Magnetic Materials: Two Marks Important Questions and Answers

Applied Physics CSIE II

UNIT I: Magnetic Materials

ANNA UNIVERSITY PART A QUESTIONS And ANSWERS

 

1. On the basis of spin how the materials are classified as para, ferro, antiferro and ferri magnetic.

(i) Paramagnetic materials have few unpaired electron spins of equal magnitudes.

(ii) Ferro magnetic materials have many unpaired electron spins with equal magnitudes.

(iii) Anti ferro magnetic materials have equal magnitude of spins but in antiparallel manner.

(iv) Ferrimagnetic materials have spins in antiparallel manner but with unequal magnitudes.

 

2. Give Curie‒Weiss law and its importance.

Curie‒Weiss law is given by

 χ_m = C / [T‒θ]

where

C→ Curie constant

T→ Absolute temperature

θ→ Curie temperature

Importance: It determines the susceptibility of the magnetic materials in terms of temperatures (i.e.,) If the temperature is less than Curie temperature, a paramagnetic material becomes diamagnetic and if the temperature is greater than Curie temperature, a ferromagnetic material becomes paramagnetic material.

 

3. What do you understand by the term, "magnetic domains" and "domain walls"?

Magnetic domains are the small regions in a ferromagnetic material which has a group of atoms. These atoms can be completely magnetised by favourable exchange spin‒spin interaction. The walls of these small regions (or) domains are called domain walls.

 

4. What are soft and hard magnetic materials? (or) Compare soft and hard magnetic materials on basis of Hysteresis loop. give examples.

Soft

1. They can be easily magnetised and demagnetised.

2. Movement of domain wall is easy and hence even for a small applied field large magnetisation occurs.

3. The Hysteresis loop is very steep as shown in Fig. (1a).

4. Loop area is less and hence the hysteresis loss is minimum.

5. Examples, Iron, silicon alloys, Ferrites, Garnets etc.

Hard

1. They cannot be easily magnetised (or) demagnetised.

2. Movement of domain wall is not easy due to the presence of impurities and hence large field is required for magnetisation.

3. The Hysteresis loop is very broad as s shown in Fig. (1b).

4. The loop area is large and hence the hysteresis loss is maximum.

5. Carbon steel, Tungsten steel, Chromium steel, Cu‒Ni‒Fe (Cunife), Cu‒Ni‒Co (Cunico), Al‒Ni‒Co (Alnico)

 

5. Draw hysteresis loop and show the retentivity and co‒ercivity in it.

The hysteresis loop, retentivity and co‒ercivity is shown in Fig. 

 

6. What are Ferrites and Ferrox cubes?

Ferrites are the modified structure of iron with no carbon in which the magnetic moments are of unequal magnitudes. They are made by two (or) more different kinds of atoms. Its general formula is given by X²⁺ Fe₂³⁺O₄

where X²⁺ is a divalent metal ion such as Mg²⁺, Zn²⁺, Fe²⁺, etc.

Ferrox cubes are the soft magnetic material for which the hysteresis loop will be in the form of a narrow rectangle.

 

7. What is domain theory of ferromagnetism?

The group of atomic dipoles organized into tiny bounded regions in the ferromagnetic materials are called magnetic domains. The boundaries separating domains are called domain walls. In Ferro‒magnets, when external magnetic field is applied, the domains align and results in large net magnetization.

 

8. Define antiferromagnetism. Mention two materials that exhibit antiferromagnetism.

In antiferromagnetism the spins are aligned in antiparallel manner (Fig. 1(d)) due to unfavourable exchange interaction among them, resulting in zero magnetic moment. Even when the field is increased, it has almost zero induced magnetic moment.

Antiferro‒magnetic materials:

(i) Ferrous oxide (FeO) (ii) Manganese Oxide (MnO₄) (iii) Manganese Sulphide (MnS) (iv) Chromium Oxide (Cr₂O₃)

These elements will be wide range of applications in magnetic storage devices.

 

9. Define magnetic lines of force and magnetic lines of induction

Magnetic lines of force: It is defined as the continuous curve in a magnetic field which travels externally in the magnet from north pole to south pole.

Magnetic lines of induction: It is the imaginary lines of forces which are supposed to travel from south pole to north pole inside the magnet.

 

10. Discuss the orientation of spin for dia, para and ferro‒magnetic substances.

Diamagnetic materials: Here the electron spins are randomly oriented and mostly they have equal and opposite spins. Thus the net magnetic moment is zero.

Paramagnetic material: Here the spins of electrons will not be equal, which leads to have some unpaired electrons. Hence there exists some resultant magnetic moment. Therefore in paramagnetic material the net magnetic moment is not zero.

Ferromagnetic material: In ferro magnetic materials the number of unpaired electrons will be more. Hence there exists a large resultant magnetic moment in it.

