Lattice dynamics and phase transitions in Fe3−xGeTe2

We present Raman spectroscopy measurements of the van der Waals bonded ferromagnet Fe3−xGeTe2, together with lattice dynamics. Four out of eight Raman active modes are observed and assigned, in agreement with numerical calculations. The energies and linewidths of the observed modes display an unconventional temperature dependence at about 150 and 220 K, followed by the nonmonotonic evolution of the Raman continuum. Whereas the former can be related to the magnetic phase transition, the origin of the latter anomaly remains an open question.