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Magnetic Materials

   Posted On :  03.12.2016 06:56 pm

1 Introduction 1.1 Basic Definitions 2 Origin of Magnetic Moments 3 Classification of Magnetic Materials 3.1 Diamagnetic materials 3.2 Paramagnetic Materials 3.3 Ferromagnetic materials 3.4 Dia, Para and Ferro magnetic materials – Comparison 4 Domain Theory of Ferromagnetism 4.1 Energies involved in the domain growth (or) Origin of Domain theory of Ferromagnetism 5 Antiferromagnetic Materials 6 Ferrimagnetic Materials 7 Hysteresis 7.1 Explanation of hysteresis on the basis of Domains 8 Hard and Soft Magentic Material 8.1 Hard Magnetic Materials 8.2 Soft Magnetic Materials 8.3 Difference between Hard and Soft magnetic materials 9 Ferrites 9.1 Properties 9.2 Structures of Ferrites 9.3 Regular spinal 9.4 Inverse spinal 9.5 Types of interaction present in the ferrites 9.6 Properties of ferromagnetic materials 9.7 Application of Ferrites 10 Magnetic Recording and Readout Memory 10.1 Magnetic parameters for Recording 10.2 Storage of Magnetic Data 10.3 Magnetic Tape 10.4 Magnetic Disc Drivers 10.5 Floppy Disk 10.6 Magnetic bubble Materials

Magnetic Materials

 

1 Introduction

1.1 Basic Definitions

2 Origin of Magnetic Moments

3 Classification of Magnetic Materials

3.1 Diamagnetic materials

3.2 Paramagnetic Materials

3.3 Ferromagnetic materials

3.4 Dia, Para and Ferro magnetic materials – Comparison

4 Domain Theory of Ferromagnetism

4.1 Energies involved in the domain growth (or) Origin of Domain theory of Ferromagnetism

5 Antiferromagnetic Materials

6 Ferrimagnetic Materials

7 Hysteresis

7.1 Explanation of hysteresis on the basis of Domains

8 Hard and Soft Magentic Material

8.1 Hard Magnetic Materials

8.2 Soft Magnetic Materials

8.3 Difference between Hard and Soft magnetic materials

9 Ferrites

9.1 Properties

9.2 Structures of Ferrites

9.3 Regular spinal

9.4 Inverse spinal

9.5 Types of interaction present in the ferrites

9.6 Properties of ferromagnetic materials

9.7 Application of Ferrites

10 Magnetic Recording and Readout Memory

10.1 Magnetic parameters for Recording

10.2 Storage of Magnetic Data

10.3 Magnetic Tape

10.4 Magnetic Disc Drivers

10.5 Floppy Disk

10.6 Magnetic bubble Materials

 

1  INTRODUCTION

 

The materials which can be made to behave like a magnet and which are easily magnetic field called as a magnetic materials.

 

1.1  Basic Definitions

 

1. Magnetic Dipole Moment (M)

 

The dipole moment is defined as the product of magnetic pole strength and length of the magnet. It is given by M = ml. Amp m2.

 

2. Magnetic Field

 

The space around which the magnetic lines of forces exist is called as magnetic field. Magnetic field is produced by permanent magnets such as a horse shoe magnet and temporarily by elelctromagnets (or) superconducting magnets.

 

3. Magnetic Lines of Force

 

The continuous curve in a magnetic field that exists from north pole to south pole is called as magnetic lines of force.


Fig. 3.1 Magnetic lines of force

 

4. Magnetic Induction (B) (or) Magnetic Flux Density

 

It is the number of magnetic lines of force passing through unit area of cross section.


 

5. Magnetic Field Strength (or) Magnetizing Field (H)

 

It is the force experienced by a unit north pole placed at a given point in a magnetic field. The magnetic induction B due to the magnetic field of intensity H applied

in vacuum is related by B =μ 0H Amp m–1.

 

6. Magnetic Flux (ϕ )

 

The total number of magnetic lines of force passing through a surface.

 

Unit : Weber.

 

7. Intensity of Magnetization (I)

 

Magnetization is the process of converting a non-magnetic material in to a magnetic material.

 

It is also defined as the magnetic moment per unit volume. I = m / V web m–2.

 

8. Magnetic Permeability (μ )

 

It is ratio of the magnetic induction (B) to the applied magnetic field intensity (H). μ= B / H

 

Unit: Henry m–1

 

It is the measure of ability of the material to permit magnetic lines of force.

9. Relative Permeability ( μr)

 

It is defined as the ratio of permeability of the medium to the permeability of the free space


 

10. Magnetic Susceptibility χ

 

It is defined as the ratio of intensity of magnetization (I) and intensity of magnetic field (H). χ= I / H.

