Unconventional magnonic surface and interface states in layered ferromagnets

Khalil Zakeri,Huajun Qin,Arthur Ernst

doi:10.1038/s42005-021-00521-7

Abstract

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.

Highlights

Electronic surface, interface and edge states are well-known concepts in low-dimensional solids and have already been utilised for practical applications
If the periodic boundary conditions are abandoned in the direction normal to the surface, the behavior of electrons will deviate from their behavior in the bulk and leads to the formation of new electronic states, that is, surface states
We provide the experimental results, obtained using spin-polarized high-resolution electron energy-loss spectroscopy (SPEELS), and combine them with accurate theoretical calculations

Summary

Introduction

Electronic surface, interface and edge states are well-known concepts in low-dimensional solids and have already been utilised for practical applications. The interface states, on the other hand, have mainly been discussed in semiconductor heterostructures, where two different materials are brought together[8] They obey somewhat the same physics as those of the surface states. Shockley states are normally states that arise due to the abrupt change in the electron potential associated solely with the crystal termination This approach is suitable to describe metals like Cu, Ag, and Au, and narrow bandgap semiconductors. Low-dimensional solids with confined geometries, for example, thin films with finite thicknesses, exhibit surface or interface states. In such structures, the effects associated with quantum confinement (quantum size effects or quantum well states) are present[1,7]

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