Magnetic structure of UO 2 and NpO 2 by first-principle methods.

The magnetic structure of the actinide dioxides (AnO(2)) remains a field of intense research. A low-temperature experimental investigation of the magnetic ground-state is complicated by thermal energy released from the radioactive decay of the actinide nuclei. To establish the magnetic ground-state, we have employed high-accuracy computational methods to systematically probe different magnetic structures. A transverse 1k antiferromagnetic ground-state with Fmmm (No. 69) crystal symmetry has been established for UO2, whereas a ferromagnetic (111) ground-state with R3m (No. 166) has been established for NpO2. Band structure calculations have been performed to analyse these results.