New magnetic model explains why delta-plutonium shrinks when heated

Researchers at Lawrence Livermore National Laboratory (LLNL) have developed a new magnetic free-energy model that explains why delta-plutonium, unlike most materials, contracts when heated above room temperature. This counterintuitive behavior, puzzling scientists for decades, is linked to the complex interplay of plutonium’s electronic structure, magnetism, and crystal arrangement. By incorporating temperature-dependent magnetic fluctuations into their calculations of the material’s free energy, the model successfully reproduces delta-plutonium’s unusual thermal contraction and offers a deeper understanding of its unique and often unpredictable properties. The study marks the first time magnetic fluctuations have been explicitly included in a free-energy model for plutonium, providing fresh insights into the subtle forces governing its behavior. Beyond plutonium, this dynamic magnetism approach could be applied to other materials where magnetic states vary with temperature. Future work aims to extend the model by including microstructures, defects, and other real-world imperfections to improve prediction accuracy. Such advancements could enhance the safe handling, material