= Weiss molecular field
{c}
{title2=$H_E=\lambda M$}
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 $k_BT_C=\mu_0\lambda n\mu^2$. This field represents a quantum exchange energy, not generally the actual magnetostatic field generated by the moments.
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