Curie heat-capacity density from an Arrott plot ($\Delta c_H=T_c{\alpha }^2/(2b)$)

For density $F=\alpha (T-T_c)M^2/2+b{M}^4/4-{\mu }_{0}HM$, 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 $X_0(T)=a(T)/{\mu }_0$. ThenThis 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.