Nonreciprocal Spin Waves in Nanoscale Hybrid Néel-Bloch-Néel Domain Walls Detected by Scanning X-Ray Microscopy in...

Spin wave nonreciprocity is crucial for signal processing in magnonic circuits. Domain walls (DWs) have been suggested as channels for nonreciprocal spin waves (magnons) with directional-dependent properties. However, the experimental investigations are challenging due to the low-damping magnetic material with DWs demanded and the nanoscale length scales involved. In this study, scanning transmission X-ray microscopy (STXM) is used to examine coherently-excited magnons when propagating in hybrid Néel-Bloch-Néel DWs in amorphous Fe/Gd multilayers with perpendicular magnetic anisotropy (PMA). Well-ordered lattices of stripe domains and DWs are created through the integration of Cobalt nanowire arrays. Their width is measured to be δ_DW = (60 ± 13) nm. Near 1 GHz magnons are detected with short wavelengths down to λ = (281 ± 44) nm which were channeled in the DWs. Consistent with micromagnetic simulations, the STXM data revealed a nonreciprocal magnon band structure inside the DWs. Bloch points are identified which disrupted the phase evolution of magnons and induced different λ adjacent to these topological defects. These observations provide direct evidence of nonreciprocal spin waves within hybrid Néel-Bloch-Néel DWs in PMA materials, serving as programmable waveguides in magnonic devices with directed information flow.