Exchange Surface Spin Waves in Type-A van der Waals Antiferromagnets

Surface waves, the evanescent solutions of the wave equation at planar discontinuities, are of fundamental importance in surface physics, optics, phononics, electronics, and magnetism. Here, we predict that van der Waals antiferromagnets support surface spin waves that are unique by their extreme (nanoscale) confinement and high group velocity with application potential in ultrafast, nanoscale magnonic devices. These excitations reside within the bulk magnon band gap of type-A van der Waals antiferromagnets (antiferromagnetically coupled ferromagnetic monolayers). In contrast to conventional magnetostatic Damon-Eshbach modes, they are pure exchange modes owing to reduced interlayer exchange coupling at the surface layers, and thus persist in ultrathin multilayer stacks and at large wave numbers. We show that they can be efficiently excited by electromagnetic waves, with absorption power comparable to or even exceeding that of bulk modes. Moreover, their emitted magnetic stray fields exhibit pronounced even-odd oscillations with the number of monolayers that should be observable by nitrogen-vacancy-center magnetometry.