A horseshoe magnet holding some safety-pins. Magnetic objects are mainly composed of ferromagnetic elements such as iron, cobalt and nickel. They have the capability to retain a certain amount of magnetization once exposed to an external magnetic field. This phenomenon is explained by the presence of domains" in the crystalline structure of these metals. They are small regions, 1-0.1mm across, in which the magnetic moment of each atom has the same direction. When an external field is applied all the domains are lined up with the field providing the high magnetization."