Heat-Capacity Measurements on Manganese Dibromide Tetrahydrate near Its Néel Temperature

Heat-capacity measurements on very small single crystals of manganese dibromide tetrahydrate have been made using the ac or oscillating-temperature method. These measurements show rounding in a temperature range of about 1 mK at the Néel temperature (2.12 K). We believe this rounding is due to chemical impurities. Outside this region the data may be fitted by a power-law singularity, within the experimental error, with $0.08\le \alpha \le 0.23$, $-0.10\mathrm{}\le {\alpha }^{\prime }\le 0.03$, and $2.1195\le T_N\le 2.1203$ K. The errors in $\alpha$, ${\alpha }^{\prime }$, and $T_N$ were obtained from a ${\chi }^2$ test for 90% probability that the true values lie within these limits. Comparison is made to the heat-capacity measurements by Dixon and Rives on Mn${\mathrm{Cl}}_2$·2${\mathrm{H}}_2$O, which is nearly isostructural to Mn${\mathrm{Br}}_2$·4${\mathrm{H}}_2$O. The heat capacity in the critical region of partially deuterated crystals with 1% deuterium was measured to obtain the effect of this impurity on the rounding. No significant effect was observed.