Spin–bath relaxation times τ of cerium ethylsulphate (CeES) immersed in liquid helium have been measured at magnetic fields up to 6 kOe. The electronic spin system of CeES has been heated by pulsed microwave radiation and the return to the equilibrium bath temperature has been monitored, using the optical Faraday rotation. At temperatures below the λ point of liquid helium, τ is a few ms and weakly dependent on magnetic field and temperature. It is assumed that the Kapitza boundary resistance Rk is the limiting factor in the energy transport between the spin system of CeES and the surrounding liquid helium. This assumption is supported by measurements of τ above the λ point between CeES and liquid or gaseous helium. Rk has been calculated from the experimental values of τ using a simple thermodynamical model. The resulting temperature dependence can be fitted with the expression Rk = 30 T−2.4 cm2 °K s (joule)−1 independent of magnetic field. The same model has been applied to PrES, which has been shown previously to exhibit a similar relaxation behavior. The results of Rk are close to those obtained for CeES.
