Substances that are attracted very strongly by a magnetic field are called ferromagnetic substances. Example: iron, cobalt, nickel, gadolinium and CrO2. Besides strong attractions, these substances can be permanently magnetised. The metal ions of ferromagnetic substances are grouped together into small regions called domains. Thus, each domain acts as a tiny magnet. In an unmagnetised piece of a ferromagnetic substance, the domains are randomly oriented and their magnetic moments get cancelled. When the substance is placed in a magnetic field all the domains get oriented in the direction of the magnetic field as shown in the figure and a strong magnetic effect is produced. This ordering of domains persists even when the magnetic field is removed and the ferromagnetic substance becomes a permanent magnet.