 

11. Classify the magnetic materials based on their magnetic moments.

The magnetic materials can be classified into two major categories based on the presence of magnetic moments as follows.

Magnetic materials are classified according to the presence or absence of the permanent magnetic dipoles. Generally, every two electrons in an energy state of an atom will form a pair with opposite spins. Thus the resultant spin magnetic moment is zero. Hence they don't have permanent magnetic moments and they are called as diamagnetic materials. Examples. gold, germanium, silicon, etc.

But in some magnetic materials like iron, cobalt, etc., there exists unpaired electrons. The spin magnetic moment of these unpaired electrons interact with the adjacent atom's unpaired electron spin magnetic moment in a parallel manner resulting in enormous permanent spin magnetic moment. These materials are classified into paramagnetic, ferromagnetic and ferrimagnetic materials with respect to the electron spins.

 (OR)

Classification of Magnetic materials:

1. Not having permanent magnet moment : Dia‒magnetic material

2. Having permanent magnetic moment: Para‒magnetic material, Ferromagnetic material, Ferrimagnetic material

 

12. Define Bohr magneton.

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

1 Bohr Magneton = e / 2m

 μ_B ⇒ 9.27 × 10²⁴ Am²

 

13. State the applications of ferrites.

(i) Ferrites are used in audio transformers, video transformers, radio receivers etc

(ii) They are used in two port devices such as gyrator, circulator and isolator

(iii) They are used in computers and data processing circuits

(iv) They are used in switching circuits and parametric amplifiers.

 

14. What are the required magnetic parameters for recording?

The basic parameters required for recording are

(i) Electromagnetic induction should occur in materials.

(ii) The material should easily acquire magnetism.

(iii) It should posses magneto‒resistance i.e., the electrical resistance should vary with respect to the magnetisation.

(iv) Soft magnets should be used for temporary storage and hard magnets should be used for permanent storage.

 

15. Define Hysteresis. What is meant hy Hysteresis loop and what do you infer from it?

When a ferromagnetic material is made to undergo a cycle of magnetistion, the intensity of magnetisation (I) and the magnetic flux density (B) lags behind the applied magnetic field (H), and this process is known as Hysteresis.

The closed curve obtained during the cycle of magnetisation of a material is known as hysteresis loop.

Inference: The area of the loop gives the energy loss (or) hysteresis loss during the cycle of magnetisation.

 

ADDITIONAL PART A QUESTIONS & ANSWERS

 

1. What is meant by magnetic materials? Give examples.

Magnetic materials are the materials which can be easily magnetised by keeping it in an external magnetic field.

Examples: Iron, Ferrites, Carbon steel etc.

 

2. Define magnetic flux density and magnetic dipole with its unit.

Magnetic flux density (B): It is defined as the number of magnetic lines of forces (ϕ_m) passing normally through unit area of cross section (A).

 (i.e.,) B = ϕ_m / A Wbm^‒2 (or) Tesla

Magnetic dipole: Two opposite magnetic poles separated by some distance is called magnetic dipole. It can be also be defined as the product of magnetic pole strength (m) and the length of the magnet. (i.e.,) M_μ = m/ Wbm^‒1

 

3. Define magnetic field intensity and intensity of magnetisation with its unit.

Magnetic field intensity (H): It is defined as the force experienced by a unit north pole placed at the given point in a magnetic field.

 (i.e.,) H = F/m Am^‒1

Intensity of magnetisation [M (or) I]: It is defined as the magnetic moment per unit volume.

(i.e.,) I = M_μ / V Wbm^‒2

 

4. Define magnetic susceptibility and magnetic permeability.

Magnetic susceptibility (χ_m): It is defined as the ratio between intensity of magnetisation (M (or) I) and the magnetic field intensity (H)

 (i.e.,) χ_m = I / H

Magnetic permeability (μ): It is defined as the ratio between the magnetic flux density (B) and the magnetic field intensity (H)

(i.e.,) μ = B / H

 

5. Prove μ_r = 1+ χ_m

When a magnetic material is kept in an external magnetic field, then flux density can be written as

 B=μ₀ (H+I)            …………(1)

We know u

μ=B/ H

⇒ B=μH            …………(2)

Equating (1) and (2) we get μH = μ₀ (H+I)

 (or)

μ₀μ_rН = μ₀H( 1+ I/H )

Here 1/H = χ_m

(or)

 χ_r = ( 1+ 1/H )

[because μ = μ₀μ_r]

(or)

μ_r = 1+χ_m

Hence Proved

 

6. Explain the term remanence and coercivity with its units.

Remanence/Retentivity: It is the residual intensity of magnetisation retained by the specimen even when the external magnetic field is cutoff.

