CHAPTER - I

INTRODUCTION

1

INTRODUCTION

The term magnetism came from Magnesia, In 470 B.C. Greeks found certain

stones in Aegean Sea, called lode stones; these stones had the unusual property of

attracting pieces of iron. In 12th century, Chinese used magnets in compass for navigation.

William Gilbert made artificial magnets in sixteenth century by rubbing pieces of iron

against lode stones. He also showed that compass always points towards north-south

direction because the earth itself has magnetic property. In 1970 John Michell of England

found that Magnetic poles obeys inverse square law, this result was confirmed by Charles

Coulomb [1]. In Hans Christian Oersted discovered that electric current produces a

magnetic field [2]. Following this discovery first electro magnet was made in 1825. Later

on Bergmann, Becquerel and Faraday invented that even liquid and gases were also

affected by magnetism, but only a few can be noticeable.

Most people know about the general properties of magnets but less familiar with

source of magnetism.

The concept of magnetism depends on magnetic field and magnetic dipole. The

term magnetic field described a volume of space where there is change in energy in the

volume which can be detected and measured. The location where a magnetic field can be

detected exiting or entering a material is called a magnetic dipole.

The magnetic poles of di-pole can’t be isolated but always occurs in pairs, hence

the dipole is defined as two equal and opposite poles separated by finite distance is

known as magnetic di-pole. A bar magnet can be viewed as a magnetic di-pole with north

and south pole at either sides.

2

If a magnet is cut into two, once again the pieces of magnet has two poles i.e we

can’t separate the magnetic poles. This sectioning and creation of di-poles can continue to

the atomic level i.e the source of magnetism lies in the building block (atom) of all matter

1.2 Sources of Magnetism:

The matter is composed of atoms; atoms are composed of electrons, protons and

neutrons. Protons and neutrons are known as nucleons, which are present inside the

nucleus. Electrons having negative charge are rotating with constant motion around the

nucleus which produces magnetic field. The strength of this field is known as magnetic

moment. This is very difficult to visualize on subatomic scale.

When electric current (electrons) are flowing through the conductor, a magnetic

field is developed around the conductor. This magnetic field can be detected using a

compass needle. The magnetic field exerts a force on the compass needle which is

another example of a di-pole. i.e. all the materials are affected by magnetic field in

different ways.

1.3 Classification of Magnetic Materials:

Magnetism in a material is due to orbital and spin motion of electrons. The way in

which electrons interact with one another is also responsible for magnetism. In few

materials the collective interaction of atomic magnetic moments is zero, where as in other

materials there is a very strong interaction between atomic moments. Depending on the

basis of interaction of electrons, the magnetic materials can be classified into five types

[3]

1. Dia magnetism

2. Para magnetism

3. Ferro magnetism

3

4. Ferri magnetism

5. Anti ferro magnetism

1.3.1 Dia magnetism: In a dia magnetic material, the resultant magnetic moment is zero

when there is no external field. When external field is applied the spinning electrons

produces magnetization (M) in the opposite direction [4], so even in presence of external

field dia magnetic materials can’t be converted to magnetic material

All the materials have a diamagnetic effect; however it is often the case that

diamagnetic effect is masked by larger para magnetic or ferro magnetic term [5].

In presence of external field, spinning electrons produces magnetic moment in

opposite direction; hence the susceptibility (χ) becomes negative and independent of

temperature. Some well known diamagnetic materials are silver (Ag), mercury (Hg),

copper (Cu) and zinc (Zn) [6]

In absence of the external magnetic field, the resultant Magnetic moment is zero

as shown in the Fig.1.1 (a) due to random orientation of electrons. But when external

field (Ho) is applied all the electrons aligned in opposite direction to applied field as

shown in Fig.1.1(b)

1.3.2 Para magnetism:

Certain atoms and molecules posses permanent magnetic moment due to orbital

and spin motion of electrons even in absence of external field. When external field is

applied few electrons are aligned in the applied field direction, hence the resultant

magnetic moment increases. These materials are known as para magnetic materials and

this magnetism are known as Para magnetism [6].This can be represented as shown in Fig

1.2(a), 1.2(b).

4

Generally para magnetism is found in atoms and molecules which have odd

number of electrons. But certain transition elements, and atoms including oxygen shows

para magnetism even they have even number of electrons. [7].

These materials show weak and positive magnetic susceptibility to external field.

The Para magnetic susceptibility is inversely proportional to temperature i.e, as

temperature increases susceptibility decreases which can be expressed as

χ =C/T--------1.1

Where

‘χ’ –susceptibility

‘C’-Curie constant

‘T’-temperature

The well known examples of para magnetic materials are Al, Cu, Mg, Na -------------[6]

1.3.3 Ferro magnetism:

Ferro magnetism is only possible when atoms are arranged in a lattice and atomic

magnetic moments can interact to align parallel to each other.

Ferro magnetic material has spontaneous magnetization due to parallel alignment

of its atomic moment even in absence of external field [6] as shown in the Fig 1.3(a),

1.3(b).

Ferro magnetism appears below certain temperature known as Curie temperature.

Above Curie temperature the moments are randomly oriented resulting the zero net

magnetization. [8]

5

Examples of ferro magnetic materials are transition metals like Fe, Co, Ni. But

other elements and alloys involving transition of rare-earth elements are also ferro

magnetic due to their unfilled 3d and 4f shells.

Ferro magnetic materials have large and positive magnetic susceptibility.

Above Curie temperature Tc, magnetic susceptibility varies according to Curie-wiess law

as shown in the fig 1.4 [6].

1.3.4 Ferri magnetism:

Ferri magnetism is mainly observed in compounds, which have more complex

structures than pure elements.

Ferri magnetic materials have spin structure of both spin-up and spin-down of

unequal magnitude but in opposite direction, hence the ferri magnetism has a net non-

zero magnetic moment [9]

If MA is magnetic moment of spin up electron (↑), MB is moment of spin down

electron (↓) then MA ≠MB. i.e., ferri magnetism is unbalanced.

Like ferro magnetism, ferri magnetism materials have spontaneous magnetization

below a critical temperature called the Curie temperature (Tc).

The magnetic susceptibility ‘χ’ for ferro magnetic materials is same but there is a

great difference in alignment of spin.

The spin alignment is of ferri magnetism is shown in the fig 1.5

1.3.5 Anti ferro magnetism:

In the periodic table chromium [2] is the only one element shows anti ferri

magnetism at room temperature.

6

Anti ferri magnetic materials have spin structure of both spin-up and spin-down of

equal magnitude but in opposite direction as shown in the fig [10].

Due to this alignment, anti ferri magnetic materials have small non zero magnetic

moment [11]

If MA is magnetic moment of spin up electron (↑), MB is magnetic moment of spin

down electron (↓) then MA = MB. i.e., Anti ferri magnetism is balanced.

Anti ferri magnetic material have weak positive magnetic susceptibility of the

order of para magnetic material. The theory of anti ferro magnetism was explained by

Neel in 1932.

Like ferro magnetic materials, anti ferri magnetic materials becomes para

magnetic above transition temperature known as Neel temperature (TN).

The variation of susceptibility of different magnets with temperature is shown in

the fig 1.7

1.4. History of ferrites:

Ferrites are mixed metallic oxides with high resistivity of semiconductor family.

Naturally occurring ferrite is magnetite, which have poor magnetic properties and not

suggestible for magnetic application. Kato and Takei [12], Kawai [13] from Japan and

Snoek [14] from Netherlands were started work on the ferri magnetic materials at the end

of first world war. The application of ferrites in science and technology was explained by

Snock. The theoretical frame work to understand to ferri magnetism was provided by

Neel [15] in 1943.

This theory gives the basic information about spin-spin interaction, taking place in

the magnetic sub lattice in ferrites, also explains experimental and theoretical

7

investigation of ferrites and other ferri magnets. The first modern ferrite was made in

1946 [16].Polder [17] derived the first ferrite permeability tensor in 1949, which gives

basic concept for understanding of ferrite at microwave frequency.

The first workable ferrite micro wave generator constructed by Hogan [18]. As

ferrites are used commercially for large scale application in television tube deflection

yokes and voltage fly back transformers so there is a demand for magnetic materials with

low core losses , radio and television applications in recent years.

From the old analog circuits to the new digital ones there is a need for high

frequency switched mode power supplies to power computer applications, biomedical

research, automotive industry and many others.

1.5 Ferrites:

The term ferrite derived from the Latin word, which has different meanings for

different area of scientist. To metallurgist, ferrite means pure iron, to geologist, the

meaning of ferrite is a group of minerals based on iron oxide. To an electrical engineer

the meaning ferrite is a group of iron oxide which has useful magnetic and dielectric

properties.

Ferrites are electrically non-conductive ferri magnetic ceramic compound

materials consisting of different mixtures of iron oxides such as Hematite (Fe203) (or)

magnetite (Fe304) and the other oxides like ZnO, MnO, CoO, NiO.

Ferrites have chemical formula Me++Fe2++O4, where ‘Me’ stands for a divalent

metal ion with an ionic radius in between 0.6 and 1 Ao. If the divalent metal ion is

occupied by transition elements like manganese (Mn++), magnesium (Mg++), nickel

8

(Ni++),copper (Cu++), cobalt (Co++), zinc(Zn++), cadmium(Cd++) then these ferrites are

known as simple ferrite.

If the divalent metal ion is replaced by a combination of two divalent metalic ions

such as nickel-zinc (Ni-Zn), cobalt-zinc (Co-Zn), manganese-magnesium (Mn-Mg) e.t.c.,

then the resultant ferrite is said to be mixed ferrite.

The divalent metal ion in the ferrite (Me2+) can be also replaced by ions which

have an average valance of two. e.g Li0.5Fe2.5O4. In this process of doping, only 50% of

the ferrous ions will convert to ferric ions to maintain electric charge neutrality.

If the trivalent iron ion (Fe3+) in Me2+Fe23+O4 is replaced by trivalent ions like

Al3+ or Cr3+ then the resultant ferrite is known as aluminates and chromites, these

compounds are also ferri magnetic at room temperature.

If the ferric ions are replaced by tetravalent iron like Ti4+, an equal part of Fe3+ is

changed in Fe2+to maintain electric charge neutrality.

The unit cell of ferrite has 8 oxygen (O2-) ions, 16 Fe3+ ions and 8 M2+ions. Out of

them 8 Fe3+ions and 8 M2+ ions occupy the octahedral sites, which means each ion is

surrounded by 6 oxygen ions. The rest 8 Fe3+ ions occupy the tetrahedral sites which

mean each ion is surrounded by 4 oxygen ions. The spin of 8 Fe+3 ions on tetrahedral site

is anti parallel to the spin of 8 Fe+3 ions on octahedral site as a result the net spin

magnetic moment of Fe+3 is zero. Hence the contribution to the magnetization is due to

only 8 M2+ ions [6].

1.6 Types of ferrites:

Ferrites are classified into three types [19]

1. Spinel ferrites (cubic ferrites)

9

2. Hexagonal ferrites

3. Garnets

Our research work is only on spinel ferrites; therefore we discuss about only

spinel ferrites only.

1.7 Spinel ferrites:

Spinel ferrites crystallize in cubic structure hence these ferrites are also known as

cubic ferrites. MgAl204 was the oldest spinel structure determined by Brages and

Nishikawa in 1915 [19].

The chemical composition of a spinel ferrite can be written as MFe2O4 where ‘M’

is divalent metal ion such as Co+2, Zn2+ , ---

The unit cell of spinel ferrite is F.C.C with 8 atoms per unit cell.

In the spinel structure metallic cations occupies two types of interstitial sites to

maintain charge neutrality, these interstitial sites are namely

1. Tetrahedral (A) sites

2. Octahedral (B) sites.

In a unit cell of ferrite there are 64 tetrahedral (A) sites, and 32 Octahedral sites

(B) as shown in Fig 1.8, and 1.9

1. Tetrahedral (A) sites:

Tetrahedral configuration is forms by keeping a metallic cation at the Centre of

cube surrounded by four anions at four corners of cube. Among the four anions three

anions touching each other are in a plane and the fourth anion sits at top of three anions

as shown in the Fig 1.10 A (b). For change neutrality of the system only 8 tetrahedral (A)

sites out of 64 are occupied by cations in spinel structure.

10

2. Octahedral (B) sites:

Octahedral (B) site configuration formed by keeping metallic cation at the Centre

of cube surrounded by six anions, among them four anions touching each other in same

plane and the remaining two anions are at above and below of Centre of plane as shown

in the Fig 1.10 B (b) For charge neutrality 16 octahedral (B) sites out of 32 are occupied

by cations in spinel structure.

