Atoms of a ferromagnetic material have a spin associated with them and hence are magnetic in nature. In the absence of external magnetic field, groups (called domains) are formed in the material in which the spins are aligned. The spin of each domain is different and the net magnetic field is zero. On the presence of an external magnetic field, the spins of the domains realign along with the magnetic field and the material shows magnetic properties. This results in strong magnetism in the material. When a strong magnetic field is applied for a longer period of time, some domains retain their spin resulting in the retention of magnetic field in the material hence converting it to a permanent magnet. Some common ferromagnetic materials are iron, nickel, cobalt and their alloys.