Proposed Absolute Temperature Scale for Cerium Magnesium Nitrate below 0.003 K

The absolute temperature scale for single-crystal cerium magnesium nitrate (CMN) has been extended to entropies as low as $\frac{S}{R}=0.002\mathrm{}$ by adiabatic demagnetization from values of $\frac{H}{T}$ up to 68 kOe/K. The temperature dependence of the highly anisotropic angular distribution of the 255-keV $\gamma$ ray from oriented ${}_{}{}^{137m}{}_{}{}^{}\mathrm{Ce}$ in the CMN provided the thermometric parameter. The nuclear-orientation results were interpreted with the spin Hamiltonian $\mathcal{H}=g_{\perp }\beta H_x{S}_x+B(S_x{I}_x+S_y{I}_y)$, where $H_x$ is a calculated dipolar field. The hyperfine-structure constant $B$ was determined by normalizing the higher-temperature nuclear-orientation results to the calorimetric results of Hudson and Kaeser and of Mess et al. at high entropies. A provisional temperature scale, based on both our nuclear-orientation results and the calorimetric work, is proposed. This scale is compared in detail with the results from earlier studies of CMN. The ${}_{}{}^{137m}{}_{}{}^{}\mathrm{Ce}$ $\gamma$-ray thermometer was also used to investigate the thermal behavior of cerium zinc nitrate (CZN). The preliminary nuclear-orientation results indicate a high degree of similarity between CZN and CMN.