= Curie constant
{c}
{title2=$C$}
The <Curie constant> is the coefficient $C$ in <Curie's law> $\chi=C/T$ for an appropriate dilute or high-<temperature> paramagnet. It sets the strength of the weak-field <magnetic susceptibility>, rather than an ordering <temperature>. In an <SI> volume-susceptibility convention for number density $n$ of identical noninteracting moments,
$$
C=\frac{\mu_0n\mu_{\mathrm{eff}}^2}{3k_B}.
$$
Here $\mu_{\mathrm{eff}}$ is the effective <magnetic moment> and $\mu_0$ the <vacuum permeability>. A mole-based susceptibility uses a correspondingly mole-based constant.
Plotting <magnetic susceptibility> against inverse <temperature> can estimate $C$ in the linear weak-field regime. Exchange interactions or thermal occupation of additional levels can change the dependence, so a fitted $C$ is meaningful only with its range and normalization stated. The factor $1/3$ reflects orientational averaging in the isotropic moment model; an anisotropic response requires more information than one scalar constant.
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