Oxygen Vacancy‐Mediated Stabilization of Antiferromagnetic Order in RuO ₂ Thin Films

ABSTRACT Recently, the 4d transition‐metal oxide RuO ₂ has attracted significant interest as a candidate altermagnet. However, its magnetic ground state remains controversial, as spin‐transport signatures of spin‐splitting torque conflict with spectroscopic evidence regarding long‐range magnetic order. Using exchange bias as a local probe, we show that antiferromagnetism in RuO ₂ is very fragile but can be stabilized by the presence of oxygen vacancies (V _o ). In epitaxial RuO ₂ ( t )/La _0.5 Sr _0.5 CoO ₃ bilayers, the exchange bias field exhibits an anomalous monotonic rise with the decrease of RuO ₂ thickness, while x‐ray absorption spectra reveal the concurrent reduction in Ru valence state (i.e., increasing in V _o content) toward the interface. Further oxygen annealing has almost extinguished the exchange bias effect in all samples, directly linking the V _o density to the antiferromagnetism strength of RuO ₂ . Density‐functional calculations confirm that oxygen vacancies, regardless of crystallographic site, enlarge the antiferromagnetism‐to‐paramagnetism energy difference by up to one order of magnitude, and thus stabilize the antiferromagnetic state of RuO ₂ . These results reconcile prior disparate reports, establishing vacancies as the dominant control parameter for RuO ₂ magnetism, and providing a practical route for engineering robust altermagnetic order in RuO ₂ thin‐film devices.