Flat band structure in chiral magnonic crystals with tunable indirect band gaps

Artificially introducing periodic patterns into magnetic materials can manipulate spin waves (SWs) with rich band structures. Incorporating a magnetochiral mechanism into magnonic crystals (MCs) unlocks nonreciprocal properties in SWs, further enhancing their potential applications. Here, we comprehensively investigate the SW band structures of bicomponent chiral MCs by periodically tailoring the intensity of the interfacial Dzyaloshinskii-Moriya interaction (iDMI) in heavy metal/ferromagnetic/heavy metal trilayer systems [${\mathrm{HM}}_{1}/\mathrm{FM}/{\mathrm{HM}}_{1(2)}$]. Notably, a flat band structure with zero group velocity, originating from the robust localization of magnon modes, can be found by tuning the effective iDMI energy. Meanwhile, the nonreciprocal properties in these MCs can be significantly enhanced with increasing the effective iDMI energy, facilitating the unidirectional propagation of SWs. Moreover, both the width and position of the indirect band gap in our MCs are easily tuned and closely related to the iDMI, lattice period, and/or external magnetic field. The wave-vector difference between the top of the first band and the bottom of the second band increases linearly with the effective iDMI energy. Both frequency-domain finite-element method calculations and finite-difference time-domain micromagnetic simulations agree well with the experimental observations. This study provides insights and methodologies for designing SW-based nanodevices.