Symmetry-breaking induced surface magnetization in non-magnetic RuO$_2$

TL;DR

This paper demonstrates that the RuO$_2$(110) surface exhibits spontaneous magnetization due to symmetry-breaking, despite the bulk being non-magnetic, revealing surface magnetic phenomena in a metallic altermagnet.

Contribution

It shows that surface symmetry-breaking induces magnetization in RuO$_2$, challenging previous assumptions of its non-magnetic bulk and highlighting surface magnetic effects.

Findings

Surface RuO$_2$(110) exhibits spontaneous magnetization.

Symmetry-breaking causes electronic redistribution and magnetic moment enhancement.

Surface magnetism leads to spin-polarized states and potential spin-dependent transport.

Abstract

Altermagnetism is a newly identified phase of magnetism distinct from ferromagnetism and antiferromagnetism. RuO$_2$ has been considered a prototypical metallic altermagnet with a critical temperature higher than room temperature. Previous interpretations of the unusual magnetic properties of RuO$_2$ relied on the theoretical prediction that local moments on two Ru sublattices, which are connected by four-fold rotational symmetry, are quite significant (approximately 1 $\mu_B$), leading to long-range antiferromagnetic order. However, accumulated experimental data suggest that local moments on Ru in RuO$_2$ are vanishingly small, indicating that the bulk material is likely non-magnetic. This observation is consistent with the delocalized nature of the 4$d$ electrons of Ru and the strong screening effect in the metallic state. In this work, we show that despite the non-magnetic bulk, the RuO$_2$(110) surface exhibits spontaneous magnetization. We attribute this effect to the breaking of local symmetry, which results 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 findings highlight the important role of surface magnetic structures in the otherwise non-magnetic bulk RuO$_2$.