Ferromagnetism

{tag=Second-order phase transition}
{wiki}

The wiki comments: https://en.wikipedia.org/w/index.php?title=Ferromagnetism&oldid=965600553#Explanation
> The Bohr-van Leeuwen theorem, discovered in the 1910s, showed that classical physics theories are unable to account for any form of magnetism, including ferromagnetism. Magnetism is now regarded as a purely quantum mechanical effect. Ferromagnetism arises due to two effects from quantum mechanics: spin and the Pauli exclusion principle.
