Plutonium is a fascinating element. Infamous as radioactive and at the core of nuclear power in both the energy-production and life-threatening sense. It was first made in the laboratory 75 years ago by Glenn Seaborg and Edwin McMillan at the University of California, Berkeley, when they bombarded uranium-238 with the nuclei of heavy hydrogen, so-called deuterons.
Aside from the (in)famous properties of its nucleus, plutonium is also rather complicated and somewhat intriguing when viewed from the perspective of the electrons that orbit its 94 protons and requisite neutrons: Plutonium's 5f electrons are on the cusp of localization and delocalization and can form bonds within the lattice as well as forming covalent bonds in the metals' molecules and complexes by virtue of their directionality. The energy difference between the 6d and 5f subshells is very low and so multiple low-energy electron configurations exist and it is this that gives rise to the odd nature of its magnetism.
One of the spinoffs [pardon the pun] of plutonium having such a complicated electronic structure is that at least as long ago as its discovery in 1940, it was predicted to have magnetic properties, but in all that time scientists had not pinned them down experimentally. Recently, however, I reported for the news section of Materials Today on the use of neutron spectroscopy to demonstrate once and for all that the heavy metal is indeed magnetic, but that its magnetic properties are in a constant state of flux because of those errant electrons. It turns out that the complicated electronic structure of plutonium does not lend itself to a "simple" magnetic field of the kind that we observe in iron, for instance, with easy transitions occurring in the electrons leading to fluctuations that meant plutonium's magnetism would remain elusive until along came the right technique for coaxing it from its shell [again, pardon the pun].
The discovery could have implications for studying plutonium superconductors and many other materials with similarly complex electronic structures. I asked lead author Marc Janoschek of Los Alamos National Laboratory, New Mexico, about the implications of the findings by he and his colleagues:
"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 (PNAS, 2015, 112, 3285-328)," he told me. "Detecting the valence fluctuations with neutron spectroscopy in the same way we did in plutonium metal would be in a way smoking gun experiment," he adds.
There are many other complex materials in which the electrons exist in what Janoschek describes as the "no man's land" between the well understood extremes of localized and delocalized configurations. Many of these complex materials are in current use and some may well have applications in the technologies of the future, among them the cuprate and iron-pnictide high temperature superconductors, which have boundless energy applications, use in levitating trains, magnetic resonance imaging (MRI) machines, for low-loss transformers and perhaps even faster computers. There are also complex oxide materials with colossal magneto-resistance that may find use in spintronics and magnetic sensing and there are also permanent magnets for electrical motors, turbines and automotive applications.
"Our group is already working on understanding the complex electronic behavior and the properties that emerge from it in detail, using various complementary experimental techniques," Janoschek told me. "Beyond our experimental advances, our work shows that modern theories for the calculation of the electronic ground state of a material (such as the dynamic mean field theory calculations that my theory colleagues used in our work) are becoming sophisticated enough that they are approaching predictive qualities." He concludes that, "Ultimately, we hope that at some point, at least certain classes of functional materials can be 'designed' using such computer codes, so that they can be tailored to have certain functional properties required for specific applications."
Of course, plutonium's 75-year history has been rather colourful and not without controversy one might say. One has to wonder what kind of spin the marketing executives would have to put on a computer or car that relied on the complexities of plutonium's electrons for its functionality...
David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the bestselling science book "Deceived Wisdom".
Read the news item titled 'Plutonium's missing magnetism found' here.