# Néel temperature

The Néel temperature (after Louis Néel , who received the Nobel Prize in Physics for the description in 1970 ) is the temperature above which an antiferromagnetic substance becomes paramagnetic ; the thermal energy here becomes large enough to destroy the magnetic order within the material. The Néel temperature is thus the analogue of the Curie temperature of ferromagnetic substances. ${\ displaystyle T _ {\ text {N}}}$ ${\ displaystyle T _ {\ text {C}}}$ Above, the following applies to the magnetic susceptibility as a function of temperature : ${\ displaystyle T _ {\ text {N}}}$ ${\ displaystyle \ chi _ {m}}$ ${\ displaystyle T}$ ${\ displaystyle \ chi _ {m} = {\ frac {C} {T + T_ {N}}}}$ with the material- specific Curie constant ${\ displaystyle C.}$ Below , the susceptibility also decreases with decreasing temperature, i. H. at it has reached its maximum. ${\ displaystyle T _ {\ text {N}}}$ ${\ displaystyle T _ {\ text {N}}}$ The Néel temperature of hematite is e.g. B. at 675 ° C.

## Derivation

The derivation takes place from the molecular field theory : d. H. a magnetic moment is considered in the mean magnetic field of its neighbors. As a consequence, the Curies law applies : ${\ displaystyle B}$ ${\ displaystyle \ mu _ {0} M = {\ frac {C} {T}} (B- \ kappa \ cdot \ mu _ {0} M).}$ It is

• ${\ displaystyle \ mu _ {0}}$ the magnetic field constant
• ${\ displaystyle M}$ the magnetization
• ${\ displaystyle \ kappa \ cdot \ mu _ {0} M}$ the exchange field, whereby the coupling regulates.${\ displaystyle \ kappa}$ So it follows:

${\ displaystyle \ Rightarrow \ chi _ {m} = {\ frac {\ mu _ {0} M} {B}} = {\ frac {C} {T + \ kappa C}}}$ and can be identified as. ${\ displaystyle \ kappa C}$ ${\ displaystyle T_ {N}}$ ## literature

• Horst Stöcker: Pocket book of physics. 4th edition, Verlag Harry Deutsch, Frankfurt am Main 2000, ISBN 3-8171-1628-4