CURIE POINT

also called Curie Temperature, temperature at which certain magnetic materials undergo a sharp change in their magnetic properties. In the case of rocks and minerals, remanent magnetism appears below the Curie pointabout 570 C (1,060 F) for the common magnetic mineral magnetite. This temperature is named for the French physicist Pierre Curie, who in 1895 discovered the laws that relate some magnetic properties to change in temperature. Below the Curie pointfor example, 770 C (1,418 F) for ironatoms that behave as tiny magnets spontaneously align themselves in certain magnetic materials. In ferromagnetic materials, such as pure iron, the atomic magnets are oriented within each microscopic region (domain) in the same direction, so that their magnetic fields reinforce each other. In antiferromagnetic materials, atomic magnets alternate in opposite directions, so that their magnetic fields cancel each other. In ferrimagnetic materials, the spontaneous arrangement is a combination of both patterns, usually involving two different magnetic atoms, so that only partial reinforcement of magnetic fields occurs. Raising the temperature to the Curie point for any of the materials in these three classes entirely disrupts the various spontaneous arrangements, and only a weak kind of more general magnetic behaviour, called paramagnetism, remains. One of the highest Curie points is 1,121 C (2,050 F) for cobalt. Temperature increases above the Curie point produce roughly similar patterns of decreasing paramagnetism in all three classes of materials. When these materials are cooled below their Curie points, magnetic atoms spontaneously realign so that the ferromagnetism, antiferromagnetism, or ferrimagnetism revives. The antiferromagnetic Curie point is called the Nel temperature in honour of the French physicist Louis Nel, who in 1936 successfully explained antiferromagnetism.