Explanation of Hysteresis on the basis of Domains

EXPLANATION OF HYSTERESIS ON THE BASIS OF DOMAINS

We know when the ferromagnetic material is subjected to external field, there is an increase in the value of the resultant magnetic moment due to two process, viz.,

(i) The movement of domain walls

(ii) Rotation of domain walls

Note: The movement and rotation of domain walls due to application of magnetic field is given in Appendix‒1.

When a small external field is applied, the domains walls is displaced slightly in the easy direction of magnetisation. This gives rise to small magnetisation corresponding to the initial portion of the hysteresis curve (OA) as shown in Fig. 1.22.

Now, if the applied field is removed, then the domains returns to its original state, and is known as reversible domains.

When the field is increased, large number of domains contribute to the magnetisation and thus the magnetisation (M) increases rapidly with H.

Now, even when the field is removed, because of the displacement of domain wall to a very large distance. The domain boundaries do not come back to their original position. This process is indicated as AB in Fig. 1.22 and this domains are called irreversible domains.

At point 'B' all the domains have got magnetised along the easy direction.

Now, when the field is further increased, the domains start rotating along the field direction and the anisotropic energy is stored in the hard direction, represented as BC in the Fig. 1.22.

Thus the specimen is said to attain the maximum magnetisation. At this position, even after the removal of external field the material posses maximum magnetisation, called residual magnetism (or) retentivity, represented by OD in Fig. 1.22.

Actually after the removal of the external field, the specimen will try to attain the original configuration by the movement of Bloch wall. But this movement is stopped due to the presence of impurities, lattice imperfections etc. Therefore to overcome this, a large amount of reverse magnetic field is applied to the specimen. The amount of energy spent to reduce the magnetisation to zero is called as coercivity represented by 'OE in the Fig. 1.22.

Hysteresis Loss

It is the loss of energy in taking a ferromagnetic specimen through a complete cycle of magnetisation and the area enclosed is called hysteresis loop.

Based on the area of the hysteresis loop, the magnetic materials are classified into soft and hard magnetic materials.