Strain-Driven Altermagnetic Spin-Splitting Effect in RuO ₂

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 RuO 2, 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 RuO 2 . Guided by recent experimental observations, we find that U is likely smaller than the value required to induce intrinsic magnetism, suggesting that bulk RuO 2 and (001)/(101) RuO 2 thin films grown on TiO 2 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 RuO 2 -based spintronic devices.