000/T (K-1) at

different frequencies (1, 10, 25,

50, 75, 100, 1,000 kHz)

a BiFeO3,

b (0.90)BiFe0.97Mn0.03O3–

(0.10)BaTiO3,

c (0.90)BiFe0.95Mn0.05O3–

(0.10)BaTiO3,

d (0.90)BiFe0.90Mn0.10O3–

(0.10)BaTiO3


123


and 0.10) composites. It has been noted from the Fig. 5a–d

that there is asymmetric variation in the lower frequency

side and becomes symmetric for higher frequency side in

entire temperatures range, revealing dominant contribution

of grain at higher frequencies It also implies the absences

of space charge effect in higher frequency region. It has

also been observed from the data that the increase in

doping concentration of Mn, decreases the magnitude of Z00

in lower frequency region, showing the effect of Mn dop-

ing on the electrical behavior of the samples. Tirupathi

et al. [31] have also found similar results in their Ca doped

BiFeO3 system and attributed to dominating the contribu-

tion of grains in high frequencies region.

It was also interesting to investigate the behavior of

grain and grain boundary effects in Mn-doped

(0.90)BiFe1–MnxO3–(0.10)BaTiO3 (x = 0.03, 0.05 and

0.10) composites. Figures 6 and 7a–c shows the plots Z0 vs

-Z00 (Nyquist plots) at room temperature (313 K) and as

function of temperature (323 to 523 K) in all Mn doped

samples. The semicircles obtained in Figs. 6 and 7a–c for

the present system may explain the contribution of grains,

grain boundaries and electrodes in electrical properties. An

equivalent circuit as shown in Fig. 8 may be used to

understand the contribution of grains, grain boundaries and

electrodes in electrical properties. In this equivalent circuit

model the distribution of electrodes is represented by the

series array of Rs and Ls and that of grains and grain

boundaries by parallel circuits of Rg, Rgb and Cg, Cgb

respectively. It has been proposed that the electrode

polarization dominates at very low frequencies and their

contribution is negligible small. The relaxation frequency

for grain and grain boundaries are inversely proportional to

their resistances and capacitances, therefore high frequency

semicircle can be identified as a bulk property from the fact

that it passes through the origin and the grain boundary

effect seems to be eliminated. The intercept of the corre-

sponding semicircle with the x axis will be used for finding

the value of the resistance for the grains (Rg). The capac-

itance (Cg) related to grain can be calculated by substitut-

ing the value of resistance of the grain (Rg) and the

frequency of maxima (fmax) in the following equation.

xs ¼ 2pfmaxRC ¼ 1 ð1Þ

The value of Cg calculated at 313 K for all the samples

is of the order of pF and decreases with the increase in Mn

concentration (Table 1). It has been observed from the

Figs. 6 and 7a–c that the diameter of the arc decreases with

the increase in Mn concentration and the temperature,

which implies decrease in resistance with increase in Mn

concentration and the temperature. Das et al. [37] also

reported similar results in Ba?2 and Gd?3 doped BiFeO3

system. According to them it may be due to thermal acti-

vated change in conductivity of the samples.Ta

ble

2A

ctiv

atio

nen

erg

y(E

a)

val

ue

for

the

Mn

do

ped

(0.9

0)B

iFe 1

-xM

nxO

3–

(0.1

0)B

aTiO

3(x

=0

.0,

0.0

3,

0.0

5,

0.1

0)

com

po

site

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Reg

ion

1R

egio

n2

Fre

quen

cy/s

ample

s1.0

0E

?04

2.5

0E

?04

5.0

0E

?04

7.5

0E

?04

1.0

0E

?05

1.0

0E

?06

1.0

0E

?04

2.5

0E

?04

5.0

0E

?04

7.5

0E

?04

1.0

0E

?05

1.0

0E

?06

BiF

eO3

0.5

90.5

70.5

60.5

50.5

40.3

50.0

98

0.0

60.0

35

0.0

25

0.0

21

0.0

15

(0.9

0)

