Discovery of magnetic superstructures and magnetic transition temperatures in UO2 and PuO2

The magnetic ground states of actinide oxides remain a persistent challenge, as both their characterisation and predictive description are hampered by discrepancies between computational models and experimental measurements. Uranium dioxide (UO2) is experimentally observed to exhibit antiferromagnetic ordering with a Néel temperature of 30.8 K, whereas the magnetic behaviour of plutonium dioxide (PuO2) remains uncertain, with limited experimental data and conflicting computational results. In this study, we use a fully computational atomistic approach to evaluate the magnetic ordering and transition temperatures of UO2 and PuO2. Both unit cell and superstructural (supramagnetic) 1k antiferromagnetic configurations were investigated to explore the stability and magnetic transition temperature variation. For UO2, Néel temperatures of 38 K (unit cell) and 18 K (superstructure) were obtained, in good agreement with experimental observations. For PuO2, consistent with several computational predictions, an antiferromagnetic ground state was identified, with transition temperatures of 4.7 K (unit cell) and 14 K (superstructure). These values are close to the lowest temperature experimentally examined for PuO2, highlighting the need for further experimental investigation at cryogenic temperatures.