Thermal Boundary Resistance between Cerium Magnesium Nitrate and LiquidHe3

The thermal boundary resistivity between cerium magnesium nitrate and liquid ${\mathrm{He}}^3$ has been measured over the temperature range 0.07-0.7 K. The resistivity can be well described as $\frac{(55\mathrm{}\pm 10)}{T^3}$ ${\mathrm{cm}}^2$ K ${\mathrm{W}}^{-1\mathrm{}}$ which compares with $\frac{150}{T^3}$ ${\mathrm{cm}}^2$ K ${\mathrm{W}}^{-1\mathrm{}}$ as deduced from the Khalatnikov acoustic-mismatch theory of thermal boundary resistance. The theoretical values includes the surface-wave contribution. This agreement with acoustic-mismatch theory is in turn compared with that obtained for other materials, and is seen to be relatively good. Finally, the thermal-boundary-resistivity result has been used to reinterpret other experiments that measure the temperature decay following a change of magnetic field. It is shown that these experiments can be described by a relaxation process consistent, as far as size dependence, temperature dependence, and magnitude are concerned, with a phonon bottleneck.