= Curie heat-capacity density from an Arrott plot
{c}
{title2=$\Delta c_H=T_c\alpha^2/(2b)$}
For <density> $F=\alpha(T-T_c)M^2/2+bM^4/4-\mu_0HM$, the zero-field ordered minimum is $-\alpha^2(T-T_c)^2/(4b)$. Its constant-field heat-capacity <density> jump is $\Delta c_H=T_c\alpha^2/(2b)$. In an <Arrott plot>, let physical slope be $S=\mu_0/b$ and horizontal intercept be $X_0(T)=a(T)/\mu_0$. Then
$$
\Delta c_H=\frac{T_c\mu_0S}{2}\left(\frac{dX_0}{dT}\right)^2.
$$
This makes numerical sensitivity explicit: uncertainty in the intercept slope is squared. A volumetric <free energy> determines a volumetric heat-capacity jump; total or molar values need further sample information.
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