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 S/R=0.002 by adiabatic demagnetization from values of H/T up to 68 kOe/K. The temperature dependence of the highly anisotropic angular distribution of the 255-keV gamma ray from oriented Ce-137m in the CMN provided the thermometric parameter. The nuclear-orientation results were interpreted with the spin Hamiltonian H=g(perpendicular to)beta HxSx+B(SxIx+SyIy), 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 Ce-137m 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.