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Chapter 3

Studies on Properties of BiFeO3 (Bulk and

Thin Film) Multiferroics

Chapter III is completely devoted to the studies on polycrystalline bulk and thin

films of BFO multiferroics. The effect of oxygen partial pressure and thickness on the

properties of BFO films has been discussed in detail in the light of oxygen vacancies and

Fe valence fluctuations. The effect of structural strain on the electric and magnetic

properties of BFO films has been explained.

Chapter 3: Studies on Properties of BiFeO3 (Bulk and Thin Film) Multiferroics

3.1 Studies on Polycrystalline Bulk BiFeO3

3.1.1 Introduction III - 01

3.1.2 Structure (XRD) III - 02

3.1.3 Microstructure (SEM) III - 02

3.1.4 Thermal (DTA) III - 03

3.1.5 Dielectric III - 04

3.1.6 Magnetization (M-H) III - 05

3.1.7 Conclusions III - 06

3.2 Studies of BiFeO3 thin films: Oxygen partial pressure dependent



3.2.3 Microstructure (AFM & MFM) III - 10

3.2.4 Rutherford Backscattering (RBS) Spectrometry III - 12

3.2.5 X-ray Photoelectron Spectroscopy (XPS) III - 13


3.2.7 Transport (I - V) and Mechanisms III - 15

3.2.8 Ferroelectric (P - E) III - 18

3.2.9 Magnetization (M - H) III - 19


3.3 Studies of BiFeO3 thin films: Thickness dependent



3.3.3 Microstructure (AFM) III - 23


3.3.5 Ferroelectric (P-E) III - 26

3.3.6 Magnetization (M-H) III - 26


References III - 29

III - 1

Studies on multiferroic property of BiFeO3 (Bulk and Thin Film)

During the course of present studies on BiFeO3 (BFO) multiferroics, an effort is

made to understand the effect of oxygen partial pressure and thickness on the

modifications in the multiferroic behavior of BFO thin films grown using Pulsed Laser

Deposition (PLD) technique. The results obtained are discussed in the context of their

structural, microstructural, electrical and magnetic properties.

3.1 Studies on Polycrystalline Bulk BiFeO3

3.1.1. Introduction

Multiferroics are known to exhibit ferromagnetic and ferroelectric properties

simultaneously as well as coupling between them. BFO multiferroic possesses high

ferroelectric transition temperature (TC) and antiferromagnetic Neel temperature (TN)

suitable for potential applications in functional devices [1]. BFO displays co-existence of

ferroelectricity (TC ~ 816 – 845˚C) and antiferromagnetism (TN ~ 375˚C) over a wide

temperature range. Prolonged sintering of bulk BFO at elevated temperature results in the

oxygen non stoichiometry and valence fluctuations between of Fe+3 and Fe+2 leading to

high conductivity making it difficult to observe ferroelectric hysteresis at room

temperature (RT). Observation of P - E loop in BFO at 80 K has been reported by Teague

et al [2]. Ferroelectric hysteresis loop at RT has been reported in BFO synthesized by

rapid thermal process. [3]. In this section, results of the structural, dielectric and magnetic

properties of polycrystalline bulk BFO synthesized using modified sol-gel route have

been discussed.

Modified sol-gel method has been employed to prepare bulk BiFeO3 (BFO)

multiferroic. Bismuth nitrate pentahydrate [Bi(NO3)3×5H2O] and Iron nitrate nanohydrate

[Fe(NO3)3×9H2O] were dissolved in acetic acid and double distilled water with 1:1 ratio

resulting in blackish red solution. The solution was stirred constantly for 6 hrs between

70˚C to 150˚C and then heated on hot plate for 24 hrs. Heating of the solution, at elevated

temperature, results in dark brownish powder of BFO. Pellets made under 4 Ton pressure

were sintered initially at 600˚C for 12 hrs and finally at 800˚C for 12 hrs. XRD

measurements were carried out to determine structure and phase purity. Scanning

Electron Microscopy (SEM) micrographs were obtained to understand the microstructure.

III - 2


Differential thermal analysis (DTA) was carried out up to 850˚C to understand the phase

transition, if any, in BFO. Frequency and temperature dependent dielectric constant and

loss were measured using Agilent L-C-R meter with Lakeshore temperature controller.

SEM images were taken at room temperature (RT) using JEOL JSM 5600 model.

Magnetization measurement was carried out at RT using vibrating sample magnetometer

(VSM).

3.1.2 Structure (XRD)

Structural studies carried out using XRD, confirm single phasic nature of the

sample. Fig. 3.1 shows the observed XRD pattern of BFO showing single phasic nature

crystallizing in hexagonal structure having R3c (no. 161) space group. Calculated values

of parameters are, a = b = 5.6229 (Å), c = 13.9120 (Å) and crystallite size is ~ 35nm.

20 30 40 50 600

500

1000

1500

2000

2500

(1 1

7)

(1 0

8)

(1 1

6)

(2 1

2)(2

0 4

)

(2 0

2)

(0 0

6)

(1 1

0)

(1 0

4)

(1 0

2)

Inte

nsit

y (a

.u.)

