Antiferromagnetic Transition in CoCl2·6H2O and Fisher's Relation

It is found that the zero-field heat capacity $C_p$ and parallel susceptibility ${\chi }_{\parallel }$ of the antiferromagnet Co${\mathrm{Cl}}_2$·6${\mathrm{H}}_2$O obey Fisher's relation $C=\frac{A\partial \left(\chi T\right)}{\partial T}$ through the Néel point $T_N\mathrm{}\left(0\right)\mathrm{}$. The coefficient $A$ is shown by a thermodynamic argument to yield the curvature of the antiferro-paramagnetic phase boundary in the $H-T$ plane for $H=0\mathrm{}$. A parabola having the curvature deduced in this way is found to fit the observed phase boundary over a wide range of $H$ and $T$.