Altermagnetism is a newly identified phase of magnetism distinct both from ferromagnetism and antiferromagnetism. ${\mathrm{RuO}}_2$ has been considered a prototypical metallic altermagnet with a critical temperature higher than room temperature. Previous interpretations of unusual magnetic properties of ${\mathrm{RuO}}_2$ relied on the theoretical prediction that local moments on two Ru sublattices, which are connected by fourfold rotational symmetry, are quite significant ($\approx 1{\mu }_{\mathrm{B}}$), leading to long-range antiferromagnetic order. However, accumulated experimental data suggest that local moments on Ru in ${\mathrm{RuO}}_2$ are vanishingly small, indicating the bulk material is likely nonmagnetic. This observation is consistent with the delocalized nature of the $4d$ electron of Ru and the strong screening effect in the metallic state. In this work, we show that despite the nonmagnetic ground state in the bulk, the ${\mathrm{RuO}}_2$(110) surface exhibits spontaneous magnetization, which we attribute to the breaking of local symmetry, 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 scanning probe microscopy images, and potentially spin-dependent transport effects. These highlight the important role of surface magnetic structures in the otherwise nonmagnetic bulk ${\mathrm{RuO}}_2$.

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• Chemical bonding
• Electronic structure
• First-principles calculations
• Flat bands
• Magnetic order
• Magnetism
• Spintronics
• Surface & interfacial phenomena
• Surface states
• Altermagnets
• Surfaces
• Band structure methods
• Density functional theory