On the basis of occupancy of cations at A and B sites, the spinel ferrites are

classified into three types as

i. Normal spinel ferrite

ii. Inverse spinel ferrite

iii. Random spinel ferrite

i. Normal spinel ferrite:

In these ferrites, the divalent cations occupy tetrahedral (A) site, while the

trivalent cations occupy octahedral (B) sites. Square brackets are used to indicate the

ionic distribution of the octahedral (B) sites.

Normal spinel ferrite is represented by [M2+]A[Me3+]BO4.

Where

‘M’ represents divalent ions and

‘Me’ for trivalent ions

The best example for normal spinel ferrite is ZnFe2O4, CdFe2O4

11

i. Inverse spinel ferrite:

In the inverse spinel, trivalent metal ion occupy all the tetrahedral (A) sites and

half of the octahedral sites (B) and the rest of octahedral sites (B) are occupied by

divalent metal ions.

Inverse spinel structure is represented by [Me3+]A[M2+Me3+]BO4

A typical example of inverse spinel ferrite is Fe3O4 in which divalent cations of

Fe occupy the octahedral (B) sites [20]

Ex: Nickel ferrite

ii. Random spinel ferrite:

If the divalent metal ions M2+ and trivalent Fe3+ ions are distributed at both

tetrahedral ‘A’ site and octahedral ‘B’ site then that ferrite is known as Random spinel

ferrite.

Random spinel ferrite is represented by [Mδ2+Me1-δ

3+] A[M1-δ2+Me1+δ

3+]BO4 ,where

δ is inversion parameter, depends on the method of preparation of ferrite and chemical

composition of ferrite.

For complete normal spinel ferrite δ =1, for complete inverse spinel ferrite δ =0,

for mixed ferrite ‘δ’ ranges between these two extreme values, for completely mixed

ferrite δ=1/3.

Typical examples for random spinel ferrite are MgFe2O4 and MnFe2O4 [5].

According to Neel the magnetic moment in ferrites are sum of the magnetic

moment of tetrahedral (A) and octahedral (B) sites.

In spinel ferrite, the exchange interaction between electrons of ions in A and B

site are different. The interaction between magnetic ions of A and B sites (AB-site

12

interaction) is strongest than A-A-site and B-B-site interaction. The interaction between

A-A-sites is almost ten times weaker than A-B site interaction where as B-B-interaction

is weakest.

The dominant A-B site interaction results into complete or partial anti ferro

magnetism known as ferri magnetism [21].

Due to dominant greatest exchange energy between A-B sites produces anti

parallel arrangement of cations between magnetic moments in the two types of sub

lattices and also parallel arrangement of cations within each sub lattice, despite of A-A or

B-B site anti ferri magnetic interaction. [22]

The following are few factors which can affect the distribution of metal ions

among A and B site

1. Ionic radii of the specific ions

2. The size of the interstices

3. Electronic configuration

4. Temperature

5. Electrostatic energy

The most important consideration for the site preference is relative size of ion

compared to size of lattice of A and B sites.

Generally the bivalent ions prefers octahedral site (B), and trivalent ions prefers

tetrahedral site (A) because bivalent ions are larger than trivalent (the larger charge

produces high electro static attraction so the outer orbit pulls inward). But there is some

exception in case of Zn2+ and Cd2+, these ions prefer A sites because the electronic

configuration is favorable for tetrahedral bonding to the oxygen ions [23].

13

1.8 Hexagonal ferrite:

Hexagonal ferrites are widely used as permanent magnets with high coercivity

[24] as shown in the Fig 1.12. The general chemical formula for Hexagonal ferrite is

M0.6Fe2O3 where M can be Ba, Sr or Pb. Hexagonal ferrite was first identified by went

Rathenau, Gorter and Van Ooster shout 1952[19] and Jonker, Wijn & Braun

1956.Hexagonal ferrite lattice is similar to the spinel structure with oxygen ions closely

packed, but layers include metal ions , which have practically the same ionic radii as

oxygen ions . Hexagonal ferrite has three different sites occupied by metals; tetrahedral,

octahedral and trigonal bi pyramid (surrounded by five oxygen ions).

1.9 Garnets:

The rare earth garnets have general formula MC3Fea

2Fed3O4 or more informatively

(3M3O3)C(2Fe3O3)a(3Fe2O3)d, where M is rare earth metal ion or an Yttrium ion, and the

super scripts c, a, d refer to dodecahedron, octahedron, and tetrahedron respectively.

Yoder and Keith reported[19] in 1951 on the synthesis of first silicon free garnet

Y3Al5O12 from mineral garnet Mn3Al2Si3O12 by substuting YIII+AlIII from MnII+SiIV

Y3Fe5O12 garnet is prepared by Bertant and Fornet in 1956 and measured their magnetic

properties.

Geller and Gileo prepared Gadelonium garnet Gd3Fe5O12 in 1957. The unit cell of

garnet is cubic with 160 atoms per unit cell and containing 8 molecules of M3Fe2Fe3O12.

Which has cubic edge length 12.5 A0 Garnets have complex structure, these are widely

used in memory devices.

14

1.10 Type of Ferrites with respect to their hardness:

Due to persistence of their magnetization and ability to be magnetized or

demagnetized ferrites are divided into two types.

1. Soft ferrites.

2. Hard ferrites.

1.10.1 Soft ferrite:

The ferrites which can be easily magnetized and demagnetized are known as soft ferrites.

E.g.: Iron nickel, Cobalt Manganese e.t.c

Properties:

The hysteresis loop area is thin and long as shown in Fig. 1.13.

As hysteresis loop area is thin hysteresis energy loss is very low.

Soft ferrites have low coercive fields.

They have low saturation magnetization

They have large values of permeability and susceptibility

Eddy current loss is more

Magnetostastic energy is very small.

Soft ferrites are free from irregularities like strains or impurities.

Applications:

In electromagnetic machines

Transformer corers

Recording heads

Microwave devices

Shift registers and many more.

15

1.10.2 Hard ferrites:

The ferrites which can’t be easily magnetized and demagnetized are known as

hard ferrites.

Eg: Alnico, rare earth metal alloys.

Properties:

The hysteresis loop area for hard ferrite is wide as shown in Fig. 1.14

As hysteresis loop area is wide hysteresis energy loss is very high.

Hard ferrites have high coercive fields.

They have high saturation magnetization

They have small values of permeability and susceptibility

Eddy current loss is low

Magneto stastic energy is high.

Hard ferrites are produced due to quenching which means sudden cooling.

Applications:

In preparation of permanent magnets[25]

In magnetic detectors

In micro phones

Voltage regulators

Magnetic separators.

1.11 Advantages over the magnetic materials:

Most of the magnetic materials such as iron and metallic alloys like ALINCO,

SMCOS, Nd-Fe-B have low dc electrical resistivity. Due to low electrical resistivity

current flows through the material produces heat, so the material becomes inefficient as

16

they waste energy. This waste of energy increases at high frequencies, hence the

magnetic materials are not suitable at high frequencies for practical application.

Ex: inductor cores in TV circuits.

But ferrites can perform much better at higher frequencies because of their high

electrical resistivity and high temperature stability, these two factors make ferrites at high

frequencies, wide band transforms delay lines, adjustable inductors and other high

frequency electric circuits.

Another important feature of ferrite is they are cheaper than other magnetic

materials and alloys, availability of raw materials for production of ferrites is also high.

Further ferrites have both magnetic and mechanical parameters.

1.12 Application of ferrites:

Ferrites have twin property of high electrical resistivity and magnetic conductor,

this best combination of electrical and magnetic properties gives variety of applications.

Because of high electrical resistivity and low eddy current loss of Ni-Zn-iron or

manganese-zinc-iron [26] are used in deflection-yoke core in TV tube enhance the

operation [27-30]

Ferrites at DC application

Power application

High frequency power supplies

Linear power supply

SMPS power transformer

Pulse transformer

Switching regulators

17

High frequency output chokes

Audio frequency applications

Telecommunication applications

Telephony applications

Applications in the area of entertainment

Ferrites are used in TV transforms because of low power high flux

Soft Ni-Zn and Mn-Zn ferrites are used in core manufacturing in

telephone system

Ferrite rods are used in radio receivers

Non volatile memory means storage of the information in computer

even the power supply fails. In the preparation of non volatile memory

devices ferrites are used.

Ferrites are used in computer memories like computer hard disks, floppy

disks, credit cards, audio, video cassettes and recorder heads

Ferrites are used in the production of ultrasonic by magneto stiction.

Iron silicon alloys are used in magnetic cores of transformer which are

operations at low line frequencies.

Ferrites are used in electromagnetic wave absorbers at low dielectric

values.

Ferro fluids as a cooling materials used in speakers, which cools the

coils with vibrations.

Nickel alloys are used in high speed relays, wide band transformers and

inductors.

18

Ferrites at DC Applications

Although the majority of ferrite applications is for high frequency there are

ferrites used as permanent magnets for D.C applications. Ferrite permanent magnets are

used in loud speakers, microphones, T.V picture tubes and most widely in D.C motors in

portable electric motors.

Power applications

Ferrites are extremely useful in the power supply circuitry used in electronics.

There is a requirement for obtaining steady 5-15 volts. Two types of power supplies are

employed known as linear power supply and the switching power supply.

Liner power supply

In the liner power supply the output choke is used to reduce the residual ripple.

Ferrites are used both in the transformer and in the choke circuit. The transformer

employs 50-60 Hz line ac. The transformer performs the job of stepping up or stepping

down the line voltage.

High frequency power supplies

The other type of power supply, namely, the switching power supply converts the

50-60 Hz.ac to a high frequency square wave through the use of transformer or similar

solid state switching device transform it to the desired voltage at high frequency using

transistor switching to produce a high frequency square wave. Transformation is done at

a high frequency, which reduces the size of the magnetic component drastically and

improves the efficiency of the device. This signal is processed to obtain the desired

voltage. In this type of power supply, high frequency is employed and hence magnetic

used are different from those used are different from used in other type of the power

supply.

19

Pulse Transformers

In some cases the transistors that act as switch to invert or form the high

frequency square wave are free running and do not require any timing mechanism.

However, some transistors must be triggered by pulses that are usually generated by pulse

transformers.

SMPS Power Transformer

The power transformer which transforms the high frequency input voltage in the

usable voltage is the heart of the smps system. It is because of this transformer that the

size of the large linear 60 Hz transformer to very much smaller one. The material must be

able to carry the high power at the very high frequency and usually at a higher than

ambient temperature.

Switching Regulators

The output of the switching power supply must have a very controllable voltage

limits or good degree of resolution. To accomplish this a switching regulator with a

magnetic core is used.

High frequency output chokes

The output DC of the inverter power supply contains a certain degree of un

wanted ripple or residual ac. A high frequency output choke is used to remove this

ripple. This device is smaller to the output choke for main or linear power supplies.

Audio frequency applications

The frequency range for such an application is from (20-20,0000) Hz. This involves

devices concerning music and voice. The increased frequency range over line frequencies

requires correspondingly improved materials, which are mostly ferrites.

20

Telecommunication applications

The next group of applications involves the primary electronic operations at

much higher frequencies and (100 KHz-100MHz). it include the areas of telephony, radio

television. In all these areas high quality magnetic materials with lower energy losses are

used.

Telephony application

A part from some of the permanent magnetic materials used in the telephone

receiver, there are many soft magnetic materials used in telephony. The inductor

functions often in conjunction with a capacitor, to shift the phase of an electrical signal.

By combining the two action , devices called filters can can selectively pass certain

frequencies while blocking others. some of the functions can be characterized as:

1. Channel filter

2. Wide band transformers

3. Loading coils

4. Touch-tone generators

Applications in the areas of Entertainment

A very large tonnage of magnetic materials goes into the consumer entertainment

market. Ferrites cores are extensively used in

Television picture tube yokes

Fly back transformers

Interstage and pincushion transformer

Antennae for radio and television

Tuning slugs

21

Television Picture tube Yokes: This application probably uses highest tonnage of

magnetic material for the entertainment segment of the business. The yokes are funnel

shaped rings placed on the neck of the picture tube. After being wonnd, their function is

to provide the horizontal and vertical deflection of the electron beam that forms the raster

on which the television signal is superimposed.

Flyback transformers : During the flyback period of the horizontal deflection cycle, the

large magnetic field stored in the deflection core is rapidly collapsed and the voltage

induced is transferred to a single turn primary winding off the flyback transformer.

Interstage and Pincushion transformer : The Interstage transformers in both audio and

television circuits are used to couple different stages with regard to isolation and

impedance matching. The pinsushion transformer of the video circuit is used to correct

the spherical aberration resulting from the use of a radial or circular sweep on a planner

television picture tube.

