chapter 5 results and discussion -...

Chapter 5

Results and discussion

This chapter is the core of thesis, which deals with results,

discussion and conclusion. Effects of calcinations temperature,

swift heavy ion irradiation on the structural, magnetic and

dielectric properties of barium hexaferrite (BaFe12O

19) are

explained in detail. A detailed analysis of the magnetic and

dielectric properties of Cu substituted M type barium hexagonal

ferrite with composition BaCuxFe

12-xO

19 (x = 0.0, 0.2, 0.4, 0.6, 0.8,

1.0) and Al substituted BaAlxFe

12-xO

19 (x = 0.0, 0.4, 0.8, 1.2, 1.6, 2.0)

hexaferrite is investigated. Nature of interactions between hard

(Ba2Ni

2Fe

12O

22) and soft (CuFe

2O

4) ferrites and incorporation of

ferroelectric material (BiFeO3) into the hexaferrite (BaFe

12O

19)

phase for tuning the magnetic and dielectric properties

components are highlighted in both Ba2Ni

2Fe

12O

22 / CuFe

2O

4 and

BaFe12O

19 / BiFeO

3 composites material. The combination of the

magnetic particles with conducting polymer leads to formation

of ferromagnetic conducting polymer composite. Structural,

magnetic and dielectric properties of Polyaniline / Ba2Ni

2Fe

12O

22

composite are given in detail.

Results and discussion Chapter - 5

- 107 - Ph.D. Dissertation

5.1 Synthesis of M-type hexaferrite (BaFe12O19) using Sol-gel auto

combustion technique

5.1.1 Effect of calcinations temperature

M type Barium hexagonal ferrite (BaFe12O19) has been synthesized by Sol-gel

auto combustion method. The combusted powders were calcined in air for 4 hours at

different temperatures 250 oC, 500

oC, 950

oC as well as 500

oC followed by 950

oC

respectively using a muffle furnace to investigate the effect of calcinations

temperature on the structural, magnetic and dielectric properties of barium hexaferrite.

The prepared hexaferrite samples were characterized using FTIR, XRD, SEM, VSM

and dielectric measurements.

5.1.1.1 Structural properties

Fig. 5.1.1. FTIR spectra of combusted and calcined BaFe12O19 powder



Fig. 5.1.1 shows the FTIR spectra of combusted and calcined BaFe12O19

samples in a wave number ranging from 4000 to 400 cm-1

. The combusted powder

and sample calcined at 250 oC show almost similar kind of spectrum. They have

dominant absorption bands at 3433 cm-1

, 1629 cm-1

, 1426 cm-1

, 628 cm-1

, 555 cm-1

and 442 cm-1

. The bands below 650 cm-1

are due to the iron-oxygen bonds which are

characteristics of hematite (γ-Fe2O3) [1]. The adsorption bands at 3433 cm-1

exhibit

the stretching vibration of hydroxyl group (O-H) [2,3] which indicate the presence of

free or absorbed water in the samples. There are two weak and broad absorptions

around 1400 and 1600 cm-1

corresponding to the presence of small amount of residual

carbon in the samples as observed usually in the case of samples prepared by citrate

precursor method [4].

The band corresponds to the O-H vibration of water molecules (3433 cm-1

)

disappear for the samples calcined at 500 oC, 950

oC, and 500

oC followed by 950

oC.

The annealing at 950 oC for 4 hours leads to the complete removal of residual carbon

from the samples. In the samples calcined at 950 oC, two strong and sharp absorption

bands at 580 and 440 cm-1

are found to be in agreement with the characteristic

infrared absorption bands for the metal oxygen stretching vibrations of hexaferrite

[5,6].

Fig. 5.1.2. XRD patterns of combusted and calcined BaFe12O19 powder



Fig. 5.1.3. XRD pattern of prepared BaFe12O19 powder calcined at 500 oC followed by 950

oC (Up),

JCPDS file - PDF#840757 of BaFe12O19 (Down)

Phase formation and the structural properties of prepared samples were

investigated using an X-ray powder diffractometer (CuKα radiation, λ=1.5405 Ǻ). The

X-ray diffraction studies were carried out on as-burnt powder, the powders calcined at

500, 950 oC and the preheated (500

oC followed by 950

oC) powder and results are

shown in Fig. 5.1.2. The XRD pattern of as-burnt powder indicates the presence of

γ-Fe2O3 as a major phase and some other minor phases such as α-Fe2O3, BaCO3 and

BaFe12O19 which confirms the completion of decomposition process and states that

released heat during self combustion process has been observed to be sufficient for

complete conversion of the metal compounds to metal oxides and carbonates [7]. The

interaction between CO or CO2 generated from citric acid decomposition and Ba2+

ions leads to formation of BaCO3.



By increasing the annealing temperature to 500 oC, α-Fe2O3 appears as a major

phase and BaFe12O19 as minor phases is detected. During annealing BaCO3

decomposes and Ba2+

librated reacts with γ or α-Fe2O3 to form a small amount of

barium monoferrite (BaFe2O4). The reaction between barium monoferrite and iron

oxide leads to formation of barium hexaferrite [8]. At 950 oC, BaFe12O19 appears as

major phase and some minor phases α-Fe2O3 and BaFe2O4 are still noticed. The

sample preheated at 500 oC followed by 950

oC shows pure single phase of barium

hexaferrite. The calculated lattice parameters a = 5.892 Ǻ and c = 23.183 Ǻ are in

close agreement with the standard JCPDS file - PDF#840757 [9] (Fig 5.1.3). From

above results it has been observed that the calcination condition plays an important

role in the formation of pure barium ferrite phase.

Table 5.1.1 Lattice parameters and unit cell volume of calcined BaFe12O19 powder

5.1.1.2 Morphology

Fig. 5.1.4. SEM images of BaFe12O19 powder calcined (a) at 950 oC (b) preheated at 500

oC

and calcined at 950 oC

Fig. 5.1.4 (a) and (b) show SEM micrographs of the sample calcined directly

at 950 oC and the sample preheated at 500

oC followed by 950

oC respectively. It can

be seen from micrograph that the powder are composed of many tiny clusters grains.

The grains of sample preheated at 500 oC followed by 950

oC are found more uniform

in both shape and size compared to the sample directly calcined at 950 oC.

Calcination

Temperature

Lattice constants Unit cell

volume (Å3) a (Å) c (Å)

500 + 950 oC 5.892 23.183 696.99



5.1.1.3 Magnetic properties

Fig. 5.1.5. Hysteresis loop of BaFe12O19 powder calcined at 500 oC followed by 950

oC

Magnetic measurement at room temperature was carried out using VSM with

a maximum applied field of 15 kOe. Fig. 5.1.5 shows Magnetic hysteresis loop of

BaFe12O19 sample calcined 500 oC followed by 950

oC. The hysteresis curve thus

obtained yielded an intrinsic coercivity (Hc) of 4625 Oe, a saturation magnetization

(Ms) of 56.24 emu/g, a remanent magnetization (Mr) of 30.0 emu/g and the squareness

ratio (Mr/Ms) is about 0.53, which confirms the formation of single domain.

5.1.1.4 Dielectric properties

Fig. 5.1.6. shows variation of dielectric constant (ε') as a function of frequency

for the sample preheated at 500 oC followed by 950

oC. It can be observed that the

sample exhibit dielectric dispersion where dielectric constant decreases with

increasing frequency. The decrease in dielectric constant is rapid at low frequency and

approach to frequency independent behaviour. This is a normal behaviour observed in

most of the ferri-magnetic materials.

This can be explained on the basis of Koop’s theory, which based on the

Maxwell–Wagner polarization model [10,11]. According to Maxwell-Wagner

polarization model, the dielectric structure of the ferrite material is assumed to be

made up of two layers. The first layer consists thick layer of ferrite grains of fairly



well conducting boundaries. The other is a thin layer of ferrite grains of relatively

poor conducting boundaries. The mobile electrons reach the grain boundaries through

hopping and if the resistance of the grain boundaries is high enough the electrons pile

up at the grain boundaries and produce induced polarization. However, as the

frequency of the applied field increases, the electrons cannot follow the applied ac

field. This results in the decrease of the probability of electrons reaching the grain

boundaries and as a result polarization decreases and hence dielectric constant

decreases [12,13].

10k 100k 1M4

5

6

7

8

9

Die

lectr

ic c

on

sta

nt

(real)

Frequency (Hz)

Fig. 5.1.6. Variation of dielectric constant (real) as a function of frequency for

BaFe12O19 hexaferrite powder preheated at 500 oC followed by 950

oC

Dielectric loss factor is represented as the energy dissipation in the dielectric

system. The variation of dielectric loss factor (tan δ) as a function of frequency for the

sample preheated at 500 oC followed by 950

oC is shown in Fig. 5.1.7. It is seen that

dielectric loss factor decreases continuously as the frequency of the alternating field

increased. In the low frequency region, which corresponds to high resistivity (due to

grain boundaries), more energy is required for electron exchange between Fe3+

and

Fe2+

ions; thus the energy loss is high. In the high frequency range, which corresponds



to low resistivity (due to grain boundaries), less energy is needed for electron transfer

between Fe3+

and Fe2+

in the grains and hence the energy loss is small [14].

10k 100k 1M0.0

0.1

0.2

0.3

0.4D

iele

ctr

ic lo

ss

fa

cto

r (t

an

)

Frequency (Hz)

Fig. 5.1.7. Variation of dielectric loss factor (tan δ) as a function of frequency for

BaFe12O19 hexaferrite powder preheated at 500 oC followed by 950

oC

5.1.1.5 Conclusions

M-type BaFe12O19 hexaferrite powders have been successfully synthesized via

Sol–gel auto-combustion technique. The results from FTIR, XRD, VSM and

dielectric measurement studies can be summarized as follows:

Two strong and sharp absorption bands observed in the FTIR spectra

attributed to the metal oxygen stretching vibrations.

XRD results show formation of to single phase of M-type BaFe12O19.

SEM micrographs show the formation of homogeneous grains.

Magnetization result of pure BaFe12O19 sample indicates that formed particles

are single domain.



Pure BaFe12O19 sample shows frequency dependent phenomena, i.e. the

dielectric constant decreases with increasing frequency and then reaches a

constant value.

5.1.2 Effect of Swift Heavy Ion (SHI) irradiation on BaFe12O19

In order to study the effects of Swift Heavy Ion irradiation on structural,

surface morphology and magnetic properties of material, pure BaFe12O19 sample

which is preheated at 500 oC followed final calcinations at 950

oC was irradiated with

200 MeV Ag16+

ions at a fluence of 1 x 1013

ions/cm2 using 15 UD Pelletron

Accelerator at Inter University Accelerator Centre, New Delhi, INDIA. The irradiated

BaFe12O19 sample was characterized using FTIR, XRD, SEM and VSM.


Fig. 5.1.8. FTIR spectra of pristine and irradiated BaFe12O19 powder

FTIR spectra of pristine and irradiated (200 MeV Ag16+

ions at a fluence of

(1 x 1013

ions/cm2) samples of BaFe12O19 powder preheated at 500

oC followed by

950 oC is shown in Fig. 5.1.8. The two absorption bands correspond to vibrations of



the bond between the metal-oxygen ions are observed in both pristine and irradiated

BaFe12O19 samples. Intensity of these bands is found to decrease in irradiated sample.