2

Figure 3.1: XRD pattern of polycrystalline bulk BFO multiferroic

3.1.3 Microstructure (SEM)

SEM micrograph (fig. 3.2) of bulk BFO obtained at RT shows well developed,

flat square and rectangular shaped grains having compact arrangement with an average

grain size ~3-10µm. There is no observed un-reacted phase of Bi2O3 present.

III - 3


Figure 3.2: SEM image of polycrystalline bulk BFO multiferroic

3.1.4 Thermal (DTA)

Fig. 3.3 shows the DTA curve of pure BFO samples obtained in heating cycles of

10 ˚C/min. Sharp peak signifies the first order phase transition observed at 816˚C while

broadened peak suggest weak magnetic transition at 340˚C due to small energy change

associated with magnetic transition. DTA analysis shows the phase transitions at 340˚C

and 816˚C which corresponds to the reported phase transition (TN and TC) for BFO [4].

Figure 3.3: DTA plot of polycrystalline bulk BFO multiferroic

III - 4


3.1.5 Dielectric

Frequency and temperature dependent dielectric measurements were carried out

on bulk BFO in the frequency and temperature range of 75 kHz to 5 MHz and 140 to

400K respectively. Fig. 3.4 shows the variation in dielectric constant (ε’) and loss (tanδ)

with frequency. With the increases in frequency, ε’ and tanδ decreases with highest value

of ε’ at lower frequencies which may be attributed to higher relaxation time [4, 5].

0 1x106

2x106

3x106

4x106

5x106

10

20

30

40

50

60

70

80

90

100

0.0

0.2

0.4

0.6

0.8

1.0

1.2

f (Hz)

Die

lect

ric

Con

stan

t (

') x

102

Dielectric L

oss

Figure 3.4: Variation in ε’ and tanδ with frequency at 300K for BFO multiferroic

Temperature dependent dielectric behavior of BFO at 0.5, 0.7, 0.9, 1.0 MHz

frequencies is shown in Fig. 3.5. Dielectric constant and loss values rapidly increase

above 250K. High values of tanδ at RT indicate that, the sintering process plays a key

role in the dielectric properties. Higher loss generally originates from the higher

conductivity, resulting higher leakage current which may arise due to fluctuations in Fe+2

and Fe+3 due to prolonged sintering. This possess difficulty in the observation of P - E

hysteresis in sol-gel grown BFO [6].

III - 5


150 200 250 300 350 400

2

4

6

8

10

12

14

150 200 250 300 350 400

0.0

0.2

0.4

0.6

0.8 0.5MHz 0.7MHz 0.9MHz 1.0MHz

Die

lect

ric

cons

tant

(10

3 )

T (K)

Dielectric loss

T (K)

0.5 MHz 0.7 MHz 0.9 MHz 1.0 MHz

Figure 3.5: Temperature dependent variations in dielectric constant and loss of BFO

multiferroic

3.1.6 Magnetization (M - H)

Fig. 3.6 shows RT M vs. H plot of sol- gel grown BFO. A weak ferromagnetic

behaviour is observed without saturation in magnetization value up to 1T which may be

due to cycloidal spiral spin arrangement of Fe3+. This linear M-H loop differentiates from

the paramagnetic material due to the observation of antiferromagnetic phase transition in

BFO. A similar magnetization character has been reported earlier for BFO [3, 5].

-10000 -5000 0 5000 10000

-1.0

-0.5

0.0

0.5

1.0

@ RT

M x

10-3

( B/f

u)

H (Oe)

Figure 3.6: M vs. H curve of polycrystalline BFO multiferroic at RT

III - 6


3.1.7 Conclusions

In summary, single phase polycrystalline bulk BFO multiferroic has been

synthesized using modified sol-gel route. Synthesis technique and sintering play a major

role in attributing ferroelectric properties. Temperature and frequency dependent

variations in dielectric constant and loss indicate good dielectric behavior and

observation of M - H loop supports the weak ferromagnetism at RT exhibited by BFO.

Non observation of P - E hysteresis loop may be attributed to the high leakage current in

sol-gel grown BFO.

III - 7


3.2 Studies of BiFeO3 thin films: Oxygen partial pressure dependent

3.2.1 Introduction

Understanding of fundamental physics of multiferroics having more than one

ferroic properties in same phase and its potential for technological application such as

data storage, multiple state memories, non - volatile memory devices, capacitors,

transducers, etc. has created large interest in materials physicist community during last

few years [7 - 9]. There is an opportunity to design novel magnetoelectric and

magnetodielectric devices using multiferroics having more applicability as compared to

conventional semiconducting/magnetic devices. It is reported that, TbMnO3, TbMn2O5,

DyMnO3, YMnO3 and GdMnO3 compounds exhibit, both multiferroic and

magnetoelectric behavior below RT [10 - 12]. In addition, BiFeO3 (BFO) is well known

multiferroic which has attracted attention of many researchers and scientists due to high

TC (~ 1100K) and TN (~ 640K) well above RT. Various factors, such as Bi-volatility,

oxygen vacancies, interface strain, leakage current, Fe valance fluctuations, etc limit the

applications of BFO compound [11, 12]. In BFO based thin films, leakage current is one

of the major problems which hinder its potential for applications in thin film based

devices. Generally, leakage current is related to the existence of oxygen vacancies and

