Limits for ordered magnetism in Pu from muon spin rotation spectroscopy

Abstract

We present $\ensuremath{\mu}\mathrm{SR}$ measurements on Pu metal which set stringent upper limits on the magnitude of the ordered moments ${\ensuremath{\mu}}_{\mathrm{ord}}$ in $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Pu}$ and $\ensuremath{\delta}$-stabilized Pu (alloyed with $4.3\phantom{\rule{0.3em}{0ex}}\mathrm{at}.%$ Ga). The magnitude of the low-temperature $(4\phantom{\rule{0.3em}{0ex}}\mathrm{K}<T<100--150\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ $\ensuremath{\mu}\mathrm{SR}$ rate in zero applied field is independent of temperature and consistent with nuclear dipolar broadening alone. The Knight shift in $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Pu}$ in $2\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ applied field is also independent of temperature below about $100\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, but increases abruptly at higher temperatures, an effect likely caused by muon diffusion to paramagnetic impurities. A rough estimate for the muon hopping rate $1∕{\ensuremath{\tau}}_{h}$ at $150\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ in $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Pu}$ yields $1∕{\ensuremath{\tau}}_{h}\ensuremath{\approx}3\ifmmode\times\else\texttimes\fi{}{10}^{7}\phantom{\rule{0.3em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$; no estimate is yet obtained for $\ensuremath{\delta}\text{\ensuremath{-}}\mathrm{Pu}$. The principal results reported here are limits for ${\ensuremath{\mu}}_{\mathrm{ord}}$ in both $\ensuremath{\alpha}$- and $\ensuremath{\delta}\text{\ensuremath{-}}\mathrm{Pu}\ensuremath{\leqslant}{10}^{\ensuremath{-}3}{\ensuremath{\mu}}_{\mathrm{B}}$ at $\mathrm{T}\ensuremath{\cong}4\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. These limits are discussed in terms of recent models of the electronic properties of $\ensuremath{\delta}\text{\ensuremath{-}}\mathrm{Pu}$, where magnetism has been previously predicted. Our results are consistent with, but cannot unambiguously distinguish between, cases where the moments in $\ensuremath{\delta}\text{\ensuremath{-}}\mathrm{Pu}$ are essentially zero, possess very weak interatomic exchange, are reduced by hybridization, or are washed out by dynamical fluctuations.