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