Incommensurate Spin Density Wave in Antiferromagnetic RuO2 Evinced by Abnormal Spin Splitting Torque

Since the discovery of antiferromagnetism, metallic oxide RuO₂ has exhibited numerous intriguing spintronics properties such as the anomalous Hall effect and anisotropic spin splitting effect. However, the microscopic origin of its antiferromagnetism remains unclear. By investigating the spin splitting torque in RuO₂/Py, we found that metallic RuO₂ exhibits a spatially periodic spin structure which interacts with the spin waves in Py through interfacial exchange coupling. The wavelength of such structure is evaluated within 14-20 nm depending on the temperature, which is evidence of an incommensurate spin density wave state in RuO₂. Our work not only provides a dynamics approach to characterize the antiferromagnetic ordering in RuO₂, but also offers fundamental insights into the spin current generation due to anisotropic spin splitting effect associated with spin density wave.