.

The Curie-Weiss law describes the magnetic susceptibility χ of a ferromagnet in the paramagnetic region above the Curie point:

\( \chi = \frac{C}{T - T_{c}} \)

where C is a material-specific Curie constant, T is absolute temperature, measured in kelvins, and Tc is the Curie temperature, measured in kelvins. The law predicts a singularity in the susceptibility at T = Tc. Below this temperature the ferromagnet has a spontaneous magnetization.

In many materials the Curie-Weiss law fails to describe the susceptibility in the immediate vicinity of the Curie point, since it is based on a mean-field approximation. Instead, there is a critical behavior of the form

\( \chi \sim \frac{1}{(T - T_{c})^\gamma} \)

with the critical exponent γ. However, at temperatures T >> Tc the expression of the Curie-Weiss law still holds, but with Tc replaced by a temperature Θ that is somewhat higher than the actual Curie temperature. Some authors call Θ the Weiss constant to distinguish it from the temperature of the actual Curie point.

.

Curie–Weiss law