Surface ferrimagnetic order in RuO2 film

TL;DR

This study reveals that RuO2 exhibits non-magnetic bulk properties but has a spontaneous ferrimagnetic order confined to its oxygen-terminated surface, clarifying previous debates about its magnetic nature.

Contribution

It provides direct experimental evidence and theoretical explanation for surface ferrimagnetism in RuO2, distinguishing it from altermagnetism.

Findings

RuO2 is non-magnetic in the bulk.

Surface ferrimagnetic order is detected on RuO2.

Magnetism is confined to the oxygen-terminated surface.

Abstract

RuO2, widely proposed as a prototypical altermagnet, remains intensely debated with regard to its magnetic nature. Here, we demonstrate that RuO2 is non-magnetic in the bulk, but possesses a spontaneous surface ferrimagnetic order. Using spin- and angle-resolved photoemission spectroscopy, we directly detect a narrow surface state with identical spin polarizations at opposite momenta and at the Brillouin-zone center, incompatible with the spin texture of any altermagnetic order. First-principles calculations identify the non-magnetic bulk state and reveal that the detected magnetism is confined to the fully oxygen-terminated surface, where the charge transfer from Ru to O at surface triggers a ferrimagnetic alignment between adjacent Ru sublattices with antiparallel moments of +0.48 uB and -0.04 uB. Our findings provide a unified explanation reconciling debating reports on the magnetism of RuO2, establishing surface ferrimagnetism as the origin of the observed magnetic signals, and distinguishing it unambiguously from altermagnetism.