Antennae for Radio and Television : The wavelengths associated with radio and

television is relatively large. To match these wavelenghts, the antennae would also be

quite large. Howeverr, since magnetic materials have the ability to concentrate the

received signal or electromagnetic wave by very large factors, antennae made of

magnetic materials can be quite small. This factor is especially important in small

portable radio or television sets.

Tuning slugs : In the tuner portion of a television sets, each channel can be fine tuned to

the proper frequency by adjusting the inductance of wound coil into which a threaded

slug of magnetic material is inserted.

22

1.13 Aim of doctoral work:

There has been tremendous interest in fine magnetic particle with possibilities of

wide range of applications in the field of science and technology from the last five years

which has been mentioned earlier [31]. The physical properties of spinel ferrites as

electrical, magnetic and dielectric properties are governed by the type of magnetic ions

occupying tetrahedral (A) and octahedral (B) sites and the relative strength on then inter

and intra-sub lattice interactions [32-35]

Ni-Mg and Ni-Co Ferrites are the most versatile magnetic materials for general

applications because of their high initial permeability, low magnetic losses, high

resistivity, low dielectric losses, mechanical hardness, high Curie temperature and

chemical stability [36-37]

Apart from these applications, Ni-Mg and Ni-Co Nano Ferrites are used in ferro

fluids to biological applications and many more ferrite structure sensitive to materials

compositions and method of preparations [38]

The objective of this thesis is to

Synthesis of Ni-Mg Nano Ferrite by citrate gel auto combustion method.

Synthesis of Ni-Co Nano Ferrite by citrate gel auto combustion method.

To study and improve magnetic properties.

To study and improve dielectric properties.

To study and improve electrical properties.

To study and improve thermoelectric power studies

23

H=0 H=H0

Fig 1.1(a) Fig 1.1(b)

Spin alignment of dia magnetic materials

H=0 H=H0

Fig1.2 (a) Fig 1.2(b)

Spin alignment of para magnetic materials

Fig 1.3(a) Fig 1.3(b)

Spin alignment of ferro magnetic materials

24

Fig 1.4 Variation of susceptibility with temperature

Fig 1.5 spin alignment of ferri magnetism

Fig 1.6 spin alignment of anti ferri magnetism

25

Fig 1.7 Variation of Susceptibility with temperature

Fig 1.8 Tetrahedral sites Fig 1.9. Octahedral sites

26

Fig 1.10. Spinel strcture

Fig 1.11 Inverse spinel strcture

27

Fig 1.12. Hexagonal ferrite structure.

Fig 1.13. Soft ferrites Fig 1.14. Hard ferrites

28

1.14 Literature Review

Spinel ferrites with twin electrical and magnetic properties have been the subject

of interest since 6 to 7 decades and are of current focus because of their new applications

in the field of medical science, environment, etc. ferrite materials are low cost,

chemically stable and possess more number of electrical, dielectric, magnetic properties

and therefore are widely studied by number of researchers.

Ferrites offer immense possibilities of tailoring its various properties for various

applications by doping with some metal ion. With a view to improve the electrical and

magnetic properties of the Ni- Mg and Ni-Co Nano ferrites for various applications is

proposed in the present work. A brief review of the earlier work done on different aspects

of various ferrites in general Ni, Mg ferrites and Co ferrites in particular is discussed

hereunder.

In the family of spinel ferrite, all the members have their own importance and

have number of applications. Nickel ferrite is one of the versatile and technologically

important soft ferrite materials because of its typical ferromagnetic properties, low

conductivity and thus lower eddy current losses, high electrochemical stability, catalytic

behavior, abundance in nature, etc. It crystallizes in a spinel structure and exhibits tunable

conducting behavior[39]. Magnesium ferrite has attracted the attention of researchers for

the past few decades [40-43]. One reason for this is the great potential of this material for

wide range of applications especially in microwave devices, which are based on higher

values of Saturation Magnetization, Curie temperature and Electrical Resistivity together

with low Dielectric losses and moderate coercive field. The ordering of the magnetic

moments of ferric ions and the strong exchange interactions explain the excellent

29

magnetic behavior of this material [44]. CoFe2O4 is interesting because of its remarkable

magnetic and electrical properties. It has good mechanical hardness, perfect chemical and

thermal stability [45].It has High electrical resistivity, Moderate saturation magnetization

and coercivity.

Ferrites offer immense possibilities of tailoring its various properties for various

applications by doping with some metal ion. With a view to improve the electrical and

magnetic properties of the Ni- Mg and Ni-Co Nano ferrites for various applications is

proposed in the present work. A brief review of the earlier work done on different aspects

of various ferrites in general Ni, Mg ferrites and Co ferrites in particular is discussed

hereunder.

Chen et al. [46] prepared the MgFe2O4 nanoparticles by coprecipitation method.

Magnetic measurements and neutron diffraction have shown the existence of a

superparamagnetic state in the synthesized system. The superparamagnetic relaxation was

studied by using Mössbauer spectroscopy and relaxation time has been correlated with

the particle size and temperature which is consistent with Neel theory.

Ravinder et al. [47] studied the dielectric properties of mixed Mg-Zn ferrites in

the frequency range of 1-100 kHz using a capacitance bridge. The dielectric constant vs.

frequency curve shows a normal dielectric behavior of ferrite materials. The frequency

dependence of a dielectric loss tangent (tanδ) curve possessed a peak at a certain

frequency for all the prepared compositions. The dielectric constant for these mixed

ferrites is almost inversely proportional to the square root of DC-resistivity. These

observations can be explained on the basis of an electron hopping between Fe2+ and Fe3+

ions.

30

Liu et al. [48] studied a correlation between the electron spin-orbital angular

momentum coupling and the superparamagnetism in MgFe2O4 and CoFe2O4

nanoparticles. The neutron diffraction studies have shown that the cation distribution and

contribution to the magnetic anisotropy from the Fe3+ lattice sites is almost the same in

both nanocrystallites. The blocking temperature of CoFe2O4 nanoparticles is 1500C

higher than that of the same sized MgFe2O4 nanoparticles due to more anisotropic nature

of Co2+ ions and confirmed by Mossbauer spectroscopic studies which demonstrate that

the magnetic anisotropy of CoFe2O4 nanoparticles is higher than that of the same size

MgFe2O4 nanoparticles, which can be controlled by adjusting the magnetic anisotropy

energy of nanoparticles.

Sepelak et al. [49] investigated the effects of high energy milling on MgFe2O4.

The crystallite size of MgFe2O4 can be reduced to nanometer range by mechanical

treatment and control the redistribution of cations between tetrahedral and octahedral

sites. The thermal stability range of mechanically induced metastable states is studied by

the change in temperature.

Rana et al. [50] prepared a series of spinel Mg1-xNixFe2O4 (x = 0.0-1.0) using

standard ceramic method. AC susceptibility measurements were performed to calculate

lande-g factor (g), effective magnetic moments (Peff), Curie temperature (TC),

paramagnetic Curie temperature (θ (K)) and exchange integral (J/k). The g-values, Peff

and θ (K) show increasing trend with the increase in Ni content up to x = 0.75 while TC

continues to decrease. The decreasing trend in magnetic moments (nB) and θ (K) for

x>0.75 could be correlated to the distribution of cations on A and B sites. The Y-K angle

for NiFe2O4 shows a gradual increase with Ni contents and approaches to 60θ.

31

Qi et al. [51] prepared Mn substituted MgCuZn ferrites (Mg0.2Cu0.2Zn0.6O)

(Fe2-xMnxO3)0.97 (x = 0.00-0.07) using nanosized precursor powders synthesized by a

sol-gel method. It has been observed that MgCuZn ferrites doped with Mn possess higher

initial permeability and better grain structure than that of NiCuZn ferrites prepared by the

same method could be ideal materials for high inductance multilayer chip inductor. The

variation of initial permeability of MgCuZn ferrites with the Mn substitution might be

attributed to the decrease of magnetostriction constant.

Sepelak et al. [52] investigated the changes in MgFe2O4 caused by high-energy

milling, using Mossbauer spectroscopy, magnetization measurements, and electron

microscopy. The observed enhancement of the magnetization in nanoscale milled

MgFe2O4 is discussed on the basis of cation redistribution and spin canting.

Radwan et al. [53] prepared the Mg1+xTixFe2-2xO4 of single phase spinel structure

assured by X-ray analysis. The octahedral preference of Mg2+ ions doped on the expense

of the Fe3+ ions increases the inversion factor of spinel. The conduction has occurred due

to mobility of thermally activated charge carriers. The mobility of charge carriers could

be explained using Verwey model of conductivity which is based on the electron hopping

between Fe2+/Fe3+ on the same sub-lattice sites. The octahedral site preference of Ti4+

ions decreases the conductivity of the samples. Unusual behavior of Ti content of 0.7 and

0.8 was observed due to the presence of impure phases.

Rabanal et al. [54] studied the magnetic properties of powdered Mg-ferrite

processed with a centrifugal mill. The starting ferrite powder was prepared by solid-state

reaction occurred at 14000C for 48 hours. The crystallite size and internal strain were

evaluated by XRD data using Williamson-Hall and Debye-Scherrer methods. The

32

nanoparticles were obtained for low milling time. The X-ray analysis indicates that the 17

hours of milling caused the appearance of two α-Fe2O3 peak. The saturation

magnetization remains nearly constant at 39.2 emu/g that indicates the lack of inversion

degree even for longer milling times, while coercivity increases up to 576.7 Oe due to

internal stresses caused by the mechanical grinding.

Chauhan et al. [55] used citrate precursor method to modify the magnetic

properties of substituted Mg-Mn ferrites successfully. The citrate precursor method is

used to obtain close composition control, better homogeneity, lower processing

temperature, higher purity, lower porosity and more uniform grain growth. The various

magnetic parameters like saturation magnetization, magneton number, and thermal

variation of AC-susceptibility, Neel temperature and initial permeability were calculated.

The saturation magnetization and initial permeability of the synthesized materials were

improved significantly with Indium and Cobalt substitution. The Neel temperature of the

samples was increased with the increase in Cobalt content. Magnetic losses were 1-2

orders lower as compared to those for Mg-Mn ferrites prepared by conventional ceramic

method, suitable for high-frequency applications.

Chhaya et al. [56] investigated the Ca2+ doped MgFe2O4 without altering the

cubic symmetry and affecting the structural, magnetic and electrical properties of

Mg1-xCaxFe2O4 (x = 0.0-0.35) spinel ferrite system studied by means of X-ray diffraction

(XRD), magnetization, AC susceptibility and DC resistivity measurements. The variation

of magneton number with Ca2+ content can be explained on the basis of Neel’s collinear

spin model. The Néel temperature determined through AC susceptibility and

33

DC resistivity measurements was in close agreement with theoretical values. The

variation in electrical resistivity coincides with the change in activation energy.

Turkin et al. [57] synthesized MgFe2O4 annealed at 1373 K using silica-tube

technique. The product is homogeneous and fine grained with the inversion parameter of

0.75. The calorimetric measurements indicate the second-order phase transition of

antiferromagnetic to paramagnetic materials at 597 K. This silica-tube technique of the

synthesis prevents the escape of oxygen at long heating.

Verma et al. [58] synthesized nanosized Magnesium ferrite using mild microwave

hydrothermal (MH) conditions. The average particle size of the ferrite is found to be ~3

nm calculated from X-ray diffraction and transmission electron microscopic analysis.

Vibrating Sample Magnetometric studies indicate the superparamagnetic nature of ferrite

particles.

Thummer et al. [59] conducted 57Fe Mossbauer spectroscopy at 300 K to

investigate the magnetic behavior of the spinel ferrite, MgAlxCrxFe2-2xO4 (x = 0.0-0.5).

The Mossbauer spectra for x = 0.0-0.2 exhibit two Zeeman sextet due to Fe3+ ions

distributed over two sites i.e. octahedral and tetrahedral sub-lattice sites, while a central

paramagnetic doublet superimposed on the magnetic sextet appeared for x = 0.3-0.5. He

discussed the variation of hyperfine interactions and the appearance of the central doublet

on the Zeeman sextet thoroughly.

Structural and electrical properties of Ni1-xMgxFe2O4 (x = 0.0, 0.3, 0.6, 0.9)

synthesized by citrate gel process were discussed by Berchman et al. [60].Lattice constant

increased whereas X-ray density decreased with the increase in Mg concentration.

Dielectric constant decreased with the rise in frequency whereas loss tangent increased up

34

to 2 kHz and then decreased with the increase in frequency. AC conductivity increased

with the rise in frequency. DC conductivity increased with the increase in temperature.

The variation in activation energy and coercivity as a function of Mg concentration were

also investigated.

