Symmetry breaking induced surface magnetization in nonmagnetic RuO₂

Altermagnetism is a newly identified phase of magnetism distinct from traditional ferromagnetism and antiferromagnetism. RuO₂ has been considered a prototypical metallic altermagnet with a critical temperature higher than room temperature. Previous interpretations of RuO₂ unusual magnetic properties relied mostly on the theoretical prediction of local moments of ~1 µB on the two Ru sublattices connected by four-fold rotational symmetry, leading to antiferromagnetic ordering. However, accumulated experimental data suggest that local moments on Ru are vanishingly small, indicating the bulk material is likely non-magnetic. This observation is consistent with the delocalized nature of 4d electron of Ru and the strong screening present in the metallic state. In this work, we show that spontaneous magnetization emerges at the surface of (110) oriented in RuO₂, independent of the bulk being magnetic or not. The developed surface magnetization is due to the breaking of local symmetry at the surface, resulting in electronic redistribution and magnetic moment enhancement. The emergence of surface magnetism gives rise to interesting spectroscopic phenomena, including spin-polarized surface states, spin-polarized contrast in scanning tunneling microscopy images, and potentially spin-dependent transport effects. These highlight the important role of surface magnetic structures in otherwise non-magnetic bulk RuO₂ in the rutile structure.