Antiferromagnetic fluctuations and orbital-selective Mott transition in the van der Waals ferromagnet Fe_3-xGeTe₂

Fe_3-xGeTe₂ is a 2D layered van der Waals (vdW) material that is a rare example of a ferromagnetic metal, with magnetism remaining robust in the bulk down to the monolayer. Despite much fundamental and applied interest, open questions exist even on basic features related to magnetism: is it a simple ferromagnet or are there antiferromagnetic regimes and are the moments local or itinerant. To address these questions we present results from neutron scattering experiments on a large single crystal with low Fe vacancy content (Fe_2.85GeTe₂). The measurements show antiferromagnetic spin fluctuations develop and coexist with ferromagnetism. To explain the underlying behavior we performed realistic dynamical mean-field theory calculations that revealed that the competing magnetic fluctuations are driven by an orbital selective Mott transition (OSMT). The detailed calculations allow the different orbital contributions to be disentangled and assigned to local and itinerant character, thereby explaining the apparently contradictory dual behavior. The results presented represent an advancement in understanding the coexistence of itinerant and local moments in a canonical quasi-2D vdW ferromagnetic material and consequences for spin and orbital dependent electronic functions within wider spintronic and topological transport research are discussed.