Flat band induced quasi-one-dimensional magnon transport in a two-dimensional spin lattice.

Flat bands in electronic systems have been widely explored for their ability to enhance electronic correlations and to induce exotic quantum states. However, the impact of flat magnon bands in spintronic devices remains largely unexplored. In this study, we experimentally demonstrate that intrinsic magnonic flat bands in the two-dimensional antiferromagnetic insulator CrOCl enable long-range, quasi-one-dimensional magnon transport within a strongly coupled two-dimensional spin lattice. We observed a magnon diffusion length exceeding 7 μm along the a-axis of CrOCl, rivaling the performance of low-damping three-dimensional magnets like α-Fe2O3 and yttrium-iron garnet (YIG). In contrast, magnon diffusion along the b-axis of CrOCl is significantly suppressed, setting it apart from α-Fe2O3 and YIG. Theoretical calculations attribute this quasi-1D spin transport to a nearly flat magnonic band along the b-axis and a highly dispersive band along the a-axis of the material, generating anisotropic magnon diffusion lengths agreeing with our experimental data. These results highlight the potential of flat magnon bands in van der Waals spin systems for the future development of energy-efficient and compact integrated spintronic devices.