Nuclear Magnetic Ordering in PrNi5at 0.4 mK

The nuclear specific heat of the hyperfine-enhanced Van Vleck paramagnet Pr${\mathrm{Ni}}_5$ has been measured from 0.2 to 100 mK in magnetic fields up to 6 T. The ${}_{}{}^{141}{}_{}{}^{}\mathrm{Pr}$ nuclei order ferromagnetically at 0.40±0.02 mK. Other results reported are hyperfine enhancement factor $1+K=12.2\pm 0.5$, enhanced nuclear magnetic moment $\mu =(0.027\mathrm{}\pm 0.004){\mu }_{\mathrm{B}}$, internal field $B_0\mathrm{}(T=0\mathrm{})=66\mathrm{}\pm 10$ mT, and nuclear exchange parameter ${\Sigma }_j\frac{{J_{\mathrm{ij}}}^N}{k_{\mathrm{B}}}=0.20\mathrm{}\pm 0.04$ mK.