Liu et al. [61] synthesized n-type nanomaterials (MgFe2O4) by convenient,

environment friendly, inexpensive solid-state reaction method. The material structure and

crystallite microstructure of samples have been evaluated by X-ray diffraction (XRD),

Transmission Electron Microscopy (TEM) and high-resolution Transmission Electron

Microscopy (HRTEM). Conductance responses of the MgFe2O4 were measured by

exposing the thick film to reducing gases like methane (CH4), hydrogen sulfide (H2S),

liquified petroleum gas (LPG) and ethanol gas (C2H5OH) and observed various sensing

responses to these gases at different operating temperature. Successive on and off

responses have been repeated and no major changes in the response signal were seen.

Pradeep et al. [62] prepared nano-particles of M0.5Mg0.5Fe2O4 (M=Ni, Cu and Zn)

using the sol-gel method. Synthesis of single phase polycrystalline ferrite materials is

assured using XRD. Lattice constant and particle size have been determined from XRD

data. Mixed CuZn-ferrites have greater values of the lattice constant as they are bigger

ions than Ni. Particle size was decreased by substitution of Cu to Zn. The samples were

subjected to VSM measurements and FTIR characterization. The magnetic moment

values are determined and the theoretical calculation was done to propose the cation

distribution. FTIR data were analyzed for the respective sites. The higher frequency band

(υ1) and lower frequency band (υ2) are attributed to the tetrahedral and octahedral

35

complexes, respectively. The difference in the trends of bond length and force constant is

able to elucidate the role of crystal field effect.

Lakshman et al. [63] prepared the Mg0.9Mn0.1InxFe2-xO4 and Mg0.9Mn0.1CryFe2yO4

by the conventional ceramic route. The influence of In3+ and Cr3+ ions on the DC

resistivity, dielectric constant and dielectric loss factor are evaluated in this paper. The

resistivity increases with the increase in dopant contents. The activation energy and

dielectric constant were found to increase with the substitution level of In3+ and Cr3+ ions.

The dielectric loss tangent (tan δ) measured at 100 kHz and 13 MHz are found to be very

small for the samples with higher dopant contents. The smaller values of loss factor

indicate that the prepared materials might have good potential for microwave devices.

A. A. Pandit et al. [64] have prepared the ferrites with the general formula

Mg1+xMnx Fe2−2xO4 (where x = 0.0 to 0.9) by the standard ceramic technique and studied

by means of X-ray diffraction, magnetization, a.c. susceptibility and dielectric constant

measurements. The X-ray analysis confirmed the single-phase formation of the samples.

The lattice parameter was found to increase up to x = 0.3 and thereafter it decreases as x

increases. The cation distribution has been studied by X-ray analysis and magnetization.

Magnetization results exhibit collinear ferrimagnetic structure for x ≤0.3 and thereafter

structure changes into non-collinear for x > 0.3. Curie temperature (TC) obtained from

AC susceptibility data decreases with increasing x. The dielectric constant (ε') and loss

tangent (tan δ) show strong frequency dependence.

Masti et al. [65] studied the magnetization and permeability of polycrystalline

materials, CdxMg1-xFe2-yCryO4 (x = 0.0-1.0; y = 0, 0.05 and 0.10). The Neel’s two sub-

lattice model exists up to x = 0.4, for y = 0, 0.05 and 0.1 and a three sub-lattice model

36

(YK-model) is predominant for x>0.4 and y = 0, 0.05 and 0.10 revealed by the

magnetization. The MS was found to decrease with the increase in Cr3+ contents, which is

attributed to the weakness of B-B site interaction. Variation of initial permeability with

temperature revealed the long-range ferromagnetic ordering in the compounds with x =

0.4. The sample with the composition x≤0.4 and y = 0, 0.05 and 0.10 possess peaking

behavior near the Curie temperature attributed to the decrease of anisotropy constant K1

to zero. Addition of Cd2+ and Cr3+ resulted in a decrease in Curie temperature and initial

permeability, respectively.

Bergmann et al. [66] reported the single-step synthesis of nanosized MgFe2O4 via

mechano-chemical processing of oxide precursors. The synthesized materials were

subjected for X-ray diffraction and 57 Fe Mössbauer spectroscopic analyses. The

transmission electron microscopy assured the nanoscale nature of the mechano-

synthesized material.

Y Atassi and M. Tally [67] have prepared Mg-Cu-Zn ferrite through a wet

synthetic method by a self-combustion reaction directly from a citrate precursor and have

sintered the samples at 750°C for only 2 h. XRD patterns and FTIR spectral analysis have

confirmed the formation of single phase Mg-Cu-Zn ferrite after combustion. They

reported that in this method Mg-Cu-Zn ferrites were sintered at such a low temperature

and the sintering process increased the crystallinity of the solid and the domain sizes.

Lakshman et al. [68] studied the effect of the substitution of Cr3+ in mixed

MgCuMn ferrites, Mg0.9Cu0.1Mn0.05CrxFe1.95-xO4 (x = 0.0-0.9) with incorporation of small

amounts of Cu2+ ions. X-ray diffraction and Mossbauer spectroscopic analyses of these

samples have been performed. Mossbauer analysis at 300 K showed two Zeeman sextet

37

for lower concentration of Cr3+ followed by relaxation phenomenon and the spectrum

shows paramagnetic doublet for x =0.9. The variation in Mossbauer parameters, viz,

isomer shift, quadruple splitting and hyperfine magnetic field with dopant concentration

has been evaluated.

Kong et al. [69] studied the densification, grain growth, and microstructure of

Mg-Cu-Co ferrite (MgFe1.98O4, Mg1-xCuxFe1.98O4) with x = 0.10-0.30 and

Mg0.90-xCoxCu0.10Fe1.98O4) with x = 0.05-0.20 fabricated for high frequency antennas.

Magnesium ferrite (MgFe1.98O4) is a promising magneto-dielectric material for high

frequency antennas. But due to its poor densification, it could not be sintered at a

temperature below 12000C. High-temperature sintering resulted in a drastic but

undesirable change in electrical and dielectric properties. The poor densification and slow

grain growth rate have been improved by the addition of Cu ions into MgFe1.98O4. While,

the presence of Co did not have any significant influence on the densification and grain

growth rate of the synthesized system.

Sepelak et al. [70] reported the magnetization enhancement in nanosized

mechano-synthesized MgFe2O4 and discussed on the basis of a modified core-shell

model. Due to random distribution of cations, the surface shell of nanoparticles exhibits

an effective magnetic moment 2 times greater than that of the particle core. The

thickness of surface shell is calculated from the size-dependent magnetization

measurements.

Ichiyanagi et al. [71] prepared the MgFe2O4 nanoparticles by using wet chemical

method. The particle size estimated from X-ray diffraction patterns were in the range of 3

to 8 nm. Magnetization measurements were performed under an applied field of 750 kOe.

38

Both the field-cooled (FC) and the zero-field-cooled (ZFC) magnetization dependent

blocking temperature, Tb was observed to be around 30 K. A high coercivity of ~1000

Oe was observed at 5 K. A big difference in magnetization was observed between the

quenched samples and annealed samples.

Cui-Ping Liu et al [72] have performed the comparative study of MgFe2O4

nanocrystallites prepared by sol–gel and co-precipitation methods. They found that at the

same calcinations temperature, the sol–gel derived sample always had bigger mean

crystalline size than the co-precipitation derived sample, implying that the sol–gel

method facilitated the formation of magnesium ferrite crystallites. Most of the sol–gel

derived magnesium ferrite particles had a lamellar structure consisting of

nanocrystallites, which were probably derived from the porous dried gel precursor. The

Magnesium ferrite particles had superparamagnetic properties at 270 C, and their

saturation magnetization increased with increasing size.

Vital et al. [73] prepared the nanoscale ZnFe2O4, Mg0.5Zn0.5Fe2O4 and

Mg0.2Cu0.2Zn0.62Fe1.98O3.99 powders by flame spray synthesis (FSS). Particle size was

estimated in the range of 6–13 nm. Compacts prepared from Mg–Cu–Zn ferrite

nanoparticles possessed an extremely high sinter-activity. A sintered density of 5.05

g cm-3 was achieved after sintering for 2 h at 9000C which is greater than that achieved

(4.91 gcm-3) from the conventional ceramic route. The permeability of the sintered Mg-

Cu-Zn ferrite compacts was reached to value of μ = 600 at 1 MHz and MS was 80 emu/g.

The significant sintering activity of the synthesized ferrite powders is attributed to their

small particle size.

39

Sharma et al. [74] studied the magnetic properties of nanosized

Mg0.95Mn0.05Fe2O4 samples and characterized by X-ray diffraction, Mossbauer

spectroscopy, DC magnetization and frequency dependent real χ'(T ) and imaginary χ"(T)

parts of AC susceptibility measurements. Mossbauer measurements show a magnetic

transition to an ordered state at 195 K. The ZFC curve shows a broad maximum at Tmean

= 195 ± 5 K, which depends upon the distribution of particle volumes in the sample. A

frequency-dependent peak is well described by Vogel-Fulcher law, giving a relaxation

time ηo = 5.8 × 10-12 s and an interaction parameter To = 195 ± 3 K. These values are

evident in strong interactions between the nanoparticle systems. On the other hand fitting

with the Néel-Brown model and the power law provide an unrealistic large value of

ηo (~6×10-69 and 1.2×10-22 s respectively).

Barati [75] synthesized the mixed ferrite with compositions

MgxCu0.20ZnxFe2O4 (x = 0.5, 0.55, 0.60 and 0.63) through nitrate-citrate gel auto-

combustion method. TG/DTA analysis was carried out to study the combustion process

of the dried gels. The obtained precursor was calcined at 8000C for 1 hour followed by

sintering at 9000C for 4 hours. The obtained materials were characterized for phase

identification, grain size and lattice parameter determination using X-ray diffraction. The

magnetic and electrical properties of synthesized materials have also been investigated

for various parameters. The initial permeability, saturation magnetization, dielectric

constant and dielectric loss were found to increase and AC-resistivity was decreased with

the increase in dopant contents. The prepared material is suitable for the application in

multilayer chip inductor due to its good magnetic properties and low loss at high

frequency.

40

Gadkari et al. [76] investigated the structural properties of Sm3+ doped

Mg1-xCdxFe2O4 (x = 0.0-1.0) synthesized by Oxalate co-precipitation method. The

samples were annealed at 10500C for 5 hours. The cubic spinel structure was assured by

X-ray diffraction analysis and evaluated the different parameters like lattice constant, X-

ray density, physical density, porosity, crystallite size, site radii and bond length on

tetrahedral and octahedral sites. The lattice constant increases with an increase in Cd2+

content and crystallite size varies from 28.69 to 32.05 nm. The surface morphology

shows an increased grain size of the samples with an increase in Cd2+ contents. The IR

spectra show two strong absorption bands at 5.87×104 m-1 and 4.27×104 m-1 for

tetrahedral and octahedral sites, respectively.

Chand and Singh [77] have synthesized the MgGd0.1Fe1.9O4 by conventional

ceramic method with enhanced electrical and dielectric properties. X-ray analysis assured

the formation of single-phase structure. The DC-resistivity is enhanced by one order of

magnitude with Gd substitution in Magnesium ferrite. High resistivity along with lower

dielectric loss is attributed to better compositional stoichiometry and nature of the

additives. Dielectric properties of the sample have been studied in the frequency range of

0.1-20 MHz at various temperatures.

Sasaki et al. [78] synthesized magnesium ferrite (MgFe2O4) by hydrothermal

route for which suspensions of Mg(OH)2 and Fe(OH)3 in appropriate ratio were prepared

and pressurized to 30 MPa by high-pressure pump followed by rapid heating to the

reaction temperature. The Mg/Fe molar ratio varied from 0.5 to 1.5 to obtain single-phase

MgFe2O4. The stoichiometric ratio of Mg/Fe = 0.5 contains both MgFe2O4 and α-Fe2O3

while, at Mg/Fe = 1.0 and 1.5, the single-phase MgFe2O4 is obtained. MgFe2O4

41

synthesized in the present study with particle size of 20 nm, exhibits superparamagnetic

behavior

Farghali et al. [79] studied the variation of physical characteristics by embedding

the ferrite materials into polymeric matrices. They prepared well dispersed

polyaniline/Co1-xMgxFe2O4 nano-composite (x = 0, 0.5, 1) with good magnetic and

electrical properties. TGA results indicated that the ferrite nano-particles could improve

the thermal stability of composite. The electrical conductivity of the pure polyaniline

decreased while the saturation magnetization (MS) and coercivity (HC) increased by

embedding the ferrite nanoparticles in composite.

Kumar et al. [80] have synthesized Magnesium-Manganese ferrites having

composition Mg0.9Mn0.1Al0.3CozFe1.7-zO4 (z = 0.3-0.7) by the citrate precursor method.

Single-phase spinel structure of the samples is assured by the X-ray diffraction analysis.

