Metallicity and anomalous Hall effect in epitaxially strained, atomically thin RuO2 films.

The anomalous Hall effect (AHE), a hallmark of time-reversal symmetry breaking, has been reported in rutile RuO₂, a debated metallic altermagnetic candidate. Previously, AHE in RuO₂ was observed only in strain-relaxed thick films under extremely high magnetic fields (~50 T). Yet, in ultrathin strained films with distinctive anisotropic electronic structures, there are no reports, likely due to disorder and defects suppressing metallicity thus hindering its detection. Here, we demonstrate that ultrathin, fully strained 2 nm TiO₂/t nm RuO₂/TiO₂ (110) heterostructures, grown by hybrid molecular beam epitaxy, retain metallicity and exhibit a sizeable AHE at a significantly lower magnetic field (< 9 T). Density functional theory calculations reveal that epitaxial strain stabilizes a noncompensated magnetic ground state and reconfigures magnetic ordering in RuO₂ (110) thin films. These findings establish ultrathin RuO₂ as a platform for strain-engineered magnetism and underscore the transformative potential of epitaxial design in advancing spintronic technologies.