Limits for ordered magnetism in Pu from muon spin rotation spectroscopy

We present mu SR measurements on Pu metal which set stringent upper limits on the magnitude of the ordered moments mu(ord) in alpha-Pu and delta-stabilized Pu (alloyed with 4.3 at.% Ga). The magnitude of the low-temperature (4 K < T < 100-150 K) mu SR rate in zero applied field is independent of temperature and consistent with nuclear dipolar broadening alone. The Knight shift in alpha-Pu in 2 T applied field is also independent of temperature below about 100 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 K in alpha-Pu yields 1/tau(h)approximate to 3x10(7) s(-1); no estimate is yet obtained for delta-Pu. The principal results reported here are limits for mu(ord) in both alpha- and delta-Pu <= 10(-3)mu(B) at T congruent to 4 K. These limits are discussed in terms of recent models of the electronic properties of delta-Pu, where magnetism has been previously predicted. Our results are consistent with, but cannot unambiguously distinguish between, cases where the moments in delta-Pu are essentially zero, possess very weak interatomic exchange, are reduced by hybridization, or are washed out by dynamical fluctuations.