Question

Explain ferromagnetism onthe basis of domain theory

Answer 1

To explain the phenomenon of ferromagnetism, Weiss proposed a hypothetical concept of ferromagnetic domains. He postulated that the neighboring atoms of the ferromagnetic materials, due to certain mutual exchange interactions, from several number of very small regions, called domains.

Weiss theory of ferromagnetism is also called domain theory of ferromagnetism. It has following points:

The domains which are aligned approximately along the direction of the applied magnetic field grow in size at the cost of unfavorably oriented domains, that is, those align opposite to the field direction get reduced. In other words, the domain boundaries move so as to expand the favorable domains.Also domains rotate and orient themselves in the direction of the external magnetic field.

In the presence of the weak external field, the magnetisation in the material occur mostly by the process of domain growing, but in the strong magnetic field the material is magnetised mostly by the process of domain alignment. When the field is removed, the domain boundaries do not recover their original positions and thus the material is not completely demagnetised, but some residual magnetism remains in it.

Answer 2

Atoms of a ferromagnetick material have a permanent non-zero magnetic dipole moment, arising mainly from spin magnetic moments of the electrons.

According to the domain theory, a ferromagnetic is composed of small regions called domains.

A domain is an extremely small region (e.g., a size of about 10−6cm310-6cm3) containing a large number of atoms (something like 10151015 atoms as in common iron). Within each domain, the atomic magnetic moments of nearest-neighbour atoms interst strongly through exchange interaction (a quantum mechanical phenomenon) and align themselves parallel to each other even in the absence of an external magnetic field. A domain is therefore, s[ontaneously magnetized to saturation.

In an unmagnetized material, however, the directions of magnetization of the different domains are so oriented that the net magnetization is zero.

When an external magnetic field is applied the resultant magnetization of the specimen increases. This is achieved in either of two ways : Either a domain that is favourably oriented grows on size at the expenses of a less favourably oriented domain, or the direction of magnetization of an entire domain changes and tends to align along the external magnetic field.

When a weak magnetic field is applied, favourably oriented domains grow in size by domain boundary displacement, Fig In strong fields, the domains changes their magnetization by rotation, Figs. and. After the external field is removed, it may be energetically favourable for a domain's direaction of magnetization to perist. Then, the specimen has a permanent magnetic dipole moment. This phenomenon, called magnetic remanence, i.e., magnetic retentivity, is the basis of the existence of permanent magnets.