Absence of Transport Altermagnetic Spin-Splitting Effect in RuO ₂

Altermagnets have attracted significant interest recently. Through the altermagnetic spin-splitting effect (ASSE), a longitudinal spin-polarized or a transverse pure spin current can be generated upon charge current injection. The ASSE is a key experimental feature for altermagnets but is often mixed with the spin Hall effect (SHE). Here, we present a comprehensive study of spin-to-charge conversion in epitaxial ruthenium dioxide (RuO₂) thin films using the ferromagnetic insulator yttrium iron garnet (YIG) as the spin current source. We conclusively show the absence of the ASSE in RuO₂ films grown with three different crystal orientations. Instead, we attribute the spin-to-charge conversion signals solely to the SHE. Moreover, we reveal a negative spin Hall angle in RuO₂ when it is adjacent to YIG, which reverses the sign when interfaced with Py. Our study provides crucial insights into the recent arguments on RuO₂ and advances the understanding of spin-to-charge conversion in low-symmetry materials.