Specific Heats of NiCl2·6H2O and CoCl2·6H2O between 1.4° and 20°K

Abstract

The specific heats of Ni${\mathrm{Cl}}_2$·6${\mathrm{H}}_2$O and Co${\mathrm{Cl}}_2$·6${\mathrm{H}}_2$O have been measured from 1.4° to 20°K. Lambda-type anomalies were observed at 5.34°K and 2.29°K for Ni${\mathrm{Cl}}_2$·6${\mathrm{H}}_2$O and Co${\mathrm{Cl}}_2$·6${\mathrm{H}}_2$O, respectively, associated presumably with antiferromagnetic-paramagnetic transitions. It has been possible in each case to separate out the magnetic contribution to the specific heat and thus to compute the total magnetic entropy gained in the transition from the fully ordered to completely disordered condition. This quantity was found to be given accurately by $R\ln \mathrm{}(2S+1)\mathrm{}$ with $S=\frac{1}{2}$ for ${\mathrm{Co}}^{++}$ and $S=1\mathrm{}$ for ${\mathrm{Ni}}^{++}$. Short range order is particularly pronounced in these substances, the fraction of the total magnetic entropy gained above the Néel point being 0.52 for Co${\mathrm{Cl}}_2$·6${\mathrm{H}}_2$O and for 0.40 for Ni${\mathrm{Cl}}_2$·6${\mathrm{H}}_2$O. Other features of the anomalies are discussed.