Altermagnets are a recently identified class of collinear antiferromagnets that host large spin-split electronic bands, offering a promising platform for efficient spin-current generation. Among proposed candidates, the metallic oxide RuO2 is predicted to exhibit strong altermagnetic spin splitting; however, whether it sustains robust magnetic order beyond the ultrathin thickness limit remains unresolved. Here, we employ optical probes to investigate charge-to-spin conversion in a 12-nm-thick (101)-oriented RuO2 film grown on sapphire. Polarization-resolved second-harmonic generation reveals nonlinear optical responses consistent with the surface symmetry and N\'eel order of RuO2. Under an applied current, both second-harmonic generation and polar magneto-optical Kerr effect measurements detect a pronounced, directionally anisotropic spin polarization, exhibiting enhanced signals for current along [010] and strongly suppressed responses for current along [-101], in agreement with the symmetry of the altermagnetic spin-splitter effect. Non-magnetic or Rashba-type mechanisms cannot explain this symmetry-selective response. Scanning transmission electron microscopy further reveals that substantial strain persists even in relatively thick films, providing a possible explanation for the observed behavior. Therefore, these results establish RuO2 as an efficient spin source and demonstrate the potential of altermagnets for field-free spintronic devices.
@article{arxiv.2605.27830,
title = {Magneto-Optical Detection of Anisotropic Spin Currents in Altermagnetic RuO2},
author = {Joongwon Lee and Jeonglyul Kim and Sreejith Nair and Seung Gyo Jeong and Changi Kim and Jae-Pil So and Bohm-Jung Yang and Bharat Jalan and Hyobin Yoo and Farhan Rana and Taekoo Oh and Hong-Gyu Park},
journal= {arXiv preprint arXiv:2605.27830},
year = {2026}
}