Question

Explain ferromagnetism on the basis of the domain theory.

Explain ferromagnetism with the help of suitable diagrams, using the concept of magnetic domain.

Answer 1

Due mostly to the electrons' spin magnetic moments, ferromagnetic material atoms have a persistent non-zero magnetic dipole moment. Little areas known as domains make up a ferromagnetic substance, according to the domain hypothesis. An incredibly small area with a high concentration of atoms—roughly 1015 atoms, or the same number as ordinary iron—is called a domain. Even in the absence of an external magnetic field, the atomic magnetic moments of nearest-neighbour atoms within each domain interact strongly through exchange interaction, a phenomenon described by quantum mechanics. As a result, the atoms align themselves parallel to one another. That is, a domain becomes spontaneously magnetised to saturation. In an unsealed substance, on the other hand, the net magnetisation is zero due to the distinct domains' magnetisation orientations being so aligned.

Domains in a single crystal of iron The arrows indicate the direction and magnitude of the magnetization of each domain.

The specimen becomes more magnetised as a result of applying an external magnetic field. One of two methods is used to do this: either a favourably oriented domain expands at the expense of a less favourably oriented domain, or the magnetisation direction of an entire domain shifts to align with the external magnetic field. Favourably oriented domains increase in size by domain border displacement when a weak magnetic field is applied, as shown in Fig. (b). Figs. (c) and (d) show how domain rotation causes the domains to shift their magnetisation under strong fields. When domains line perfectly, as they do in Fig. (d), they combine to form a single, huge domain.

It might be energetically advantageous for a domain's direction of magnetisation to continue after the external field is eliminated. A persistent magnetic dipole moment is thus present in the specimen. The existence of permanent magnets is based on a phenomenon known as magnetic remanence.

Answer 2

Ferromagnetism based on domain theory:

1. Individual atoms in ferromagnetic materials are linked to significant magnetic moments.
2. The magnetic moments of atoms close to each other interact and naturally line up in the same way over large areas known as domains.
3. Each domain typically measures approximately 1 mm and comprises around 10¹¹ atoms. So, each domain possesses a significant magnetic moment.
4. Without an external magnetic field, these domains are arbitrarily oriented, resulting in a net magnetic moment of zero.
5. 
   
6. When subjected to a magnetic field, all of the domains align with one another along the direction of the field being applied.
7. Magnets aggressively attract ferromagnetic materials.
8. The alignment of domains may occur in either of the two ways.
   1. By displacing the boundaries of domains.
   2. By rotation of domains.