 

The sign and magnitude of χ are used to determine the nature of the magnetic materials.

 

11. Bohr Magnetron ( μB)

 

The orbital magnetic moment and the spin magnetic moment of an electron in an atom can be expressed in terms of atomic unit of magnetic moment called as Bohr magnetron.

 

 

 

12. Relation between susceptibility (χ ) and Relative permeability μr)

 

We know that when a current is supplied through a coil, magnetic field is developed. When a magnetic material is placed inside a external magnetic field, the magnetic flux density (B) arises due to applied magnetic field (H) and also due to the induced magnetization (I).

 

i.e., the total flux density,


13. Retentivity or Remanence

 

When an external field is applied to the specimen it is magnetized and when the field is removed it is demagnetized. But some materials do not completely demagnetize when field is removed. There is some magnetism left out in the specimen. “This residual magnetism which is left out even after the removal of the external magnetic field” is called as the Retentivity or Remanence.

 

14. Coercivity

 

The residual magnetism can be removed completely from the material by applying a reverse magnetic field. “The reverse magnetic field which is used to completely remove the residual magnetism” is called as the coercivity.

 

 

2  ORIGIN OF MAGNETIC MOMENTS

 

The macroscopic magnetic properties of a substance are a consequence of magnetic moments associated with individual electrons. Each electron in an atom has magnetic moments that originate from the following two sources

 

Orbital magnetic moment of electrons

 

Spin magnetic moment of electrons.

 

Magnetic moments associated with an orbiting electron and a spinning electron is shown in Fig.3.3 (a and b).


 

We know that the electrons in an atom revolve around the nucleus in different orbits. Basically, there are three contributions for the magnetic dipole moment of an atom.

 

The orbital motion of electrons (the motion of electrons in the closed orbits around the nucleus). It is called as orbital magnetic moment. Its magnitude is always small. Spin motion of the electrons (i.e. due to electron spin angular momentum) and it is called as spin magnetic moment.

 

The contribution from the nuclear spin (i.e., due to nuclear spin angular momentum). Since this is nearly 103 times smaller than that of electron spin, it is not taken into consideration.

 

For all practical purposes, we assume that the magnetic moment arises due to the electron spin ignoring the orbital magnetic moments and the nuclear magnetic moments as their magnitudes are small.

 

We may note that permanent magnetic moments can also arise from spin magnetic moments of the nucleus. Of all the three, the spin dipole moments of electrons are important in most magnetic materials.

 

1. Orbital angular momentum

 

This corresponds to permanent magnetic dipole moments. Let us consider an electron describing a circular orbit of radius ‘r’ with a stationary nucleus at the centre as shown in Fig 3.3.(a). Let the electron rotate with a constant angular velocity of ‘w’ radians per second.

 

Electron revolving in any orbit may be considered as current carrying circular coil producing magnetic field perpendicular to its plane. Thus the electronic orbits are associated with a magnetic moment. The orbital magnetic moment of an electron in an atom can be expressed in terms of atomic unit of magnetic moment called Bohr Magnetron, defined as


 

2. Electron spin magnetic moment

 

The concept of the electron having an angular momentum has been introduced in order to explain the details of atomic spectra. This angular momentum of the electron is referred to as the spin of the electron. Since the e- has a charge, its spin produces a magnetic dipole moment.

 

According to quantum theory, the spin angular momentum along a given direction is


In a many electron atom, the individual spin magnetic moments are added in accordance with certain rules. Completely filled shells contribute nothing to the resultant spin moment.

 

3. Nuclear magnetic moment

 

The angular momentum associated with the nuclear spin is also measured in units of h/2π . The mass of the nucleus is larger than that of an e- by the order of 103. Hence nuclear spin magnetic moment is of the order of 10–3 Bohr magnetrons.

 

3  CLASSIFICATION OF MAGNETIC MATERIALS

 

The magnetic materials are classified into two categories:

 

          The materials without permanent magnetic moment Example: 1. Diamagnetic materials.

 

          The materials with permanent magnetic moment. Example: 1. Paramagnetic materials

 

Ferromagnetic materials

 

Anti-Ferromagnetic materials

 

Ferrimagnetic materials.

 

3.1  Diamagnetic materials

 

Definition

In a diamagnetic material the electron orbits are randomly oriented and the orbital magnetic moments get cancelled. Similarly, all the spin moments are paired i.e., having even number of electrons. Therefore, the electrons are spinning in two opposite directions and hence the net magnetic moment is zero.

 

Effect of magnetic field

 

When an external magnetic field is applied, the electrons re-orient and align perpendicular to the applied field, i.e., their magnetic moment opposes the external magnetic field.

 

 

Fig. 3.4 Effect of magnetic field in Diamagnetic material

 

In the above diagram, there is no penetration of magnetic lines through the diamagnetic material.

