  Fe3-xGeTe2 is a layered magnetic van der Waals material of interest for both
fundamental and applied research. Despite the observation of intriguing
physical properties, open questions exist even on the basic features related to
magnetism: is it a simple ferromagnet or are there antiferromagnetic regimes
and are the moments local or itinerant. Here, we demonstrate that
antiferromagnetic spin fluctuations coexist with the ferromagnetism through
comprehensive elastic and inelastic neutron scattering and thermodynamic
measurements. Our realistic dynamical mean-field theory calculations reveal
that the competing magnetic fluctuations are driven by an orbital selective
Mott transition, where only the plane-perpendicular a1g orbital of the Fe(3d)
manifold remains itinerant. Our results highlight the multi-orbital character
in Fe3-xGeTe2 that supports a rare coexistence of local and itinerant physics
within this material.