Fe2+ in BFO [13 - 15]. Several synthesis factors, such as, preparation technique, nature

of substrate, substrate temperature, oxygen partial pressure, film thickness, etc affect the

properties of BFO films [16 - 18]. Recent studies have shown that, oxygen vacancies

have more impact on leakage current than Fe valance fluctuations. Also, various chemical

substitutions (such as, La, Ti, Zr and Mn), different oxygen partial pressures and post

annealing studies have been reported in order to reduce leakage currents in BFO films [19

- 22]. Bea et al have explored the effect of growth mechanism, temperature and oxygen

partial pressure on BFO films and explained the pressure – temperature phase diagram

for BFO [23]. You et al. have reported that, leakage current in Bi-deficient samples are

much smaller than Bi-rich ones [24]. In general, oxygen concentration plays an important

role in governing the electrical and magnetic properties of BFO films because they are

responsible for the formation of conducting Bi2O3 and ferromagnetic Fe2O3 impurities.

III - 8


To understand the role of oxygen content in multiferroic behavior of BFO films,

in this chapter, an attempt is made to study the tunability of multiferroic properties in

PLD grown BFO/SNTO films with varying oxygen partial pressure. Oxygen

stoichiometry and Bi: Fe ratio calculated from Rutherford Backscattering (RBS) data are

found to play an important role in affecting the transport, electrical polarization and

magnetic properties of BFO films, which have been understood on the basis of oxygen

vacancies and Fe valence state fluctuations. Also, different theoretical models have been

studied to understand dominant charge-transport mechanism for BFO films. In addition,

for studying the modifications in structure, microstructure, electric and magnetic

properties of BFO films synthesized at varying oxygen partial pressure, XRD, atomic

force microscopy (AFM), magnetic force microscopy (MFM), RBS, X-ray photoelectron

spectroscopy (XPS), dielectric, I - V, P - E hysteresis loop and M - H loop measurements

has been carried out and discussed in detail.

BFO target prepared by modified sol-gel method (details in section 3.1) was used

for thin film deposition. BFO films were deposited on n-type SNTO (100) single crystal

substrates using pulsed laser deposition (PLD) technique. Experimental conditions used

during laser ablation are given in Table 3.1 Single phasic structure of the films was

confirmed using XRD. AFM and MFM images were obtained at RT using Nanoscope

IIIa scanning probe microscope (SPM). Chemical stoichiometry and film thickness were

measured by performing the RBS experiment using 3.037 MeV He+ ion beam, in Nuclear

Resonant Backscattering (NRBS) mode at IUAC, New Delhi. XPS measurements were

performed using Al-Kα (λ = 0.834 nm) lab-source to study the valence state of Fe in BFO

samples. To carry out the transport and electrical measurements, ~1mm Au contacts were

deposited on to film surface using DC coating unit. I - V and P - E loop measurements

were performed using current perpendicular to plane (CPP) geometry, at RT, using

Keithley 2612A source meter and Radiant technologies precession ferroelectric loop

tracer, respectively. Magnetization measurements at RT were performed using Quantum

design 7 Tesla SQUID-VSM system.

III - 9


Table 3.1: BFO film deposition parameters


XRD patterns of BFO/SNTO films grown under different oxygen partial pressures

are shown in fig. 3.7.

20 30 40 50 60

*

Inte

nsi

ty (

a.u.

)

30mT

50mT

K

SN

TO

SN

TO

100mT

SN

TO

*

SN

TO

(20

0)

BF

O (

200)

BF

O (

104)

SN

TO

(100)

BF

O (

100)

300mT

2

SNTO Substrate

Figure 3.7: XRD patterns of BFO/SNTO films grown under 30, 50, 100 and 300mT.

(* indicates XRD peaks of Bi2O3)

Laser used KrF Excimer

Targets BFO Polycrystalline bulk

Substrate SrNb0.002Ti0.998O3; Single crystalline (100)

Laser energy ~ 250mJ

Repetition rate 5Hz

Substrate temperature 670C

Oxygen Partial Pressure 30, 50, 100 and 300mT

Substrate to target distance 5.5cm

III - 10


It can be seen that, the films grown under 50, 100 and 300mT partial pressure are

single phasic while the film grown under 30mT has Bi2O3 impurity phase (marked as *).

There is no evidence of un-reacted Fe2O3 phase present in the films. All the films show

Bragg reflections due to (h00) plane of BFO along with the (h00) peaks of SNTO

substrate. Presence of (104) reflection suggests polycrystalline growth of BFO in film.

3.2.3 Microstructure (AFM and MFM)

Surface topography of BFO films was investigated using atomic force microscopy

(AFM). Fig. 3.8 shows the AFM micrographs of BFO films (10×10 µm2) grown under

30, 50, 100 and 300mT oxygen partial pressure. It can be seen from the AFM

micrographs that, films grown under 30 and 50mT oxygen partial pressure possesses less

developed granular structure having barely smooth surface and larger rectangular grain

growth like outcome on surface which may be due to Bi- rich phase occurring during the

growth process while 100 and 300mT films possesses compact well developed granular

structure without any impurity phases. Bi2O3 impurities typically have rectangular shapes

and different growth rate; therefore, they usually appear as protuberance in the smoother

BFO matrix [25]. Values of surface roughness of 30, 50, 100 and 300mT films are ~

6.19, 12.74, 17.82 and 19.32 nm respectively. Increasing in roughness with in oxygen

partial pressure can be attributed to increase in BFO phase formation, making the films

more granular in structure.

