Possible Tricritical Behavior and Anomalous Lattice Softening in van der Waals Itinerant Ferromagnet Fe_3GeTe_2 under High Pressure

We present a high-pressure study of van der Waals ferromagnetic metal Fe_3GeTe_2 through electrical transport and Raman scattering measurements in diamond anvil cells at pressures up to 22.4 GPa. Upon compression, the ferromagnetic transition temperature T_\rm c manifested by a kink in resistance curve decreases monotonically and becomes undiscernable around P_\rm c = 10 GPa, indicative of suppression of the itinerant ferromagnetism. Meanwhile, by fitting the low temperature resistance to the Fermi liquid behavior of R =R_0 + AT^2, we found that R_0 shows a cusp-like anomaly and the coefficient A diverges around P_\rm c. These transport anomalies imply a tricritical point as commonly observed in itinerant ferromagnets under pressure. Unexpectedly, the Raman-active E_2g and A_1g modes soften remarkably after an initial weak hardening and the peak widths of both modes broaden evidently on approaching P_\rm c, followed by complete disappearance of both modes above this critical pressure. A possible underlying mechanism for such anomalous lattice softening near P_\rm c is discussed.