Strain-Driven Altermagnetic Spin-Splitting Effect in RuO₂

The non-relativistic spin-momentum locking in altermagnets gives rise to a time-reversal-odd spin Hall effect, known as the altermagnetic spin-splitting effect (ASSE). Although ASSE was first reported in RuO₂, subsequent experiments have yielded inconsistent results, leaving its spin transport mechanism unclear. Here, we systematically investigate how strain, crystal orientation, and the Hubbard U parameter influence the magnetic ground state and spin Hall response of RuO₂. Guided by recent experimental observations, we find that U is likely smaller than the value required to induce intrinsic magnetism, suggesting that bulk RuO₂ and (001)/(101) RuO₂ thin films grown on TiO₂ are non-magnetic in the absence of extrinsic effects. In contrast, (100) and (110) films exhibit strain-induced altermagnetic spin splitting, leading to a strong ASSE even without Hubbard U corrections. These results reconcile previous experimental discrepancies and provide design guidelines for RuO₂-based spintronic devices.