Each of the two methods that has been used to cool 3 He to temperatures in the millikelvin region has important limitations: Compressional or Pomeranchuk cooling produces a mixture of solid and liquid at a pressure that is determined by the details of the compression and which cannot be varied independently, and adiabatic demagnetization of cerium magnesium nitrate (CMN) cannot produce temperatures below its ordering temperature. The interest in the properties of 3 He at temperatures of 1 mK and lower provides an incentive for the development of other methods of cooling, and the most obvious alternative is by adiabatic demagnetization of nuclei in a metal. Attainment of lower temperatures than those accessible by demagnetization of CMN, however, would require a substantial reduction in the Kapitza or thermal boundary resistance, R B , at the metal-3 He interface. In the usual case of heat transfer by phonons, R B <*T~3. 6 S. Ezekiel and R. Weiss, Phys. Rev. Lett. _20, 91 (1968); L. O. Hocker, M. A. Kovacs, C. K. Rhodes, G. W. Flynn, and A. Javan, Phys. Rev. Lett. JU, 233 (1966). 7 W. Heitler, The Quantum Theory of Radiation (Oxford Univ. Press, London, 1954), 3rd ed. An exact solution to the two-level problem would not contain the 6 function but rather would have a sharply peaked function whose width near resonance is related to the inverse duration of a collision. Since this is small compared with the expected distortion of the upper electronic states, we may regard the peaked function as a <5 function. 8 If one takes the ratio of T at two different r i , it is possible to get the relative size of the matrix elements at these points.For Cu the proportionality constant can be reduced by work hardening 1 or precipitation hardening. 2 For Pt, RJQOZT" 2 and increases with the addition of low concentrations of 4 He, and it has been suggested that the conduction electrons may be directly coupled to the 3 He nuclei. 3 However, the lowest values of R B in the millikelvin region have been found for the CMN-3 He interface for which R B ccT. 4 In that case it has been shown that the energy transfer can be accounted for by a magnetic dipole coupling between the Ce +3 ions and the 3 He nuclei. 5 These results suggest that the low values of R B that would be necessary to make nuclear cooling in a metal an attractive method for cooling 3 He to temperatures near 1 mK might be achieved by the introduction of impurities with localized magnetic moments. We have made measurements on several systems to test this possibility. The initial results are encouraging, and are reported here.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 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...