Fig. 5.1.9. XRD patterns of pristine and irradiated BaFe12O19 powder

Table 5.1.2. Estimation of lattice constants and unit-cell volume of pristine

and irradiated BaFe12O19 powder

XRD patterns of both pristine and irradiated BaFe12O19 samples are shown in

Fig. 5.1.9. XRD results show pure single phase barium hexaferrite. The basic

hexagonal crystal structure remains almost same after SHI irradiation. The peak

intensities of irradiated BaFe12O19 sample are found to be decreased where as the peak

widths are found to be increased, which indicates that SHI irradiation have generated

point/cluster of defects which cause certain amount of amorphization in the materials

[15,16]. The calculated values of lattice constants and unit-cell volume of both

Samples Lattice constants Unit cell volume

(Å3) a (Å) c (Å)

Pristine 5.892 23.183 696.99

Irradiated 5.890 23.188 696.68



pristine and irradiated BaFe12O19 samples are given in Table 5.1.2. There is no much

change in lattice constants and unit-cell volume values.

5.1.2.2 Morphology

Fig. 5.1.10. SEM images of pristine and irradiated BaFe12O19 powder

The SEM micrographs of pristine and irradiated BaFe12O19 samples are shown

in Fig. 5.1.10. The uniformity of the grain size is slightly changed with irradiation; the

pristine sample shows more uniform grains while SHI irradiated sample show slight

non uniform clusters.


Fig. 5.1.11. Hysteresis loops of pristine and irradiated BaFe12O19 powder



Table 5.1.3. Magnetic parameters of pristine and irradiated BaFe12O19 measured under maximum

applied field of 15 KOe.

Samples Ms (emu/g) Mr (emu/g) Mr / Ms Hc (Oe)

Pristine 56.24 30.00 0.533 4625

Irradiated 53.34 30.48 0.571 4250

Room temperature hysteresis loops of pristine and irradiated BaFe12O19

hexaferrites are shown in Fig. 5.1.11 saturation magnetization and coercivity of

irradiated sample are slightly less than that of pristine sample. On irradiation,

amorphous tracks are produced which lead to the suppression of the atomic and

ferrimagnetic long-range orders [17]. SHI irradiation may be affected more

effectively Fe3+

site 4fIV (tetrahedral) besides 4fVI (octahedral) site than the 12k sites

[18]. Therefore, this decrease in magnetization can be attributed to the reduction of

Fe3+

ions, both at the octahedral as well as at the tetrahedral sites.

5.1.2.4 Conclusions

The results of the comparative study of pristine and irradiated BaFe12O19

hexaferrite particles lead to the following conclusions:

The strong absorption bands of FTIR confirm the metal oxygen stretching

vibrations of hexaferrite.

XRD result reveals the formation of single phase M-type barium hexaferrite

for the both pristine and irradiated samples.

SHI irradiated sample show slight non uniform clusters.

The irradiated sample shows lesser value of magnetisation as well as

coercivity.

5.1.3 Preparation of BaFe12O19 in presence of surfactant CTAB

M-type BaFe12O19 hexaferrite was prepared in the presence of cationic

surfactant cetyltrimethylammonium bromide (CTAB) using Sol-gel auto combustion

technique. The prepared sample was pre-calcined at 500 °C in a furnace for 4 hours

followed by final calcination at 950 °C for 4 hours. The effect of surfactant on the



phase purity, microstructure and magnetic properties were studied using FTIR, XRD,

SEM and VSM.


Fig. 5.1.12. FTIR spectra of BaFe12O19 powder with and without surfactant CTAB

Fig. 5.1.12 shows FTIR spectra of calcined BaFe12O19 hexaferrite with and

without surfactant CTAB. Two major absorption bands correspond to the metal

oxygen vibrations are found in the range 600-550 cm-1

and 450-400 cm-1

for both the

samples.

The indexed XRD patterns of barium hexaferrite samples with and without

surfactant are shown in Fig. 5.1.13. XRD analysis of normal sample shows mono

phase of barium hexaferrite while surfactant added sample revealed two phases;

hexagonal barium ferrite BaFe12O19 and small amount of intermediate phases

(α - Fe2O3 and γ - Fe2O3). Most of the peaks were ascribed to diffraction planes of

hexagonal barium ferrite BaFe12O19 and the intermediate phases also can be

eliminated by selecting proper molar ratio of metal nitrates to citric acid in the auto

combustion method [7,19].



Fig. 5.1.13. XRD patterns of BaFe12O19 powder with and without surfactant CTAB

Table 5.1.4. Estimation of lattice parameters and cell volume of calcined

BaFe12O19 powder with and without surfactant CTAB

Samples Lattice parameters Volume of

unit cell (Å3) a (Å) c (Å)

Normal 5.892 23.183 696.99

CTAB 5.893 23.190 697.43

5.1.3.2 Morphology

Fig. 5.1.14 (a,b) show SEM micrographs of calcined BaFe12O19 hexaferrite

samples prepared without and with surfactant CTAB. Morphology of BaFe12O19

sample was changed in the presence of surfactant; Shape of grains was changed from

spherical to the flakes type and grain size is also increased in presence of surfactant.

More agglomeration was observed in BaFe12O19 sample prepared in presence of

cationic surfactant CTAB.



Fig. 5.1.14. SEM images of BaFe12O19 (a) without surfactant and (b) with surfactant CTAB


-15000 -10000 -5000 0 5000 10000 15000

-60

-40

-20

0

20

40

60

M (

em

u/g

)

H (Oe)

Normal

CTAB

Fig. 5.1.15. Hysteresis loops of BaFe12O19 powder with and without surfactant CTAB

Table 5.1.5. Magnetic parameters of BaFe12O19 with and without surfactant CTAB

measured under maximum applied field of 15 KOe.

Sample Ms(emu/g) Mr (emu/g) Mr / Ms Hc (Oe)

Normal 56.24 30.00 0.533 4625

CTAB 34.65 22.58 0.652 375



The magnetization curves of both the samples were measured on a VSM at

room temperature under an applied field of 15 kOe. The experimental results (Table

5) show that the low values of the saturation magnetization and coercivity in the

sample prepared in the presence of cationic surfactant CTAB. The squareness ratio,

Mr / Ms for both samples clearly indicates that this typical behaviour is due to the

transition from single domain to multi-domain structure with increasing particle size

[20]. The presence of small amount of intermediate phases (α - Fe2O3 and γ - Fe2O3)

which are weak ferromagnetic, leads to a reduction in magnetization of surfactant

added samples.

5.1.3.4 Conclusions

M-type barium hexaferrite hexaferrite powders have been successfully

synthesized via Sol–gel auto-combustion technique in the presence of surfactant

CTAB. Effect of surfactant on phase purity, microstructure and magnetic properties

can be summarized as follows:

XRD analysis reveals the formation of intermediate phases along with pure

phase in the sample prepared in presence of surfactant

Morphology of the sample was changed in the sample prepared in presence of

cationic surfactant; shape of grains were changed from spherical to the flakes

type.

The presence of small amount of intermediate phases (α - Fe2O3 and γ - Fe2O3)

which are weak ferromagnetic, leads to a reduction in magnetization of

surfactant added sample.



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5.2 Synthesis of Copper doped M-type barium hexaferrite,

BaCuxFe12-xO19 (x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0)

Series of Cu substituted M-type barium hexagonal ferrite with chemical

composition BaCuxFe12-xO19 (x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0) have been prepared using

Sol-gel auto combustion technique. The combusted powders were pre-calcined at

500 °C in a furnace for 4 hours followed by final calcination at 950

°C for 4 hours in a

furnace and then slowly cooled to room temperature. The prepared hexaferrite

samples were characterized using FTIR, XRD, SEM, VSM and dielectric

measurements.

5.2.1 Structural properties

Fig. 5.2.1. FTIR spectra of BaCuxFe12-xO19 (x = 0.0, 0.6, 1.0) hexaferrite powder samples



FTIR spectra of copper substituted barium hexaferrite (BaCuxFe12-xO19) with

x = 0.0, 0.6 and 1.0 compositions are shown in the Fig. 5.2.1. Two absorption bands,

namely, high frequency band ν1 (600 cm-1

) and low frequency band ν2 (430 cm-1

) can

be seen in all spectra. These IR bands are assigned to the metal - oxygen stretching

vibrations of hexaferrite [1, 2]. The higher splitting and shifting of these bands

towards higher frequency were observed with Cu substitution. The intensity ratio

between these two bands may give the fraction of occupancy of cations to the lattice

sites while the difference in the band positions is because of the difference in the

Fe3+

- O2-

distances for five distinct sites i.e. 2a, 2b, 4f1, 4f2 and 12k [3].

Fig. 5.2.2. XRD patterns of BaCuxFe12-xO19 (x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0) powder samples preheated

at 500 °C followed by final calcinations at 950 oC for 4 hours.



XRD patterns of the copper substituted BaCuxFe12-xO19 (x = 0.0, 0.2, 0.4, 0.6,

0.8, 1.0) hexaferrite samples are shown in Fig. 5.2.2. XRD analysis of all the samples

confirms the formation of M-type BaFe12O19hexaferrite as major crystalline phase,

which indicates that the Fe3+

ions are substituted by Cu2+

ions in the crystallography

sites of the BaFe12O19 structure. The peaks for Cu substituted hexaferrite appear

approximately at the same position as for pure (un-doped) barium hexaferrite. Some

minor Spinel ferrite (CuFe2O4) phase is also noticed in the XRD patterns. Spinel

ferrite may be attributed to the distribution of Cu and Fe cations between the two non-

equivalent lattice tetrahedral and octahedral sites [4]. The lattice parameters a and c

were calculated from the value of dhkl corresponding to the most intense (114) peak.

The lattice constants of BaCu0.6Fe11.4O19 were a = 5.895 Å and c = 23.215 Å, which

are slightly larger than that of BaFe12O19. These slight changes in the lattice constant

may have been caused by the difference between the ionic radius of Cu2+

(0.73 Å) and

Fe3+

(0.645 Å).

Table 5.2.1: Estimation of Lattice parameters and unit cell volume of BaCuxFe12-xO19

(x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0).

5.2.2 Morphology

The morphology of Copper substituted hexaferrite particles were examined by

SEM. Fig. 5.2.3 shows SEM images of BaCuxFe12-xO19 (x = 0.2, 0.4, 0.6, 0.8 and 1.0)

hexaferrite powder samples. Grains of samples BaCu0.2Fe11.8O19 and BaCu0.6Fe11.4O19

show flake-like structure, while the samples BaCu0.4Fe11.6O19, BaCu0.8Fe11.2O19 and

BaCu1.0Fe11.0O19 show agglomerated clusters. The grains size decreased with

increasing in Cu content except for the sample BaCu0.6Fe11.4O19 where, grain size is

larger among all the samples.

