Discovery of magnetic superstructures and magnetic transition temperatures in UO₂and 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 UO₂and 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.