Zero-Field Magnetic Relaxation in Cerous Magnesium Nitrate below 0.2 K

The magnetic relaxation of ${\mathrm{Ce}}^{3+}$ in cerous magnesium nitrate (CMN) has been measured in zero applied field at temperatures in the range 0.02-0.2 K. For pure CMN, the relaxation is phonon bottlenecked, with some evidence for resonant trapping. Addition of Pr impurity to CMN reduces the severity of the bottleneck, apparently by excitation of the ${\mathrm{Pr}}^{3+}$ hyperfine levels via a two-phonon process. With sufficient Pr content, the relaxation becomes dominated by the thermal boundary resistance between the bath and the crystal lattice.