Absence of Altermagnetic Spin-Splitting Effect in Epitaxial (100)-RuO₂Films

Recently a class of magnetic materials known as altermagnets has been proposed that exhibit momentum-dependent spin-splitting effect (SSE) despite their compensated magnetic moments [1]. Interestingly, spin polarization direction and spin current flow direction can be controlled in altermagnets by tuning the Néel vector and the electric current directions due to anisotropic spin-splitting effect. Previously regarded as a classic Pauli paramagnet, rutile RuO₂ has been recently reclassified as an altermagnet. It was reported that a strong spin-splitting effect occurs in (100)-oriented RuO₂ films, where the spin polarization direction is dependent on the Néel vector [2]. However, conflicting results have also been reported, indicating only conventional spin current generation on (100)-oriented RuO₂ films [3]. It is also noted that all these reports are based on RuO₂ films which have multiple in-plane crystal orientation, complicating the studies of SSE that depends on the crystalline direction. Using single-crystal films, here we report a study of spin current generation from (100)-RuO₂ films deposited on TiO₂ substrate via spin-torque ferromagnetic resonance (STFMR) measurements. Comparative studies have also been conducted on (100)-oriented RuO₂ films with multiple in-plane orientations, grown on Al₂O₃ substrates. Spin-torque efficiency for y-polarized spin current (ξ^Y_ST) are extracted by varying in-plane external magnetic field direction in STFMR measurements. Notably, the value of ξ^Y_ST is obtained ~ 0.04 which is significantly smaller than the value reported in [2]. Additionally, ξ^Y_ST shows minimal variation when current is applied along different crystal directions in both set of samples. Our results suggest the absence of SSE in (100)-oriented RuO₂, raising question about the altermagnetic behavior of RuO₂.