Nuclear Cooling in PrPt5and PrTl3to below 5 mdeg K

Nuclear adiabatic demagnetization experiments have been carried out on the Van Vleck paramagnets Pr${\mathrm{Pt}}_5$ and Pr${\mathrm{Tl}}_3$. Starting from 60 mdeg K and 20 kOe, end temperatures of 5 and 3 mdeg K, respectively, are obtained. In Pr${\mathrm{Pt}}_5$, the end temperature seems to be limited by a residual nuclear level splitting due to the hexagonal structure. No sign of a nuclear ordering phenomenon can be detected in Pr${\mathrm{Tl}}_3$ down to 3 mdeg K. The suitability of these materials for magnetic cooling purposes is pointed out.