 

Properties

 

They repel the magnetic lines of force, if placed in a magnetic field as shown in figure (3.4).

 

The susceptibility is negative and it is independent to temperature and applied field strength. (X = –ve)

 

The permeability is less than one

 

There is no permanent dipole moment.

 

When the temperature is greater than the critical temperature diamagnetic becomes normal material.

 

It has superconducting property.

 

Examples : Gold, germanium, silicon, antimony, bismuth, silver, lead, copper, hydrogen, Water and alcohol.

 

3.2 Paramagnetic Materials

 

Definition

Para magnetism is due to the presence of few unpaired electrons which gives rise to the spin magnetic moment. In the absence of external magnetic field, the magnetic moments (dipoles) are randomly oriented and possess very less magnetization in it.

 

Fig.3.5 Paramagnetic Material

 

Effect of magnetic field

 

When an external magnetic field is applied to paramagnetic material, the magnetic moments align themselves along the field direction and the material is said to be magnetized. This effect is known as paramagnetism.

 

Fig. 3.6 Effect of magnetic field in paramagnetic material

 

Thermal agitation disturbs the alignment of the magnetic moments with an increase in temperature, the increase in thermal agitation tends to randomize the dipole direction thus leading to decrease in magnetization.

This implies that the paramagnetic susceptibility decreases with increase in temperature. It is observed that the paramagnetic susceptibility varies inversely with temperature.


There is a permanent magnetic moment.

 

When the temperature is less than the Curie temperature, paramagnetic materials become diamagnetic materials.

 

It spin alignment is random in nature.

 

Examples : Platinum, CuSO 4 , MnSO4 , Aluminum, etc

 

3.3  Ferromagnetic materials

 

Definition

 

Ferromagnetism is due to the presence of more unpaired electrons. Even in the absence of external field, the magnetic moments align parallel to each other. So that it has large magnetism. This is called spontaneous magnetization.


Fig. 3.7 Ferromagnetic materials

 

Effect of magnetic field

 

If a small external magnetic field is applied the magnetic moments align in the field direction and become very strong magnets.


Fig.3.8 Effect of magnetic field in ferromagnetic material

 

Properties of ferromagnetic materials

 

1. All the magnetic lines of force pass through the material.


The permeability is very much greater than one.

 

They have enormous permanent dipole moment.

 

When the temperature is greater than the Curie temperature, the Ferromagnetic material becomes paramagnetic material.

 

The ferromagnetic material has equal magnitude dipole lying parallel to each other.

 

Examples: Nickel, iron, Cobalt, Steel, etc.

 

(Curie temperature - The temperature below which a material can acts as ferromagnetic material and above which it can acts as paramagnetic material is called Curie temperature.)

 

3.4  Dia, Para and Ferro magnetic materials – Comparison


 

 

4  DOMAIN THEORY OF FERROMAGNETISM

 

This theory was proposed by Weiss in 1907. It explains the hysteresis and the properties of ferromagnetic materials.

 

Magnetic Domains

 

A ferromagnetic material is divided into a large number of small region is called domains. (0.1 to 1 of area), each direction is spontaneously magnetized. The direction of magnetization varies from domain to domain and the net magnetization is zero, in the absence external magnetic field. The boundary line which separates two domains is called domain wall or Block wall. When the magnetic field is applied to the Ferromagnetic material, the magnetization is produced by two ways.

 

          By the motion of domain walls.

 

          By the rotation of domains.

 

Process of Domain magnetization

 

There are two ways to align a random domain structure by applying an external magnetic field.

 

1. By the motion of Domain walls

 

When a small amount of magnetic field is applied, the domains having dipoles parallel to the applied magnetic field increases in area by the motion of domain walls. (Fig. 3.9 (2)).

 

2. By the rotation of Domains

 

If the applied magnetic field is further increased, the domains are rotated parallel to the field direction by the rotation of domains. (fig. 3.9 (3)).

 

Fig. 3.9 Domin theory of ferromagnetism

        Energies involved in the domain growth (or) Origin of Domain theory of Ferromagnetism

 

We can understand the origin of domains from the thermodynamic principle i.e., in equilibrium, the total energy of the system is minimum.

 

The total internal energy of the domain structure in a ferromagnetic material is made up from the following contributions.

 

          Exchange energy (or) Magnetic field energy.

 

          Crystalline energy (or) Anisotropy energy.

 

          Domain wall energy (or) Bloch wall energy.

 

          Magnetostriction energy

 

          Exchange energy (or) Magnetic Field energy

 

“The interaction energy which makes the adjacent dipoles align themselves” is the called exchange energy (or) magnetic field energy.

