Magnetic excitations in the quasi-2D ferromagnet Fe3-xGeTe2 measured with inelastic neutron scattering

Fe3-xGeTe2 is an itinerant ferromagnet composed of two-dimensional layers weakly connected by van der Waals bonding that shows a variety of intriguing phenomena. Inelastic neutron scattering measurements on bulk single crystals of Fe2.75GeTe2 were performed to quantify the magnetic exchange interaction energies and anisotropy. The observed inelastic excitations are indicative of dominant in-plane correlations with negligible magnetic interactions between the layers. A spin-gap of 3.9 meV is observed allowing a measure of the magnetic anisotropy. As the excitations disperse to their maximum energy (~65 meV) they become highly damped, reflective of both the magnetic site occupancy reduction of 25{\%} on one Fe sublattice and the itinerant interactions. A minimal model is employed to describe the excitation spectra and extract nearest neighbor magnetic exchange interaction values. The temperature evolution of the excitations are probed and correlations shown to persist above Tc, indicative of low dimensional magnetism.