  We report high-pressure low-temperature Raman studies of the Verwey
transition in pure and Al-doped magnetite (Fe_3O_4). The low temperature phase
of magnetite displays a number of additional Raman modes that serve as
transition markers. These transition markers allow one to investigate the
effect of hydrostatic pressure on the Verwey transition temperature. Al-doped
magnetite Fe_2.8Al_0.2O_4 (TV=116.5K) displays a nearly linear decrease of the
transition temperature with an increase of pressure yielding dP/dT_V = -0.096
GPa/K. In contrast pure magnetite displays a significantly steeper slope of the
PT equilibrium line with dP/dT_V = -0.18 GPa/K. The slope of the PT equilibrium
lines is related to the changes of the molar entropy and molar volume at the
transition. We compare our spectroscopic data with that obtained from the
ambient pressure specific heat measurements and find a good agreement in the
optimally doped magnetite. Our data indicates that Al doping leads to a smaller
entropy change and larger volume expansion at the transition. Our data displays
the trends that are consistent with the mean field model of the transition that
assumes charge ordering in magnetite.