The synthesized materials have been investigated for their electrical and magnetic

properties. Fairly constant value of initial permeability in a wide frequency range of 0.1-

20 MHz along with a low loss factor of the order of 10-4-10-5 are the superb achievements

of the present study. Moreover, initial permeability increased with an increase in

temperature while RLF was found to be low at measurement temperatures. The DC

resistivity and Curie temperature were observed to increase with dopant content.

Ghatak et al. [81] measured the alternate current conductivity, direct current

conductivity and dielectric properties of Mg-Zn ferrite below room temperature. The

nanocrystalline ferrite powder was prepared from oxides of magnesium, zinc and iron by

using a solid state processing method. The X-ray diffraction analysis was used to

determine the structure and composition of synthesized ferrite, while impedance analyzer,

42

liquid nitrogen cryostat and electromagnet were used for conduction and dielectric

properties of ferrite. The frequency exponent (s) of conductivity is found to be highly

temperature dependent. The temperature exponent (n) of dielectric permittivity decreases

with increasing frequency. The grain boundaries have more contribution as compared to

the grains contribution in conduction phenomena and resistance due to grain and the

grain boundary contribution possesses two activation regions.

Bharti et al. [82] synthesized Magnesium ferrite, Zinc ferrite and Magnesium Zinc

ferrite using an economic wet chemical synthesis technique. Phase formation of the

synthesized powders has been confirmed by infrared spectroscopy and X-ray Rietveld

refinement analysis. The structural features of these materials have been correlated with

their magnetic properties. Single phase Magnesium Zinc ferrite nano-particles were

investigated for Carbon monoxide and Hydrogen gas sensing properties. The response

and recovery transients of conductance were modelled using Langmuir adsorption

kinetics having two active sites in the sensing elements named as CO sensors. The

activation energies for response and recovery behavior of these two adsorption sites were

found to be different. This difference in activation energies for response and recovery is

due to different chemical-adsorbed oxygen species in these two sites.

Dalt et al. [83] investigated the synthesis of nanostructured MgFe2O4 through

solution combustion technique. The 30% fuel-deficient formulation was selected to

synthesized powders at different furnace temperatures. The structural and morphological

characterizations were performed by using X-ray diffraction and Transmission Electron

Microscopy (TEM). Mössbauer spectroscopy and vibrating sample magnetometer (VSM)

were employed for magnetic measurements. Crystallite sizes of MgFe2O4 around 42.8 nm

43

calculated from the XRD pattern were consistent with the results obtained from TEM

analysis.

Sadhana et al. [84] have prepared the nanocrystalline MgCuZn-ferrites with

particle size ~30 nm using microwave-hydrothermal method. The structural and

morphological features are investigated using X-ray diffraction and Scanning Electron

Microscopy. The grain sizes of the samples are in the range of 60-80 nm. The ultrasonic

velocities measured on MgCuZn ferrites are found to decrease with an increase of

temperature. The anomaly observed in the thermal variation of longitudinal velocity and

attenuation is explained on the base of magneto-crystalline anisotropy constant.

Mansour [85] prepared the Manganese-Magnesium ferrite nanoparticles with

general formula Mn1-xMgxFe2O4; 0≤x≤0.25 using the co-precipitation route. The XRD

analysis confirms the single phase spinel structure. The crystallite size calculated from

Scherrer formula was found to be in the range of 3-6 nm and observed to increase with

the increase in dopant contents. TEM was also utilized to analyze the microstructure of

nanosized Mn1-xMgxFe2O4. Hysteresis loops measured at room temperature revealed the

lower value of saturation magnetization associated with Mn1-xMgxFe2O4 nanoparticles.

Shah et al. [86] intended to improve the humidity sensing properties of Mg ferrite

by doping with Pr in 0.1 mol% and 0.3 mol% concentrations. The spin density calculated

from electron paramagnetic resonance (EPR) increased from 8.15×1020 to 15.6×1020 for

0.3 mol% Pr doped compared to undoped Mg-ferrite. The bulk porosity of the samples

increases from 8.4 to 34% with Pr contents. Pr doping also caused the increase in

sensitivity factor Z10%/Z90% from 24 to 113. Impedance gradient |dlog Z/dRH| at low

10-30% RH and high 70-90% RH was determined corresponding to spin density and

44

porosity of the samples. A maximum drift in humidity hysteresis of 22% RH is mainly

reduced to 2% RH by a 0.1 mol% Pr doping in Magnesium ferrite. Such significant

improvements make this material as a strong candidate for humidity sensors.

Patil et al. [87] reported the synthesis of spinel MgFe2O4 by a simple, inexpensive

combustion method and applied as a gas sensor for reducing gases (LPG, Acetone,

Ethanol, Ammonia). The reducing gas sensing properties as a function of structural and

surface morphological properties have been studied. The structural and morphological

features were analyzed by X-ray diffraction and Scanning Electron Microscopy

respectively. The porous morphology revealed by SEM analysis owed to decrease with

the grain growth by an increase in sintering temperature. The maximum response of 71%

to 2000 ppm of LPG was observed at 698 K with the synthesized material.

Mounkachi et al. [88] prepared the Mg0.6Cu0.4Fe2O4 ferrites using the solid-state

reaction technique. The structural properties have been studied using X-ray diffraction

analysis. While magnetic measurements were carried out using mean field theory and

high-temperature series expansions (HTSE), extrapolated with the padé approximants

method. The Mossbauer data were dealt to compute nearest neighbor super-exchange

interactions for intra- and inter-site using the probability approach. The obtained

experimental results are in good agreement with the theoretical ones obtained by the

magnetic measurements.

Singh et al. [89] investigated the DC and AC electrical resistivity of Mg

substituted Ni-Cu ferrites as a function of temperature, frequency and composition. The

Ac-resistivity of the samples decreases with an increase in frequency, having

ferrimagnetic behavior. The dielectric loss tangent showed maximum frequency

45

dependence in between 10Hz and 1 kHz in all the samples. The conductivity relaxation of

charge carriers has been examined using the electrical module formulism, and results

indicate the presence of the non-Debye relaxation in the synthesized samples. Similar

values of Ea both for DC conduction and conductivity relaxation indicate that the

mechanism of electrical conduction and dielectric polarization is the identical in these

ferrites. The saturation magnetization and coercivity calculated from the hysteresis loops

show striking dependence on composition.

Sujatha et al. [90] examined the effect of Mg substitution on structural and

magnetic properties of Ni-Cu-Zn ferrites prepared by sol-gel process. Various parameters

were studied using X-ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy

(FT-IR), Field Emission Scanning Electron Microscopy (FE-SEM) and Vibrating Sample

Magnetometer (VSM). XRD analysis was used for the phase identification, unit cell

parameter and crystallite size determination. The lattice constant was found to decrease

with the increase in Mg content. Saturation magnetization and coercivity showed an

opposite behavior with the Mg content. The Curie temperature (TC) increases while initial

permeability (μi) decreases with the increase in Mg contents. This is because of reduced

magnetization, grain size and enhanced magneto-crystalline anisotropy. The permeability

is also found to be constant up to 30MHz frequency ensuring the compositional stability

and quality of the material. The synthesized materials have attraction for applications in

Multilayer Chip Inductors because of their invariable permeability and high thermal

stability.

Chen et al. [91] synthesized the Mg-ferrite nanoparticles with high saturation

magnetization using microwave assisted ball milling process. The as-milled materials

46

were characterized by X-ray diffraction, Transmission Electron Microscopy and

Vibrating Sample Magnetometer. The obtained results revealed that the average size and

the saturation magnetization of Mg-ferrite nanoparticles were 30 nm and 43.40 emu/g

respectively. The microwave assisted ball milling is a simple and environment friendly

approach as compared to conventional milling process and could be considered as a

promising approach for the synthesis of nanoparticles of high performance in the future.

Bobade et al. [92] synthesized the nanocrystalline Ni2+ substituted Mg-Zn ferrites

with general formula Mg0.7-xNixZn0.3Fe2O4 (x = 0.0-0.6) by using sol-gel auto combustion

method. The Citric acid was used as a fuel and the metal nitrate to citric acid ratio was

taken as 1:3. The structural and morphological features of Mg-Ni-Zn ferrites were studied

by X-ray diffraction, scanning electron microscopy, and FT-IR spectroscopy. The lattice

parameters were determined from the XRD data. The FTIR spectroscopy was used to

deduce the cation distribution between tetrahedral and octahedral sites of spinel Mg-Ni-

Zn ferrites. Micrographs indicate the grain growth formation with the increase in Ni2+

contents. The M-H loops were utilized to find out the saturation magnetization (MS) and

magneton number of the synthesized materials. The value of Ms increases with the

increase in Ni2+ contents of Mg-Zn ferrite.

S.M.Rathod et al [93] have synthesized MgxNi0.6-xCo0.2Fe2O4 (where x = 0.2, 0.4,

0.6) using low temperature Sol-gel Auto-Combustion method. They found that there is a

decrease in Lattice constant of the material as percentage of Magnesium increases. They

reported that as the percentage of Magnesium increases the magnetization, Coercivity and

retentivity decreases. From UV spectroscopy they found that band gap energy of the

material is 2.72 eV and conferred that sample is semiconductor in nature.

47

Lee et al [94] synthesized Co1-xMnxFe2O4 spinels in air as bulk phases. The lattice

parameters increased with the addition of Mn cation, which was closely related to the

effective substitution of Mn2+ cation. From the measurements of the magnetic moment, it

is shown that Mn contributes to the canted magnetic moment between tetrahedral (A) and

octahedral (B) sites. The n-type conduction was observed from Seebeck coefficient

measurements: this was ascribed the formation of Co3+ and Mn3+ from Co2+ and Mn2+

cations on A and B sites. The electrical conductivities increase with Mn substitution. It

was suggested that the possibility of charge transfer between 2+ and 3+ cation in A as well

as B sites contributed to electrical conductivity.

Vasamber et al [95] prepared polycrystalline compounds of the series CdxCo1-xFe2-yCryO4

where x = 0, 0.25, 0.50, 0.75 and 1.00; y =0, 0.15 and 0.30 by a standard ceramic

technique. The crystallographic data were obtained using X-ray diffraction showed that

all the compounds have fcc symmetry. The ionic radii on A and B sites, rA and rB,

respectively and the bond lengths on A and B sites (A-O and B-O, respectively) were

calculated. The values of rB and B-O were found to be greater than rA and A-O, except

for the Cd2+ and Cr3+ substituted Cd ferrites. The activation energies (Ea) were found to

be higher in the para-region than in the ferri-region. The resistivity of the samples was

found to be dependent on the saturation magnetic moments of the samples, the resistivity

of Co ferrite was found to be higher than that of Cd ferrite at 475 K.

Liu et al [96] synthesized CoFe2O4 nanoparticles by microemulsion method using

a stable ferric salt (FeCl3). The normal micelles were formed by Sodium Dodecyl

Sulphate (NaDS) in aqueous solutions. The mean size of the nanoparticles could be

controlled from less than 4 nm to about 10 nm through controlling the concentrations of

48

the reagents. CoFe2O4 nanoparticles had a high degree of inversion with 66% of the

tetrahedral sublattice occupied by Fe3+ and are superparamagnetic in nature. The blocking

temperature and coercive field of the nanoparticles increased with increasing size of the

nanoparticles.

Li et al [97] synthesized Cobalt ferrite nanoparticles in water-in-oil microemulsions

reversed micelles with varying cation composition. Transmission Electron Microscopy

revealed that the particles were nanospheres with particle size ranging from 12 to 18 nm.

X-ray diffraction results indicated that at low Co2+:Fe2+ ratio 1.10 and 1.5 in the

precursor, the particles retained an essentially ferrite structure (α - Fe2O3). However, the

Cobalt–ferrite phase (CoFe2O4) formed upon further increase of the Co2+ content. The

materials were found to exhibit superparamagnetism. The blocking temperatures and

coercivities were dependent on the Co2+:Fe2+ ratio in the system.

Kahn and Zhang [98] doped Lanthanide ions into Cobalt spinel ferrites using an

oil-in-water micellar method to form CoLn0.12Fe1.88O4 nanoparticles with Ln = Ce, Sm,

Eu, Gd, Dy, or Er. Doping with lanthanide ions (LnIII) modulated the magnetic

properties of Cobalt spinel ferrite nanoparticles. In particular cases of Gd3+ or Dy3+ ions,

a dramatic increase in the blocking temperature and coercivity was observed. Indeed, the

introduction of only 4% of Gd3+ ions increased the blocking temperature 100 K and the

coercivity 60%.

Chae et al [99] fabricated the Ti0.2Co1.2Fe1.6O4 ferrite films by a sol–gel method.

