Nonreciprocal Spin Waves in Nanoscale Hybrid Néel–Bloch–Néel Domain Walls Detected by Scanning X‐Ray Microscopy in Perpendicular Magnetic Anisotropic Fe/Gd Multilayers

Abstract

Abstract 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.