Sample Lattice constants Unit cell volume

(Å3) a (Å) c (Å)

BaFe12O19 5.892 23.183 696.99

BaCu0.2Fe11.8O19 5.892 23.198 697.44

BaCu0.4Fe11.6O19 5.892 23.183 696.99

BaCu0.6Fe11.4O19 5.895 23.215 698.54

BaCu0.8Fe11.2O19 5.892 23.183 696.99

BaCu1.0Fe11.0O19 5.892 23.183 696.99



Fig. 5.2.3. SEM images of BaCuxFe12-xO19 (x = 0.2, 0.4, 0.6, 0.8, 1.0) hexaferrite powder samples

5.2.3. Magnetic properties

Fig. 5.2.4 and Fig. 5.2.5 show initial magnetization curves and hysteresis

loops of BaCuxFe12-xO19 (x = 0.0, 0.2, 0.6, 0.8) hexaferrite samples respectively

measured at room temperature under applied field of 15 KOe. Magnetic parameters -

saturation magnetization (Ms), coercivity (Hc), and remanent magnetization (Mr) were

calculated and listed in Table 5.2.2. The variation of the saturation magnetization

(Ms), coercivity (Hc), and remanent magnetization (Mr) as a function of Cu

concentration are shown in Fig. 5.2.6. Magnetic properties of BaFe12O19 hexaferrite

depend on 12 Fe3+

ions distributed over five distinct sites i.e. 2a, 2b, 4f1, 4f2 and 12k.



Out of these five, 2a, 4f2 and 12k are octahedral, 4f1 is tetrahedral and the last 2b is

trigonal bipyramid. The 12 Fe3+

are arranged as: 6 Fe3+

are in 12k site having the spin

up, 2 ions in 4f2 and 4f1 having spin down and 1 ion in 2a and 2b site having spin up.

So the 8 Fe3+

are in the upward direction and 4 in the downward direction. So 4

upward spin and downward spin cancel each other and the net magnetic moment is

obtained of 4 Fe3+

per formula units. According to the configuration of Fe3+

, there are

5 unpaired electrons in the 3d orbital, each Fe3+

ion has the magnetic moment of 5μB

and the total moment is 20 μB per formula unit [5-7].

Fig. 5.2.4. Initial magnetization curves of BaCuxFe12-xO19 (x = 0.0, 0.2, 0.6, 0.8) hexaferrite samples

Table 5.2.2: Estimation of magnetic parameters of BaCuxFe12-xO19 (x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0)

hexaferrite samples measured under maximum applied field of 15 KOe.

Sample

Saturation

magnetization

Ms (emu/g)

Remanent

magnetization

Mr (emu/g)

Mr / Ms coercivity

Hc (Oe)

BaFe12O19 56.24 30.00 0.533 4625

BaCu0.2Fe11.8O19 61.10 26.60 0.435 1063

BaCu0.6Fe11.4O19 53.70 20.10 0.374 750

BaCu0.8Fe11.2O19 39.30 20.70 0.527 3250

0 3000 6000 9000 12000 150000

10

20

30

40

50

60 x = 0.0

x = 0.2

x = 0.6

x = 0.8

M (

em

u/g

)

H (Oe)



-15000 -10000 -5000 0 5000 10000 15000

-60

-40

-20

0

20

40

60

M (

em

u/g

)

H (Oe)

X = 0.0

x = 0.2

x = 0.6

x = 0.8

Fig. 5.2.5. Hysteresis loops of BaCuxFe12-xO19 (x = 0.0, 0.2, 0.6, 0.8) hexaferrite samples

0.0 0.2 0.4 0.6 0.8

40

45

50

55

60

65

Value of x in BaCuxFe

12-xO

19

Ms

20

22

24

26

28

30

Mr

0

1000

2000

3000

4000

5000

Hc

Fig. 5.2.6. Variation of Ms, Mr and Hc with copper concentration.



The saturation magnetization increases and decreases alternatively with the

addition of copper content. The variation of Ms can be explained on the basis of

microstructure and distribution of cations at different sites in the crystal structure of

hexaferrite [8]. Usually large size and non-magnetic dopant ions prefers to occupy

octahedral sites [9]. Since ionic radius of Cu2+

is larger than that of Fe3+

, Cu ion

occupies the octahedral sites (2a, 4f2 and 12k). Increase in the value of Ms is due to

the occupancy of 4f2 site by Cu2+

which enhances the hyperfine fields at 12k and 2b

sites and strengthening the Fe3+

– O – Fe3+

super-exchange interaction giving higher

net magnetization [3,8,10]. Hence, the value of Ms reaches a maximum of 61.1 emu /

g for x = 0.2 sample. The decrease in MS can be caused because of the occupancy of

2b site with the increase of Cu2+

content [11].

Copper substitution in barium hexaferrite led to a significant decrease of Hc

compared to that of pure BaFe12O19 hexaferrite. Saturation magnetization (Ms), play a

direct role in decreasing coercivity through Brown’s relation [12] which is given by

(5.2.1)

Where, μ' is initial permeability and k1 anisotropy energy. According to this relation,

Hc is inversely proportional to Ms. In the copper substituted composition, the sample

x = 0.8 shows least saturation magnetization and highest coercivity. It can be seen

from SEM images, grain size is getting smaller with increase of copper content and so

numbers of grain boundary are increased. The increase of Hc may be due to the

increasing grain boundary existed as defects during magnetization process. The

sample x = 0.6 shows larger grain size, so coercivity is decreased in larger amount

[8].

5.2.4 Dielectric properties

Fig. 5.2.7 and 5.2.9 show variation of dielectric constant (real) and loss factor

(tan δ) as a function of frequency of BaCuxFe12-xO19 (x = 0.0 to 1.0). All samples

exhibit frequency dependent phenomenon, dielectric constant decreases with

increasing frequency. At low frequency electron hopping occurs between Fe3+

and

Fe2+

. The electrons reach the grain boundary through hopping and are piled up at the

grain boundaries, which results in the interfacial polarization. However, as the



frequency is increased, the probability of electrons reaching the grain boundary

decreases, which results in a decrease in the interfacial polarization. Therefore, the

dielectric constant decreases with increasing frequency [13-15].

10k 100k 1M

3

4

5

6

7

8

9

10 x = 0.0

x = 0.2

x = 0.4

x = 0.6

x = 0.8

x = 1.0

Die

lec

tric

co

ns

tan

t (r

ea

l)

Frequency (Hz)

Fig. 5.2.7. Variation of dielectric constant (real) as a function of frequency for BaCuxFe12-xO19

0.0 0.2 0.4 0.6 0.8 1.0

4

6

8

10

0.05

0.10

0.15

0.20

0.25

0.30

tan

D

iele

ctr

ic c

on

sta

nt

(re

al)

Cu content (x) in BaCuxFe

12-xO

19

Fig. 5.2.8. Dielectric parameters Vs Copper concentration in BaCuxFe12-xO19



10k 100k 1M0.00

0.05

0.10

0.15

0.20

0.25

0.30 x = 0.0

x = 0.2

x = 0.4

x = 0.6

x = 0.8

x = 1.0

Die

lec

tric

lo

ss

fa

cto

r (t

an

)

Frequency (Hz)

Fig. 5.2.9. Variation of loss factor (tan δ) as a function of frequency for BaCuxFe12-xO19

The compositional dependence of dielectric constant (real) and dielectric loss

factor (tan δ) at frequency of 3170 Hz is given in Fig. 5.2.8. Since Cu ion occupies the

octahedral site (2a, 4f2 and 12k) then the total number of iron ions at that site is

decreased. As a result the hopping of electrons between ferric and ferrous ions also

decreases; consequently the values of dielectric constant and loss factor are decreased

[16]. The above assumption is valid up to x = 0.2; after which the increase in

dielectric constant is due to decrease in grain size with copper substitution. When

grain size is decreased, the resistivity increases and hence dielectric constant is

increased [17]. Sudden increase in dielectric constant with Cu ion concentration at x =

0.6 may be due to corresponding changes in microstructure or grain boundary as seen

in the SEM image.

5.2.5 Conclusions

Cu doped M-type barium hexaferrite with chemical composition

BaCuxFe12-xO19 (x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0) have been successfully synthesized

using Sol-gel auto combustion technique. Effects of Cu2+

ion on crystallography,

magnetic properties and dielectric properties can be summarized as follows:



In all XRD patterns, M type hexaferrite (BaFe12O19) appears as major

crystalline phase, which indicates that the Fe3+

ions are substituted by Cu2+

ions in the crystallography sites of the BaFe12O19 structure.

SEM images show flake-like and agglomerated clusters.

The saturation magnetization increased and decreased alternatively with

substitution of copper content. It was found that coercivity values of all doped

samples were lower than those of undoped samples.

All samples exhibit frequency dependent phenomenon, dielectric constant

decreases with increasing frequency. There were no systematic changes in the

values of dielectric constant with Cu substitution; it was due to corresponding

changes in microstructure or grain boundary and grain size.



References

[1] R. Waldron, Phys. Rev., 99 (1955) 1717.


[3] F. Khademi, A. Poorbafrani, P. Kameli, H. Salamati, J. Supercond Nov. Magn., 25

(2012) 525.

[4] S.A. Farag, M.A. Ahmed, S.M. Hammad, A.M. Moustafa, Cryst. Res. Technol. 36

(2001) 85.

[5]. P. Wartewig, M.K. Krause, P. Esquinazi, S. Rosler, R. Sonntag, J. Magn.

Magn.Mater., 192 (1999) 83.

[6] J. Li, T.M. Gr, R. Sinclair, S.S. Rosenblum, H. Hayashi, J. Mater. Res., 9 (1994)

1499.

[7] Y. Liu, M.G.B. Drew, Y. Liu, J. Magn. Magn.Mater., 323 (2011) 945.

[8] A. Haq, M. Anis-ur-Rehman, Physica B, 407 (2012) 822.

[9] S. Thongmee, P. Winotai, I.M. Tang, Sci. Asia, 29 (2003) 51.

[10] A.L. Geiler, Y. He, S.D. Yoon, A. Yang, Y. Chen, V.G. Harris, C. Vittoria, J.

Appl. Phys., 101 (2007) 09M510.

[11] W. Li, X. Qiao, M. Li, T. Liu, H.X. Peng, Mater. Res. Bull., 48 (2013) 4449.

[12] M. Ahmad, R. Grössinger, M. Kriegisch, F. Kubel, M.U. Rana, Curr. Appl.

Phys., 12 (2012) 1413.


[14] J.C. Dyre, T.B. Schroder, Rev. Mod. Phys., 72 (2000) 873.

[15] C.G. Koops, Phys. Rev., 83 (1951)121.

[16] G. Sathishkumar, C. Venkataraju, R. Murugaraj, K. Sivakumar, J Mater Sci:

Mater. Electron., (2011) 395.

[17] S. M. Reda, International Journal of Nano Science and Technology, 5 (2013) 17.