MFM study was carried to investigate the magnetic domain structure of BFO

films. Fig. 3.9 shows the MFM images of 30, 50, 100 and 300mT oxygen partial pressure

grown BFO films at lift height 40nm, 60nm and 80nm. It can be seen from the MFM

micrographs that, the contrast of granular structure changes with the lift height suggesting

the magnetic response of the BFO films. No other magnetic reflection observed in all

MFM micrographs confirms the BFO films possess magnetic domain structure without

any magnetic impurities such as γ-Fe2O3. Also, angle of vertical distance of grains

decreases with increases in lift height which is possibly due to magnetic signal, coming

out from the grains and grain boundaries of BFO samples grown at different oxygen

partial pressures. Observed variations in angle of vertical distance are given in Table 3.2.

III - 11


Figure 3.8: AFM micrographs of BFO/SNTO films grown under 30, 50, 100 and

300mT

Figure 3.9: MFM images of BFO/SNTO films grown under 30, 50, 100 and 300mT

III - 12


Table 3.2: Angel of vertical distance observed in MFM micrograph of BFO films

Oxygen partial pressure

Lift height

Vertical distance

40nm 60nm 80nm

30mTorr 1.667˚ 0.742˚ 0.313˚

50mTorr 1.131˚ 0.388˚ 0.344˚

100mTorr 1.931˚ 0.723˚ 0.434˚

300mTorr 1.244˚ 0.645˚ 0.399˚

3.2.4 Rutherford Backscattering (RBS) Spectroscopy

Fig. 3.10 shows the RBS spectra of all the BFO / SNTO films obtained using

3.037MeV He+ ion beam, in Nuclear Resonant Backscattering (NRBS) mode, using the

RBS facility at IUAC, New Delhi. Use of high energy ions, results in strong resonance

and better sensitivity for the detection of lighter elements, such as, B, C, N and O present

in the sample [26]. Oxygen stoichiometry and film thickness was determined by fitting

the RBS data using SIMNRA software [27]. Calculated values of thickness are ~76, 55,

31 and 42nm for films grown under 30, 50, 100 and 300mT, respectively. Variation in

film thickness may be attributed to the Gaussian shape of plasma plume changing with

oxygen partial pressure inside the PLD chamber. Calculated Bi/Fe ratios are ~ 1.78, 2.0,

1.0 and 1.0 for films grown under 30, 50, 100 and 300mT pressures, respectively,

suggesting that the Bi/Fe atomic stoichiometry is maintained in the BFO/SNTO films

grown at higher partial pressures (~ 100 and 300mT). Oxygen stoichiometry values, in

the film and substrate combinely, obtained from the fitting the RBS data are ~ 61, 67, 72

and 68% for 30, 50, 100 and 300mT grown films, respectively. It can be observed that,

BFO / SNTO film grown under 30mT is more oxygen deficient as compared to others.

Using NRBS mode for RBS measurement, it is difficult to distinguish the oxygen

contents of BFO films and SNTO substrates.

III - 13


500 1000 1500 20000

2000

4000Bi

Sr

FeTi

O

N

orm

aliz

ed Y

ield

(C

ounts

)

Channel Numbers

30mT 50mT 100mT 300mT

Figure 3.10: RBS plots of BFO films grown at 30, 50, 100 and 300mT

3.2.5 X-ray Photoelectron Spectroscopy (XPS)

To understand Fe valence fluctuations due to oxygen vacancies in BFO films,

XPS measurements were performed using Al-Kα (λ = 0.834 nm) lab-source at INDUS - 1

(UGC DAE, CSR beam line). Fig. 3.11 shows the fitted of the XPS spectra of BFO films

grown at 30, 50, 100 and 300mT.

704 706 708 710 712 714704 706 708 710 712 714

300mT

B.E. (eV)

Intensity (arb. unit)

30mT

Inte

nsit

y (a

rb. u

nit)

Fe3+

Fe2+ 50mT

100mT

B.E. (eV)

Figure 3.11: Fe 2P3/2 XPS spectra of BFO films grown at 30, 50, 100 and 300mT

III - 14


Fitting of observed Fe 2P3/2 peak using Gaussian peak related to Fe2+ and Fe3+ at

binding energy (B.E.) ~709 and 711eV having Fe2+/Fe3+ ratio ~1.72, 1.71, 1.54 and 1.59

for 30, 50, 100 and 300mT films respectively, suggest that, films grown at 30mT and

50mT possess more oxygen vacancies resulting in more conversion of Fe valance state

(+3 to +2) as compared with 100 and 300mT films.

3.2.6 Dielectric

Fig. 3.12 shows the frequency dependent dielectric behaviour of BFO / SNTO

films grown under 30, 50 and 300mT oxygen partial pressures, while inset of fig. 3.12

shows the same for 100mT film. Dielectric constant (ɛ׳) was measured in CPP geometry

at RT in the frequency range 75 KHz - 1MHz.

