Explain ferromagnetism and ferromagnetic substances.

(N/A) Ferromagnetic substances are those which get strongly magnetized when placed in an external magnetic field.
They have a strong tendency to move from a region of weak magnetic field to a region of 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,similar to those in a paramagnetic material.
$A$ key feature of ferromagnetic substances is the existence of domains. In each domain,atoms are arranged such that their dipole moments are aligned in the same direction. Hence,each domain has a net magnetization. However,if the entire substance is considered,the different domains have randomly oriented dipole moments,and thus the net magnetization of the bulk substance is zero. This is shown in Figure $(a)$.
The typical domain size is $1 \ mm$ and each domain contains about $10^{11}$ atoms.
When an external magnetic field $\vec{B}_{0}$ is applied,the domains orient themselves in the direction of $\vec{B}_{0}$,and the domains grow in size,causing the whole substance to become strongly magnetized. This is shown in Figure $(b)$.
In a ferromagnetic material,the magnetic field lines are highly concentrated.
In a non-uniform magnetic field,the sample tends to move towards the region of high field intensity.
Examples of ferromagnetic substances include Iron $(Fe)$,Cobalt $(Co)$,Nickel $(Ni)$,Gadolinium ($Gd$,$Z = 64$),and Dysprosium ($Dy$,$Z = 66$).