 

The interaction energy makes the adjacent dipoles align themselves. It arises from interaction of electron spins. It depends upon the inter atomic distance. This exchange energy also called magnetic field energy is the energy required in assembling the atomic magnets into a single domain and this work done is stored as potential energy. The size of the domains for a particular domain structure may be obtained from the principle of minimum energy. The volume of the domain may very between say, say, 10–2 to 10–6 cm3.

 

Fig. 3.10 Exchange energy in ferromagnetism

 

2. Anisotropy energy

 

The excess energy required to magnetize a specimen in particular direction over that required to magnetize it along the easy direction is called the crystalline anisotropy

 

energy.

 

In ferromagnetic materials there are two types of directions of magnetization namely,

 

          Easy direction and

 

          hard directions.

 

In easy direction of magnetization, weak field can be applied and in hard direction of magnetization, strong field should be applied.

 

Crystalline anisotropy energy is energy of magnetization which is the function of crystal orientation. As shown in figure magnetization curves for iron with the applied field along different crystallographic direction crystallographic directions have been drawn. For example, in BCC iron the easy direction is [100], the medium direction is [110], and the hard direction [111]. The energy difference between hard and easy direction to magnetize the material is about. This energy is very important in determining the characteristic domain boundaries.


Fig. 3.11 Anisotropy energy in ferromagnetism

 

3. Domain wall energy or Bloch wall energy

 

A thin boundary or region that separates adjacent domains magnetized in different directions is called domain wall or Bloch wall.


Fig. 3.12 The change of electron spin in the transition region of Bloch wall

 

The size of the Bloch walls is about 200 to 300 lattice constant thickness. In going from one domain to another domain, the electron spin changes gradually as shown in figure. The energy of domain wall is due to both exchange energy and anisotropic energy.

 

Based on the spin alignments, two types of Bloch walls may arise, namely

 

Thick wall: When the spins at the boundary are misaligned and if the direction of the spin changes gradually as shown figure, it leads to a thick Bloch wall. Here the misalignments of spins are associated with exchange energy.


Fig. 3.13 The change of electron spin in the transition region of thick wall

 

Thin wall: When the spins at the boundaries changes abruptly, then the anisotropic energy becomes very less. Since the anisotropic energy is directly proportional to the thickness of the wall, this leads to a thin Bloch wall.


Fig.3.14 The change of electron spin in the transition region of thin wall

4. Magnetostriction energy

 

When a material is magnetized, it is found that it suffers a change in dimensions. This phenomenon is known as Magnetostriction. This deformation is different along different crystal directions. So if the domains are magnetized in different directions, they will either expand or shrink. This means that work must be done against the elastic restoring forces. The work done by the magnetic field against these elastic restoring forces is called magneto-elastic energy or Magnetostrictive energy.

 

5 ANTIFERROMAGNETIC MATERIALS

 

Definition

In this material, the spins are aligned in anti-parallel manner due to unfavorable exchange interaction among them, resulting in zero magnetic moment. Even when the magnetic field is increased, it has almost zero induced magnetic moment.


Fig. 3.15 Spin alignment of antiferromagnetic materials

 

Properties

 

1. It susceptibility is very small and it is positive.


Examples : Ferrous oxide, Fe Cl4 ,Mn O4 ,MnS and some ionic compounds etc.

 

 

6  FERRIMAGNETIC MATERIALS

 

Definition

Ferrimagnetic materials or Ferrites are much similar to Ferromagnetic materials. The magnetic dipoles are aligned anti-parallel with unequal magnitudes. If small value of magnetic field is applied, it will produce the large value of magnetization.

 

Ferrimagnetic materials are widely used in high frequency applications and computer memories.

 

Fig. 3.16 Spin alignment of Ferrimagnetic materials

 

Properties


          These materials have low eddy current loss and low hysteresis losses.

 

Examples: Ferrous Ferrites and Nickel Ferrites

 

7  HYSTERESIS

 

Hysteresis means “Lagging” i.e., The Lagging of intensity of magnetization (I) behind the intensity of magnetic field (H).

 

Experimental Determination

 

A graph is drawn between the intensity of magnetization [I] and the intensity of magnetic field [H], for a cycle of magnetization. The experimental setup consists of solenoid coil through which current is passed and the material is magnetized. By varying the value of current we can get different values of Intensity of magnetization [I] due to the magnetic field (H) in the solenoid.


When the intensity of magnetic field ‘H’ is increased from O to F, the value of Intensity of magnetization T if also increases from O to A, at ‘A’ the material reaches the saturation value of Intensity of magnetization.

 

Then the value of I is constant.

