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

• Received 11 November 2005

DOI:https://doi.org/10.1103/PhysRevB.73.094453