100000 1000000

600

700

800

900

1000

1100

1200

100000 1000000

3600

3900

4200

4500

4800

5100

@ RT

Frequency (Hz)

Die

lect

ric

cons

tant

('

)

300mT 50mT 30mT

'

Frequency (Hz)

100mT

Figure 3.12: Frequency vs. dielectric constant plots of BFO/SNTO films grown under

30, 50, 100 (inset) and 300mT

BFO / SNTO films show the higher value of ɛ׳ at low frequencies, mainly due to

higher relaxation time. Observed lower ɛ׳ value of 30mT film may be attributed to the

III - 15


presence of conducting Bi2O3 phase in film. BFO / SNTO films grown under 50, 100 and

300mTorr oxygen partial pressures show higher values of dielectric constant, which can

be correlated with the oxygen content in present films calculated from RBS data analysis.

It is well established fact that, oxygen vacancies are responsible for the conduction in

BFO films. In the present case, 100mT grown films show high dielectric constant as

compared to other films due to high oxygen content resulting in the less conductivity.

Also, 100 and 300mT grown films show homogenously distributed grain growth (from

AFM micrograph, fig. 3.8) as compared to 30 and 50mT grown films suggesting that,

microstructural density of grains play an important role because of the interfacial

polarization is responsible for dielectric behaviour in BFO films.

3.2.7 Transport (I - V) and Mechanisms

I - V measurements at RT were performed on all the BFO films for understating

the mechanism of leakage current. Fig. 3.13 shows I - V plots of all the films depicting

rectifying behaviour due to BFO acting as p - type and n - type SNTO substrate.

-6 -4 -2 0 2 4 6

-2

0

2

4

6

8

10

12

14

16

18

I (m

A)

V(Volt)

30mT 50mT 100mT 300mT

@ RT

Figure 3.13: I - V curves of BFO films taken at 300K

III - 16


In forward bias, slope of I - V curve decreases in films grown under 30 to 50,

300mT and 100mT. Film grown under 30mT, possess conducting Bi2O3 impurity phase,

resulting in higher conduction while other films having single phasic nature with some

oxygen vacancies show the decrease in conductivity. To understand the role of leakage

currents in conduction of charge carrier across bulk and oxide film-substrate interface,

two sets of mechanisms, have been reported, namely, (i) bulk limited: Space-Charge-

Limited-Conduction (SCLC) and Poole - Frankel (P - F) emission and (ii) interface

limited: Schottky barrier and Fowler - Northeim (F - N) tunneling [28-31].

Figs. 3.14 (a - d) show the fitting of I - V data using different charge transport

mechanisms. Non-linear data fitting using F - N tunneling model [ln (I/V2) vs. 1/V; IFN =

CV2 exp (– D2ψ3/2V-1)] [fig. 3.14 (a)] suggests that, the observed I - V beahviour cannot

be explained using this model. Figs. 3.14 (b) and (c) show the I - V data fitting using

Schottky barrier [ln(I/T2) vs. V1/2; IS = AT2 exp(– {ψ – (q3V/4πε0K)1/2} / KBT)] and Pool-

Frankel model [ln(I/V) vs. V1/2; IPF = BV exp(–{VI – (q3V/4πε0K)1/2} / KBT)]

respectively. Calculated values of optical dielectric constant (K) using Schottky barrier

mechanism are 1.46, 2.02, 2.23 and 2.65 and using P - F emission are 0.45, 0.97, 1.18 and

2.65 for BFO films grown under 30, 50, 100 and 300mT respectively which are much

lower than reported (K = 6.25) for BFO films using ellipsometric data [32]. It can be seen

that, I - V data fits well using both these mechanisms, in high voltage region only

suggesting that, role of leakage currents cannot be understood completely in high and low

voltage regions using Schottky barrier and P – F emission mechanisms. Fig. 3.14 (d)

depicts the fitting of I-V data using SCLC model [I vs. V2, ISCLC = 9μεrε0V2 / 8d3, where

d is the film thickness and is the ratio of free electron density to trapped electron

density at the interface] showing the best fits throughout the voltage range studied,

indicating the validity of SCLC model to explain the leakage current mechanism in BFO /

SNTO films. Observation of slight curvature (non-linearity) in low voltage region for

30mT film may be attributed to high leakage current across the film -substrate interface

due to the presence of conducting Bi2O3 impurity. Further, the applicability of SCLC

model can be verified by I vs. 1/d3 plots [fig. 3.14 (e)] confirming the decreases in current

with thickness, suggesting that, all the films satisfy I α 1/d3 condition.

III - 17


0 5 10 15 20 25-202

4

68

10

1214

1618

0.4 0.8 1.2 1.6 2.0 2.4 2.8

-18

-16

-14

-12

-10

-8

0.4 0.8 1.2 1.6 2.0 2.4 2.8-7

-6

-5

-4

-3

-2

-1

0

1

2

0.00 1.50x10-5

3.00x10-5

0

3

6

9

12

15

18

0.0 0.5 1.0 1.5 2.0 2.5-7

-6

-5

-4

-3

-2

-1

0

1

2

3

(d)

SCLC

I(m

A)

V2 (Volt)

30mT 50mT 100mT 300mT

(b) K=1.46385

K=2.02114

K =3.46107

K=2.23086

Schottky barrier

Ln(

I/T

2 )(m

A/K

2 )

V1/2

(Volt)

30mT 50mT 100mT 300mT

(c) K=0.453

K=0.97747

K=1.18792

K= 2.652

V1/2

(volt)

P-F emission

Ln(

I/V

) (m

A/V

olt)

30mT 50mT 100mT 300mT

(e)

I (m

A)

1/d3 (nm

-3)

5V 4V 3V

(a)

F-N Tunneling

Ln(

I/V

2 ) (m

A/V

olt)

1/V (1/volt)

30mT 50mT 100mT 300mT

Figure 3.14: I - V data of BFO/SNTO films fitted using (a) F-N Tunneling (b) Schottky

emission (c) P-F emission (d) SCLC and (e) verification of SCLC model.