 

          When intensity of magnetic field ‘H’ is decreased from G to O, the value of Intensity of magnetization ‘I’ also decreases from A to B, but not to zero (0). Now the material retains [stores] some amount of magnetism known as Retentivity, even though the intensity of magnetic field ‘H’ is zero. It is represented as ‘OB’ in the graph.

          When intensity of magnetic field ‘H’ is increased in reverse direction from O to C, the value of Intensity of magnetization ‘I’ decreases from B to C. i.e., the value of ‘I’ reaches zero.


Fig.3.17 Hysteresis curve

 

The amount of intensity of magnetic field ‘H’ applied in the reverse direction to remove the retentivity is known as Coercivity or Coercive force. It is represented as ‘OC’ in the graph.

 

Further repeating the process the remaining portion [CDEFA] in the graph is obtained. The closed loop [OABCDEFA] is called Hysteresis loop (or) (I – H) curve. For one cycle of magnetization.

 

Now the material is taken out. After a cycle of magnetization, there is some expenditure (loss) of energy.

 

This loss of energy is radiated in the form of heat energy in the material.

 

This loss of energy is directly proportional to the area of the loop.

 

From the Hysteresis graph, we can select soft and hard magnetic materials depending upon the purpose.

 

Energy product

 

It is the product of residual magnetism Br and coercivity which HC gives the maximum amount of energy stored in the specimen.

 

Energy product = Br × HC

 

Hysteresis loss

 

When the specimen is taken through a cycle of magnetization. There is a loss of energy in the form of heat. This loss of energy is known as Hysteresis loss.

 

7.1  Explanation of hysteresis on the basis of Domains


Fig.3.18 Hysteresis curve on the basis domain theory

 

OA    -  Due to smaller reversible domains wall movement.

AB    -        Due to larger irreversible domain wall movement.

BS     -        Due to smaller irreversible domain rotation.

S    -  Point of saturation.

 

When a field is applied, for small H, the domain walls are displaced and gives rise to small value of magnetization. [OA in the graph]. Now, the field is removed, the domains return to its original state known as reversible domains.

When the field is increases, a large number of domains contribute to the magnetization and I increases rapidly with H.

 

Now, when the field is removed the domain boundaries do not come back to the original position due to the domain wall movement to a very large distance (AB in the graph). These domains are called irreversible domains.

 

Now if the field is further increased, domains start rotating along the field direction and anisotropic energy is stored and it is represented as BC in the graph.

 

Thus the specimen is said to attain maximum magnetization at this position even after the removal of the field. The material is having magnetism called Retentivity. This Retentivity can be destroyed by applying a high reverse magnetic field called coercivity.

 

Thus the reversible and irreversible domain wall movements give rise to hysteresis in the Ferromagnetic materials.

 

8 HARD AND SOFT MAGENTIC MATERIAL

 

In Hysteresis, after a cycle of magnetization, there is some expenditure (loss) of energy. This loss of energy is radiated in the form of heat energy in the material and it is directly proportional to the area of the loop. From the Hysteresis graph, we can select the soft and hard magnetic materials.

 

8.1  Hard Magnetic Materials

 

The materials which are very difficult to magnetize and demagnetize are called hard magnetic materials. These materials can be made by heating the magnetic materials and then cooling it suddenly. It can also be prepared by adding impurities.


Fig.3.19 Hysteresis loop of a hard magnetic material

 

The above hysteresis loop is very hard and has a large loop area for a hard magnetic material, therefore the loss is also large. Domain wall does not move easily and require large value of H for magnetization. Its coercivity and retentivity values are large. Its eddy current loss is also high due to its low resistivity, the magnetostatic energy is large. It has low susceptibility and permeability. The hard magnetic materials have large amount of impurities and lattice defects.

 

Examples : Tungsten steel, Carbon steel, Chromium steel, Alnico etc.,

 

Properties

 

It is easilly magnetised and demagnetised.

 

They hysteresis area is very small and hence, the hysteresis loss is also small, as shown in figure.

 

The coercivity and rentenivity are very small.

 

These materials have large values of susceptibility and permeabilty.

 

Their magnetostatic energy is very small.

 

The eddy current loss is very small.

 

Applications

 

Iron-Silicon alloys are used in electrical equipment and magnetic cores of transformes.

 

Cast iron is used in the structure of electical machinery and the frame work of D.C machine.

 

Nickel alloys are used to manufacture inductors, relays and small motors.

 

It is also used for computer and data storage devices.

 

8.2  Soft Magnetic Materials

 

The materials which are easily magnetized and demagnetized are called soft magnetic materials. These materials can be made by heating the magnetic materials and then cooling it slowly to attain an ordered structure of atoms.