5.3 Synthesis of trivalent (Al) doped M-type barium hexaferrite,

BaAlxFe12-xO19 (x = 0.0, 0.4, 0.8, 1.2, 1.6, 2.0)

Series of Al substituted BaAlxFe12-xO19 (x = 0.0, 0.4, 0.8, 1.2, 1.6, 2.0)

hexaferrite samples have been prepared using Sol-gel auto combustion technique. The

combusted powders were pre-heated at 500 oC for 4 hours and followed by 950

oC for

4 hours in a muffle furnace and then slowly cooled to room temperature. The prepared

hexaferrite samples were characterized using FTIR, XRD, SEM, VSM and dielectric

measurements.


Fig. 5.3.1. FTIR Spectra of BaAlxFe12-xO19 (x = 0.0, 0.4, 0.8, 1.2, 1.6, 2.0) hexaferrite samples



Fig. 5.3.1 shows FTIR spectra of BaAlxFe12-xO19 (x = 0.4, 0.8, 1.2, 1.6 and

2.0) hexaferrite samples. The spectra of pure and Aluminium substituted barium

hexaferrite samples show two bands in the range 400 - 700 cm-1

corresponding to Al-

O bending vibrations and Fe-O stretching vibrations [1]. These bands are

characteristic infrared absorption bands for the metal oxygen stretching vibrations of

hexaferrite [2, 3]. The intensities of bands are decreased with Al substitution. This

may be due to replacement of heavier element (Fe) with lighter element (Al) and the

contraction in the unit cell [1].

Fig. 5.3.2. XRD patterns of BaAlxFe12-xO19 (x = 0.0, 0.4, 0.8, 1.2, 1.6, 2.0) hexaferrite samples



X-ray diffraction patterns of BaAlxFe12-xO19 samples with x = 0.0, 0.4, 0.8,

1.2, 1.6, 2.0 are shown in Fig. 5.3.2. XRD analysis show hexagonal structure with

space group P63/mmc and matches well with the literature [4]. It should be noted that

Al3+

substitution for Fe3+

in the concentration range reported in this work do not affect

the hexagonal structure of BaFe12O19. A similar result was obtained in a study of the

preparation of doped barium ferrite when Fe3+

was replaced by Al3+

[5]. The result

could be rationalized by the fact that for the synthesis of pure barium ferrite, a

significant amount of Fe3+

must be present alongside Fe2+

. But for Al-doped barium

ferrite, the presence of Fe3+

is not necessary as its role is taken by Al3+

. All aluminium

ions enter the lattice of barium ferrite [6].

The lattice constants (a and c) and unit cell volume are calculated from the

XRD data are listed in Table 5.3.1. The value of lattice parameters ‘a’ and ‘c’

decreases with increase in Al3+

substitution, The maximum reduction in lattice

parameters is about 0.7 – 0.8%. These change in lattice constant results from the

difference in ionic radii of Al3+

ion (0.535 Å) and Fe3+

ion (0.645 Å). The smaller

Al3+

ion, replacing Fe3+

ion leads to lattice contraction of the unit cell [7,8].

Table 5.3.1 Lattice parameters and unit cell volume of BaAlxFe12-xO19 (x = 0.0, 0.4, 0.8, 1.2, 1.6, 2.0)

powder

Sample Lattice parameters Volume of

unit cell (Å3) a (Å) c (Å)

BaFe12O19 5.892 23.183 696.99

BaAl0.4Fe11.6O19 5.872 23.207 692.91

BaAl0.8Fe11.2O19 5.858 23.106 686.75

BaAl1.2Fe10.8O19 5.857 23.113 686.52

BaAl1.6Fe10.4O19 5.849 23.040 682.63

BaAl2.0Fe10.0O19 5.842 23.020 680.75

5.3.2 Morphology

The surface morphology and microstructure of BaAlxFe12-xO19 (x = 0.0, 0.4,

0.8, 1.2, 1.6, 2.0) powder samples were observed by the scanning electron microscopy

(SEM) technique and images are shown in Fig.5.3.3. The surface morphology of pure

sample shows well crystalline nature with an almost uniform grain size distribution



but it changes dramatically with Al substitution. The grain size of the pure sample

appears to be smaller than that of the doped samples. This is probably caused by the

Al3+

doping which could inhibit the development of grain size. This result agrees well

with X-ray data, where the values of the lattice parameter (a- Å) found to decrease

gradually with increasing Al content.

Fig. 5.3.3. SEM images of BaAlxFe12-xO19 (x = 0.0, 0.4, 0.8, 1.2, 1.6, 2.0) hexaferrite powder

5.3.3 Magnetic properties

The measured hysteresis loops for the Al substituted samples as a function of

applied magnetic field are shown in Fig. 5.3.5. The values of saturation magnetization

(Ms), remenance magnetization (Mr), squareness ratio (Mr/Ms) and coercivity (Hc) for

all BaAlxFe12-xO19 (x = 0.0, 0.4, 0.8, 1.2, 1.6, 2.0) hexaferrite samples are listed in



Table 5.3.2. An increase in the coercivity from 4625 Oe to 8500 Oe and decrease in

the saturation magnetization and remenance magnetization from 56.24 to 26.21 emu/g

and from 30.0 to 15.79 emu/g, respectively were observed for the Al-substituted

barium ferrite. The increase in coercivity and decrease in the magnetizations with Al

substitution is in agreement with the observation made for Al-substituted barium

hexaferrite prepared by glass-ceramic method [9].

Fig. 5.3.4. Initial magnetization curves of BaAlxFe12-xO19 (x = 0.0, 0.4, 0.8, 1.2, 1.6, 2.0)

hexaferrite powder

Table 5.3.2 Estimation of magnetic parameters of BaAlxFe12-xO19 (x = 0.0, 0.4, 0.8, 1.2, 1.6, 2.0)

hexaferrite powder measured under maximum applied field of 15000 Oe.

Sample Ms(emu/g) Mr (emu/g) Mr / Ms Hc (Oe)

BaFe12O19 56.24 30.00 0.533 4625

BaAl0.4Fe11.6O19 37.52 21.07 0.562 5500

BaAl0.8Fe11.2O19 19.18 11.55 0.602 6500

BaAl1.2Fe10.8O19 35.69 21.02 0.589 7375

BaAl1.6Fe10.4O19 26.84 13.19 0.491 7500

BaAl2.0Fe10.0O19 26.21 15.79 0.602 8500



Fig. 5.3.5. Hysteresis loop of BaAlxFe12-xO19 (x = 0.0, 0.4, 0.8, 1.2, 1.6, 2.0) hexaferrite powder

Fig. 5.3.6. Variation of Ms, Mr and Hc with increase in Aluminium concentration.



The magnetic moment in M-type hexaferrite is due to the distribution of iron

on five non equivalent sublattices of which three are octahedral (2a, 12k 4f2), one

tetrahedral (4f1) and one trigonal bipyramidal (2b) [10,11]. The total magnetic

moment (i.e., 20 µB) is due to uncompensated upward spins. In the present case Fe3+

ions having magnetic moment of 5 µB are replaced by Al3+

ions having zero magnetic

moment. Owing to the magnetic moment of Al3+

(0 µB) ion it is not able to cancel out

with spin down moments of Fe3+

ions (5 µB), which is responsible for the reduction in

saturation magnetization and remanence of the synthesized materials. The

replacement of Fe3+

with diamagnetic Al3+

also reduces the super-exchange

interaction between Fe3+

- O - Fe3+

[1, 7, 12]. This decrease in exchange interaction

also leads to a non-collinear spin arrangement [13]. The sample BaAl0.8Fe11.2O19

(x = 0.8) shows different behaviour, it has lowest value of saturation and remenance

magnetization among all the samples. It may be due to various causes such as small

size character, lattice defects. Amorphous aluminum oxide also may be present in the

samples, which are not observable in the XRD patterns; it may be attributed to the

occurrence of local combustion during annealing [14].

This increase in coercivity is thus attributed to the increase in anisotropy field

Ha, since the coercivity is proportional to magnetic anisotropy field [15].

Hc α Ha (3.3.1)

Hc α 2K / MS (3.3.2)

Where, MS is the magnetic saturation and K is the magneto-crystalline anisotropy.

According this equation, the decrease in the MS upon Al3+

doping, leads to an increase

in the intrinsic coercivity Hc.


Figs. 5.3.7 and 5.3.8 show the effect of frequency on the dielectric constant

(real) and the dielectric loss factor (tan δ) of BaAlxFe12-xO19 (x = 0.0, 0.4, 0.8, 1.2, 1.6,

2.0) samples. It is clear from the figure that dielectric constant decrease with increase

in frequency. The decrease in the values of both dielectric constant and loss factor

with the frequency is a normal behaviour for ferrites and can be explained on the basis

of charge polarization [16,17].



Fig. 5.3.7. Variation of dielectric constant (real) as a function of frequency for BaAlxFe12-xO19

hexaferrite samples

Fig. 5.3.8. Variation of loss factor (tan δ) as a function of frequency

for BaAlxFe12-xO19 hexaferrite samples



0.0 0.4 0.8 1.2 1.6 2.0

10

20

30

40

50

60

70

0.4

0.6

0.8

1.0

1.2

tan

D

iele

ctr

ic c

on

sta

nt

(re

al)

Al concentration (x) in BaAlxFe

12-xO

19

Fig. 5.3.9. Dielectric parameters Vs Aluminium concentration in BaAlxFe12-xO19 hexaferrite samples

It has been concluded that the electron exchange between Fe2+

and Fe3+

results

in the local displacement of charges, and this is responsible for polarization in ferrites.

The magnitude of exchange, which also controls the conduction in ferrites, depend

upon the concentration of Fe3+

/ Fe2+

ion pairs present in lattice sites. The dielectric

loss represents the phase lag of the dipole oscillations with respect to the applied

electric field. The decrease of dielectric loss with an increase in frequency is due to

the fact that the dipole oscillations cannot follow the changes of the external field at

high frequencies [1,18]

The compositional dependence of dielectric constant (real) and dielectric loss

factor (tan δ) at 1000 Hz is given in Fig. 5.3.9. It can be seen that the values of

dielectric constant (real) and tan δ increase with the substitution level of Al3+

ions.

The increase of dielectric constant with composition can be attributed to reduction of



Fe3+

and the excess amount of Fe2+

. Since Fe 2+

ions are easily polarizable, therefore,

the samples with higher substitution level (large number of Fe 2+

ions) will have high

dielectric constant [19]. The sudden decrease in the dielectric parameters for

BaAl1.2Fe10.8O19 (x = 0.8) might be due to the existence of an insulating secondary

phase on grain boundaries, which may reduce the Fe2+

ions in the lattice sites and also

it is true that extra phase at grain boundaries limits the reduction of Fe2+

ions

concentration [20].

5.3.5 Conclusions

A series of Al substituted BaAlxFe12-xO19 (x = 0.0, 0.4, 0.8, 1.2, 1.6, 2.0)

hexaferrites have been prepared using Sol-gel auto combustion technique. Role of

Al3+

ion in place of Fe3+

in the preparation of M-type barium hexaferrite can be

summarized as follows:

FTIR spectra of pure and Aluminium substituted barium hexaferrite samples

show two bands corresponding to Al-O bending vibrations and Fe-O

stretching vibrations.

