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 $μ_B$), 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$_2$.
Disentangling bulk and surface electronic structure using targeted cleave planes in RuO$_2$
Max Planck Institute for Chemical Physics of Solids（马克斯·普朗克固体化学物理研究所）；SUPA, School of Physics and Astronomy, University of St Andrews（圣安德鲁斯大学物理学与天文学学院（SUPA））；MAX IV Laboratory, Lund University（隆德大学MAX IV实验室）
Exploration of Altermagnetism in $\mathrm{RuO_{2}}$
China Three Gorges University（三峡大学）；Quantum Science Center of Guangdong-Hong Kong-Macao Greater Bay Area (Guangdong)（粤港澳大湾区量子科学中心（广东））；Southern University of Science and Technology (SUSTech)（南方科技大学）；Hefei National Laboratory（合肥国家实验室）；University of Science and Technology of China（中国科学技术大学）
Observation of mirror-odd and mirror-even spin texture in ultrathin epitaxially strained RuO2 films
University of Minnesota-Twin Cities（明尼苏达大学双城分校）；Kyung Hee University（庆熙大学）；Rice University（莱斯大学）；Paul Scherrer Institute（保罗谢勒研究所）；Gwangju Institute of Science and Technology (GIST)（光州科学技术院）
Strain-induced Dynamic Spin-Phonon Coupling in Epitaxial RuO2 Films
Gwangju Institute of Science and Technology (GIST)（光州科学技术院）；University of Minnesota−Twin Cities（明尼苏达大学双城分校）；Hankuk University of Foreign Studies（韩国外国语大学）；Seoul National University（首尔大学）；Sejong University（世宗大学）
All optical excitation of spin polarization in d-wave altermagnets
RPTU University Kaiserslautern-Landau（凯泽斯劳滕-兰道 RPTU 大学）；Johannes Gutenberg University Mainz（美因茨约翰内斯·古腾堡大学）；Universidade de Santiago de Compostela（圣地亚哥-德孔波斯特拉大学）；Tohoku University（东北大学）