BiF

e 0.9

7M

n0.0

3O

3

–(0

.10)

BaT

iO3

0.7

40.6

80.6

20.5

80.5

50.2

60.0

89

0.0

58

0.0

49

0.0

41

0.0

37

0.0

1

(0.9

0)

BiF

e 0.9

5M

n0.0

5O

3

–(0

.10)

BaT

iO3

0.6

0.5

80.5

50.5

30.5

10.2

90.0

38

0.0

29

0.0

21

0.0

17

0.0

15

0.0

06

(0.9

0)

BiF

e 0.9

0M

n0.1

0O

3

–(0

.10)

BaT

iO3

0.5

40.5

30.5

10.5

0.4

90.2

50.2

40.2

30.1

20.0

84

0.0

50.0

44


123


3.4 Temperature dependent conductivity

To relate the macroscopic measurement to the microscopic

movement of the ions, the conductivity measurement is

most prominent. The total conductivity in solids is gener-

ally analyzed using Jonsher power law [26] and is repre-

sented as.

r xð Þ ¼ rdc þ Axs ð2Þ

where rdc is the d.c. conductivity, ‘A’ is pre-exponential

factor and ‘s’ is the frequency component, both ‘A’ and ‘s’

are temperature and material dependent [32] .

The d.c. conductivity corresponds to the drift of charge

carriers and is well defined by Arrhenius relation [33, 34]

given below.

LnðrdcÞ ¼ Lnðr0Þ �Ea

KBTð3Þ

Where r0: pre-exponential factor and Ea: activation

energy for charge conduction.

Ln(rdc) was plotted against 1,000/T (on K scale) at

different frequencies from 10 to 1,000 kHz and the plots

are presented in Fig. 9a–d. Each plot in figures may be

divided into two regions. Region I show that the d.c.

conductivity depends on temperature but almost indepen-

dent of frequency, whereas region II shows strong depen-

dence of d.c. conductivity on frequency as compared to

temperature. The activation energy of the samples can be

calculated from the slope of the curves in both the regions

and is tabulated in Table 2. It has been observed that

activation energy decreases with increase in frequency. The

value of activation energy of the samples in region I vary

from 0.25 to 0.74 eV, and in region II the variation in

activation energy vary from 0.006 to 0.24 eV, which is

7 8 9 10 11 12-8.5

-8.0

-7.5

-7.0

-6.5

-6.0

-5.5

Ln(

σ ac)

Ln(f)

473K 498K 523K 548K 573K 598K

(b)

7 8 9 10 11 12

-9.0

-8.5

-8.0

-7.5

-7.0

-6.5

Ln (

σ ac)

Ln(f)

473K 498K 523K 548K 573K 598K

(a)

7 8 9 10 11 12

-7.5

-7.0

-6.5

-6.0

-5.5

Ln(

σ ac)

Ln(f)

473K 498K523K 548K573K 598K

(c)

Fig. 10 a–b Frequency

dependence of ac conductivity

(rac) at various temperatures

(473, 498, 523, 548, 573,

598 K) for

a (0.90)BiFe0.97Mn0.03O3–

(0.10)BaTiO3,

b (0.90)BiFe0.95Mn0.05O3–

(0.10)BaTiO3

c (0.90)BiFe0.90Mn0.10O3–

(0.10)BaTiO3

Table 3 Values of exponent ‘s’

at different temperature for

(0.90)BiFe1-xMnxO3–

(0.10)BaTiO3 (x = 0.03, 0.05

and 0.10)

