The Solid State

Ferromagnetic substances would make better permanent magnets.In solid state, 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  and a strong magnetic effect is produced. This ordering of domains persist even when the magnetic field is removed and the ferromagnetic substance becomes a permanent magnet.

 In ccp structure of a sphere there is 2A tetrahedral voids and A octahedral voids.

 there is 8 tetrahedral voids and 4 octhahedral  voids in a face centred cubic unit cell.

Frenkel defect.