Unconventional magnonic surface and interface states in layered ferromagnets

Electronic surface, interface and edge states are well-known concepts in low-dimensional solids and have already been utilised for practical applications. It is expected that magnons-the bosonic quasiparticles representing the magnetic excitations- shall also exhibit such exotic states. However, how these states are formed in layered magnetic structures is hitherto unknown. Here we bring the topic of magnonic surface and interface states in layered ferromagnets into discussion. We provide experimental examples of synthetic layered structures, supporting our discussions and show that these states can be tailored in artificially fabricated structures. We demonstrate that the magnonic surface or interface states may show peculiar features, including "standing" or "ultrafast" states. We argue that these states can drastically change their electronic and magnonic transport properties. In this way one can design layered ferromagnets which act as magnon conductor, semiconductor and insulator of specific states. Magnons are the quantized spin waves, which describe the collective magnetic excitations and the long-range magnetic order in a solid. Here, the authors describe how to engineer magnonic band structures at the interface and surface of ferromagnetic layered structures and how such magnonic states alter the transport properties.