Material Science -Unit 4 | MAGNETIC, DIELECTRIC & SUPERCONDUCTING MATERIALS

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Important Questions For Material Science: 
*Unit-01     *Unit-02    
*Unit-03    *Unit-04 
*Unit-05    *Short-Q/Ans
*Question-Paper with solution 21-22 

Q1. What do you understand by ferromagnetism, anti-ferromagnetism and ferrimagnetism ?

Ans. A.  Ferromagnetism :

• 1. Certain materials have a constant atomic dipole moment (Fig. (a)).
• 2. They are placed in a strong magnetic field and strongly magnetised in the direction of the applied field; such materials are known as ferromagnetic materials. As an example, consider the iron, nickel, and cobalt alloy.
• 3. TDespite the randomizing nature of their atom’s thermal agitation, they have a tendency to align themselves in the direction of the field.
• 4. These material features are known as ferromagnetism.

B. Anti-Ferromagnetism :

• 1. Magnetic moment coupling between nearby atoms or ions happens in materials other than ferromagnetic materials.
• 2. This connection results in an antiparallel alignment in one such group (Fig. (b)).
• 3. Anti-ferromagnetism is defined as the alignment of the spin moments of neighbouring atoms or ions in completely opposing orientations.

C. Ferrimagnetism :

• 1. Some ceramics also exhibit ferrimagnetism, which is a permanent magnetization.
• 2. Ferromagnets and ferrimagnets have comparable macroscopic magnetic properties; the difference is in the source of the net magnetic moments.
• 3. The opposing moments are uneven in ferrimagnetism (Fig. (c)), and a spontaneous magnetization remains. This occurs when the populations are made up of distinct materials or ions.
• 4. Ferrimagnetism is exhibited by ferrites and magnetic garnets.

Q2. Explain domain theory of ferromagnetism.

Ans.

• 1. According to domain theory, quantum mechanical exchange forces cause neighbouring atoms’ magnetic moments to tend to point in the same direction. Magnetic domains are groups of neighbouring atoms that point in the same direction.
• 2. Domains point in random directions at extended distances, cancelling each other out and leaving the substance unmagnetized. When an external magnetic field is introduced, the domains align in the direction of the field and contribute to it.
• 3. Although an external force has little effect on individual atoms, it has a greater impact on atoms in a domain because they all point in the same direction. This explains why a ferromagnetic material can become substantially magnetized in the presence of a weak external field.
• 4. Below a specific temperature, known as the Curie temperature, quantum mechanical exchange forces overcome thermal energy, which would otherwise randomise individual atoms magnetic moments.
• 5. Some materials are ferromagnetic due to the presence of adequate exchange forces.
• 6. Every ferromagnetic substance has its own Curie temperature. Thermal energy prevails above the Curie temperature, and the material becomes paramagnetic.
• 7. Other magnetic features, such as the existence of permanent magnets, are also explained by domain theory.

Q3. What is hysteresis and explain retentivity and coercivity in hysteresis curve ?

Ans.

• 1. Magnetic induction lags behind the magnetizing field when a ferromagnetic material is placed in one. This is referred to as hysteresis.
• 2. When a ferromagnetic material, such as iron, is gradually magnetised by gradually increasing the intensity of the magnetic field H, magnetic induction begins to increase.
• 3. Fig. shows a graph plot of B-H. When the magnetic field is increased from O to G, the fraction ‘Oa’ corresponds to initial magnetization.
• 4. The variation in magnetic induction is provided by the curve abcdefa when the value of the magnetiszng field is adjusted from G to – G and – G to G. This closed curve is referred to as a hysteresis curve or a hysteresis loop.

• 5. At point b, where H = 0, B ≠ 0 but B → B_r known as residual induction or retentivity of the magnetic field applied in the negative direction.
• 6. At point c, B = 0, H ≠ 0 but H → H_c known as coercive force or coercivity.

Q4. Discuss some applications of hysteresis curve.

Ans.

• 1. The shape of the hysteresis curve gives a wealth of information on a material’s magnetic properties, including coercivity, retentivity, susceptibility, and permeability.
• 2. The hysteresis curve can be used to select magnetic materials for permanent magnets, electromagnets, transformer cores, and other applications.
• 3. Permanent magnets require materials with high retentivity and coercivity.
  • Example: Cobalt steel is suitable material for permanent magnets.
• 4. A good material for producing electromagnets should have a low hysteresis loss, a high initial permeability, and a high flux density.
  • Example : Soft iron is suitable material for making electromagnets.
• 5. For transformer cores, dynamo armatures, and motors, the material should have a low initial permeability and a high specific resistance to reduce eddy current losses.
  • Example: Silicon iron or transformer steel is most suitable for the core purpose.
• 6. Magnetic shield materials have a high saturation induction and a low coercivity.
  • Example: Permalloys are widely used for this purpose.

Q5. Differentiate between hard and soft magnetic materials.

Ans.

Q6. What is Meissner effect ? Discuss in detail.

Ans.

• 1. When a superconductor is cooled under a magnetic field below its critical temperature, the lines of induction are expelled from the material. This is known as the Meissner effect.
• 2. Fig.(a) shows a superconductor in normal state and the magnetic lines of force passing through it.
• 3. However, when the specimen is cooled below its transition temperature, the magnetic lines of force are evacuated, as illustrated in Fig (b).
• 4. The Meissner effect refers to the emission of magnetic lines of force from a superconducting material when it is cooled below the transition temperature in a magnetic field.
• 5. Meissner effect is reversible. When the temperature is increased above T_c,the flux suddenly penetrates through the specimen and substance comes to its normal state.

• 6. Because the magnetic induction B in a superconductor is zero, it is a perfect diamagnetic. So, for superconductors

• 8. We know that when temperatures are dropped, the susceptibility of noble metals does not vary.
• 9. When metals become ideal conductors, their susceptibility does not alter.
• 10. On the other hand, in case of superconductors, the susceptibility change e.g., in case of lead, in normal state. It has susceptibility – 0.12 x 10^-6 while its value in superconducting state is – 1.0.
• 11. The distinction between a perfect conductor and a superconductor is that the former is merely an ideal conductor, whereas the latter is an ideal conductor and an ideal diamagnetic at the same time.

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