Sample/

temperature

(0.90)BiFe0.97Mn0.03O3–

(0.10)BaTiO3

(0.90)BiFe0.95Mn0.05O3–

(0.10)BaTiO3

(0.90)BiFe0.90Mn0.10O3–

(0.10)BaTiO3

473 0.78 0.5 0.36

498 0.74 0.43 0.3

523 0.69 0.36 0.24

548 0.63 0.29 0.19

573 0.53 0.22 0.14

598 0.43 0.16 0.09


123


very small. Mukherjee et al. [35] reported that the value of

activation energy lies between 0.35 to 0.50 eV in Y doped

BiFeO3 system. Pradhan at el. [36] has reported that the

activation energy decreases from 0.4 to 0.2 eV with the

doping of Mn in Bi0.90La0.10FeO3 system. It has been

explained that the conduction mechanism in region I may

be due to long range motion in oxygen vacancies and in

region II it may be due to short range hopping motion of

oxygen vacancies. It has also been observed that the acti-

vation energy decreases with the increase in Mn concen-

tration. Pradhan et al. [36] has reported that the electron

hoping between Mn?3 and Mn?4 ions is possible in high

permittivity perovskite ceramics. So it may be possible that

the electron hopping between Mn?3 and Mn?4 ions is one

of the explanations for this decrease in activation energy.

3.5 Frequency dependent conductivity

The frequency dependent contribution can be calculated by

subtracting the d.c. contribution from the total conductivity

represented in Eq. 2. Natural Logarithm of ac conductivity

(rac) [Ln(rac)] was plotted against natural logarithm of

frequency [Ln(f)] for all samples at temperature range from

473 to 598 K, and shown in Fig. 10a–c. It can be seen from

the figures that the conductivity remains almost constant at

lower frequencies but exhibits dispersion for higher

frequencies, this behavior is in accordance with Eq. 2. It

has also been observed that the frequency at which dis-

persion takes place shift toward higher (hopping frequency)

value with increase in temperature. The value of frequency

exponent ‘s’ was calculated from the slope of Ln(rac) vs

Ln (f) curves and is presented in Table 3. One may notice

here that this value of ‘s’ is less than unity

(0.09 \ ‘s’ \ 0.78) and decreases with increase in Mn

doping and temperature. Das et el. [37] have reported that

the value of exponent‘s’ lies between 0 \ ‘s’ \ 1 for Ba?2

and Gd?3 doped BiFeO3 systems. The electron transport

mechanism can usually be observed by the nature of var-

iation of ‘s’ and can be explained on the basis of correlated

barrier height (CBH) [38]. In CBH model, the conduction

occurs via polaron hopping process where two polarons

simultaneously hop over the potential barrier between two

charged defect states and the barrier height is correlated

-30000-20000-10000 0 10000 20000 30000

-4

-3

-2

-1

0

1

2

3

4

M (

emu/

gm)

H(Oe)

(c)

-2000 -1000 0 1000 2000

-3

-2

-1

0

1

2

3

-30000-20000-10000 0 10000 20000 30000

-3

-2

-1

0

1

2

3

M (

emu/

gm)

H(Oe)

(b)

-2000 -1000 0 1000 2000

-2

-1

0

1

2

-40000 -20000 0 20000 40000

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

M(e

mu/

gm)

H(Oe)

(a)

-2000 -1000 0 1000 2000

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

Fig. 11 a–c Magnetic hysteresis loops (M–H) at 5 K for a (0.90)BiFeO3–(0.10)BaTiO3 b (0.90)BiFe0.97Mn0.03O3–(0.10)BaTiO3

c (0.90)BiFe0.90Mn0.10O3–(0.10)BaTiO3 [Inset shows the M–H loop with the maximum applied field of 1,000 Oe.]

Table 4 Spontanous magnetization and magnetic coercivity for

(0.90)BiFeO3–(0.10)BaTiO3, (0.90)BiFe1-xMnxO3–(0.10)BaTiO3

(x = 0.03, 0.10)

Samples Ms (emu/gm) Hc (Oe)

(0.90)BiFeO3–(0.10)BaTiO3 1.30 870

(0.90)BiFe0.97Mn0.03O3–(0.10)BaTiO3 2.44 980

(0.90)BiFe0.90Mn0.10O3–(0.10)BaTiO3 3.40 1,383


123


with the inter-site separation via a Coulombic interaction

resulting gradually decrease of ‘s’ The conduction in Mn

doped (0.90)BiFeO3–(0.10)BaTiO3, may be attributed to

the translational type hopping of charge carriers.

