Direct Observation of Propagating Spin Waves in the 2D van der Waals Ferromagnet Fe<inf>5</inf>GeTe<inf>2</inf>

Magnetism in reduced dimensionalities is of great fundamental
interest while also providing perspectives for applications of materials with novel
functionalities. In particular, spin dynamics in two dimensions (2D) have become
a focus of recent research. Here, we report the observation of coherent
propagating spin-wave dynamics in a ∼30 nm thick flake of 2D van der Waals
ferromagnet Fe5GeTe2 using X-ray microscopy. Both phase and amplitude
information were obtained by direct imaging below TC for frequencies from 2.77
to 3.84 GHz, and the corresponding spin-wave wavelengths were measured to be
between 1.5 and 0.5 μm. Thus, parts of the magnonic dispersion relation were
determined despite a relatively high magnetic damping of the material.
Numerically solving an analytic multilayer model allowed us to corroborate the
experimental dispersion relation and predict the influence of changes in the
saturation magnetization or interlayer coupling, which could be exploited in
future applications by temperature control or stacking of 2D-heterostructures.