[ealloc]

This explains it: https://en.wikipedia.org/wiki/Curie_temperature

In the ferromagnetic phase, the atoms are magnetically aligned and their magnetic fields adds up. If you increase the temperature past a certain point though, the atoms are jiggled enough that they can no longer stay aligned and they 'point' in random directions, and their magnetic fields cancel out.

This is a big problem in hard drives: If you increase the temerature of a hard drive the magnetic regions no longer maintain their magnetization and your data gets erased. A lot of hard drive technology (eg 'perpedicular recording') has been developed to solve this problem.