The X-ray diffraction patterns reveal the formation of hexagonal structure with

space group P63/mmc for all level of Al3+

substitutions without any trace of

secondary phases. A decrease in the lattice parameters with increasing Al3+

doping level is due difference in ionic radii of Al3+

ion (0.535 Å) and Fe3+

ion

(0.645 Å).

Increasing the amount of aluminium affected the morphology of the particles;

agglomeration is increased with increase of Aluminium content.

It was found that replacement of Fe3+

with diamagnetic Al3+

leads to a

decrease in saturation magnetization and to a significant increase in the

coercive field.

The substitution of Al3+

ions leads to an increase in the polarization on the

sample, and results in the increase of dielectric constant. Also, with the

increase of Fe2+

ions, the chance of electron transfer increased between Fe3+

and Fe2+

leading to higher dielectric loss.



References

[1] S.M. El-Sayed, T.M. Meaz, M.A. Amer, H.A. ElShersaby, Physica B, 426 (2013)

137.



[4] Obradors, J. Solid State Chem., 56 (1985) 171.

[5] J. Qiu, Q. Zhang, M. Gu, H. Shen, J. Appl. Phys., 98 (2005) 103905.

[6] Y. Liu, M.G.B. Drew, J. Wang, M. Zhang, J. Magn. Magn. Mater., 322 (2010)

366.

[7] V.N. Dhage, M. L. Mane, A.P. Keche, C.T. Birajdar, K.M. Jadhav, Physica B, 406

(2011) 789.

[8] T.T.V. Nga, N.P. Duong, T.D. Hien, J. Magn. Magn. Mater., 324 (2012) 1141.

[9] T. Suzuki, K. Kani, K. Warri, S. Kawakami, Y. Torii, J. Mater. Sci. Lett., 11

(1992) 895.

[10] L. Lechevallier, J.M.L. Breton, J.F. Wang, I.R. Harris, J. Magn. Magn. Mater.,

269 (2004) 192.

[11] X. Liu, P. Hernandez-Gomez, K. Huang, Sh. Zhou, J. Magn. Magn. Mater., 305

(2006) 524.

[12] H. Luo, B.K. Rai, S.R. Mishra, V.V. Nguyen, J.P. Liu, J. Magn. Magn. Mater.,

324 (2012) 2602.

[13] G. Albanese, J. Magn. Magn. Mater., 147 (1995) 421.

[14] D. Mishra, S. Anand, R.K. Panda, R.P. Das, Mater. Lett., 58 (2004) 1147.

[15] P. Gornert, W. Schüppel, E. Sinn, F. Schumacher, K.A. Hempel, G. Turilli, , J.

Magn. Magn. Mater., 114 (1992) 193.

[16] I.T. Rabinkin, Z.I. Novikova, “Ferrites”, Izv Acad. Nauk USSR Minsk 146

(1960).



[17] S. Shakoor, M.N. Ashiq, M.A. Malana, A. Mahmood, M.F. Warsi, M. Najam-ul-

Haq, N. Karamat, J. Magn. Magn. Mater., 362 (2014) 110.

[18] P. Singh, T.C. Goel, S.L. Srivastava, Indian J. Pure Appl. Phys., 42 (2004) 221.

[19] Z. Haijun, L. Zhichao, M. Chengliang, Y. Xi, Z. Liangying, W. Mingzhong,

Mater. Sci. Eng. B, 96 (2002) 289.

[20] K.M. Nair, “Advances in Electroceramic Materials”, Ceramic Transactions,

John Wiley & Sons,Inc., Publicaiton, 204 (2008).



5.4 Preparation of hexa-spinel (Ba2Ni2Fe12O22 / CuFe2O4) ferrite

composites

Ba2Ni2Fe12O22 (Y) and CuFe2O4 (S) ferrite powder samples were prepared

separately using Sol-gel auto combustion technique and then physically mixed in

different mass ratios (1:0, 1:1, 1:2, 1:5,1:8 and 0:1 coded as Y,Y:S, Y:2S, Y:5S,Y:8S,

S). Then mixed powders were calcined at 800 oC for 4 hours in a muffle furnace and

then slowly cooled to room temperature. The structural properties on prepared

composite samples were investigated using FTIR and XRD measurements. Scanning

electron micrographs (SEM) were obtained to observe surface morphology of the

samples. Field dependent magnetization behaviour of composite samples was

examined at room temperature using VSM. The dielectric measurements were carried

out at room temperature in a frequency range of 20 Hz to 2 MHz using inductance

capacitance resistance Meter Bridge.


Fig. 5.4.1. FTIR spectra of Ba2Ni2Fe12O22 / CuFe2O4 ferrite composites

with different mass ratios (1:0,1:1, 1:2, 1:5, 1:8, 0:1)



Fig. 5.4.1 shows FTIR spectra of Ba2Ni2Fe12O22 / CuFe2O4 composites

samples with different mass ratios (1:0,1:1, 1:2, 1:5, 1:8, 0:1). It is clear from Fig.

5.4.1 that all samples show two absorption bands around 430 and 600 cm-1

, which are

due to metal oxygen stretching vibrations [1, 2]. The intensity of low frequency band

(around 430 cm-1

) is slightly decreased and the high frequency band (600 cm-1

) is

broadening with increasing in spinel ferrite in the composite. These are due to

variation in the number of metal ions in the lattice sites.

Fig. 5.4.2. XRD patterns of CuFe2O4 (S) and Ba2Ni2Fe12O22 (Y) ferrites

Fig. 5.4.2 shows XRD patterns of Ba2Ni2Fe12O22 (Y) and CuFe2O4 (S) ferrite

powder samples prepared using Sol-gel technique. The XRD analysis of

Ba2Ni2Fe12O22 sample confirms formation of mono phase with high crystallinity. The

lattice parameters and unit cell volume for all the samples were calculated from XRD

and listed in Table 5.4.1 are in close agreement with the standard JCPDS file -



PDF#511880 [3]. The XRD analysis of CuFe2O4 has confirmed formation of

tetragonal spinel phase with lattice parameters a = 5.802 Ǻ and c = 8.701 Ǻ match

with the standard JCPDS file – PDF#340425 [4].

Table 5.4.1: Lattice parameters and unit cell volume of Ba2Ni2Fe12O22 (Y) and

CuFe2O4 ferrite samples.

Fig. 5.4.3. XRD patterns of Ba2Ni2Fe12O22 / CuFe2O4 composite ferrites

Fig. 5.4.3 shows the XRD patterns for the composite samples with different Y-

type hexaferrite - Ba2Ni2Fe12O22 to spinel ferrite - CuFe2O4 mass ratio 1:1, 1:2, 1:5

and 1:8. The figure clearly shows that the characteristic peaks for both the hexagonal

(Y) and spinel (S) ferrite are present in all XRD patterns. As the spinel content

increased in relation to Y phase the reflections at 2θ = 34.5o becomes stronger and the

Samples Lattice constants Unit cell volume

(Å3) a (Å) c (Å)

Ba2Ni2Fe12O22 (Y) 5.839 43.386 1281.012

CuFe2O4 (S) 5.802 8.701 292.911



hexaferrite reflections decrease in relative intensities indicating that the composites

contained only two phases spinel and hexaferrite. The absence of any new phase

proved that there was no chemical reaction between spinel and hexagonal ferrites

[5,6].

5.4.2 Morphology

Fig. 5.4.4. SEM micrographs of the Ba2Ni2Fe12O22 / CuFe2O4 composite ferrites



Fig. 5.4.4 shows series of SEM micrographs of single and composites of hexa

(Y) and spinel (S) ferrites. Micrograph of pure Ba2Ni2Fe12O22 hexaferrite powder

sample represents hexagonal plate-like grains and micrographs of pure CuFe2O4

ferrite sample shows cluster of grains, they are irregular in shape. But for composites,

non-homogeneity nature was observed. With increasing ratio of spinel phase in the

composite ferrite, the average grain size decreased.


0 3000 6000 9000 12000 150000

10

20

30

40

50 Y

Y:S

Y:5S

Y:8S

S

M (

em

u/g

)

H (Oe)

Fig. 5.4.5. Initial magnetization curves of Ba2Ni2Fe12O22, Ba2Ni2Fe12O22 / CuFe2O4 composite and

CuFe2O4 samples

Fig. 5.4.5 and Fig. 5.4.6 show initial magnetization curves and magnetic

hysteresis loops respectively of Ba2Ni2Fe12O22, CuFe2O4 and their composites with

different mass ratio (Y:S, Y:5S and Y:8S). The magnetic parameters are listed in

Table 5.4.2. Crystallographically two phased composites showed a good single-phase

magnetic behaviour suggesting that the magnetic hard and soft phases are exchange

coupled in these composites irrespective of size irregularity and large size ratio of

hard and soft ferrite grains [7]. If it is not exchange coupled, then the magnetization

could have been shown as the superimposition of two loops corresponding to the soft

and the hard ferrites instead of a single one [8].



-15000 -10000 -5000 0 5000 10000 15000

-40

-20

0

20

40

M (

em

u/g

)H (Oe)

Y

Y:S

Y:5S

Y:8S

S

Fig. 5.4.6. Hysteresis loops of Ba2Ni2Fe12O22, Ba2Ni2Fe12O22 / CuFe2O4

composite and CuFe2O4 samples

Table 5.4.2: Estimation of magnetic parameters of Ba2Ni2Fe12O22 / CuFe2O4 composites measured

under maximum applied field of 15 KOe.

Generally, the exchange coupling interaction and dipolar interaction play a

major role for the determination of magnetic properties of the composite materials. As

the composite is a mixture of the hard and soft phases, there are three types of

exchange interactions. The major one is the interaction between soft–hard grains

whereas the other two are the hard–hard and soft-soft [9]. So the relative strength of

the exchange interaction between the soft and hard phases acts as a tuning factor for

Sample Ms (emu/g) Mr (emu/g) Mr / Ms Hc (Oe)

Y 46.84 22.05 0.471 750

Y:S 18.11 6.41 0.354 330

Y:5S 22.39 9.11 0.407 450

Y:8S 41.09 19.16 0.466 500

S 10.08 5.10 0.506 500



determining the magnetic property of the composite. But if the dipolar interaction is

also considered, then along with the hard and soft exchange interaction, the competing

dipolar interaction also decides the magnetization in the soft grains [10].

10

20

30

40

50

5

10

15

20

25

300

400

500

600

700

800

Ms

Mass ratios of Ba2Ni

2Fe

12O

22 / CuFe

2O

4 composites

Mr

Hc

SY:8SY:5SY Y:S

Fig. 5.4.7. Variation of Ms, Mr and Hc with increase in Spinel (CuFe2O4) content

From the Table 5.4.2, it is noticed that saturation magnetization (Ms) value of

Y (Ba2Ni2Fe12O22) sample is maximum and S (CuFe2O4) is minimum. The hard

barium ferrite (Ba2Ni2Fe12O22) phase is having large anisotropy; therefore, it is

difficult to achieve magnetization reversal with the lower applied field. So

Ba2Ni2Fe12O22 ferrite possesses high saturation magnetization compared to CuFe2O4

ferrite. In case of composite, saturation magnetization (Ms) increases with increasing

in soft phase. Coercivity values of composites are less than that of hard phase

Ba2Ni2Fe12O22 and nearly equal to that of soft phase CuFe2O4.

