Confined magnons

The common belief is that excitations of magnets are always deconfined ``bulk'' spin waves spread out in space, whose quanta are called magnons. In reality all materials have surfaces and interfaces that break translation symmetry, giving rise to excitations that are confined to them, in addition to the deconfined ones. The conditions for the existence of these confined magnons are still unknown. The authors show that extra magnetic anisotropy at the surface or interface plays a key role in magnon confinement, and obtain a characteristic length scale for their spatial localization. Calculations of the spin-scattering function show how confined magnons can be observed with neutron, electron, and photon scattering experiments in a wide variety of materials. The accompanying image shows the heat map for magnon energy versus wave vector in ferromagnetic and antiferromagnetic materials. The confined magnon appears as a flat dispersion (delocalized in $q$ space, localized in real space) with energy below the lowest bulk mode.