Fig.3.20 Hysteresis loop of a soft magnetic material

 

The above hysteresis loop is very small and has a less loop area for a soft magnetic materials. Therefore the loss is also small. Domain wall move easily and require small value of H for magnetization. Its coercivity and retentivity values are small, its eddy current loss is small due to its high resistivity. The magnetostatic energy is less, it has high value of susceptibility and permeability. The soft magnetic materials do not have impurities and lattice defects.

 

Examples: Iron-Silicon alloys, Nickel-Iron alloys and Iron-cobalt alloys.

 

Properties

 

It is very hard to magnetize and also demagnetize.

 

The hysteresis cure is very broad and has a large as shown in figure.

 

The coercivity and retentivity values are large.

 

These materials have low value of susceptibility and permeability.

 

The magnetostatic energy is large.

 

The eddy current loss is very high.

 

Applications

 

Magnets made by carbon steel are used for manufacturing the toys and compass needle.

 

Tungsten steel is used in making permanent magnets for D.C motors.

 

It is also used for making a small size of magnets.

 

8.3  Difference between Hard and Soft magnetic materials


Hard Magnetic Materials

1. Cannot be easily magnetized

2. It can be produced by heating and sudden cooling

3. Domain wall does not move easily and require large value of H for magnetization.

4. Hysteresis loop area is large Susceptibility and Permeability values are small.

5. Retentivity and Coercivity are large

6. High eddy current loss

7. Impurities and defects will be more

8. Examples: Alnico, Chromium steel, tungsten steel, carbon steel.

9. Uses: Permanent magnets, DC magnets.

 

Soft Magnetic Materials

1. Can be easily magnetized.

2. It can be produced by heating and slow cooling.

3. Domain wall move easily and requires small value of H for magnetization.

4. Hysteresis loop area is small Susceptibility and Permeability values are high.

5. Retentivity and Coercivity are small.

6. Low eddy current loss

7. No impurities and defects

8. Examples: Iron-silicon alloy, Ferrous nickel alloy, Ferrites Garnets.

9. Uses: Electro magnets, computer data storage. Transformer core.

 

9  FERRITES

 

Definition

 

Ferrites or Ferrimagnetic materials are the modified structure of iron without carbon. In Ferities the spins of adjacent ion is the presence of a magnetic field are in opposite directions with different magnitudes.

 

9.1 Properties

 

These are made from ceramic ferromagnetic compounds.

 

It has low tensile strength and it is brittle and soft.

 

In these materials all valence electrons are tied up by ionic bonding.

 

These are bad conductors with high resistivity of the order of 1011   m

 

Ferrites have low eddy current loss and low hysteresis loss.

          The general formula for Ferrites is X²+ (Fe2)3+ O4 where X-may is a metal (divalent metal) such as Mg, Ni, Mn, Zn, etc.

 

Ferrites are manufactured by powder metallurgical process by mixing, compacting and then sintering at high temperatures followed by age hardening in magnetic fields.

 

9.2 Structures of Ferrites

Ferrites are the magnetic compounds consisting to two or more different kinds of atoms. Generally ferrites are expressed as X²+ (Fe2)3+ O 4 where X²+ stands for suitable divalent metals ions such etc Mg2+, Zn2+, Fe2+, Mn2+, Ni2+ etc.

Normally, there are two types of structures present in the ferrites

1. Regular spinel 2. Inverse spinel

 

9.3 Regular spinal

 

In the regular spinal type, each metal atom (divalent) is surrounded by fourions in a tetragonal fashion.

 

For example in Mg2+, Fe23+, O42+, the structure of Mg2+is given in the Fig. 3.21 and it is called “A’ site. Totally in a unit cell, there will be 8 tetrahedral (8A) sites.


 

Each Fe3+ (trivalent) is surrounded by ‘6’ O2 ions and forms an octahedral fashion as show in Fig.3.20. Totally there will be 16 such octahedral sites in the unit cell. This in indicated by ‘B’ site.

 

Thus in a regular spinal, each divalent metal ion (mg2+) exists in a tetrahedral form (A site) and each trivalent metal ion (Fe2+) exists in an octahedral form (B site). Hence, the sites A and B combine together to form a regular spinal ferrite structure as shown in Fig.3.21.

 

9.4 Inverse spinal

 

In this type, we consider the arrangement of dipoles of a single ferrous ferrite molecule Fe3+ [Fe2+ Fe3+] O42– Fe3+ , ions (trivalent) occupies all A sites (tetrahedral) and half of the B sites (octahedral) also.

 

Thus the left out B sites will be occupied by the divalent (Fe2+). The inverse spinal structure is shown in the Fig. 3.22.


Fig.3.22 Structure of ferrites (inverse Spinal)

 

9.5  Types of interaction present in the ferrites

 

The spin arrangement between the A site B site is in an ant parallel manner and it was explained by Neel. According to him, in ferrites, the spin arrangement is ant parallel and there exists some interaction between the A and B sites which is represented as AB interaction.

