Neutron scattering investigation of the magnetic interactions in the quasi-2D itinerant ferromagnet Fe_3-xGeTe₂

Reducing a materials dimensionality to topologically constrained two-dimensional (2D) layers yields phenomena beyond well-established classical behavior. The resultant material properties are of interest for fundamental studies as well as having the potential to drive new functionality in electronic/spintronic devices. Neutron scattering provides a powerful probe of bulk quasi-2D compounds. The quantitative extraction of the spin Hamiltonian from neutron scattering can in turn provide routes to understanding behavior in the bulk down to a single 2D-monolayer and allow the targeting of novel fundamental and applied phenomena. Here we present neutron scattering results on the quasi-2D itinerant ferromagnetic Fe_3-xGeTe₂, an exfoliable material with weakly connected van der Waals bonded 2D layers. As part of our study we investigate the influence of the high Fe site vacancy of up to 25% which controls the magnetic ordering temperature. By probing the inelastic excitations and elastic diffuse scattering we are able to reveal short range correlations in the 2D-layers associated with the Fe vacancies. Despite the appreciable disorder robust and gapped magnetic excitations are observed. The implications of the neutron data are discussed and models presented.