The growths of particles, crystallographic and magnetic properties of the films were

investigated by X-ray diffraction, Atomic Force Microscopy and Vibrating Sample

Magnetometry. Ferrite films annealed at and above 873K had only a single spinel

49

structure. The grain sizes and the surface roughness increased as the annealing

temperature increased. The coercivity perpendicular to the plane was higher than that

parallel to the plane. The coercivity of the samples annealed at and above 673K increased

as the annealing temperature increased. The maximum coercivity of our ferrite films

annealed at 1073K was 1566 Oe.

Yamamoto and Nissato [100] investigated the effect of NiO substitution on the

magnetic and physical properties of CoFe2O4 prepared by the chemical co-precipitation

method without post annealing. They found that the single-phase Co–Ni spinel ferrite

fine particles could be prepared by the chemical co-precipitation method without post

annealing. The typical magnetic and physical properties were saturation magnetization

M= 56.3x106 Wb m/kg (44.8 emu/g), coercivity = 506.9 kA/m (6.37 kOe), Curie

temperature = 557.30 C, the lattice constant = 0.8384 nm, and the average particle size =

30 nm. The rotational hysteresis integral Rh, which was related to the magnetization

mechanism of these fine particles, was 1.57.

Mahajan et al [101] prepared CoFe2O4 –BaTiO3 composites using conventional

ceramic double sintering process with various compositions. Presence of two phases in

the composites was confirmed using X-ray diffraction. The dc resistivity and thermo-emf

as a function of temperature in the temperature range 300 K to 600 K were measured.

Variation of dielectric constant (εo) with frequency in the range 100Hz to 1 MHz and

with temperature at a fixed frequency of 1 kHz was studied. The ac conductivity was

derived from dielectric constant (εo) and loss tangent (tan δ).The nature of conduction

was discussed on the basis of small polaron hopping model. The static value of magneto-

electric conversion factor had been studied as a function of magnetic field.

50

V.K. Sankaranarayanan, [102] Om Prakash, R.P. Pant, Mohammad Islam have

synthesized Nanoparticles of Lithium ferrite for ferrofluid applications with a size in the

range of 10nm by a Citrate precursor method. They found that sample decomposed at

2000C has the β-LiFe5O8 type (a disordered type of spinel) structure which on annealing

at 3500C transforms to the α-LiFe5O8 type (an ordered type spinel) structure as shown by

both IR spectra and XRD studies. The reported Magnetization studies of these ferrites

show that these particles are in the superparamagnetic regime and have adequate

properties for application in low magnetization ferrofluids

D. Ravinder [103] and P. Shalini have investigated the superparamagnetism in

Co–Zn ferrite thin films produced by pulsed-laser deposition. They found the blocking

temperature (TB) of Cu-Zn ferrite thin film at 40K above which the material was

superparamagnetic in nature. They also observed that below TB the material showed

hysteresis and above this temperature there was no hysteresis found which is the criteria

for superparamagnetism.

Panda et al [104] prepared the magnetic properties of nano-crystalline

CoMxFe2xO4 (where M=Gd and Pr and x = 0, 0.1 and 0.2) powders by a Citrate precursor

technique and studied by using Vibrating Sample Magnetometer (VSM). The crystallite

sizes of the materials were within the range of a minimum of 6.8nm and a maximum of

87.5 nm. TG study indicated the formation of the spinel ferrite phase at 2200C. The room

temperature saturation magnetization of the ferrite materials decreased with the reduction

of size due to the presence of superparamagnetic fractions in the materials and spin

canting at the surface of nano-particles. Insertion of rare-earth atoms in the crystal lattice

inhibited the grain growth of the materials. The improved coercivity compared with those

51

for the pure cobalt ferrites was attributed to the contribution from the single ion

anisotropy of the rare-earth ions present in the crystal lattice and the surface effects

resulting in alteration of magnetic structures on the surface of nano-particles.

Li and Kutal [105] synthesized CoFe2O4 nanoparticles having dimensions varying

from 6.3 to 10.5nm by a micelle chemical control method. The average diameter of

cobalt ferrite particles ranged from several nanometers to tens of nanometers, which

could be controlled by the value of x. For the fine particle, a diffused electron pattern was

observed. The Mossbauer absorption patterns consisted of a ferromagnetic component

superposed on a superparamagnetic doublet. The intensity of the superparamagnetic

doublet was found to be larger for particles having small average diameter. The magnetic

hyperfine field showed size dependence and was bigger for very fine particle. They

decreased with increasing particle size for all the two sublattice sites.

Choi et al [106] synthesized CoFe2O4 nanoparticles by a microemulsion method.

All peaks of X-ray diffraction patterns could be attributed to a cubic spinel structure with

the lattice constant a= 8.39oA. The average size of the particles, determined by

Transmission Electron Microscopy, was 7.8 nm. Superparamagnetic behavior of the

particles was confirmed by the coincidence of plots of the magnetization versus field

divided by temperature. As the temperature approached toward the Neel point,

Mossbauer line broadening and a pronounced central doublet appeared, suggesting

superparamagnetic relaxation. As the temperature increased, the relaxation rate increased

rapidly as the seventh power of the absolute temperature.

Vestal and Zhang [107] developed a method for coating silica on CoFe2O4 and

MnFe2O4 spinel ferrite nanoparticles by using a reverse micelle microemulsion approach.

52

The ability to controllably synthesize magnetic nanoparticulate cores independent of

encapsulation provided great flexibility in tuning the magnetic properties of this magnetic

nanocomposite system by controlling the magnetic properties of nanoparticulate cores.

For these spinel ferrite nanoparticles, the saturation and remnant magnetizations

decreased upon silica coating. The coercivity of silica-coated CoFe2O4 nanoparticles did

not show any change after coating, while the coercivity of MnFe2O4 nanoparticles

decreased by 10% after they were coated with silica.

Gabal and Ata-Allah [108] synthesized polycrystalline samples with the general

formula Co1-xCdxFe2O4 (0 ≤ x≤ 1) by calcination of the respective oxalates mixtures at

10000C for 5 h. Their structural, electrical and magnetic properties were studied using X-

ray diffraction, Fourier Transform Infrared and Mossbauer spectroscopy, and electrical

conductivity and magnetic susceptibility techniques. With Cadmium ion substitution, the

lattice parameter, X-ray density, oxygen parameter, inversion factor and radii of

tetrahedral and octahedral sites were calculated. The Fourier transform infrared spectra

showed two dominant bands in the high- and low frequency range which were assigned

to the tetrahedral and octahedral complexes, respectively. The relationship between bands

position and cadmium content was also investigated. Mossbauer spectroscopic study

revealed that ions at octahedral site moved to the tetrahedral site, and that this system

varied from an inverse to a normal spinel structure. The temperature variation of the

conductivity showed a definite kink, except for the CdFe2O4 sample, which corresponds

to the ferrimagnetic to paramagnetic transitions. The effective magnetic moment of the

samples and their Curie temperature were observed to decrease by the substitution effect.

53

Yang et al [109] prepared the mixed transition-metal spinel oxide CoFe2O4

through a co- precipitation method. The electrochemical performances of CoFe2O4 as

active material for lithium ion battery were tested in the Teflon cells. It was found that

the first discharge capacity was close to 882mAh g-1 at a current density of 0.2mAcm-2,

corresponding to the reaction of 7.7 Li+ per CoFe2O4. And the mechanism of the reaction

of lithium with cobalt ferrite spinel was discussed.

Lattice parameter, Scanning Electron Micrographs, grain size, transmittance and

FTIR vibrations were studied by Bahout et al. [110] for Zn1-xNixFe2O4 ferrites prepared

by Soft-chemistry method based on citrate-ethylene glycol precursors, at a relatively low

temperature (6500C) in the range 0 ≤ x ≤ 1 with the step increment of 0.25. The lattice

parameter decreased with the rise in nickel concentration whereas the grain size increased

with the increase in annealing temperature.

Abo El Ata et al [111] prepared a series of polycrystalline spinel ferrite with

composition Li0.5xCoxFe2.5-0.5xO4 where (x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0) by the well known

double sintering process to investigate their spectral, initial magnetic permeability and

transport properties. The X-ray diffraction analysis showed that all samples have single

cubic spinel phase. The lattice parameter “a” was increased with increasing Co2+ ion

substitution. The IR spectrum shows four main absorption bands. The differential thermal

analysis (DTA) reveals three principal peaks at 90, 3500C and at Curie temperature,

respectively. The thermoelectric power coefficient has negative sign indicating that the

majority charge carriers were electrons. The DC electrical conducting increases with

increasing temperature ensuring the semiconducting nature of the samples. The Curie

54

temperature determined from DC electrical conductivity was found in satisfactory

agreement with that determined from initial magnetic permeability measurements.

Arulmurugan et al [112] prepared Co1-xZnxFe2O4 and Mn1-xZnxFe2O4 (x = 0.12-

0.5) nanoparticles less than 12nm by chemical co-precipitation method which could be

used for ferrofluidpreparation. The saturation magnetization of the Co–Zn substituted

ferrite nanoparticles decreased continuously with the increase in Zn concentration,

whereas for the Mn–Zn substituted ferrite nanoparticle the saturation magnetization was

maximum for x=0.2 and decreased on further increase in Zn concentration. The particle

size decreased with the increase in the Zn concentration for both Co–Zn and Mn–Zn

ferrites. The estimation of associated water content, which increased with the Zn

concentration, played a vital role for the correct determination of cation contents The

Curie temperature and the temperature at which maximum value of thermomagnetic

coefficient was observed simultaneously decreased with the increase in the initial

substitution degree of Zinc.

Khedr et al [113] used various preparation techniques to produce Cobalt ferrite

nanoparticles namely ball milling, co-precipitation and ceramic method. Thermal analysis

(TGA and DTA), X-ray diffraction, SEM, TEM, magnetic and surface area

measurements have been used for characterization of the prepared samples. Results

showed that saturation magnetic flux density (Bs) and remnant magnetic flux density (Br)

varied with crystallite size from 6.929-14.91×10-3 and 2.73-8.146×10-3 Am2/ kg

respectively. The measured surface area (SBET) for the prepared Co-ferrite particles

ranged from 5.3 - 47×103 m2/ kg. Nanocrystalline CoFe2O4 showed a catalytic activity

towards CO2 decomposition with the formation of carbon nanotubes.

55

Tahar et al [114] prepared pure nanoparticles of the CoFe2-xRExO4 (RE = Gd, Sm;

x = 0.0, 0.1) system by forced hydrolysis in polyol. X-ray diffraction (XRD) evidences a

cell size increase with slight distortions in the spinel-like lattice indicating the entrance of

RE3+ ions. Micro-Raman spectroscopy confirms the cubic inverse spinel structure and

rules out the existence of impurities like hematite. Magnetic measurements (SQUID)

show important differences in the magnetic properties of the unsubstituted and

substituted particles. All the particles are superparamagnetic at room temperature and

ferrimagnetic at low temperature. However, their main magnetic characteristics appear to

be directly dependent on the RE content.

Lavela and Tirado [115] have prepared CoFe2O4 and NiFe2O4 by a Sol–gel

process based on a vacuum sublimation of a Citrate precursor. Several samples of

CoFe2O4 were obtained by varying the conditions of Citrate precursor formation and

further annealing. The formation of layered flake-like aggregates defining a macroporous

system is assumed to improve the electrolyte–electrode contact in iron containing

samples. An enhanced electrochemical performance was achieved for samples annealed

at high temperatures; especially for CoFe2O4 heated at 10000C for 24h. 57Fe Mossbauer

spectroscopy was used to clarify aspects of the mechanism of the electrochemical

reaction.

S. J. Lee et. al [116] have investigated the magnetic and magnetoelastic properties

of a series of Cr-substituted cobalt ferrite CoCrxFe2−xO4, x=0.0–0.79 samples prepared by

standard powder ceramic techniques. Substitution of Cr for some of the Fe in Cobalt

ferrite reduced the Curie temperature, and the effect is more pronounced than that

observed in Mn substituted cobalt ferrite samples. Cr substitution also caused the

56

maximum magnetostriction to decrease at a greater rate than substitution of the same

amount of Mn. The maximum of the strain derivative was reached for x=0.38. They

found that small amounts of substituent, both Cr substituted and Mn-substituted Cobalt

ferrites have maximum strain derivatives greater than that of pure Cobalt ferrite, and

therefore show promise for magnetoelastic sensing applications.

Co1-xNixFe2O4 ferrites with (x = 0.0, 0.1, 0.2, 0.3, 0.4, 0.5), were prepared by Gul

et al. [117] by chemical co-precipitation method. The grain size was in the range of 14 to

21nm. The lattice constant, sintered density, X-ray density, porosity, specific surface

area, susceptibility, Curie temperature and correlation coefficient were also investigated.

