Measurements of the Specific Heats of Three Magnetic Salts at Low Temperatures

New experimental results are presented of the heat capacity of chromium potassium alum, ferric ammonium alum, and manganese ammonium Tutton salt below 0.7°K. They have been obtained using two different experimental methods which differ in the way in which the samples are placed in thermal contact with the cerium magnesium nitrate thermometer and the electrical heater. $\lambda$-type transitions were found at 0.0260°K in FeN${\mathrm{H}}_4$ alum and at 0.173°K in Mn${\left(\mathrm{N}{\mathrm{H}}_4\right)}_2$ Tutton salt. The heat capacity of CrK alum, measured to 0.017°K, is a good fit to the theory of O'Brien using paramagnetic resonance parameters suggested by Bleaney. The entropy of each salt has been calculated from the specific-heat data, and a comparison with previous work is presented. An estimate of the thermal resistance between the paramagnetic crystals and ${\mathrm{He}}^4$ is obtained.