Nuclear Cooperative Ordering in Single-Crystal Praseodymium at Low Temperatures

We have observed a $\lambda$ anomaly in the nuclear heat capacity of a single crystal of praseodymium known not to order down to 1.8 K. The anomaly is associated with a cooperative magnetic ordering of the hexagonal-site nuclei at 25-30 mK of the type discussed by Murao. This result is in contrast to earlier calorimetric measurements where ionic order has precluded cooperative nuclear processes. The cooperative anomaly is suppressed by a field of 1.5 T.