The so-called Néel temperature was named after his discoverer Louis Néel, a French physicist who received the Nobel Prize in 1970. It describes a temperature limit at which an antiferromagnetic substance becomes a paramagnet. The Néel temperature can thus be viewed as an analogue to the Curie temperature of ferromagnetic materials.
What is antiferromagnetism?
Antiferromagnets consist of many individual elementary magnets. These have the same magnetic moment in terms of their magnitude, but their orientation is antiparallel, i.e. opposite to each other. For this reason, no permanent external magnetic field can be measured. Because the spins of the atoms have a fixed angle of rotation, they compensate for each other in the entire material. In the case of ferromagnets, however, these spins are aligned parallel to one another.
Ferro and antiferromagnets only have a stable order up to a certain temperature. Above a special phase transition temperature, the permeability of antiferromagnets decreases and the material behaves paramagnetically. This temperature is the Néel temperature.
Deriving the Néel temperature
The Néel temperature is assigned the formula symbolTN, the Curie temperature the symbolTC. Above TN, the following applies to the susceptibility χm as a function of the temperature T ith the material-specific Curie constant C:
(Exchange field in which the coupling regulates ) needs:
This results in:
κC can be identified as TN, the material-specific Néel temperature.