The substances in which atoms cooperate with each other in such a way that they exhibit a strong magnetic effect and are strongly attracted by magnetic fields are called ferromagnetic substances. For example, Nickle, Cobalt, Alnico, etc.
The small magnetized regions in a ferromagnetic substance are called domains.
In 1907 A.D., Weiss proposed domain theory to describe the ferromagnetic phenomenon in materials. According to this theory, a solitary crystal of ferromagnetic strong consists of a lot of tiny areas, and each region is automatically allured to saturation level called a domain. The dimension of the domain might vary from 10 − 6 to the entire volume of the crystal.
The spin magnetic moments of whole atoms restricted within a domain are oriented in a certain direction. The directions of magnetizations of various domains of the sampling are random to make sure that the resultant of magnetizations of all domains in the material is zero in the absence of an applied magnetic field.
These domains arise since the energy is not minimum when a large sampling has a uniform magnetization. When the external magnetic field is applied, these domains align in the direction of the applied field and the specimen shows magnetization and also becomes a magnet.
In the presence of the weak external field, the magnetization in the material takes place mostly by the process of domain growing, however, in the strong magnetic field the material is magnetized primarily by the procedure of domain placement.
When the field is removed, the domains orientation does not recover its initial positions, and also thus the product is not completely demagnetized, yet some residual magnetism remains in it.
Domains in Soft Magnetic Material
In soft materials, domains can easily be oriented on applying an external magnetic field and on removing the field, also return to the original position. Iron is the best example.
Domains in Hard Magnetic Material
In hard substances, domains cannot be easily oriented on applying the external magnetic field. Steel is a good example of hard magnetic material. The domain in the steel can be aligned by a very strong external magnetic field, but once oriented, retain their positions.
Effect of Temperature
Thermal vibrations tend to disturb the orderliness of the domains. Ferromagnetic substances preserve the orderliness at an ordinary temperature which begins to lose their orderliness due to an increase in temperature, this process starts at a certain temperature.
The temperature above which a ferromagnetic substance becomes paramagnetic is called curie temperature. The Curie temperature for iron is about 750 ᵒC.