Two-state Curie law
= Two-state Curie law
{title2=$C=\mu_0n\mu^2/k_B$}
For independent magnetic moments constrained to $\mu_z=\pm\mu$, the one-site <partition function> is $2\cosh[\mu\mu_0H/(k_BT)]$. The weak-field <magnetization> is $M=\mu_0n\mu^2H/(k_BT)$, so the SI <magnetic susceptibility> obeys $\chi=C/T$ with $C=\mu_0n\mu^2/k_B$. If $\mu=gS\mu_B$, this involves $S^2$, not the unrestricted-spin factor $S(S+1)/3$. The constraint on allowed moment orientations must be specified before using a <Curie law> coefficient.