As molten iron cools down it crystallises at 1538°C (2800°F)into its delta allotrope, which has a body-centred cubic (BCC) crystal structure.
As it cools further its crystal structure changes to face centred cubic (FCC) at 1394°C, when it is known as gamma-iron, or austenite.
At 912°C (1674°F) the crystal structure again becomes BCC as alpha-iron also known as ferrite, is formed, and at 770°C (1418°F) (the Curie point, Tc ) the iron becomes magnetic as alpha-iron, which is also BCC, is formed.
As the iron passes through the Curie temperature there is no change in crystalline structure, but there is a change in the magnetic properties.
In unmagnetised iron, all the electron spins of the atoms within one domain are in the same direction.
However, in neighbouring domains they point in various directions and thus cancel out.
In magnetised iron, the electronic spins of all the domains are all aligned, so that the magnetic effects of neighbouring domains reinforce each other.
Although each domain contains billions of atoms, they are very small; each domain is about one thousandth of a centimetre across.
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