Measurements of the heat capacity of manganese ammonium Tutton salt as close as 4.8×${10}^{-5\mathrm{}}$ °K to $T_c\left(\approx 0.176{}_{}{}^{\circ }{}_{}{}^{}\mathrm{K}\right)$ are presented. The measurements were made using both a cerium magnesium nitrate and a chromium potassium alum thermometer. Power- and logarithmic-law fits to the data are possible above $\epsilon \approx {10}^{-3\mathrm{}}$ for a limited range of $\epsilon =\frac{(T-T_c)}{T_c}$. In the power-law fit the heat capacity varies as ${\epsilon }^{-\frac{1}{8}}$. For smaller $\epsilon$, a pronounced rounding of the heat capacity is observed below and above $T_c$. In every series of measurements, a $T_c$ could be defined with an accuracy in $\epsilon$ better than 3×${10}^{-5\mathrm{}}$. A comparison with measurements on other magnetic transitions and a simple hypothesis to explain the "rounding" of the peak are also presented.

• Received 7 August 1967

DOI:https://doi.org/10.1103/PhysRev.165.692