Ferromagnetism is the phenomenon by which materials in an external magnetic field become magnetized and remain magnetized for a period after the material is no longer in the field. Examples of ferromagnetic substances are iron, cobalt, nickel, gadolinium, and dysprosium. All ferromagnetic materials are made up of domains, microscopic regions within which all magnetic moments are aligned. In an unmagnetized sample, the domains are randomly oriented. When the material is placed in an external magnetic field the domains become oriented according to the alignment of the magnetic field submitted.