the Curie temperature

Heat a magnet hot enough and it stops being a magnet. There is a sharp temperature, the Curie temperature, above which a ferromagnet simply loses its ferromagnetism and turns into a weak, ordinary paramagnet. Below it, an iron magnet clings to your fridge; carry that same iron above 770 degrees Celsius and it will no longer stick to anything, until it cools back down and the magnetism returns.

The reason is a contest between order and chaos. The exchange interaction tries to lock neighboring atomic moments parallel, while thermal vibration tries to shake them into random directions. As temperature rises the shaking wins more and more, until at the Curie temperature it finally overpowers the coupling and the long-range alignment collapses all at once. It is a genuine phase transformation of the magnetic order, not a slow fading. Different materials have very different Curie points: iron at about 770 degrees C, cobalt at about 1121 degrees C, nickel at about 358 degrees C, and gadolinium right around room temperature, so a piece of gadolinium is a magnet in your fridge but not in your warm hand.

The Curie temperature is a hard design limit for anything magnetic that runs hot, an electric motor, a transformer, a magnetic sensor, must stay comfortably below it or its magnetism will fade and it will fail. It is also useful: the transition is so sharp that some rice cookers use a Curie-point magnet as a thermostat, releasing when the pot passes a set temperature. Ferrimagnets have their own analogous transition. One caveat: the change is reversible for a simple ferromagnet, cool it back and the magnetism returns, but a permanent magnet driven near its Curie temperature may lose its stored magnetization for good even after cooling, because heat let its domains scramble.