  Van-der-Waals (vdW) ferromagnets have enabled the development of
heterostructures assembled from exfoliated monolayers with spintronics
functionalities, making it important to understand and ultimately tune their
magnetic properties at the microscopic level. Information about the magnetic
properties of these systems comes so far largely from macroscopic techniques,
with little being known about the microscopic magnetic properties. Here, we
combine spin-polarized scanning tunneling microscopy and quasi-particle
interference imaging with neutron scattering to establish the magnetic and
electronic properties of the metallic vdW ferromagnet Fe3GeTe2. By imaging
domain walls at the atomic scale, we can relate the domain wall width to the
exchange interaction and magnetic anisotropy extracted from the magnon
dispersion as measured in inelastic neutron scattering, with excellent
agreement between the two techniques. From comparison with Density Functional
Theory calculations we can assign the quasi-particle interference to be
dominated by spin-majority bands. We find a dimensional dichotomy of the bands
at the Fermi energy: bands of minority character are predominantly
two-dimensional in character, whereas the bands of majority character are
three-dimensional. We expect that this will enable new design principles for
spintronics devices.
