Explain ferromagnetism and ferromagnetic substance.

Explanation

Ferromagnetic substance are those which gets strongly magnetised when placed in an external magnetic field.
They have strong tendency to move from a region of weak magnetic field to strong magnetic field. They get strongly attracted to a magnet.
The individual atoms (or ions or molecules) in a ferromagnetic material possess a dipole moment as in a paramagnetic material.
Domain is found in paramagnetic substance. In that domain atoms arranged so that their dipole moment can be found in the same direction. Hence, each domain has net magnetisation. But, if the entire substance is considered then the different domain have randomly dipole moment and hence net magnetisation will be zero. This is shown in figure.

Typical domain size is 1 mm and the domain contains about `10^11` atoms.
When we apply an external magnetic field `overset(to) (B_0)` the domains orient themselves in the direction of `overset(to) (B_0)` and domain grow in big size and whole substance have been magnetised.
In a ferromagnetic material the field lines are highly concentrated.
In non-uniform magnetic field, the sample tends to move towards the region of high field.
Iron, Cobalt, Nickel, Gadolinium Gd (Z = 64) and Dysprosium (Dy) Z = 66 are ferromagnetic substances.