Improved Thermal Contact at Ultralow Temperatures betweenHe3and Metals Containing Magnetic Impurities

For Pd containing 30 ppm Fe, and for Au containing 0.2% Gd, we have found a thermal boundary resistance that is approximately proportional to $T^{-1\mathrm{}}$ below 20 mK. The improvement in heat transfer is, at 4 mK, a factor of 50 for the Pd sample and a factor of 20 for the Au sample over that expected for phonon conduction in the same samples. The mechanism for thermal transfer is thought to be a direct coupling between the magnetic moments of the impurities and the ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ nuclei.