Point Curie

Short answer: The Curie point, or Curie temperature, is named after Pierre Curie. His 1895 thesis, 'Propriétés magnétiques des corps à diverses températures' (Magnetic properties of substances at various temperatures), showed how magnetism changes with temperature.

In a ferromagnet such as iron, a quantum effect called exchange interaction lines up the atoms' magnetic moments within small regions called domains. Above the Curie temperature, heat scrambles that alignment. The material loses its spontaneous magnetization and becomes paramagnetic: it's only weakly attracted to a magnet, and only while a field is applied. The Curie temperature is about 770 °C for iron, 358 °C for nickel and about 1,115 °C for cobalt. That's why a red-hot piece of iron stops sticking to a magnet and is attracted again as it cools. Being below the Curie point doesn't automatically make something a permanent magnet. In an ordinary iron nail the domains point in different directions and cancel out, which is why the nail isn't a magnet until it's magnetized. Antiferromagnets have a similar transition called the Néel temperature. The footage shows a torch heating a small metal piece until it glows, while a magnet on a rod hangs above it.

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