Magnetic Transition in Thiourea-Coordinated Cobalt Chloride

Specific-heat measurements have been made on a single crystal of Co${\mathrm{Cl}}_2$.${\left[{\left(\mathrm{N}{\mathrm{H}}_2\right)}_2\mathrm{C}\mathrm{S}\right]}_4$ in the temperature range 0.42°-13.9°K. A $\lambda$-type anomaly was observed at $T_N=0.92\mathrm{}°$K and is associated with a paramagnetic-antiferromagnetic transition. The total entropy change associated with the magnetic transition is 1.38 cal/mole deg and is completely accounted for from the expected value of $R$ $log\mathrm{}(2S+1)\mathrm{}$ where $S=\frac{1}{2}$ for the ${\mathrm{Co}}^{++}$ ion. Approximately 47% of this entropy change occurs above 0.92°K. The specific-heat results are in reasonable agreement with those calculated from magnetic susceptibility measurements from 0.42°K to 0.92°K but are somewhat lower above 0.92°K. Furthermore, the specific-heat measurements give slightly higher magnetizations than those expected from the molecular-field theory. These comparisons indicate that cobalt thiourea chloride is considerably anisotropic and that a two-sublattice model of nearest-neighbor interactions does not suffice to explain the spin arrangements for the ${\mathrm{Co}}^{++}$ ions.