RuO2: a puzzle to be solved

Altermagnetism is a topic that has lately been gaining attention and the RuO2 compound is among one of the most studied altermagnetic candidates. However, the survey of available literature on RuO2 properties suggests that there is no consensus about the magnetism of this material, especially about the size of the ordered moments. By performing density functional theory calculations, we show that the stoichiometric RuO2, for reasonable values of Hubbard U in DFT+U, is not, or only very weakly magnetic, in agreement with spin-polarized neutron scattering experiments. We further argue that Ru vacancies can actually aid the formation of a magnetic state in RuO2, and that may be the reason why other, indirect probes are consistent with sizeable magnetic moment. This. in turn. suggests that a characterization of the amount of Ru vacancies in experimental samples might help to resolve of the controversy between the different experimental results.