Hyperfine Interactions, Magnetic Impurities, and Ordering in Praseodymium

The antiferromagnetic ordering in Pr due to the coupling of the $4f$ electronic system to the nuclei and to magnetic Nd impurities has been studied by neutron diffraction. A pure monocrystal of Pr develops true long-range order at about 50-60 mK. The ordering in both this crystal and a $P\mathcal{r}\mathrm{Nd}$ alloy is, however, presaged by unusually intense and longrange magnetic fluctuations, which are ascribed to the special form of the exchange interactions.