Complex ac susceptibility, $\chi ={\chi }^{\prime }-i{\chi }^{\prime \prime }$, measurements of the clathrate compound ${\mathrm{Pr}}_3{\mathrm{Pd}}_{20}{\mathrm{Ge}}_6$ were performed in static fields up to 10 mT for $H\parallel \left[001\right]$ and at temperatures down to $500 \mu \mathrm{K}$. Praseodymium (Pr) nuclear magnetic moments at the $8c$ site, where quadrupole moments of $4f$ electrons order at ${\mathit{\text{T}}}_{\text{Q1}}=250$ mK, were found to order antiferromagnetically at 9 mK, as shown by a peak in ${\chi }^{\prime }$ and a substantial increase in thermal relaxation time. The large enhancement factor $\left(1+K_{8c}\right)$ obtained by calculation of the hyperfine-enhanced nuclear susceptibility of Pr at the $8c$ site accounts for the high transition temperature of Pr nuclear magnetic moments and the large ${\chi }^{\prime }$ below 30 mK. From analysis of the crystalline electric field and the mean-field approximation, we conclude that a $\chi$ peak at 77 mK can be ascribed to an antiferromagnetic ordering of magnetic moments of $4f$ electrons at the $4a$ site. We found that nuclear and $\mathit{\text{f}}$-electron moments order separately on two sublattices in this compound. The temperature and magnetic field dependence of ${\chi }^{\prime }$ and ${\chi }^{\prime \prime }$ between 30 and 60 mK are discussed in terms of dissipation phenomena.

• Received 6 March 2014
• Revised 6 August 2014

DOI:https://doi.org/10.1103/PhysRevB.90.100402