Plutonium's missing magnetism found

Scientists have long thought that plutonium should be magnetic but observing that property experimentally seemed impossible. Now,  a neutron scattering study by researchers in the USA has revealed that this electronically complex and unstable heavy metal does indeed display magnetism, but it is in constant flux, hence the difficulties in attempting to observe it since the metal was first produced 75 years ago.

Plutonium famously is a fissile material and was first produced in 1940 by Glenn Seaborg and Edwin McMillan at the University of California, Berkeley, by bombarding uranium-238 with deuterons. Not only is it radioactive, but its 5f electrons sit in a state between delocalized and localized and the energy difference between this shell and the 6d shell is very low, which gives rise to anomalous chemical behavior. Theories abound as to why plutonium should have such a complex electronic structure and predict that the metal should have magnetic order.

Marc Janoschek and colleagues at Los Alamos and at Oak Ridge national laboratories have detected the ever-changing magnetism of plutonium. Plutonium exists in a state of quantum mechanical superposition, Janoschek explains, in which the electrons are completely localized in one state giving rise to a magnetic moment and at the other extreme are entirely delocalized and no longer associated with the same ion in the bulk.

The team's neutron measurements revealed that the fluctuations give rise to different numbers of electrons in plutonium's outer valence shell; an observation that also explains volume changes observed in different phases. "This is a big step forward, not only in terms of experiment but in theory as well." Janoschek says. "We successfully showed that dynamical mean field theory more or less predicted what we observed. It provides a natural explanation for plutonium's complex properties and in particular the large sensitivity of its volume to small changes in temperature or pressure."

The work was painstaking not least because of regulatory approval required for research with this infamous metal but also because earlier neutron spectroscopy measurements by others had revealed that the plutonium-239 isotope is too neutron absorbent, so the team had to work with plutonium-242 instead. Additionally, plutonium adsorbs hydrogen leading to strong but spurious signals in the same spectroscopic region as the team anticipated magnetic signals would be observed. Nevertheless, the team developed a technique to remove hydrogen from their sample.

The new observations on plutonium and the techniques developed to make them and understand them could have broader applications, perhaps offering new insights into other functional materials with similar electronic dichotomies. Janoschek suggests that the methods could open the door to future investigations for materials critical to future computing and energy applications. For instance, he told us, "We are hoping to perform similar measurements on the plutonium superconductor PuCoGa5. In this material, recent work from some of my colleagues at LANL suggests that the valence fluctuations of plutonium could mediate the unconventional superconductivity in this compound."

David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the bestselling science book "Deceived Wisdom".