Question

a way to demagnetise something is to heat it right? why does it become magnetised again when it's cooled?

Answer 1

Heating a magnet past its Curie point will destroy the long range ordering resulting in demagnetization......
As you heat a magnet you supply it with more thermal energy, so the individual electron spins become more likely to be in high-energy states, pointing oppositely to their neighbors. That means that they’re less lined up so the total magnetism is reduced.
At some point, in between the weakening of the overall magnetism and the availability of extra thermal energy, it becomes easy for domain walls- the boundaries between regions that are lined up pointing different directions- to slide around. Then the domains will rearrange so that they reduce the large-scale field energy by pointing different directions. That means that your permanent magnet is no longer overall magnetized.

Cooling the material will cause magnetic domains to form again at the Curie temperature, but unless an external field is applied as the material cools, the domains will point all different directions, so you won’t have a net magnetized permanent magnet.