Unit: Wbm^‒2

Coercity: It is the strength of reverse magnetic field required to completely remove the residual magnetisation (or) demagnetise the material.

Unit: Ampere turn / metre

 

7. What is the origin of the presence of magnetic moments in magnetic materials?

The origin of presence of magnetic moments is due to orbital and spin motion of electrons in atom. Generally, every two electrons in an energy state of an atom will form a pair of opposite spins, but in some materials there exists unpaired electron spins also. These gives rise to a resultant spin magnetic moment, which plays a vital role in the classification of magnetic materials.

 

8. Why diamagnetic materials are called weak magnets and ferromagnetic materials are called strong magnets?

Weak magnets: If a diamagnetic material is kept in an external magnetic field, the electrons spins in the material reorient in such a way that they align perpendicular to the field direction and hence the materials will not be easily magnetised. Thus diamagnetic materials are called weak magnets.

Strong magnets: When a ferro magnetic material is kept in an external magnetic field, the electrons which are already aligned parallel to the direction of magnetic field acquires a very strong magnetic moment in it. Hence ferromagnetic materials are called strong magnets.

 

9. What is Curie temperature?

Curie temperature is the critical temperature below which a material can behaves as ferromagnetic material and above which it can behave as paramagnetic material.

 

10. Compare the properties of dia, para and ferro magnetic materials.

Dia‒magnetic material

1. In diamagnetic material there are equal number of electron spins which are randomly oriented and hence the net magnetic moment is zero.

2. When the external magnetic field is applied, the electrons will align perpendicular to the field direction and hence it reduces the magnetic induction present in the material. Thus they are name as weak magnets.

3. When the material is placed in the magnetic field, the magnetic flux lines are repelled away from the material.

4. The susceptibility negative ( χ = ‒ve).

5. The susceptibility is independent of temperature.

6. Permeability is less than 1.

7. When the temperature is less than critical temperature, the diamagnetism suddenly disappears and becomes a normal material.

8. Examples: Gold, antimony, bismuth, water, hydrogen, alcohol, germanium, silicon etc.

Para‒magnetic material

1. In paramagnetic material there are unequal number of electron spins and hence there exists a permanent magnetic moment.

2. When the external magnetic field is applied, the electrons will align parallel to the field direction and hence the material is magnetised. Thus they are named as strong magnets.

3. When the material is placed in the magnetic field, the magnetic flux lines passes through the material.

4. The susceptibility is positive and small (χ =+ve)

5. The susceptibility varies inversely with the absolute temperature.

6. Permeability is greater than 1.

7. When the temperature of the material is less than Curie temperature, para‒magnetic material converted into diamagnetic material.

8. Examples: Platinum, chromium, aluminium, copper sulphate, manganese sulphate etc.

Ferro‒magnetic material

1. In ferromagnetic material there will be large number of unequal electron spins and hence there exists enormous amount of permanent magnetic moment.

2. When the external magnetic field is applied, the electrons which are already aligned parallel will reorient itself along the field direction and will be very easily magnetised. Thus they are named as very strong magnets.

3. When the material is placed in the magnetic field, the magnetic flux lines are highly attracted towards the centre of the material.

4. The susceptibility is positive and large (χ=+ve)

5. The susceptibility depends upon the temperature.

6. Permeability is very much greater than 1.

7. When the temperature is greater than Curie temperature, the ferromagnetic material is converted into paramagnetic material,

8. Examples: Iron, nickel, cobalt, steel, etc.

 

11. Distinguish the properties of soft and hard magnetic materials.

Soft

1. They can be easily magnetised and demagnetised.

2. Loop area is less and hence the hysteresis loss is minimum.

3. Susceptibility and permeability is high

4. Retentivity and Coercivity are small

5. They have low eddy current loss.

6. These materials are free from irregularities like strain or impurities.

Hard

1. They cannot be easily magnetised (or) demagnetised.

2. The loop area is large and hence the hysteresis loss is maximum.

3. Susceptibility and permeability is low.

4. Retentivity and Coercivity are large.

5. They have high eddy current loss.

6. These materials have large amount of impurities and lattice defects.

 

12. What is meant by reversible and irreversible domains?

When the external magnetic field applied to a domain is increased, it starts expanding. Now when the external magnetic field is removed, if the domain returns to its original position it is called reversible domains and if the domain doesn't returns to its original position it is known as irreversible domains.

 

13. What are the requirements (required properties) of a transformer core material and electromagnets?

(I) A transformer core material should have the following requirements (properties)

(i) High resistivity

(ii) Low eddy current losses

(II) An electromagnet should have the following requirements (properties)

(i) High initial permeability

(ii) Low coercivity.

 

14. What is meant by eddy current and eddy current losses?

When an alternating magnetic field is applied to the material, it induces an e.m.f and sets up a large current in the material. This current is known as eddy current and the power loss is called eddy current losses.