 

Apart from this, there are two more interactions (i.e.,) AA and BB interaction which is negative and considerable weaker than AB interaction.

The tendency of AB interaction is to align all A spins parallel to each other and anti parallel to all B spins, but the tendency of AA and BB interaction is to spoil the parallel arrangement of AB spins respectively.

 

Since AB is very strong as compared with AA and BB, the effect of AB interaction dominates and gives rise to anti parallel spin arrangement.

 

9.6 Properties of ferromagnetic materials

 

          Ferromagnetic materials posses’ net magnetic moment.

 

          Above Curie temperature, it becomes paramagnetic while it behaves as ferromagnetic material below Curie temperature.

 

          The susceptibility of Ferrimagnetic material is very large and positive. It is temperature dependent and is given by


          Beyond Neel temperature,decreases.

 

          Spin alignment is anti parallel of different magnitudes.

 

          Mechanically, it has pure ion character.

 

          They have high permeability and high resistivity.

 

          They have low eddy current losses and low hysteresis.

 

9.7 Application of Ferrites

 

          Ferrites are used to produce ultrasonic wave by Magnetostriction principle.

 

          Ferrites are used in audio and video transformers.

 

          Ferrites rods are used in radio receivers to increase the sensitivity.

 

          Ferrites are widely used in non-reciprocal microwave devices such as gyrator, circulator and Isolator.

Gyrator : It transmits the power freely in both directions with a phase shift of radians.

Circulator : It provides sequential transmission of power between the ports.

Isolator : It is used to display differential attenuation.

          They are also used for power limiting and harmonic generation.

 

          Ferrites are used in parametric amplifiers so that the input can be amplified with low noise.

 

          They are used in computers and data processing circuits.

 

          Bi-stable elements, Ferro cube (Ferrite with square hysteresis loop), magnetic shift register, and magnetic bubbles are also examples for Ferrites.

 

10 MAGNETIC RECORDING AND READOUT MEMORY

 

Nowadays, large number of information are stored in (or) retrieved from the storage devices, by using devices is magnetic recording heads and they function according to the principle of magnetic induction.

 

Generally Ferro or Ferrimagnetic materials are used in the storage devices because in this type of materials only the magnetic interaction between only two dipoles align themselves parallel to each other.

 

Due to this parallel alignment even if we apply small amount of magnetic field, a large value of magnetization is produced. By using this property information are stored in storage devices.

 

In the storage devices, the recording of digital data (0’s and 1’s) depends upon the direction of magnetization in the medium.

 

10.1 Magnetic parameters for Recording

 

          When current is passed through a coil, a magnetic field is induced. This principle called “Electromagnetic Induction” is used in storage devices.

 

          The case with which the material can be magnetized is another parameter.

 

          We know the soft magnetic materials are the materials which can easily be magnetized and demagnetized. Hence a data can be stored and erased easily. Such magnetic materials are used in temporary storage devices.

 

          Similarly, we know hard magnetic materials cannot be easily magnetized and demagnetized. So such magnetic materials are used in permanent storage devices.

 

          In soft magnetic materials, the electrical resistance varies with respect to the magnetization and this effect is called magneto-resistance. This parameter is used in specific thin film systems.

 

The magnetic medium is made of magnetic materials (Ferro or Ferric oxide) deposited on this plastic.

 

The magnetic medium move across the read / write heads and either logic 1’s and logic 0’s are written on the medium. The magnetized spots on the medium generate small electrical signals and this different direction signals represents logic is and 0’s on the medium.

 

10.2 Storage of Magnetic Data

 

Memory units are the devices used to store the information (Input and Output) in the form of bits [8bits = 1 Byte].

 

The memory units are classified into two categories.

 

          Main memory (Primary) or Internal Memory.

 

          Auxiliary Memory (Secondary) or External Memory.

 

          Main Memory

 

The memory unit of the central processing unit (CPU) is called as main memory. Compare a black beard main memory. We can write many data on memories and finally erase it if we want.

 

Example : RAM, ROM, EPROM etc.

 

2. Auxiliary Memory

 

Since storage capacity of primary memory is not sufficient secondary memory units are developed to store the large volume of data. Separately and hence called as extra (or) additional (or) external memory.

 

This type of memory is also referred to as back up storages because, it is used to store large volume of data on a permanent basis.

 

Example: 1. Magnetic tapes

 

Magnetic disk (Floppy and Hard disc)

 

          Ferrite core memories

 

          Magnetic bubble memories.

 

10.3 Magnetic Tape

 

It is one of the most popular storage medium for data. The tape is a plastic ribbon with metal oxide material coated on one side which can be magnetized. In this, information can be written and also can be read by read / write heads.

