Curie temperature
/ KYOOR-ee TEM-pruh-cher /
Heat a magnet hot enough and it simply stops being a magnet. There is a sharp threshold — different for each material — above which the magnetism vanishes entirely, and below which it springs back. That tipping point is the Curie temperature, named for Pierre Curie, who first charted it.
It is a contest between two opposing influences. The exchange interaction tries to keep neighboring atomic moments aligned and orderly, while heat tries to shake them into random chaos. Below the Curie temperature, order wins and the material is a ferromagnet. As you approach it, heat gains the upper hand and the alignment weakens; right at the Curie temperature the order collapses to zero. Above it, the moments still exist but tumble randomly, and the material becomes an ordinary paramagnet.
The Curie temperature matters because it is a genuine phase transition — as crisp and real as ice melting into water — and it sets where a magnetic material can actually be used. For iron it is a glowing 770 degrees Celsius; for some materials it is below room temperature, which is why they seem 'non-magnetic' on your bench. The common misconception is that heating gradually drains a magnet's strength; instead, magnetization holds on and then drops away abruptly as the threshold nears.
Hang a steel nut from a magnet, then heat the nut with a flame. As it passes its Curie temperature it abruptly loses its grip and drops — and as it cools back below the threshold, it becomes attracted again. A simple flame turns magnetism on and off.
Heated past its Curie temperature, steel loses its magnetism and falls; cooled, it sticks again.
The Curie temperature is the threshold for ferromagnets and ferrimagnets, where the order is the kind that gives net magnetism. The matching threshold for antiferromagnets, whose order hides itself, has a different name: the Neel temperature.