Curie temperature is the temperature below which a paramagnetic substance suddenly becomes ferromagnetic, that is, when the atoms spontaneously align themselves in the bulk material such that their individual magnetic fields re-inforce each other. This usually occurs within small boundaries called domains, but each domain takes up a randomly orientated magnetisation with the effect that there is no net magnetic field outside the substance. A ferromagnetic material can be 'permanently magnetised', that is generate a net non-zero external magnetic field, when all its internal domains are each persuaded to take up the same magnetic orientation, by the temporary application of an external magnetic field. All such ferro-magnetism is 'forgotten' if the temperature of the substance is taken above its Curie temperature when the substance becomes paramagnetic. See also 'Magnetic susceptibility'.
Iron, cobalt and nickel are the only elements which are ferromagnetic at room temperature because their Curie temperatures of 770░C, 1131░C and 358░C respectively are above room temperature. The so-called rare earth elements gadolinium, terbium, dysprosium, thulium, holmium and erbium all have much lower Curie temperatures. Gadolinium with a Curie temperature of 20░C becomes ferromagnetic just below room temperature, the others at cryogenic temperatures. Alloys incorporating either gadolinium or terbium are finding use as the magnetic coating for magneto-optical re-recordable CDs and computer discs because a laser can easily supply enough local heating to temporarily take a small dot above the Curie temperature.
The Neel Temperature is the temperature below which a substance becomes antiferromagnetic, that is, orients the magnetic axis of its' neighbouring atoms alternately anti-parallel then parallel. Antiferromagnetic substances have no net magnetisation. Chromium is antiferromagnetic below its Neel temperature of 202░C and so is manganese, but its Neel temperature is -173░C. Dysprosium exhibits both properties, but not simultaneously. It is antiferromagnetic below its Neel temperature of -94░C and ferromagnetic below its Curie temperature of -188░C.
When the TABLE chart is displaying Curie temperatures, the thermometer also shows the Curie/Neel temperatures (in Kelvin).
