Limits for ordered magnetism in Pu from muon spin rotation spectroscopy

Abstract

We present SR measurements on Pu metal which set stringent upper limits on the magnitude of the ordered moments ord in ␣-Pu and ␦-stabilized Pu ͑alloyed with 4.3 at. % Ga͒. The magnitude of the low-temperature ͑4 KϽ T Ͻ 100-150 K͒ SR rate in zero applied field is independent of temperature and consistent with nuclear dipolar broadening alone. The Knight shift in ␣-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 / h at 150 K in ␣-Pu yields 1/ h Ϸ 3 ϫ 10 7 s −1 ; no estimate is yet obtained for ␦-Pu. The principal results reported here are limits for ord in both ␣and ␦-Puഛ 10 −3 B at T Х 4 K. These limits are discussed in terms of recent models of the electronic properties of ␦-Pu, where magnetism has been previously predicted. Our results are consistent with, but cannot unambiguously distinguish between, cases where the moments in ␦-Pu are essentially zero, possess very weak interatomic exchange, are reduced by hybridization, or are washed out by dynamical fluctuations.

Nature of the5fstates in actinide metals

“…Both techniques, however, must be guided by four facts about Pu: the metal has approximately a 5f 5 configuration, it exhibits intermediate coupling near the jj limit, it has no bulk magnetism in any of the six allotropic phases, and electron correlation effects are important in both the α and δ phases. It is up to future experiments and theory to resolve why Pu has a 5f 5 configuration and exhibits intermediate coupling near the jj limit, but shows no long-range magnetic order (Lashley et al, 2005;Heffner et al, 2006). Approaches that can discern between Kondo shielding (Shim et al, 2007), electron pairing correlations , and a Mott transition (Johansson, 1974) will be paramount.…”

“…Experimental and theoretical results on Pu metal clearly show: The 5f states contain approximately five electrons (Söderlind 1998, van der Laan et al, 2004Moore et al, 2003;Shim et al, 2007;Zhu et al, 2007), intermediate coupling near the jj limit is the appropriate angular momentum coupling scheme for the 5f states (Moore et al, 2007b), electroncorrelation effects are present, to varying degrees, in both the α and δ phases (Shim et al, 2007), and all six allotropic phases of Pu metal are nonmagnetic (Lashley et al, 2005, Heffner et al, 2006. In order to understand this, let us look at the spectroscopic data and bulk measurements in turn.…”

“…Why then is there no experimentally observed magnetism in any of the six allotropic phases of the metal (Lashley et al 2005, Heffner et al 2006? The lack of magnetism for Am is obvious, since it has a nearly filled j= 5/2 level (total angular momentum J = 0), but Pu, which is ~f 5 and has at least one hole in the j = 5/2 level, is positively vexing!…”

“…While magnetism in actinide compounds is widely studied and accepted (Santini et al, 1999), magnetism in pure Pu is often debated, even though there is no convincing experimental evidence of moments in any of the six allotropic phases of the metal (Lashley et al, 2005;Heffner et al, 2006). EELS and x-ray absorption (XAS) clearly show that Pu is at or near a 5f 5 configuration with at least one hole in the j = 5/2 level (Moore et al, 2003;.…”

Emergent magnetic moments produced by self-damage in plutonium

“…The lack of significant magnetic moments is a central issue among theorists and inspired a recent experimental review (8) painstakingly describing the evidence against the existence of magnetic moments in plutonium. Recent SR ϩ studies for ␣-and ␦-phase Pu further support this absence of magnetic moments, placing the upper bound on frozen moments at 0.001 Bohr magneton ( B ) (9). However, all solid Pu phases possess large magnetic susceptibilities, suggesting they border on becoming magnetic.…”