In low contents of the soft ferrite, the exchange force on the soft magnetic

moments excreted by the hard ferrite is strong, so initially the coercive field will

increase with increase in soft ferrite, but saturation magnetization is decreased due to

low anisotropy of soft phase CuFe2O4. In high contents of the soft ferrite, the

exchange force on the soft grains is enervated and dipolar interaction among soft



moments becomes significant [11]. The reverse domains in the soft ferrite with low

nucleation field could be nucleated easily [12]. So there is no significant change in the

coercivity values with further increase in soft ferrite and dipolar interaction among

soft moments leads to a higher value of magnetization.


Fig. 5.4.8 shows the variation of dielectric constant (ε') of all the composites

and individual hard and soft ferrite samples with frequency (20 Hz – 2 MHz) at room

temperature. It is seen that dielectric constant decreases with increasing frequency and

then reaches a constant value at higher frequency. Dielectric nature of pure Y-type

hexaferrite (Ba2Ni2Fe12O22) can be explained based on Verwey mechanism [13] and

Heiks model [14]. Two exchange interactions: electron hopping between Fe2+

and

Fe3+

and hole transfer between Ni3+

and Ni2+

in the octahedral sites are responsible for

the dielectric behaviour. The first exchange give rise to the displacement of the local

charges in the direction of the external field leading to the main source of polarization

in the ferrite, while the latter exchange gives rise to displacement of the holes in

opposite direction of the external field leading to the second source of polarization in

the ferrite [15].

10k 100k 1M0

10

20

30

Y

S

YS1

YS2

YS5

YS8

Die

lectr

ic c

on

sta

nt

(re

al)

Frequency (Hz)

Fig. 5.4.8. Variation of dielectric constant (ε') with frequency for Y - Ba2Ni2Fe12O22, S - CuFe2O4 and

their composites at room temperature



Dielectric constant of spinel ferrite (CuFe2O4) is higher than that of hexaferrite

(Ba2Ni2Fe12O22). The high value of dielectric constant of the spinel ferrite may be due

to the structural changes associated with the copper ferrite. Cu2+

ions occupy

tetrahedral site and Fe3+

ions are equally divided between the tetrahedral (A) and

octahedral (B) sites in spinel lattice. All composite samples have high dielectric

constants than pure Y type hexaferrite; it seems that dielectric constants increase with

spinel ferrite. But some abnormal behavior was also observed in the composite

samples, where one has higher and lower values of dielectric constants compared to

the pure state of spinel ferrite. It is probably associated with the sizes of the grains in

both phases and to the interface between these grains [16].

5

10

15

20

25

30

Die

lec

tric

co

nsta

nt

(rea

l)

0.25

0.30

0.35

0.40

0.45

0.50

0.55

SY:8SY:5SY:2SY:SY

Mass ratio of Y/S composites

tan

Fig. 5.4.9. Variation of dielectric constant and loss factor with mass ratio of Ba2Ni2Fe12O22 / CuFe2O4

composites at 3170 Hz.



Fig. 5.4.10. Variation of loss factor (tan δ) with frequency for Y - Ba2Ni2Fe12O22, S - CuFe2O4 and their

composites at room temperature

Fig. 5.4.10 shows the variation of dielectric loss as a function of frequency at

room temperature. The dielectric loss gives the loss of energy from the applied field

into the sample. At higher frequencies, the losses are found to be low. From Fig. 8, it

is seen that dielectric losses for composites Y:2S, Y:5S, Y:8S and pure S show the

peaking nature and a slight shift in these maxima is also observed. The maxima shift

towards high frequency region with increasing spinel ferrite in hexa ferrite.

The condition for having maxima in the dielectric losses of a dielectric

material is given by the relation [8].

ωτ = 1 (5.4.1)

where ω = 2πfmax and τ is the relaxation time.

A relation relates the relaxation time to the jumping probability per unit time is as

follow as

τ =1/2p (5.4.2)

fmax α p (5.4.3)



Therefore, from the above relation it is clear that, maxima can be observed

when the jumping or hoping frequency of electrons between Fe2+

and Fe3+

becomes

approximately equal to the frequency of the applied field [17].

5.4.5 Conclusions

The composites with different ratios of Ba2Ni2Fe12O22 and CuFe2O4 (1:0, 1:1,

1:2, 1:5, 1:8 and 0:1) have been successfully synthesized via Sol-gel auto-combustion

technique. The results from FTIR, XRD, SEM, VSM and dielectric measurement

studies can be summarized as follows:

Two absorption bands observed in FTIR spectra due to metal oxygen

stretching vibrations.

XRD patterns of composites exhibited the characteristic peaks of both hard

ferrite and soft ferrite showing the coexistence of both the phases in composite

material.

SEM images of composites consist of both larger and smaller grains which

also confirm the coexistence of both the phases.

Magnetic measurements suggest that the competition between exchange and

dipolar interactions of soft and hard ferrite grains determines the magnetic

properties of magnetic composites.

All the samples show the frequency dependent phenomena, i.e. the dielectric

constant decreases with increasing frequency and then reaches a constant

value. All composite samples have high dielectric constants than pure

Ba2Ni2Fe12O22 hexaferrite, the dielectric constants increase with spinel ferrite.



References

[1] R. Patil, S. Kakatkar, A. Sanlepal, S. Sawant, J. Pure Appl. Phys., 2 (1994) 193.


[3] L. Fang, R. Yuan, H. Zhang, State Key Lab. of Advanced Technology for

Materials Synthesis & Processing, Wuhan Univ. of, ICDD Grant-in-Aid (2000).

[4] Natl. Bur. Stand. (U.S.) Monogr. 25 (1983).

[5] W. Qu, X. H. Wang, L. Li, J. Magn. Magn. Mater., 257 (2003) 284.

[6] C. Sudakar, G.N. Subbanna, T.R.N. Kutty, J. Magn. Magn. Mater., 268 (2004) 75.

[7] J. Xiang, X. Zhang, J. Li, Y. Chu, X. Shen, Chem. Phys. Lett., 576 (2013) 39.

[8] R.K. Kotnala, S. Ahmad, A.S. Ahmed, J. Shah, A. Azam, J. Appl. Phys., 112

(2012) 054323.

[9] F. Song, X. Shen, M. Liu, J. Xiang, J. Solid State Chem., 185 (2012) 31.

[10] D. Roy, P. S. Anil Kumar, J. Appl. Phys., 106 (2009) 073902.

[11] D. Roy, C. Shivakumara, P.S.A. Kumar, J. Magn. Magn. Mater., 321 (2009)

L11.

[12] M.A. Radmanesh, S.A. SeyyedEbrahimi, J. Magn. Magn. Mater., 324 (2012)

3094.

[13] E. J.W. Verwey, J. H. de Boer, J. Less Common Met., 106 (1985) 257.

[14] R.R. Heikes, W.D. Johnston, J. Chem. Phys., 26 (1957) 582.


[16] F.M.M. Pereira, C.A.R. Junior, M.R.P. Santos, J. Mater. Sci. Mater. Electron.,

19 (2008) 627.

[17] M.A. Dar, K.M. Batoo, V. Verma, W.A. Siddiqui, and R. K. Kotnala, J. Alloys

Compd., 493 (2010) 553.



5.5 Preparation of magnetoelectric (BaFe12O19 / BiFeO3) composites

In present study, composites BaFe12O19 / BiFeO3 with different mass

percentage of BiFeO3 ( 0 to 100 %) were prepared. BaFe12O19 (BHF) and BiFeO3

(BiF) ferrites were prepared separately using Sol-gel auto combustion technique and

then physically mixed. The mixed samples were again calcined 500°C for 4 hours and

slowly cooled to room temperature. Prepared composite samples were characterized

using FTIR, XRD, SEM, VSM, dielectric measurements and PE loop tracer. The

composite samples with the mass percentage mentioned above were coded as 100%

BHF, 25% BiF, 50% BiF, 75% BiF and 100% BiF.


Fig. 5.5.1. FTIR spectra of BaFe12O19 / BiFeO3 ferrite composites



Fig. 5.5.1 shows FTIR spectra of indivitual BaFe12O19 and BiFeO3 ferrites

and the composites with different mass percentage (25%, 50% and 75%) of BiFeO3.

FTIR spectra of pure and composite samples show two strong absorption bands

around 580 and 440 cm-1

, which are due to metal oxygen stretching vibrations [1].

The intensity and sharpness of these peaks decreased with increase in BiFeO3 amount.

In case of BiFeO3 ferrite, the FTIR peaks at 410, 547, 649 and 750 cm-1

are

corresponding to the vibrations bonds of Bi – O or Fe – O [2].

Fig. 5.5.2. XRD patterns of BaFe12O19 (BHF) and BiFeO3 (BiF) ferrites

Table 5.5.1. Lattice parameters and unit cell volume of BaFe12O19

and BiFeO3 ferrites

Sample Lattice constants Unit cell

volume (Å3) a (Å) c (Å)

BaFe12O19 (BHF) 5.892 23.183 696.99

BiFeO3 (BiF) 5.577 13.861 431.12



Fig. 5.5.2 shows XRD patterns of BaFe12O19 and BiFeO3 ferrites prepared

using Sol-gel auto cumbustion technique.Tthe XRD pattern of BaFe12O19 (BHF)

sample heated at 500 oC followed by 950

oC shows pure single phase M-type barium

hexaferrite. In case of BiFeO3 (BiF) heated at 500 oC, the prominent peaks in XRD

plot are indexed to the pure rhombohedral structure of BiFeO3 of space group R3c

with lattice parameters of a = b = 5.577 Ǻ and c = 13.861 Ǻ , which are in good

agreement with the standard JCPDS file- PDF# 861518 [3]. Besides these prominent

peaks, some other peaks are also observed, which assined to Bi2O3. The high

temperature synthesis of BiFeO3 usually results in the evaporation loss of bismuth as

well as the formation of impurity phase like Bi2O3 [4]. Hence the calcination

temperature was restricted to 500 oC for this sample.

Fig. 5.5.3. XRD spectra of composites BaFe12O19/ BiFeO3 with different mass percentage of BiFeO3



Fig. 5.5.3 shows the XRD spectra of composites BaFe12O19 / BiFeO3 with

different mass percentage (25%, 50% and 75%) of BiFeO3. The XRD patterns of

these three compositions reveal that mixed hexaferrite and bismuth ferrite phases have

been formed in each case without any detectable strong third phase. This indicates

that no significant chemical interaction has occurred between the two phases and

provides a wide range of composition to study their properties. In the sample with

50% of BiF, the intensity of XRD peaks of both hexaferrite and bismuth ferrite is high

compared to other two compositions. The intensity of BHF peaks from the planes

such as (1 1 4), (2 0 1), (2 0 5), (2 0 6), (2 2 0), (2 3 8) and (3 1 7) has been decreased

with the increase of BiF content in the composition.

