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.
@article{arxiv.2604.10659,
title = {Surface ferrimagnetic order in RuO2 film},
author = {Jiahua Lu and Huangzhaoxiang Chen and Zhe Zhang and Xinyue Wang and Donghang Xie and Bo Liu and Liang He and Yao Li and Jun Du and Zhi Wang and Junwei Luo and Rong Zhang and Yongbing Xu and Xuezhong Ruan},
journal= {arXiv preprint arXiv:2604.10659},
year = {2026}
}