There are some materials when placed in the magnetic field acquire strong magnetic field towards the magnetizing field. Again when these materials are placed in a magnetic field the magnetic field inside these materials increases manifold. Again, magnetism disappears if a fixed temperature exceeds. These materials are ferromagnetic materials. Example – iron, nickel, cobalt etc.
Theory: The atoms of ferromagnetic substances may be thought of as petite magnets with an N-pole and an S-pole. These atomic magnets, or dipoles, interact with their adjacent neighboring dipoles and a group of them line up with their magnetic axes in the similar direction to form a magnetic domain.
There are innumerable, very small regions in a ferromagnetic material, whose dimension is of the order 10-2 cm (nearly) and in each region, there are 1015 to 1017 atoms. These are spontaneously magnetized. These small regions are called magnetic domains or ferromagnetic domains. This means that the individual magnetic moments of the atoms are aligned with one another and they point in the same direction.