5.5.2 Morphology

Fig. 5.5.4. SEM micrographs of BiFeO3 ferrites and the composite with 50% of BiFeO3



Fig.5.5.4 shows the morphology of BaFe12O19, BiFeO3 and the composite with

50% of BiFeO3 powder calcined at 500 °C for 4 hours. The surface morphology of

calcined BaFe12O19 powder sample revealed formation of dense grains. SEM

micrograph of calcined BiFeO3 ferrite shows the formation of porous clusters of non

uniform grains. The SEM image of composite sample (50% of BiFeO3 powder

calcined at 500 °C for 4 hours) shows irregular, porous grains, which are randomly

distributed.


-12000 -9000 -6000 -3000 0 3000 6000 9000 12000

-60

-40

-20

0

20

40

60M

(em

u/g

)

H (Oe)

100% BHF

25% BiF

50% BiF

75% BiF

Fig. 5.5.5. Hysteresis loops of BaFe12O19 and composites BaFe12O19 / BiFeO3

with different mass percentage

The variation of magnetization M (emu/g) versus the applied magnetic field H

(Oe), for BaFe12O19 and composites BaFe12O19 / BiFeO3 at room temperature, is given

in Fig. 5.5.5 and the magnetic parameters are listed in Table 5.5.2. It is noticed that

saturation magnetization (Ms) decreases gradually with increasing in BiFeO3 content.

Similar results were also reported for CrFe2O4 - BiFeO3 and MnFe2O4 - BiFeO3

nanocomposites [5,6]. This may be due to the ferroelectric material incorporated into

the ferrite phase which acts as pores in the presence of applied magnetic field and

break the magnetic circuit, resulting in the decrease of magnetic behavior with

increasing ferroelectric content [7]. And also the individual ferrite grains act as

centres of magnetic moment and the Ms of the composites is the vector sum of all



these individual contributions [8]. Thus the net magnetic moment of the BaFe12O19/

BiFeO3 composites decreases with increase of BiFeO3 content and result in decrease

of the net magnetization. However, there were no drastic changes in coercivity values

for composite samples with increase in BiFeO3 content. Coercivity slightly increased

for the 25% and 50% BiF composites and then decreased for 75% BiF composition

compared to BaFe12O19 ferrite.

Table 5.5.2 Estimation of magnetic parameters of BaFe12O19 / BiFeO3 composites

Sample

Ms

(emu/g)

Mr

(emu/g) Mr / Ms

Hc

(Oe)

100% BHF 56.24 30.00 0.533 4625

25% BiF 34.90 20.20 0.579 4860

50% BiF 27.30 15.60 0.571 4860

75% BiF 15.40 8.20 0.532 4540

10

20

30

40

50

60

75 %

BiF

50 %

BiF

25 %

BiF

100

% B

HF

Ms

5

10

15

20

25

30

Mr

4500

4600

4700

4800

4900

Hc

Fig. 5.5.6. Variation of Ms, Mr and Hc with increase in BiFeO3 content




Fig. 5.5.7. Variation of (a) dielectric constant and (b) dielectric loss versus frequency for BaFe12O19

(BHF) and BiFeO3 (BiF) and their composites at room temperature



Fig.5.5.7 shows the frequency dependence of dielectric constant (ε') and

dielectric loss (tan δ) of the composites at room temperature. Both dielectric constant

(ε') as well as dielectric loss (tan δ) decrease at lower frequencies and they remain

constant at higher frequencies. It is due to the different polarization mechanisms at

different frequencies. At low frequency, electron displacement polarization, ion

displacement polarization, turning direction polarization and space charge

polarization contribute to dielectric constant. At high frequency, dielectric constant

just results from the electron displacement polarization [9]. In additional, the high

value of dielectric constant at lower frequency is due to the predominance of species

like Fe2+

ions, interfacial dislocation pile-ups, oxygen vacancies and grain boundary

defect [10]. Dielectric constant of bismuth ferrite (BiF) is higher than that of

hexaferrite (BHF). Because of oxygen vacancy in bismuth ferrite materials, which is

mostly generated during the heat treatment, is the major origin of the deterioration of

insulation and the formation of Fe2+

[11]. Hence space charge polarization is always

present.

10

15

20

25

30

35

0.10

0.15

0.20

0.25

0.30

0.35

0.40

100

% B

iF

75 %

BiF

50 %

BiF

25 %

BiF

100

% B

HF

tan

D

iele

ctr

ic c

on

sta

nt

(re

al)

Fig. 5.5.8. Variation of dielectric constant and loss factor with mass percentage of BiFeO3 at 1002 Hz



In case of BaFe12O19 / BiFeO3 composite samples, the dielectric constant

increases with BiFeO3 content. The high values of dielectric constant at lower

frequencies may be explained on the basis of space charge polarization due to

inhomogeneous dielectric structure in the ferromagnetic and ferroelectric system [12].

When the composites of two phases BaFe12O19 and BiFeO3 with different

conductivity and permittivity are subjected to an electric field, the space charge

carriers provided by BiFeO3 accumulate in the interface of two phases, thereby

intensifying the interfacial polarization that contributes to a higher value of dielectric

constant permittivity [7]. Dielectric loss factor is represented as the energy dissipation

in the dielectric system. Dielectric loss decreases with increase in BiFeO3, which is

right opposite that of dielectric constant; similar result was reported for multiferroic

composites [7]. It may be due to density, oxygen vacancy concentration and

secondary phase of Bismuth ferrite.

5.5.5 Ferroelectric properties

-20 -10 0 10 20 30

-1

0

1

2

Po

lari

za

tio

n (C

/cm

2)

Electric field (kV/cm)

100 % BHF

25 % BiF

50 % BiF

75 % BiF

100 % BiF

Fig. 5.5.9. Polarization-electric field (P-E) loop for BaFe12O19 (BHF) and BiFeO3 (BiF)

and their composites at room temperature



Table 5.5.3 Estimation of ferro-electric parameters of BaFe12O19 / BiFeO3 composites

Sample Emax

(kV/cm)

Pmax

(µC/cm2)

Ec (kV/cm) Pr (µC/cm2)

100% BHF 17.646 1.188 16.834 1.134

25%BiF 23.148 0.999 22.034 0.969

50%BiF 17.912 0.784 17.113 0.747

75%BiF 14.092 0.411 12.633 0.353

100%BiF 19.621 0.270 13.467 1.134

Fig.5.5.9 shows the polarization-electric field (P-E) hysteresis loop measured

at room temperature for the BaFe12O19/ BiFeO3 composites. Due to conductivity of

the samples at high electric filed, measurement of the polarization data beyond 25

kV/cm has been restricted. The P-E loop of BaFe12O19 displays elliptical shape, which

means the sample has serious electrical leakage [13-15]. The loop is not fully

saturated because of the low applied electric field. High leakage current indicates the

highly conductive nature of BaFe12O19 ferrite at high electric field. The P-E loop for

pure BiFeO3 displays well-saturated rectangular shape-like loop revealing the good

ferroelectric properties. Since the particle size of BiFeO3 seems to be small, which

results in lower space charge density, smaller leakage current density and this

increases spontaneous polarization [16]. Normally a single crystal or a specimen with

100% c-oriented grains exhibits nearly a square loop with high remanent polarization

close to the spontaneous polarization value [17].

The P-E loop of the BaFe12O19 / BiFeO3 composites is elliptical in shape, but

much improved in comparison with that of BaFe12O19 ferrite. With increasing BiFeO3

content in the composite, the P-E loops became more and more typical, which means

the electric leakage is greatly reduced in the presence of BiFeO3. However, the

ferroelectric parameters such as remanent polarization, maximum polarization and the

coercive electric field decreased with BiFeO3 amount. This may be due to the

randomly oriented BiFeO3 grains in the composite system which affects the domain

reversal, as a result remanent polarization value decreases [17]. Low coercivity values

indicate that there was no sufficient growth of ferroelectric domains due to the

random orientation of grains and further due to the masking effect of BaFe12O19

grains in the microstructure.



5.5.6 Conclusions

The composites BaFe12O19 / BiFeO3 with different mass percentage of BiFeO3

(100% BHF, 25% BiF, 50% BiF, 75% BiF and 100% BiF) have been successfully

synthesized via a Sol-gel auto-combustion technique. The results from FTIR, XRD,

SEM, VSM, dielectric measurement and PE loop studies can be summarized as

follows:

FTIR specta of BaFe12O19 / BiFeO3 composites reveal the strong two

absorption bands corresponding the metal – oxygen vibrations.

XRD spectra of BaFe12O19 / BiFeO3 composites reveal that mixed hexaferrite

and bismuth ferrite phases have been formed.

Micrographs of BaFe12O19 / BiFeO3 composites show that irregular, porous

and randomly distributed grains.

Saturation magnetization (Ms) of BaFe12O19/ BiFeO3 composites decreased

gradually with increasing in BiFeO3 content. There were no drastic changes in

coercivity values for composite samples with increase in BiFeO3 content.

Dielectric constant of bismuth ferrite is higher than that of Barium hexaferrite.

In case of BaFe12O19 / BiFeO3 composites, the dielectric constant increases

with BiFeO3 content.

The P-E loop of BaFe12O19 displays elliptical shape, which means the sample

has serious electrical leakage. The P-E loop for pure BiFeO3 displays well-

saturated rectangular shape-like loop revealing the good ferroelectric

properties. With increasing BiFeO3 content in the composite, the P-E loops

became more and more typical, which means confirms the electric leakage is

greatly reduced in the presence of BiFeO3.



References


[2] Y. Hu, L. Fei, Y. Zhang, J. Yuan, Y. Wang, H. Gu, Journal of Nanomaterials ,

Hindawi Publishing Corporation, 2011.

[3] Sosnowska, J. Magn. Magn. Mater., 160 (1996) 384.

[4] H. Yang, T. Xian, Z.Q. Wei, J.F. Dai, J Sol-Gel Sci Technol, 58 (2011) 238.

[5] A. Kumar, K.L. Yadav, H. Singh, R. Pandu, P.R. Reddy, Physica B, 405 (2010)

2362.

[6] A. Kumar, K.L. Yadav, Physica B, 406 (2011) 1763.

[7] Y. Liu, Y. Wu, D. Li, Y. Zhang, J. Zhang, J. Yang, J. Mater. Sci. Mater. Electron.

24 (2013) 1900.

[8] R.C. Kambale, K.M. Song, N. Hur, Current Applied Physics, 13 (2013) 562.

[9] W. Cai, C. Fu, W. Hu, G. Chen, X. Deng, J. Alloys Compd., 554 (2013) 64.

[10] M.N. Ashiq, M.J. Iqbal, M. Najam-ul-Haq, P.H. Gomez, A.M. Qureshi, J. Magn.

Magn. Mater., 324 (2012) 15.

[11] A. Kumar, K.L. Yadav, Mater. Sci. Eng. B, 176 (2011) 227.

