Reversible Spin Splitting Effect in Altermagnetic RuO ₂ Thin Films

Spin-dependent anisotropic Fermi surfaces in altermagnets can give rise to nonrelativistic spin-charge conversion, known as the altermagnetic spin splitting effect (ASSE). While several studies have reported the inverse ASSE (IASSE) in altermagnetic RuO₂ thin films, its magnitude and sign relative to the spin Hall effect (SHE) remain controversial. Here, we demonstrate reversible IASSE in RuO₂ thin films by controlling the Néel vector orientation and accounting for the anisotropic transport characteristic. Spin Seebeck measurements in CoFeB/RuO₂ heterostructures reveal polarity reversal of the IASSE-induced spin charge conversion (SCC) upon Néel vector switching. This indicates that IASSE can either enhance or suppress SCC depending on the Néel orientation. These results provide compelling evidence for the symmetry-governed and directionally reversible nature of IASSE in RuO₂.