Static and dynamical properties in the Pr-based filled skutterudite compoundPrFe4P12revealed by aP31-NMR study

Abstract

$^{31}\mathrm{P}$-NMR studies have been carried out to investigate magnetic properties in the Pr-based filled skutterudite compound $\mathrm{Pr}{\mathrm{Fe}}_{4}{\mathrm{P}}_{12}$. This compound shows an unusual phase transition at ${T}_{A}\ensuremath{\sim}6.5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, which is now regarded as antiferroquadrupole (AFQ) ordering from neutron-diffraction experiments. Splitting of the P-NMR spectrum due to the appearance of two P sites with different hyperfine fields was observed below ${T}_{A}$. From the field dependence, the splitting seems to disappear in zero magnetic field, indicating that the different hyperfine fields are not due to magnetic order, but to the appearance of two inequivalent P sites below ${T}_{A}$. This is ascribed to the distortion of the P cage surrounding a Pr ion below ${T}_{A}$, which is associated with the Pr orbital ordering. The nuclear spin-lattice relaxation rate $(1∕{T}_{1})$ shows the typical behavior of Kondo systems, where onset of local-moment screening due to the coupling between conduction and localized $\mathrm{Pr}\text{\ensuremath{-}}4f$ electrons is observed below $50\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Far above a critical field ${H}_{A}\ensuremath{\sim}50\phantom{\rule{0.3em}{0ex}}\mathrm{kOe}$, the Korringa behavior ${T}_{1}T=\mathrm{const}$ is observed below $2\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ in $100\phantom{\rule{0.3em}{0ex}}\mathrm{kOe}$. The Korringa value below $2\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ is one order of magnitude larger than that in $\mathrm{La}{\mathrm{Fe}}_{4}{\mathrm{P}}_{12}$, which has no $4f$ electrons. Our NMR experiment shows that the heavy-fermion state is realized far above ${H}_{A}$ in $\mathrm{Pr}{\mathrm{Fe}}_{4}{\mathrm{P}}_{12}$. The magnetic fluctuations in $\mathrm{Pr}{\mathrm{Fe}}_{4}{\mathrm{P}}_{12}$ are discussed from a comparison with typical heavy-fermion compounds. In magnetic fields below ${H}_{A}$, $1∕{T}_{1}$ shows a sharp decrease below ${T}_{A}$, however, in smaller magnetic fields less than $10\phantom{\rule{0.3em}{0ex}}\mathrm{kOe}$, $1∕{T}_{1}$ stays constant far below ${T}_{A}$ with a relatively large value. The temperature and field dependence of $1∕{T}_{1}$ reveals the presence of low-energy spin fluctuations in the low-temperature and low-field region. These unusual magnetic fluctuations are considered to originate from the magnetic dynamics of Pr-nuclear spins since the fluctuating magnetic field $\ensuremath{\sim}1.4\phantom{\rule{0.3em}{0ex}}\mathrm{Oe}$ and frequency $\ensuremath{\sim}3.5\phantom{\rule{0.3em}{0ex}}\mathrm{MHz}$ are so small. The nuclear ordering temperature is estimated to be $\ensuremath{\sim}0.08\phantom{\rule{0.3em}{0ex}}\mathrm{mK}$ using the nuclear exchange fluctuations derived from the observed $1∕{T}_{1}$ of P. We show that $\mathrm{Pr}{\mathrm{Fe}}_{4}{\mathrm{P}}_{12}$ is a quite unique compound in which nuclear magnetism can be detected by P NMR thanks to the nonmagnetic ground state of the $\mathrm{Pr}\text{\ensuremath{-}}4f$ moments.