3.6 Magnetic properties

The magnetization behavior of (0.90)BiFe1-xMnxO3–

(0.10)BaTiO3 (x = 0.0. 0.03. 0.10) were evaluated using a

SQUID magnetometer. M-H hysteresis loops of

(0.90)BiFe1-xMnxO3–(0.10)BaTiO3 (x = 0.0. 0.03. 0.10)

crystalline powders measured at 5 K and is shown in

Fig. 11a–c. The value of spontaneous magnetization (Ms)

has been obtained by linear extrapolation of high magne-

tization to zero field and its intercepts on the magnetization

axis. The value of spontaneous magnetization (Ms) and

coercivity (Hc) for the samples (0.90)BiFe1-xMnxO3–

(0.10)BaTiO3 (x = 0.0. 0.03. 0.10) are tabulated in

Table 4. The samples showed typical M-H hysteresis loops

for samples Mn (x = 0.0, 0.03 and 0.10). The Ms and Hc

increases with the increase in doping percentage of Mn

(x = 0.0, 0.03, 0.10). The pure BiFeO3 have antiferro-

magnetic ordering [39], whereas the ferromagnetism in

BaTiO3- rich compositions may be due to structural dis-

tortion [40] and statistical distributions of Fe3? and Ti4?

ions in the octahedral sites of BiFeO3–BaTiO3 [41]. Sos-

nowska et al. [42] have also reported increase in magne-

tization in BiFe1-xMnxO3 polycrystalline samples, by

claiming that the spiral G-type antiferromagnetic spin

structure of BiFeO3 shifts towards a collinear G-type

antiferromagnetic structure with increasing Mn concentra-

tion. Such suppression of the spiral spin structure will

subsequently result in weak ferromagnetism in such com-

posites [36].

4 Conclusion

A polycrystalline BiFeO3 and Mn doped (0.90)BiFe1-x-

MnxO3–(0.10)BaTiO3 (x = 0.0, 0.03 and 0.10) were suc-

cessfully synthesized. X-ray structural analysis shows the

phase formation of the materials. SEM images shows

decrease of grain size with the increase in Mn concentra-

tion. Both dielectric constant and dielectric loss decreases

with frequency following Maxwell–Wagner type relaxa-

tion. The anomaly in dielectric constant versus temperature

plot at around 600 K shows magnetodielectric coupling.

The calculated value of activation energy were found to

decrease with Mn doping, and may attribute due to electron

hopping between Mn?3 and Mn?4 ions. The grain resis-

tance and capacitance observed to decreases with the

increase in temperature for all the composites and an

equivalent circuit was used to explain the phenomena. The

value of frequency exponent ‘s’ decreased with increase in

temperature for all Mn doped samples which corresponds

to a large polaron hopping type conduction mechanism. Mn

(x = 0.0, 0.03, 0,10) doped samples show magnetization

and the value of spontaneous magnetization increases with

the increase in Mn concentration. This may be due to the

further suppression of spin cycloid in BiFeO3 and different

superexchange pathways owing to the occupation of both

Mn ion and Fe ion at the same lattice sites.

Acknowledgments M.S is greatful to the Sharda University of India

for providing financial support and leave for carrying out this work at

Motilal Nehru National Institute of Technology Allahabad. N.K is

thankful to Department of Science and Technology, Government of

India for funding (SR/FTP/PS-04/2008) and director MNNIT for the

support. N.K also acknowledges consistent support provided by the

Director MNNIT Allahabad and Centre for Interdisciplinary Research

(CIR) MNNIT Allahabad for the access of centered research facilities.

M.K is thankful to National Facility installed in Magnetics &

Advanced Ceramics Laboratory at IIT Delhi for magnetic

measurements.

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