K is the optical dielectric constant

III - 18


3.2.8 Ferroelectric (P - E loop)

Verification of the ferroelectric properties of BFO films grown under various

oxygen partial pressures has been done using, polarization vs. electric field (P - E)

measurements performed at RT. Fig. 3.15 depicts the P - E loops of all the BFO/SNTO

films obtained at RT with 50Hz, showing distinct hysteresis in P - E behaviour. Various

drive fields were applied to the films due to different breakdown fields exhibited.

-200 -100 0 100 200-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0 @ RT, f= 50Hz

Pr(

c/cm

2 )

E (kV/cm)

30mT 50mT 100mT 300mT

Figure 3.15: P - E hysteresis loops of 30, 50, 100 and 300mT grown BFO films

It can be seen that, for the films grown under 30, 50, 100 and 300mT, the values

of saturation polarization (PS) are ~ 0.12, 0.15, 1.69 and 0.5µC/cm2 and coercive fields

(EC) are ~ 38, 60, 88 and 44kV/cm, respectively, indicating that, with increasing oxygen

partial pressure, PS and EC increase up to 100mT film while for 300mT film, they

decrease due to the lower oxygen content (~ 68%). For films grown at 30 and 50mT,

higher leakage current (which can be seen in reverse bias I - V curves) leads to

suppression in polarization due to conversion of Fe3+ to Fe2+. Our observed values of PS

are lower as compared to those reported for other conducting substrates [26], due to

lower conductivity (~ 5.21×10-6Ω-1cm-1) of SNTO substrate and lower measurement

frequency (~ 50Hz) used.

III - 19


Observation of increase in PS and EC in 30mT to 100mT BFO/SNTO film can be

corroborated with the change in slopes of the I-V plots fitted with SCLC model [Figure

3.2.8 (d)], wherein, slope is proportional to “” i.e. the ratio of free electron density to

trapped electron density at the interface. Initially, the slope decreases from 30-100mT

while for 300mT, there is slight increase, confirming the reliability of SCLC mechanism

for charge transport in BFO/SNTO films.

3.2.9 Magnetization (M - H loop)

Fig. 3.16 shows M - H loops of 100 and 300mT BFO/SNTO films, obtained at RT

using SQUID-VSM measurements under maximum applied field of 10kOe. Both the

films possess complete 1:1 Bi:Fe ratio (RBS results). Inset of fig. 3.2.10 shows the

presence of narrow hysteresis loop (0 - 1.4kOe) suggesting weak ferromagnetic nature of

films. Values of saturation magnetization (Ms) are ~10 and 5emu/cc while coercive fields

(HC) are ~ 0.05 and 0.1KOe for 100 and 300mT films respectively. Observation of

higher magnetization in 100mT film can be attributed to the presence of lower oxygen

vacancies and hence reduced Fe2+ conversion possibility.

-12 -9 -6 -3 0 3 6 9 12

-10

-8

-6

-4

-2

0

2

4

6

8

10

-1.6 -1.2 -0.8 -0.4 0.0 0.4 0.8 1.2 1.6

-6

-3

0

3

6

@ RT

M (

emu/

cc)

H (KOe)

100mT 300mT

M (

emu/

cc)

Figure 3.16: M - H curves of BFO/SNTO films grown under 100 and 300 mTorr

III - 20


3.2.10 Conclusions

In conclusion, it is shown that, oxygen partial pressure used during the BFO /

SNTO film deposition, affect the nucleation of BFO and microstructure which modify

multiferroic properties. It is shown that, SCLC mechanism governs the transport

behaviour and explains the leakage current in these films. Observation of improved

polarization and magnetization in BFO films grown under 100 and 300mTorr oxygen

partial pressures, may be attributed to the higher oxygen content (calculated from RBS

data ) present in the films resulting in the reduction of Fe+3 to Fe2+ (XPS studies). Also,

the observation of 1:1 Bi: Fe ratio in 100 and 300mToor grown films suggest that, higher

oxygen partial pressure helps to maintain the BFO stoichiometry (fig. 3.17). Hence, it is

concluded that, structure, microstructure, transport, electrical and magnetic properties are

highly sensitive to oxygen partial pressure used during the film deposition.