[12] M. Kumar, K.L. Yadav, J. Phys. Chem. Solids, 68 (2007) 1791.

[13] G.S. Lotey, N.K. Verma, J. Nanopart. Res., 14 (2012) 742.

[14] Z.X. Cheng, A.H. Li, X.L. Wang, S.X. Dou, K. Ozawa, H. Kimura, S.J. Zhang,

T.R. Shrout, J. Appl. Phys., 103 (2008) 07E507.

[15] A. Srinivas, R. Gopalan, V. Chandrasekharan, Solid State Commun., 149 (2009)

367.

[16] G.S. Lotey, N.K. Verma, J. Nanopart. Res., 15 (2013) 1553.

[17] A. Srinivas, T. Karthik, R. Gopalan, V. Chandrasekaran, Mater. Sci. Eng. B, 172

(2010) 289.



5.6 Polyaniline / Ba2Ni2Fe12O22 composites prepared by in situ

polymerization method

Polyaniline (PANI) / Ba2Ni2Fe12O22 composites with different aniline /

Ba2Ni2Fe12O22 hexaferrite weight ratios (1:1, 1:2 and 2:1) have been prepared by in

situ polymerization method. In order to study the effect of surfactants on the particle

size, microstructure and dielectric properties, Polyaniline / Ba2Ni2Fe12O22 composites

were prepared in the presence of different surfactants using in situ polymerization

method. Three different surfactants; cetyl trimethyl ammonium bromide (CTAB),

Sodium dodecyl sulfate (SDS) and Polyethylene glycol sorbitan monooleate (Tween-

80) (cationic, anionic and non-ionic, respectively) were used for preparing three

Polyaniline / Ba2Ni2Fe12O22 composites in which the aniline:Ba2Ni2Fe12O22 ratio was

1:1. The obtained composites were characterized by Fourier transform infrared

spectra (FT-IR), X-ray diffraction (XRD), Scanning Electron Microscope (SEM) and

dielectric measurements were carried out at room temperature in a frequency range of

20 Hz to 2 MHz using inductance capacitance resistance Meter Bridge.


Fig. 5.6.1. FTIR spectra of Ba2Ni2Fe12O22 and Polyaniline/ Ba2Ni2Fe12O22 hexaferrite composites with

different aniline/ Ba2Ni2Fe12O22 ferrite weight ratios (1:1, 1:2 and 2:1).



Fig.5.6.1 shows the FTIR spectra of Y-type Ba2Ni2Fe12O22 hexaferrite and

polyaniline/Ba2Ni2Fe12O22 hexaferrite composites powder with different aniline /

Ba2Ni2Fe12O22 hexaferrite weight ratios (1:1, 1:2 and 2:1). Two strong and sharp

absorption bands at 580 and 440 cm-1

are found in Ba2Ni2Fe12O22 sample which are

characteristic infrared absorption bands for the metal oxygen stretching vibrations

[1,2]. The characteristic peaks of polyaniline appear at 1587, 1496, 1303, 1130 and

802 cm-1

. The peaks at 1587 and 1496 cm-1

are attributed to the characteristic C = C

stretching of the quinoid and benzenoid rings [3,4], the peak at 1303 cm-1

is assigned

to C – N stretching of the benzenoid ring [5], the broad and strong peak around 1130

cm-1

is associated with vibrational modes of N = Q = N (Q refers to the quinonic-

type rings) [6,7]. The peak at 802 cm-1

is attributed to the out-of-plane deformation

vibration of the p-disubstituted benzene ring [8,9]. With an increase in the polyaniline

content, the intensity of the bands corresponding to polyaniline characteristics

increases distinctively.

Fig. 5.6.2. FTIR spectra of polyaniline / Ba2Ni2Fe12O22 composites in the presence of surfactant



Fig. 5.6.2 shows FTIR spectra of polyaniline / Ba2Ni2Fe12O22 composite

samples prepared in the presence of surfactant. The spectra show the characteristics

peaks of Polyaniline (1587, 1510, 1311 and 800 cm-1

) but some notable changes in

positions of peaks are observed in the presence of surfactants. Characteristics peaks of

Ba2Ni2Fe12O22 hexaferrite (metal-oxygen vibrations) have also been observed at 580

and 440 cm-1

in the spectrum but these peaks disappear in presence of SDS surfactant

and there is no significant change in the presence of other two CTAB and Tween-80

surfactants.

Fig. 5.6.3. XRD patterns of Ba2Ni2Fe12O22 and polyaniline / Ba2Ni2Fe12O22 composites samples

with different aniline/ Ba2Ni2Fe12O22 weight ratios (1:2, 1:1, 2:1)



XRD patterns of polyaniline / Ba2Ni2Fe12O22 composite samples exhibit both

the characteristic peaks of Ba2Ni2Fe12O22 and polyaniline (Fig. 5.6.3). Polyaniline is

an amorphous nature, and has a certain degree of crystallinity. Due to presence of

Y-type Ba2Ni2Fe12O2 ferrite, XRD patterns of composites show high degree of

crystalline order. For polyaniline phase, the crystalline peaks appeared at 2θ: 22.7,

23.6, 29.0, 34.1, 37.9, 40.2, 46.4, 52.5 and 57.6o corresponding to (021), (112), (211),

(221), (132), (310), (115), (006) and (421) reflections, respectively [10]. The relative

intensity of these peaks might differ, depending on the amount of aniline /

Ba2Ni2Fe12O22 ferrite compositions. The same phase of the metal oxides are found in

their respective composites with polyaniline [11,12].

Fig. 5.6.4. XRD patterns of polyaniline / Ba2Ni2Fe12O22 composite

prepared in the presence of surfactant.



Fig. 5.6.4 shows XRD patterns of polyaniline / Ba2Ni2Fe12O22 composite

prepared in the presence of surfactant. The characteristic peaks of both Ba2Ni2Fe12O22

and polyaniline are observed for cationic and non-ionic surfactants CTAB and

Tween-80 added the polyaniline / Ba2Ni2Fe12O22 ferrite composites. It was similar to

the polyaniline / Ba2Ni2Fe12O22 ferrite composite in the absence of surfactant. In case

of anionic surfactant SDS added composite, Ba2Ni2Fe12O22 appears as a major phase.

5.6.2 Morphology

Fig. 5.6.5. SEM images of polyaniline / Ba2Ni2Fe12O22 composites with and without surfactants.



Fig. 5.6.5 shows series of SEM micrographs of pure Ba2Ni2Fe12O22 hexaferrite

and Polyaniline / Ba2Ni2Fe12O22 hexaferrite composites with and without surfactants.

Micrograph of pure Ba2Ni2Fe12O22 hexaferrite represents hexagonal plate-like

structure. Different morphologies were observed when hexaferrite is dispersed in

polyaniline. The SEM images of the composites with aninline / Ba2Ni2Fe12O22 ferrite

weight ratios 1:1 and 2:1 reveal that polyaniline is deposited on the surface of ferrite

particles which possesses coral like structure [5]. It is observed from the pictures that,

compared to the composite samples (1:1, 2:1) and the composite sample prepared in

presence of CTAB; SDS and Tween-80 composite samples have smaller grain size

and less agglomeration. It seems that in the surfactants (SDS, Tween-80) control the

microstructure of the formed composites.


Fig. 5.6.6 shows variation of dielectric constant (ε') as a function of frequency

of Ba2Ni2Fe12O22 and Polyaniline / Ba2Ni2Fe12O22 hexaferrite composites with and

without surfactants. It can be observed that dielectric constant of all the samples

decreases with increase in frequency. Dielectric nature of pure Y-type hexaferrite

(Ba2Ni2Fe12O22) can be explained based on Verwey mechanism [13,14] and Heiks

model [15]. In case of polyaniline, there are two types of charged species, one

polaron/bipolaron system which is mobile and free to move along the chain, the others

are bound charges (dipoles) which have only restricted mobility and account for

strong polarization in the system. When the frequency of the applied field is

increased, the dipoles present in the system cannot reorient themselves fast enough to

respond to applied electric field and as a result, dielectric constant decreases [16,17].

Polyaniline/Ba2Ni2Fe12O22 hexaferrites are having high values of dielectric

constants compared to the pure ferrite. Initially dielectric constant reaches highest

value with little amount of polyaniline and then decreases with increase in polyaniline

amount in the composite. With more amount of polyaniline the dielectric behaviour is

similar to the pure Ba2Ni2Fe12O22 hexaferrite. Surfactant added polyaniline /

Ba2Ni2Fe12O22 hexaferrite composites also have similar nature like pure ferrite due to

large amount of polyaniline.



Fig. 5.6.6. Variation of dielectric constant (ε') as a function of frequency for polyaniline /

Ba2Ni2Fe12O22 hexaferrite composites (a) without and (b) with surfactants.



Fig. 5.6.7. Variation of dielectric loss tangent (tan δ) as a function of frequency for Polyaniline/

Ba2Ni2Fe12O22 composites (a) without and (b) with surfactants.



Fig. 5.6.8. Dielectric parameters Vs (a) aniline:ferrite ratio and

(b) different surfactant in the composites

Fig. 5.6.7 shows the variation of dielectric loss as a function of frequency at

room temperature. The dielectric loss tangent is found to decrease with increase of

frequency as a similar trend to that of dielectric constant for all the composition at low

frequency. This is usually associated with ion drift, dipole polarisation or interfacial

polarisation [18]. It can be observed that for the composites with aninilie /

Ba2Ni2Fe12O22 hexaferrite ratio 1:2 and 1:1, the loss tangent increases with frequency

followed by appearance of the broad dielectric relaxation peak at 89.3 kHz and 28.2

kHz respectively. Surfactant CTAB added polyaniline composite shows highest value

of loss factor at low frequency among other surfactant added composites.

5.6.4 Conclusions

Polyaniline / Ba2Ni2Fe12O22 composites with different aniline/Ba ferrite

weight ratios (1:1, 1:2 and 2:1) showing remarkable magnetic and dielectric properties

have been obtained by in situ polymerization of aniline in the presence of

Ba2Ni2Fe12O22 particles. Effect of aniline/ferrite ratios and role of different



surfactants; cetyl trimethyl ammonium bromide (CTAB), Sodium dodecyl sulfate

(SDS) and Polyethylene glycol sorbitan monooleate (Tween-80) in the preparation of

Polyaniline / Ba2Ni2Fe12O22 composites can be summarized as follows:

FTIR spectra of polyaniline / Ba2Ni2Fe12O22 composites show absorption

bands correspond to both metal-oxygen stretching vibrations and stretching of

the quinoid and benzenoid rings.

XRD patterns of polyaniline / Ba2Ni2Fe12O22 composites exhibit both the

characteristic peaks of Ba2Ni2Fe12O22 and polyaniline.

The SEM images of the composites reveal that polyaniline is deposited on the

surface of ferrite particles which possesses core shell structure.

Polyaniline embedded ferrites are having high values of dielectric constants

compared to the pure ferrite. Surfactant added polyaniline / Ba2Ni2Fe12O22

ferrite composites also have similar nature like pure ferrite due to large

amount of polyaniline.



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chapter 5 results and discussion -...

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