Thermal resistance between a paramagnetic salt (CMN) and liquid helium at millikelvin temperatures

We report new measurements of the thermal resistance between cerium magnesium nitrate (CMN) and liquid helium. AtT=100 mK, the thermal resistanceR for powdered CMN in contact with liquid³He-⁴He or liquid³He is about 2000 times the Kapitza boundary resistance. From the field and temperature dependences ofR we conclude that it is dominated by the phonon-bottlenecked spin-lattice resistance. However, if high-purity liquid³He is in contact with CMN, we observe, below 20 mK, a magnetic boundary resistance in parallel to the spin-lattice resistance. This resistance is two orders of magnitude smaller than the spin-lattice resistance atT=10 mK, and is described byR T^1.4. The absolute values of the magnetic boundary resistance, as well as the spatially limited phonon-bottlenecked resistance, are extremely sensitive to the surface treatment of the CMN crystals.