 

Information recorded in the tape is in the form of tiny magnetized and non magnetized spots on the metal oxide coating.

 

The magnetized spot represents ‘I’ and un magnetized spot represent ‘0’ in binary code. The information can be accessed, processed, erased and can be again stored in the same area.

 

Advantages

 

          Its storage capacity is large

 

          It is easy to handle and is portable

 

          Its cost is less than other storage devices.

 

          It can be erased and reused may times.

 

Disadvantage

 

1. It consumes lot of time.

 

10.4 Magnetic Disc Drivers

 

These disks are direct access storage devices. These disks are magnetically coated. There are two types of disks.

 

          Hard disc

 

          Floppy disc

 

          Hard disc

 

The hard disc is made of hard aluminum platters. The platter surface is carefully machined until it is flat (or) plane. The platter surface is coated with magnetic material (magnetic oxides). The platter is built into a box.

 

Similar such disks are mounted on a vertical shaft, forming a disk pack and it is shown in fig.


 

Fig.3.23 Hard disc

 

The disc pack is placed is a drive mechanism called hard disk drive. The drive mechanism driver the disc pack with the spindle. The data is written (or) ready by R/ W beads in the horizontal sensing arms by moving in an out between the platters with the precaution that the R/W head doesn’t touch the surface instead, it fly over the disk surface by a fraction of an mm.

 

Advantages

 

          It has very large storage capacity.

 

          Thousands of files can be permanently stored

 

          Very high speed is reading and writing the information.

 

          This is prevented from dust particles, since they are seated in special chamber.

 

Disadvantages

 

          It is very costly.

          It data is once corrupted, there is a heavy loss of data.

10.5    Floppy Disk

 

The hard disc is suitable only for large and medium sized computers and often are too expensive for small computers systems. Floppy disc are the latest development in secondary storage devices.

 

It is made up of a flexible plastic material and hence called as floppy disc. It is also called as diskette. It acts both as an input and output device.



Fig.3.24 floppy disc

 

The disk is provided with a central hole. This hole is used for mounting the disc in the floppy derive unit. The envelope prevents the disk from dust and moisture.

 

There is a small index hole in the cover and there will be a hole in the drive disk. When these two holes match then only the storage operation can be started. Write protect notch is used to prevent writing on the disc by other users. This can be done by covering the notch with a sticker. A 5.25° floppy is shown in fig.

 

Writing operation

When the floppy disk moves over the gap the CPU flow through the write will of the head and magnetizes the iron oxide coating in the disk to the proper pattern.

 

Reading Operation

When the data are to be read, the magnetized patterns induces pulses of current in the read coil and is amplified then fed to the CPU. Thus data can be stored and accessed from the floppy disc on both sides (or) single side. Reading / writing data on the magnetic medium using frequently modulated wave.

 

Special features

 

          The cost is very low

 

          It can be easily handled

 

          It can be taken to any place

 

          It has high storage capacity

 

          Many types of floppies are available, depending on their storage capacities.

 

Disadvantage

 

Here the magnetic disk is moved (rotated) mechanically.

 

10.6 Magnetic bubble Materials

 

Magnetic bubble is direct access storage medium, magnetic bubbles are soft magnetic materials with magnetic domains of a few micrometers in diameter. These bubbles are the electrical analogue of the magnetic disk memories used in computers.

 

The magnetic disk in the hard disk memory is moved mechanically where as the bubbles in a bubble memory device are moved electronically at very high speeds, so the read out time or storing time is greatly reduced in bubble memory device. The bubble units are made with solid state electronic chips.

 

A magnetic bubble can be thought of as a positively charged and in a negatively charged magnetic film.The presence of a bubble is on ‘ON’ condition and the absence of a bubble is an ‘off’ condition. ie., [1 or 0].


Fig. 3.25 Magnetic bubble memory

 

Figure shows the schematic diagram of a magnetic bubble memory. It consists of one major loop and 157 minor loops. Each minor loop has 641 bubble sites.


Writing operation

 

When a data is to be stored, the bubbles from the minor loops are transferred to the major loop, and it goes to the write station. In write station the data is entered and the bubble again comes to minor loop.

 

Reading operation

 

To read the data from the storage, the bubble from minor loops are transferred to the major loop and it goes to the read station, then it comes to the minor loop. The data can be altered by the erase station, if we need to erase it.

 

Advantage

 

          It is non-volatile.

 

          It has high storage capacity than the magnetic hard disk.

 

          It has high across speed.

 

Construction

 

A Bubble memory consist of materials such as magnetic garnets and store the data as microscopic magnets. A thin film of these garnets is deposited on a non-magnetic substrate of Gadolinium Gallium Garnet in Integrated Circuit [IC] form.


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