 Weiss molecular field

The Weiss molecular field replaces exchange coupling to neighbouring moments by an effective local field proportional to the average magnetization. For two-state moments and $B_E={\mu }_0\lambda M$, self-consistency gives $m=\tanh (m/t)$ at zero applied field, where $m=|M|/(n\mu )$ and $t=T/T_C$, with $k_B{T}_C={\mu }_0\lambda n{\mu }^2$. This field represents a quantum exchange energy, not generally the actual magnetostatic field generated by the moments.