Thermal resistance between the electronic spin system of CuK2(SO4)2⋅6H2O (powder) and 4He I I below 1.8 K to magnetic fields of 15 kG

Thermal resistance measurements were made on randomly oriented small crystals of high purity CuK2(SO4)2⋅6H2O [CPS] in contact with 4He ii. Three magnetothermodynamic methods are described for measuring the thermal resistance. The data have the temperature independence and approximate field dependence expected from a rate determining spatial phonon bottleneck. Various heat exchange mechanisms for the CPS–4He ii system are described, and the limitations of the standard phonon bottleneck model are discussed.