Flat-band localization and chiral texture formation in magnonic crystals with graded Dzyaloshinskii–Moriya interaction

Spatially graded magnetic nanostructures offer a powerful route to control collective excitations beyond the limitations of simple periodic materials. In this work, spin-wave localization and flat-band formation are investigated in chiral magnonic crystals engineered through spatially graded interfacial Dzyaloshinskii–Moriya interaction (DMI). By combining plane-wave calculations, micromagnetic simulations, and analytical modeling, it is demonstrated that a continuous DMI gradient within a single stripe induces a robust accumulation of low-frequency flat bands accompanied by strongly localized modes. The localization mechanism is shown to originate from the inhomogeneous interfacial DMI, which creates a position-dependent resonance landscape and a spectral mismatch across the structure. This graded chiral coupling not only enhances modal confinement in the dynamical regime but also establishes a direct connection between soft spin-wave modes and the emergence of chiral textures at remanence. The transition between these regimes follows a simple relation that links the local DMI strength to the exchange interaction. These results establish graded interfacial interactions as a versatile design strategy for flat-band engineering and spin-wave localization in magnetic metamaterials, with direct implications for reconfigurable magnonic devices and, more broadly, for graded-interaction architectures in wave-based functional materials.