It was found that resistivity increased from 1.981x106(Ω-cm) to 8.323x106(Ω-cm) at

temperature of 393K with the increase in zinc concentration from 0.0 to 0.5. Dielectric

constant and dielectric loss factor both decreased with the increase in frequency.

Single-phase well-crystallized ferrite nanoparticles of CoFe2−xAlxO4 (for x = 0.00,

0.25, 0.50) were successfully synthesized using the sol–gel route by I.H. Gul, A.

Maqsood. [118] they found that the crystallite size and the lattice parameter “a” also

decreased with increase in Aluminum concentration. The observed decrease of Curie

temperature from 667 to 528K with increase in the substitution of non-magnetic Al3+

concentration has been explained by the A–B exchange interaction strength due to the

change of Fe3+ distribution between A and B sites. They observed a decrease in DC

electrical resistivity with increase in temperature with which they ensured the

semiconductor like behavior of the samples. Activation energy calculated from the DC

electrical resistivity versus temperature for all the samples increased with the increase of

Al3+ concentration. The dielectric constant decreased with increasing frequency for all the

57

prepared Cobalt ferrite nanoparticle compositions. They reported that the values of ε',

tanδ and ε" decreased from 11 to 5, 0.41 to 0.24 and 5.27 to 1.21 respectively at 1 MHz.

They concluded that lower dielectric constants obtained for the ferrites warrant their

application at high frequencies as microwave absorbers.

Ruiting MA etal [119] has synthesized Nanocrystalline Co0.5 Zn0.5Fe2O4 ferrite by

polyacrylamide gel method. They have investigated the electromagnetic and microwave

absorption properties of the ferrite. The results indicated that calcining temperature of the

ferrite had a significant influence on the effective properties of the ferrite. When the

calcining temperature was 500, 600 and 7000 C, the average size of particles was 10, 30

and 80 nm, respectively. The dielectric loss, magnetic loss and the reflection loss of the

Co0.5 Zn0.5Fe2O4 obviously increased with increasing the calcining temperature. With

these results they proposed that the prepared composites can fruitfully be utilized for

suppression of electromagnetic interference and reduction of radar signatures.

Nanoparticles of Mn, Co, and Ni ferrites with average crystal sizes between 6 and

9 nm dispersed in a highly porous aerogel silica matrix were prepared by D. Carta et al,

[120] which has resulted to have the spinel structure with an increasing degree of

inversion. They found that the degree of inversion in nanosized ferrites was increasing

from 0.20 for MnFe2O4 to 0.68 for CoFe2O4 and to 1.00 for NiFe2O4which were in

accordance with the values commonly found in the corresponding bulk spinels. From this

result they have reported that particle size does not influence significantly the degree of

inversion. Magnetic characterization further supported these findings.

A T Raghavender and K M Jadhav [121] have reported the effect of Al-

substitution on structural and dielectric properties of Cobalt ferrites by synthesizing

58

CoFe2–xAlxO4 (0 ≤ x ≤ 1) via sol–gel method. X-ray diffraction analysis revealed the

nanocrystalline nature in the prepared ferrite samples. The particle size (D) decreases

with increase in Al-content. The lattice parameter (a) and X-ray density (dx), decreased

with increase in Al-content. It was found that the sol–gel prepared samples exhibit lower

values of the dielectric constant and losses as compared to those obtained in ferrites

prepared by the conventional ceramic method. They reported that Dielectric constant and

loss decreases rapidly with increasing frequency, and then reaches a constant value

Sonal Singhal et al [122] have synthesized Zinc substituted cobalt ferrite

nanoparticles (CoxZn1-xFe2O4, with x = 0.0, 0.2, 0.4, 0.8 and 1.0) via sol-gel route and the

effect of zinc concentration on saturation magnetization and lattice parameter were

investigated. The particle sizes of the as obtained samples were found to be ~10 nm

which increases up to ~92 nm on annealing at 10000C. The frequency bands near 564-

588 cm-1 and 425-442 cm-1 are assigned to the tetrahedral and octahedral clusters which

confirm the presence of M-O stretching band in ferrites. The unit cell parameter ‘a’

increases linearly with increasing concentration of Zinc due to larger ionic radii of Zn2+

ion. It was found that this substitution allows tunable changes in the magnetic properties

of Cobalt ferrite. Interestingly, saturation magnetization first increases up to x = 0.4 and

then decreases for higher Zn substitution, thus tunable changes in magnetic properties of

cobalt ferrite are possible. Source of such behavior could be the variation of exchange

interaction between the tetrahedral and the octahedral sites.

J.Balavijayalakshi et al[123] studied the effect of concentration on dielectric

properties of Co-Cu ferrite nano particle.

59

M.A. Amer et al[124] studied the structural and physical properties of Nano-

crystalline Al substituted Cr-Cu ferrite.

M.Eshraghi et al[125] studied Structural and magnetic characterizations of Cd

substituted nickel ferrite nanoparticles.

I.H.Gu et al[126] studied the High frequency AC response, DC resistivity and

magnetic studies of holmium substituted Ni-ferrite: A novel electromagnetic material.

A.B.Salunkhe et al[127] Studied the Low temperature combustion synthesis and

magneto structural Properties of Co–Mn nano ferrites.

A.M. Escamilla-Pérez et al[128] studied the Crystal structure of

superparamagnetic Mg0.2Ca0.8Fe2O4 nanoparticles synthesized by sol-gel method.

In recent years, nano materials have emerged as a rapidly advancing field,

providing vast avenues for research. Rapid growth in the demand of materials for

telecommunication and high-frequency magnetic devices encourages the production of

materials with significant improvements in their performance with lower fabrication

costs. The literature of the last 10-15 years reveals that spinel magnesium ferrites and

Cobalt ferrites of different compositions have been extensively studied and used in

technological products. However, the search for a better product with lowest energy

consumption and optimum performance is still going on. The various properties of spinel

Mg-ferrite systems have been investigated by doping with single metal e.g. Cr, Al, Gd,

Cu as well as co-substitution of two different metals e.g. Cu-Zn, Gd-Co, Nd-Co and Zr-M

(M = Mn and Zn). The electrical and magnetic properties were found convincing for

various electromagnetic applications. On the basis of literature survey carried out for

Magnesium spinel ferrite and Cobalt spinel ferrite it is observed that the efforts of the

60

researchers have been mainly concentrated on the synthesis of single crystals, bulk

materials, thin films and nanosized structures by using a variety of well known methods

including the ceramic method, co-precipitation method, double sintering method, solid

state method, micro emulsion method etc. The physical, magnetic and electrical

properties of ferrite materials depend upon the synthesis routes, chemical composition,

annealing temperature and distribution of metal ions at A- and B-sites. The conventional

ceramic method of preparation involves the mixing of suitable metallic oxides with

appropriate grinding followed by a solid-state reaction at high annealing temperatures of

1573-1973 K. Though the route is quite simple yet it has several drawbacks, such as; high

sintering temperature, large reaction time, large particle size and limited degree of

homogeneity. To correlate size effects with changes in magnetic and electric properties, it

is essential to select a synthesis method which allows controlling the size of nanoparticles

with a narrow size distribution. However, chemical route like sol-gel method is preferable

as it offers many advantages over other conventional methods such as low temperature

processing, better homogeneity, production of ultra-fine particles with narrow size

distribution, short processing time, low sintering temperature, etc. It is also easier and

cost effective than the other chemical methods. Under the Sol-gel method Citrate-gel

method has been known to successfully prepare nanocrystalline spinel ferrites with the

particle sizes less than 30 nm.

61

1.15. References:

[1] P.G Hewitt, Conceptual physics, 7th edition, Harper Collins college publishers,

Newyork, 1993.

[2] B. D. Cullity Introduction to magnetic materials, Addision Wesley publishing

company, 1972.

[3] K. J. Stalledly Oxide magnetic materials,2nd edition (1972)

[4] J. D. Jackson, Classical electrodynamics, 3rd John wisely & son, Inc, Newyork,

1998.

[5] D. Halliday, R. Resnick, J. Walker, fundamentals of physics, 6th edition John

Wiley & sons, Newyork 2002.

[6] M.S. Vijaya, G. Rangarajan Material science, MC Graw-Hill publishing company

limited, New Delhi, 1999-2000, P.447

[7] C. Kittel Introduction to solid state physics, 7th edition, John Wiley & sons.

[8] M.A Omar, Elementary solid state physics (principles & applications).

[9] J. R. Reitz, F. J. Milford, R.W.Chrisly, Foundation of electromagnetic theiry,3rd

edition, Addison –Wesley publishing company , London 1979.

[10] M.A. Omar, Elementary solid state physics (principles & applications) Addison-

Wesley publishers Company, Amsterdam, 1962.

[11] A.H. Morish, The physical plrinciples of magnetism, John Wiley and sons,1965.

[12] V.Kato, T. Takei, J.Inst.Elect.Engrs.53(1933).408

[13] M.Kawai, J,Soc.Che.Ind.Jpn.37,(1934),392

[14] J.L. Snock, physics 3, (1936)463

[15] L.Neal Ann.phys(paris) 3, 1948,137.

[16] J.L.Snock “non-metalic magnetic materials for high frequency”,

Philips.Tech.Rev.,8(Dec 1946)353.

[17] D.Polder “on the theory of ferromagnetic resonance” phil.mag., 40(1949)99.

[18] C.L.Hogan , “The microwave gyrator” J.Bell system tech., 31(1952).

[19] K.J.Standely, “Oxide magnetic materials” 2nd edi., Oxford university press,

(1972)

[20] F.S.li, L.Wang, J.B.Wang, Q.G.Zhou, H.P.Kunkel, G.Williams, J.Magn, Magn-

mater.268 (2004)332

62

[21] L. Neel, magnetic properties of ferrites, Ferrimagnetism and antiferromagnetism,

Ann.phys.paris, 3(1948)137

[22] G.Mumcu, K. Sertel, J.L.Volkais, A.Figotin, I-vitebsky, R.F propagation in

ferrite thickness non reciprocal magnetic photonic crystals, IEEE Antenna

propagant.soc.symi.,2(2004)1395

[23] Cullity BD Introducting to magnetic materials 1972

[24] N.Spaldin, magnetic materials, fundamental and device applications, Cambridge:

Cambridge university press,2003.

[25] J.D.Livingston, “A review of coercivity mechanism”, Appl.phys(1981),522-541.

[26] Mistho sugimoto J.Am.Ceram.soc [2] 269-80 (1999)

[27] Alex goldman Springer Modern ferrite technology second edition.

[28] Chandana rath, S.Anand and R.P.Das J.Appl,phys 91 2002 4

[29] Aralmuragan R, Jeyadevan B, Vaidyanath G and Sendhilnathan S, J.Magn.

Magn.mater., 288 2005 470

[30] Yimin Xuan Quiang Li Gang Yang J.Magn.Magn.mater 312, 2007 464-469

[31] Mahmoud MH, Hamdeh HH, HOJC, O’Shea MJ &Walker JC, J Magn.Mater.,220

2000 139

[32] Mostafia A, El.shibaky GA& E.Girgis, J.Phys.D; Appl phys 39 2006 2007

[33] Herger G, Vaznez M, Knobel M, Zhokov A, Reininger Tm Davies HA Grossinger

R & anchez Li JL, J Magn.magn,mater.,294(2005)252

[34] Grimes NW, physics in technology, January 1975

[35] Vandenberghe R.E, Vandenberghe R, Degrave E & Robbrecht G, J Magn Magn

Mater, 117 1980 15.

[36] Durushotham yadoji, Ramesh peelamedum , Dinesh Agrawal and Rustum Roy,

Mater.sci.Eng ‘B’. 98 2003 269-278

[37] R.V. Mangalaraja, S.Anantha kumar, P. Manohar, F.D.Gnanam and M.Awano,

Mater.sci.Eng ‘A’ 3672004 301-305

[38] R.V.Mangalaraja, S.Ananth kumar, P. Manohar and F.D.Gnanam Mater.sci.eng

‘A’.335 2003 320-324.

[39] D.S.Mathew,R.S.Juang,Chem.Eng.J.129(2007)51–65.

[40] Modi. K.B., Joshi. H.H., Kulkarni. R.G., J. Mater. Sci. 31 (1996) 1311.

63

[41] Chen. Q., Zhang. Z., J. Appl. Phys. Lett. 73 (1998) 3156.

[42] Maensiri. S., Sangmanee. M., Wiengmoon. A., Nanoscale Res. Lett. 4 (2009) 221.

[43] Hankare. P.P., Vader. V.T., Patil. N.M., Jadhav. S.D., Sankpal. U.B., Kadam.

M.R., Chougule. B.K., Gajbhiye. N.S., Mater. Chem. Phys. 113 (2009) 233.

