Anisotropic Generation of Spin Torque in Rutile RuO₂Films

Previous reports of spin-torques produced by RuO₂ have been cited as evidence for the altermagnetic spin-splitting effect (ASSE). However, the possible roles of an anisotropic spin Hall effect (SHE) and other unconventional torques originating from low-symmetry crystal facets have not been thoroughly examined or disentangled. Since the generation of spin torques from ASSE depends on both the in-plane charge current direction and the out-of-plane crystal direction, it is essential to use isostructural samples and compare crystal orientations such as (100)- and (110)- that allow and forbid ASSE, respectively. Here, we present a comparative study of spin torques in (100)- and (110)-oriented RuO₂ films deposited by reactive magnetron sputtering on TiO₂ substrates. High-resolution X-ray diffraction confirms that the films are epitaxial and of high crystalline quality. The films were patterned into devices along various in-plane crystal directions and characterized via spin-torque ferromagnetic resonance (ST-FMR), measuring field-angle dependence to extract the damping-like spin-torque efficiencies ξ^DL of the x-, y-, and z-components. Both (100)- and (110)- films exhibit significant dependence of ξ_x^DL and ξ_y^DL on the current direction, while ξ_z^DL remains negligible. Notably, no enhancement of ξ_y^DL in (100) over (110) indicates no detectable ASSE contribution, suggesting RuO₂ behaves as a normal metal rather than an altermagnet with anisotropic SHE.