Nuclear-induced ferromagnetism below 50 mK in the Van Vleck paramagnet PrCu5

In the intermetallic compound Pr${\mathrm{Cu}}_5$ the ${}_{}{}^3{}_{}{}^{}H_4^{}{}_{}{}^{}$ multiplet of the Pr ion is crystal field split such that the lowest state is a singlet, leading to a nearly temperature-independent Van Vleck susceptibility below 4.2 K. At very low temperatures (below ∼ 50 mK) however, we observe ferromagnetic self-polarization of the singlet due to hyperfine and exchange interactions. Specific-heat and magnetization measurements down to 5 mK are presented and discussed in the molecular-field model of a coupled electron-nuclear system.