[44] Chen. C., Magnetism and Metallurgy of Soft Magnetic Materials, Dover Pubs.

(1986) 26.

[45] Muzquiz-Ramos. EM, Cortes-Hernandez. DA, Herrera-Romero. OA, Escobedo-

Bocardo. JC, Mater Sci Forum 644 (2010) 39.

[46] Chen. Q., Rondinone. A.J., Chakoumakos. B.C., Zhang. Z.J., J. Magn.

Magn.Mater. 194 (1999) 1.

[47] Ravinder. D., Latha. K., Mater. Lett. 41(1999) 247.

[48] Liu. C., Zou. B., Rondinone. A.J., Zhang. Z.J., J. Am. Chem. Soc. 122 (2000)

6263.

[49] Sepelak V., Schultze. D., Krumeich. F., Steinike. U., Becker. K.D., Solid State

Ionics 14 (2001) 677.

[50] Rana. M.U., Abbas. T., Mater. Lett. 57 (2002) 925.

[51] Qi, X., Zhou. J., Yue. Z., Gui. Z., Li. L., J. Magn. Magn. Mater. 251 (2002) 316.

[52] Sepelak. V., Baabe. D., Mienert. D., Litterst. F.J., Becke. K.D., Scr. Mater. 48

(2003) 961.

[53] Radwan. F.A., Ahmed. M.A., Abdelatif. G., J. Phys. Chem. Solids 64 (2003)

2465.

[54] Rabanal. M.E., Várez. A., Levenfeld. B., Torralba., J.M. J. Mater.

Process.Technol. 143 (2003) 470.

[55] Chauhan. B.S., Kumar. R., Jadhav. K.M., Singh. M., J. Magn. Magn.Mater. 283

(2004) 71.

[56] Chhaya. S.D., Pandya. M.P., Chhantbar. M.C., Modi. K.B., Baldha. G.J., Jo.

H.H., J. Alloys Compd. 377 (2004) 155.

[57] Turkin. A.I., Drebushchak. V.A., J. Cryst. Growth, 265 (2004) 165.

[58] Verma. P.A., Khollam. Y.B., Potdar, H.S., Deshpande. S.B., Mater.Lett. 58

(2004) 1092.

64

[59] Thummer. K.P., Chhantbar. M.C., Modi. K.B., Baldha. G.J., Joshi. H.H.,

Mater.Lett. 58 (2004) 2248.

[60] A. Pradeep, C. Thangasamy, G. Chandrasekaran, J. of Matr., Sci: Matrs. In Elec.

15 (2004) 797.

[61] Liu. Y., Liu. Z., Yang. Y., Yang. H., Shen. G., Yu. R., Sens. Actuators, B 107

(2005) 600.

[62] Pradeep. A., Chandrasekaran. G., Mater. Lett. 60 (2006) 371.

[63] Lakshman. A., Rao. P.S.V.S., Rao. B.P., Rao. K.H., J. Phys. D: Appl. Phys. 38

(2005) 673.

[64] A. A. Pandit, A. R. Shitre, D. R. Shengule, K. M. Jadhav, J. Mater. Sci. 40 (2005)

423.

[65] Masti. S.A., Sharma. A.K., Vasambekar. P.N., Vaingankar. A.S., J. Magn.

Magn.Mater. 305 (2006) 436.

[66] Bergmann. I., Šepelák. V., Becker. K.D., Solid State Ionics 177 (2006) 1865.

[67] Y. Atassi, M. Tally, J. of Iranian Chemical Society 3 (2006) 242.

[68] Lakshman. A., Rao. P.S.V.S., Rao. K.H., Mater. Lett. 60 (2006) 7.

[69] Kong. L.B., Li. Z.W., Lin. G.Q., Gan. Y.B., J. Am. Ceram. Soc. 90 (2007) 3106.

[70] Šepelák. V., Bergmann. I., Menzel. D., Feldhoff. A., Heitjan. P., Litterst.

F.J.,Becker. K.D., J. Magn. Magn. Mater. 316 (2007) e764.

[71] Ichiyanagi. Y., Kubota. M., Moritake. S., Kanazawa. Y., Yamada. T., Uehashi.T.,

J.Magn. Magn. Mater. 310 (2007) 2378.

[72] Cui-Ping Liu ,Ming-Wei Li , Zhong Cui, Juan-Ru Huang ,Yi-Ling Tian , Tong

Lin , Wen-Bo Mi., J Mater Sci 42 (2007) 6133.

[73] Vital. A., Angermann. A., Dittmann. R., Graule. T., Topfer, J. Acta Mater. 55

(2007) 1955.

[74] Sharma. S.K., Kumar. R., Kumar. S., Kumar. V.V.S., Knobel. M., Reddy.

V.R.,Banerjee. A., Singh. M. Solid State Commun. 141 (2007) 203.

[75] Barati. M.R., J. Alloys Compd. 478 (2009) 375.

[76] Gadkari. A.B., Shinde. T.J., Vasambekar. P.N., Mater. Charact. 60 (2009) 1328.

[77] Chand. J., Singh. M., J. Alloys Compd. 486 (2009) 376.

65

[78] Sasaki. T., Ohara. S., Naka. T., Vejpravova. J., Sechovsky. V., Umetsu.

M.,Takami. S., Jeyadevan. B., Adschiri. T., J. Supercrit. Fluids 53 (2010) 92.

[79] Farghali. A.A., Moussa. M., Khedr. M.H., J. Alloys Compd. 499 (2010) 98.

[80] Kumar. G., Chand. J., Dogra. A., Kotnala. R.K., Singh. M., J. Phys. Chem. Solids

(2010) 71 375.

[81] Ghatak. S., Sinha. M., Meikap. A.K., Pradhan. S.K., Mater. Res. Bull. 45(2010)

954.

[82] Bharti. D.C., Mukherjee. K., Majumder. S.B., Mater. Chem. Phys. 120 (2010)

509.

[83] Dalt. S.D., Takimi. A.S., Volkmer. T.M., Sousa. V.C., Bergmann. C.P., Powder

Technol. 210 (2011) 103.

[84] Sadhana. K., Praveena. K., Murthy. S.R., J. Magn. Magn. Mater. 323 (2011)

2977.

[85] Mansour. S.F., J. Magn. Magn. Mater. 323 (2011) 1735.

[86] Shah. J., Arora. M., Purohit. L.P., Kotnala. R.K., Sens. Actuators A 167 (2011)

332.

[87] Patil. J.Y., Khandekar. M.S., Mulla. I.S., Suryavanshi. S.S., Curr. Appl. Phys.12

(2012) 319.

[88] Mounkachi. O., Hamedoun. M., Belaiche. M., Benyoussef. A., Masrour. R.,

ElMoussaoui. H., Sajieddine. M.,Physica B 407 (2012) 27.

[89] Singh. N., Agarwal. A., Sanghi. S., Khasa. S., J. Magn. Magn. Mater. 324 (2012)

2506.

[90] Sujatha. C., Reddy. K.V., Babu. K.S., Reddy. A.R.C., Rao. K.H. Physica B 407

(2012) 1232.

[91] Chen. D., Zhang. Y., Tu, C., Mater. Lett. 82 (2012) 10.

[92] Bobade. D.H., Rathod. S.M., Mane. M.L., Physica B 407 (2012) 3700.

[93] S.M.Rathod, S.V. Gaikwad, S.S.Jagtap, International Journal of Engineering

Research & Technology (IJERT) 1(2012).

[94] Lee. D. H., Kim. H. S., Yo. C. H., Ahn. K., Kim. K. H., Mater. Chem. Phys 57

(1998) 169.

66

[95] Vasamber. P. N., Kolekar. C. B., Vaingankar. A. S., J. Mater. Sci: Mater. Electr.

10 (1999) 667.

[96] Liu. C., Zou. B., Rondinone. A. J., Zhang. Z., J. Pure Appl. Chem. 72 (2000) 37.

[97] Li. S., John.V. T., OConnor. C., Harris. V.,Carpenter.E., J. Appl. Phys 87

(2000) 6224.

[98] Kahn. M. L., Zhang. Z. J., Appl.Phys. Lett. 78 (2001) 3651.

[99] Chae. K. P., Lee. J. G., Kim. W. K., Lee. Y. B. J., Magn. Magn. Mater. 248

(2002) 236.

[100] Yamamoto. H., Nissato. Y., IEEE Tran.Mag. 38 (2002) 3488.

[101] Mahajan. R. P., Patankar. K K, Kothale. M. B, Chaudhari. S. C., Mathe. V. L.

Patil, S. A. Parmana J. Phys. 58 (2002) 1115.

[102] V.K. Sankaranarayanana, Om Prakasha, R.P. Panta, Mohammad Islamb, J. Magn.

Magn. Mater 252 (2002) 7.

[103] D. Ravinder , P. Shalini , Appl. Phys. Lett. 82 (2003) 4738.

[104] Panda. R. N., Shih. J. C., Chin. T. S., J. Magn. Magn. Mater. 257 (2003) 79

[105] Li. X., Kutal. C. J. Alloy. Comp. 349 (2003)264

[106] Choi. E. J., Ahn. Y., Kim. S., An. D. H., Kang. K. U., Lee. B. G., Baek. K. S.,

Oak. H. N., J. Magn. Magn. Mater. 262 (2003) L198.

[107] Vestal. C. R., Zhang. Z., J. Nano Lett. 3 (2003) 1739.

[108] Gabal.M. A., Ata-Allah. S. S., Mater. Chem. Phys. 85 (2004) 104.

[109] Yang . X, Wang. X., Zhang. Z, J. Cryst. Growth 277 (2005) 467.

[110] M. M. Bahout, S. Bertrand, O. Peña, J. Solid State Chem. 178 (2005) 1080.

[111] Abo El Ata. A. M., Attia. S. M., El Kony. D., Al-Hammadi. A. H. J. Magn.

Magn.Mater. 295 (2005) 28.

[112] Arulmurugan, R., Jeyadevan, B.; Vaidyanathan, G.; Sendhilnathan, S. J.

Magn.Magn. Mater. 288 (2005) 470.

[113] Khedr. M. H., Omar. A. A., Abdel-Moaty. S. A., Colloid Surface A:

Physicochem. Eng. Aspects 281(2006) 8.

[114] Tahar. L. B., Smiri. L. S., Artus. M., Joudrier. A. -L., Herbst. F. Vaulay. M. J.,

Ammar. S., Fievet, F. Mater. Res. Bull. 42 (2007)1888.

[115] Lavela, P.; Tirado, J. L., J. Power Sour. 172 (2007) 379.

67

[116] S. J. Lee, and C. C. H. Lo, P. N. Matlage and S. H. Song, Y. Melikhov, J. E.

Snyder, and D. C. Jiles, j. app. Phys.102 (2007) 073910.

[117] I. H. Gul, F. Amin, A. Z. Abbasi, M. Anis-ur-Rehman, A. Maqsood, Scrip. Mater.

56 (2007) 497.

[118] I.H. Gul, A. Maqsood, J. Alloys and Compounds 465 (2008) 227.

[119] Ruiting MA, Yanwen TIAN, Haitao ZHAO, Gang ZHANG and Hui ZHAO, J.

Mater. Sci. Technol., 24 (2008) 4.

[120] D. Carta, M. F. Casula, A. Falqui,D. Loche, G. Mountjoy, C. Sangregorio, and A.

Corrias, J. Phys. Chem. C, 113 (2009) 8606.

[121] A T Raghavender, K M Jadhav , Bull. Mater. Sci., 32 (2009) 575.

[122] Sonal Singhal, Tsering Namgyal, Sandeep Bansal, Kailash Chandra, J.

Electromagnetic Analysis & Applications, 2 (2010) 376.

[123] J.Balavijayalakshmi,N.suriyanarayanan,Jayaprakash,V.Gopalkrishnan,Physics

procedia 49(2013)49-57.

[124] M.A. Amer,T.M.Meaz,A.G. Mostafa, H.F. El-Ghazally J. Magn. Magn. Mater

343(2013)286-292.

[125] M.Rahimi,etal., Structural and magnetic characterizations of Cd substituted nickel

ferrite nano particles, Ceramics International (2014), http://dx.doi.org/10.1016/

j.ceramint.2014.07.033.

[126] E.PervaizI.H.Gu Journal of Magnetismand Magnetic Materials349 (2014)27–34

[127] A.B.Salunkhe, V.M.Khot, M.R.Phadatare, N.D.Thorata, R.S.Joshi b,

H.MYadavand S.H.Pawar Journal of Magnetis mand Magnetic

Materials352(2014)91–98

[128] A.M. Escamilla-Pérez, D.A. Cortés-Hernández, J.M. Almanza - Robles, D.

Mantovani, P. Chevallier Journal of Magnetism and Magnetic Materials.