Ferromagnetic Domains

FERROMAGNETIC DOMAINS

We can observe that ferromagnetic materials such as iron does not have magnetisation unless they have been previously placed in an external magnetic field. But according to Weiss theory, The molecular magnets in the ferromagnetic material are said to be aligned in such a way that, they exhibit a magnetisation even in the absence of an external magnetic field. This is called Spontaneous magnetisation. i.e., it should have some internal magnetisation due to quantum exchange energy.

Thus according to Weiss hypothesis, a single crystal of ferromagnetic material is divided into large number of small regions called domains. These domains have spontaneous magnetisation due to the parallel alignment of spin magnetic moments in each domain. But the direction of spontaneous magnetisation varies from domain to domain and are oriented in such a way that the net magnetisation of the specimen is zero as shown in Fig. 3.13. Due to this reason the iron does not have any magnetisation in the absence of an external field.

Now, when the magnetic field is applied, then the magnetisation occurs in the specimen by two ways

(i) By the movement of domain walls

(ii) By rotation of domains walls.

(i) By the movement of domain walls

The movement of domain walls takes place in weak magnetic fields. Due to this weak field applied to the specimen the magnetic moment increases and hence the boundary of domains are displaced, so that the volume of the domains changes as shown in Fig. 3.14.

(ii) By rotation of domains walls

The rotation of domain walls takes place in strong magnetic fields. When the external field is high (strong) then the magnetisation changes by means of rotation of the direction of magnetisation towards the direction of the applied field as shown in Fig. 3.15.

Experimental Evidence of Domain Structure (Bitter Powder Pattern)

The direct experimental evidence of domain structure is observed from the microphotographs of domain boundaries obtained by the technique of magnetic powder pattern which discovered by Bitter.

In this method, a drop of colloidal suspension of finely divided ferromagnetic powder is allowed to spread over the surface of ferromagnetic material under investigation.

Now, when the colloidal particles are observed through microscope, it is found that the colloidal particles are collected along the domain boundaries because of the strong local magnetic field, which exists near the domain boundaries as shown in Fig. 3.16.

Thus the particles are attracted about well defined lines which represents the domain boundaries. Now when the external field is applied, the domain walls will start moving and that can be viewed through the microscope.