Oxygen Vacancy-Mediated Stabilization of Antiferromagnetic Order in RuO₂ Thin Films

Recently, the 4d transition-metal oxide RuO₂ 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 RuO₂ is very fragile but can be stabilized by the presence of oxygen vacancies (V_o). In epitaxial RuO₂(t)/La_0.5Sr_0.5CoO₃ bilayers, the exchange bias field exhibits an anomalous monotonic rise with the decrease of RuO₂ 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 RuO₂. 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 RuO₂. These results reconcile prior disparate reports, establishing vacancies as the dominant control parameter for RuO₂ magnetism, and providing a practical route for engineering robust altermagnetic order in RuO₂ thin-film devices.