0 50 100 150 200 250 30060

62

64

66

68

70

72

1.0

1.2

1.4

1.6

1.8

2.0

Bi:F

e ratio

Oxy

gen

Con

tent

(%

)

Oxygen Partial Pressue

Figure 3.17: Variation in oxygen content and Bi: Fe ratio with oxygen partial pressure

in BFO films

III - 21


3.3 Studies of BiFeO3 thin films: Thickness dependent

3.3.1 Introduction

It is reported that, polycrystalline bulk BFO exhibit weak multiferroic properties

due to inhomogeneous spiral spin structure and Fe valance state fluctuations [11, 12]

which limits its applicability. However, to overcome this aspect and study the practical

applications, BFO is required to be studied in thin film form. Various film synthesis

techniques and parameters are used during film growth, namely, nature of substrate,

substrate temperature, oxygen partial pressure, film thickness which play an important

role in controlling the properties of BFO films. Films grown by pulsed laser deposition

(PLD), rf - sputtering, chemical solution deposition show improved multiferroic

properties as compared to polycrystalline bulk BFO [10, 34 - 35]. For electrical

characterization, conducting substrate or thin buffer layer is required. Several reports

exist on effect of buffer layer such as SrRuO3, LaNiO3, La0.7Sr0.3MnO3, etc on electrical

properties of BFO films [16, 19]. Film - substrate interface plays an important role in

achieving the desire properties of BFO, but, by using the conducting buffer layer, the

chances of diffusion and intermixing of buffer layer, at nanoscale, can be improved. Also,

reports on BFO films grown on conducting substrate such as polycrystalline

Pt/TiO2/SiO2/Si and single crystal Nb doped SrTiO3 (SNTO), and

(LaAlO3)0.3(SrAlTaO6)0.7 (LSAT) substrate exhibit better electrical properties [17, 36].

Physical properties of BFO films are highly dependent on structural strain.

In this section, the effect of film thickness on the multiferroic behaviour of PLD

grown BFO films have been discussed. Oxygen partial pressure ~100mTorr was fixed

during all the depositions for maintaining the Bi, Fe and O stoichiometry in BFO.

Thickness dependent modifications in the structural strain and its effect on the

multiferroic properties of BFO/SNTO have been investigated. Modifications in the

structural, microstructural, electrical and magnetic properties by varying the thickness of

BFO films have been using XRD, AFM, dielectric, I - V, P - E hysteresis loop, C - V and

M - H loop measurements.

Thin film deposition was carried out using BFO target prepared by modified sol-

gel method (details in section 3.1) [37]. N-type conducting SrNb0.002Ti0.998O3 (SNTO)

III - 22


single crystal substrate was used for the growth of BFO/SNTO films having 50, 100 and

200nm BFO thickness. The experimental conditions used during laser ablation are given

in Table 3.3 Structure of films was confirmed using XRD. I - V and P - E loop

measurements were carried out using two probe techniques, in current perpendicular to

plane (CPP) geometry, at RT, using Keithley 2612A and Radiant technologies precession

ferroelectric loop tracer, respectively. For carrying at these measurements, 1mm Au-

contacts were deposited on to film using DC coating unit. AFM were taken at RT using

Nanoscope scanning probe microscope. Frequency dependent dielectric constant was

measured using Agilent L-C-R meter. Magnetization measurements at RT were

performed using Quantum design 7 Tesla SQUID-VSM system.

Table 3.3: BFO film deposition parameters


Fig. 3.18 depicts XRD patterns of 50, 100 and 200nm BFO/SNTO films showing

that, all films exhibit single phasic nature and are grown according to substrate

orientation. There is no evidence of un-reacted Fe2O3 and Bi2O3 phase present in the

films. XRD patterns clearly show that, BFO peak is shifted towards the lower 2θ angle in

films having 200 to 50nm thickness. The presence of lattice mismatch between the film

and substrate is evident from the separation between the film and substrate peaks and

Laser used KrF Excimer

Target BFO Polycrystalline bulk

Substrate SrNb0.002Ti0.998O3; Single crystalline (100)

Laser energy ~ 250mJ/

Repetition rate 5Hz

Substrate temperature 670C

Oxygen Partial Pressure 100mTorr

Thickness of films 50, 100 and 200nm

Substrate to target distance 5.5cm

III - 23


quantified as δ (%) = [(dsubstrate – dfilm) / dsubstrate] × 100, where, ‘d’ is the lattice parameter

of film / substrate. Values of δ are ~ -4.46%, -4.30% and -3.25% for 50, 100 and 200nm

films respectively. Negative value of δ suggests that, all films possess compressive strain.

20 30 40 50 60 70 80

SN

TO

(30

0)

BF

O (

300

)

SN

TO

(20

0)

BF

O (

200)

SN

TO

(100

)

BF

O (

100)

200nm

2

100nm

Inte

nsi

ty (

a.u.)

50nm

Figure 3.18: XRD patterns of 50, 100 and 200nm BFO/SNTO films

3.3.3 Microstructure (AFM)

Surface topography of BFO films was investigated using AFM. Fig. 3.19 shows

the 2D and 3D AFM micrographs (10×10 µm2) of 50, 100 and 200nm BFO films. It can

be clearly seen that, 50nm film possesses island like granular structure which gets

converted into compact structure having bigger rectangular grains in 100 and 200nm

films with calculated rms roughness values are ~ 4.88, 4.53 and 3.10nm, respectively.

Decreases in rms roughness with increase in film thickness may be attributed to the effect

of larger deposition time required in higher thickness film, resulting in grain

III - 24


agglomeration and barely smooth surface. Improved surface morphology in 100 and

200nm films, suggests that, the films become more crystalline according to substrate

orientation which can be correlated with the decrease in the structural strain (observed

from XRD results).

