Final Report for Plutonium and Quantum Criticality LDRD 03-ERD-077

Abstract

Plutonium possesses the most complicated phase diagram in the periodic table, driven by the complexities of overlapping 5f electron orbitals. Despite the importance of the 5f electrons in defining the structure and physical properties, there is no experimental evidence that these electrons localize to form magnetic moments in pure Pu and the {sup +}{mu}SR measurements included here place an upper limit of <0.001{micro}{sub B} for the magnetic moment on Pu. Instead, a large temperature independent Pauli susceptibility indicates they form narrow conduction bands. Radiation damage from the {alpha}-particle decay of Pu creates numerous defects in the crystal structure which produce a significant temperature dependent magnetic susceptibility {chi}(T), in {alpha}-Pu, {delta}-Pu(4.3at%Ga), and Pu{sub 1-x}Am{sub x} alloys ({delta}-Pu phase). This effect can be removed by thermal annealing above room temperature. By contrast, below 35K the radiation damage is frozen in place permitting the evolution in {chi}(T) with increasing damage to be studied systematically. This leads to a two component model consisting of a Curie-Weiss term and a short-ranged interaction term consistent with disorder induced local moment models. Thus it is shown that self-damage creates localized magnetic moments in previously nonmagnetic plutonium. This effect is greatly magnified in some Pu{sub 1-x}Am{sub x} alloys where an apparent damage-induced phase transition occurs at low temperatures near Stage I annealing which results local moments on the order of 1 {micro}{sub B}/Pu. The phase is metastable, and anneals away at higher temperatures.