We point out the suitability of rare-earth ions in singlet ground states for the production of very low temperatures by means of nuclear adiabatic demagnetization. We have observed nuclear cooling in PrBi from an initial temperature of 0.026°K to a final temperature of 0.01°K.

• Received 11 September 1968

DOI:https://doi.org/10.1103/PhysRevLett.21.1221