Figure 3.19: Surface topography (2D & 3D images) of 50, 100 & 200nm BFO/SNTO

films

3.3.4 Dielectric

Figs. 3.20 (a) and (b) show the frequency response of dielectric properties of 50,

100 and 200nm BFO films. Dielectric constant (ɛ’) and loss (tanδ) were measured in CPP

geometry at RT in the frequency range (75 kHz – 1MHz). BFO films show higher value

of ɛ’ at low frequencies, due to higher relaxation time. Dielectric constant increases with

increases film thickness while loss tanδ decreases, suggesting the presence of high

leakage current in higher thickness film. Also, higher ɛ’ and lower tanδ value in 200nm

film, as compared with100 and 50nm, may be due to dense microstructure, (bigger

rectangular grains) evident from the AFM micrographs.

III - 25


0.0 2.0x105

4.0x105

6.0x105

8.0x105

1.0x106

1.2

1.6

2.0

2.4

2.8

3.2

3.6

4.0

4.4 @ RT(a)

Die

lect

ric

const

ant

(')

f (Hz)

50nm 100nm 200nm

0.0 2.0x105

4.0x105

6.0x105

8.0x105

1.0x106

0.0

0.1

0.2

0.3

0.4

0.5 @ RT(b)

Die

lect

ric

loss

(ta

n)

f (Hz)

50nm 100nm 200nm

Figure 3.20: Frequency vs. (a) Dielectric constant and (b) loss of 50, 100 and 200 nm

BFO films

III - 26


3.3.5 Ferroelectric (P - E hysteresis loop)

To study the thickness dependent ferroelectric behaviour of BFO films,

polarization vs. electric field measurements were carried out at 50Hz and RT. Figs. 3.21

(a - c) show the P - E loops obtained for all the films displaying partially saturated

hysteresis loop under varying applied voltage. It can be seen that, value of saturation

polarization (PS) increases with applied voltage and decreases with film thickness. Values

of PS are ~1.28, 1.15 and 0.7µC/cm2 and coercive fields (EC) are ~90, 40 and 5kV/cm for

50, 100 and 200nm films, respectively at higher applied voltage. These values of Ps are

lower as compared to those reported for BFO on other conducting substrates [33]

because, SNTO substrate used, is less conducting (~5.21×10-6Ω-1cm-1) and lower

measurement frequency (~50Hz) has been used.

In BFO films, ferroelectric and ferromagnetic properties are highly dependent on

structural strain. Oxygen vacancies and Fe valence fluctuations (+3 to +2) are responsible

for increasing the leakage current in BFO films. Observation of saturated hysteresis loop

50nm film may be attributed to the higher structural strain (~ 4.46%) as compared to in

100 and 200nm films (from XRD results) and low leakage currents.

3.3.6 Magnetic Properties (M - H hysteresis loop)

M - H measurements on 50, 100 and 200nm films were recorded using SQUID-

VSM and plots are shown in fig. 3.22. Appreciable enhancements in saturation

magnetization (Ms) for 50nm BFO film have been observed ~25 emu/cc. Moreover, as

the film thickness increases, values of MS decrease to ~6.7 and 5.2 emu/cc for 100 and

200nm films, respectively, suggesting weak ferromagnetic nature in higher thickness

films. Bulk BFO shows weak antiferromagnetism due to inhomogeneous spiral spin

structure while in films, epitaxial strain plays an important role to suppress spiral spin

structure resulting in the enhanced magnetic property.

III - 27


-150 -100 -50 0 50 100 150-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5(a) 50nm

0.55V 0.6V 0.65V 0.7V 0.75V 0.8V 0.85V 0.9V

@ RT, 50Hz

P( C

/cm

2 )

E(kV/cm)

0.1V 0.15V 0.25V 0.3V 0.35V 0.4V 0.45V 0.5V

-100 -80 -60 -40 -20 0 20 40 60 80 100

-1.2

-0.8

-0.4

0.0

0.4

0.8

1.2 (b) 100nm

@ RT, 50Hz

P(

C/c

m2 )

E(kV/cm)

0.1V 0.2V 0.3V 0.4V 0.5V 0.6V 0.7V 0.8V

-60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8(c) 200nm

@ RT, 50Hz

P( C

/cm

2 )

E(kV/cm)

0.1V 0.2V 0.3V 0.4V 0.6V

Figure 3.21: P - E hysteresis loops of (a) 50nm (b) 100nm and (c) 200nm BFO films

III - 28


-20000 -10000 0 10000 20000-30

-20

-10

0

10

20

30

@ RT

M (

emu/

cc)

H (Oe)

50nm 100nm 200nm

Figure 3.22: M - H loops of 50, 100 and 200nm BFO films

3.3.7 Conclusions

In this section, the effect of film thickness on the multiferroic properties of BFO

has been discussed. XRD studies show that, all the BFO films are grown according to

substrate orientation and possess single phasic nature. AFM studies reveal that grain size

increases with film thickness while rms roughness and average height decreases.

Electrical polarization and magnetic properties of 50nm BFO film are more pronounced

as compared to higher thickness films. Finally, it is concluded that, structural strain plays

an important role in the modification of multiferroic properties of BFO films which is

responsible for suppression in spiral spin structure and reduction in Fe valence

fluctuations.

III - 29


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