High-Resolution Specific Heat of CoCl2· 6H2O

The specific heat of a single crystal of Co${\mathrm{Cl}}_2$ · 6${\mathrm{H}}_2$O has been measured with high resolution near $T_N=2.29\mathrm{}$ K. The data were fitted to the usual asymptotic form $C=A{\epsilon }^{-\alpha }+B$, where $\epsilon =|1-\frac{T}{T_N}|$, using several different techniques. An examination of the gradient $\frac{\partial C}{\partial T}$ showed that the critical region was limited to $\epsilon \lesssim 7\times {10}^{-2\mathrm{}}$ and that for $\epsilon \lesssim 4\times {10}^{-3\mathrm{}}$ the specific heat was rounded. The data were found to fit the asymptotic form for $4\times {10}^{-3\mathrm{}}\lesssim \epsilon \lesssim 7\times {10}^{-2\mathrm{}}$ with ${\alpha }^{\prime }=-0.19\mathrm{}\pm 0.04$ for $T<\mathrm{}{T}_N$ and $\alpha =0.34\mathrm{}\pm 0.05$ for $T>\mathrm{}{T}_N$. It was also found that the observed rounding could be predicted by treating the sample as a system of microcrystals having a finite distribution of ordering temperatures. It was thus possible to produce a reasonable fit to all the data for $\epsilon <7\mathrm{}\times {10}^{-2\mathrm{}}$ using the above values of ${\alpha }^{\prime }$ and $\alpha$ which were determined from the data in the "unrounded critical region." It was further found that the results of a previous experiment in another laboratory could also be fitted over the entire critical region using the above values of ${\alpha }^{\prime }$ and $\alpha$ with a somewhat broader distribution of ordering temperatures.