 

15. What is meant by Garnet? Give examples.

Garnet is a ferrimagnetic material with a typical formula Me₃Fe₅O₁₂

where, Me₃ → Trivalent metal ion

Fe₅ → Trivalent ferric ion

Examples: Gadolinium Gallium Garnet, Yttrium Iron Garnet.

Properties

(i) They have high resistivity

(ii) They have low hysteresis loss

 

16. What is GMR?

If the charge in electrical resistance is very high compared to the magnetisation, it is called as Giant Magneto‒Resistance (GMR) and this effect is called GMR effect.

 

17. What are ESD magnets? Give its properties.

ESD magnets are Elongated Single Domain magnets, which are made by very small particles with very high magnetisation. Hence these ESD magnets possess the following properties

Properties

(i) They are highly stable.

(ii) They have single domain structure.

(iii) They possess large magnetisation.

 

18. What is a magnetic storage device? Give examples.

Ferro and ferri magnetic materials which are used to store the data in form of zeros and ones are called magnetic storage devices.

Examples: Floppy disk, Audio cassettes, magnetic tapes etc.

 

19. What are the advantages and disadvantages of magnetic disks?

Advantages

1. It has very large storage capacity.

2. Thousands of files can be permanently stored.

3. Very high speed in reading and writing the informations.

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

Disadvantages

1. It is very costly.

2. If data is once corrupted, there is a heavy loss of data.

 

20. What is meant by magnetic bubble? How they are formed?

Magnetic bubbles are soft magnetic materials with magnetic domains of few micrometer in diameter.

Formation: When a magnetic field is applied to magnetic garnets like Gadolinium Gallium garnet, small cylindrical domain area known as magnetic bubble is formed. These bubbles has a magnetic region of one polarity (either north (or) south) surrounded by the other polarity.

 

21. Distinguish between magnetic and optical storage devices.

Magnetic storage devices

1. Datas are stored using magnetic principle.

2. Writing and reading the data can be done using electro magnets

3. Access time is slow.

4. Example: Floppy disks.

Optical storage devices

1. Datas are stored using optical principle.

2. Writing and reading the data is made using Laser.

3. Access time is very fast.

4. Example: Compact disk (CD's)

 

22. What is meant by Magnetic HDD? Give the principle of storing data in HDD.

Hard disk drives [HDD] made up of magnetic garnet materials are called magnetic hard disk drives. It is a mass data storage device recently used for storing data to a very high level in terms of Tera bytes.

Now‒a‒days GMR [Giant Magneto Resistive] sensors, which has a very high magnetic sensitivity are used to read the data at greater speed.

Principle

In Hard disk drives, the binary data in terms of zero's (0) and one's (1) are stored by inducing magnetic moment in a thin magnetic layer and GMR effect is used as the principle to read the data in HDD.

Here zero(0) represents missing transition and one (1) represents transition in the medium.

 

23. What are the advantages and disadvantages of HDD?

Advantages

1. HDD's can store data interms of Terabytes and in future it can store data interms of Petabytes and Exabytes.

2. It is has very large storage capacity.

3. It is compact in size and can be easily transferred from one place to another.

4. The size of the recording medium in HDD shall be reduced upto few nano‒metres using nanotechnology.

5. GMR sensors are non diffusive and are very sensitive in reading.

Disadvantages

1. HDD is slower than SSD [solid state drives].

2. They consume more power and will damage, when dropped even at a smaller distance.

3. Sometimes the data in HDD may be corrupted, due to thermal radiation.

4. HDD has bulkier form factor.

5. The GMR noise ratio is high for the nano size recording media as it is temperature dependent.

 

24. What is the role of GMR sensor in Magnetic HDD

1. Giant magnetoresistive (GMR) effect is the principle used to read/retrieve the data from the recording medium.

2. When the GMR sensor is made to move near the recorded medium, then, the resistance of the GMR sensor varies, with respect to the orientation of the magnetic moments as follows.

3. When the layers are magnetised in parallel manner, then the resistance in the GMR sensor is minimum and therefore maximum current flows through the sensor, which represents the data as one (1), as shown in Fig.

4. When the layers are magnetised in antiparallel manner, then the resistance in the GMR sensor will be maximum and therefore minimum (or) no current will flow through the sensor, which represents the data as zero (0), as shown in Fig.

5. Therefore, with the help of the reading current, the zero's (0's) and one's (1's) can be retrieved from the magnetic hard disk drive.

 

25. Give any two applications of Magnetic Hard disk drives.

Applications

1. HDD's are used as storage devices in cloud applications.

2. They are used in coding and signal processing units.

3. It is used in many Engineering fields such as control systems, nanoelectronics, etc.

4. The GMR and spin valve sensors are used in modern HDD by IBM.