The existence of domains leads to strongest magnetism in ferromagnetics. Due to the presence of cohesive forces in ferromagnetics the unpaired electron spins to line up parallel with each other in a region called a domain. A ferromagnetic domain is a region of crystalline matter whose volume may be between 10–12 m3 to 10–8 m3. Each domain contains about 1017 – 1021 atoms. The various atomic magnets are aligned in the same direction in a domain even in the absence of external magnetising field but different domains have magnetic moments in different directions such that within the domain, the magnetic field is intense, but in a bulk sample the material will usually be unmagnetized because the many domains will themselves be randomly oriented with respect to one another. When a ferromagnetic material is kept in external magnetising field, its domain experience force and torque which tends to align them along a definite direction, till magnetic saturation. This is called Magnetostriction effect. The extent of alignment is found to depends directly upon the intensity of the magnetising field & inversaly upon the temperature of the material.