Raman study of the Verwey transition in magnetite at high-pressures and low-temperatures

Recently magnetite (Fe_3O_4) attracted a lot of attention on the account of its charge carriers' strong spin polarization at the Fermi level. As a result, this compound has a potential as candidate material for spintronics. Several research groups attempt to grow high quality films of this compound. At the same time many of the bulk properties of magnetite remain unexplained. In particular, the mechanism of the metal- ``semiconductor'' (Verwey) transition at TV =120K remains puzzling. We report Raman spectra of magnetite that provide several distinct markers of the Verwey transition. This includes new Raman modes in the low temperature phase as well as a singularity in the temperature dependence of the intensity and frequency of the dominant Raman modes. We used these markers to determine the pressure dependence of the Verwey transition. We find that at 20 GPa the TV decreases to 20 K yielding dT_V/dp= -5 K/GPa, a value which is about twice the value predicted by the Coulomb-interaction-only model of the transition.