We've discussed the loss of magnetism before: . The key is that there's a competition between the tendency of things to fall into low-energy states and the tendency to explore all different states. Low temperatures mean, by definition, more tendency to fall into low-energy states. In magnets, the states with all the little magnetic spins lined up together are the lowest energy states. So at low temperatures they settle into something close to one of those states.
Ypu might think that the magnetism would just gradually fade as you heated the magnet up. Instead, at the Curie point, all the long-range magnetism is lost. The reason is that as the fraction of spins that are lined up together falls, the field that makes it pay (by lower energy) to line up falls. That leads to more reduction in the magnetism. So there's a sort of positive feedback effect. Above the Curie point, it just doesn't pay to line up.
(published on 02/09/2015)