Laser-driven first-order spin reorientation and Verwey phase transitions in the magnetite Fe$_3$O$_4$ beyond the range of thermodynamic equilibrium

Abstract: Ultrafast photo-induced phase transitions occurring under the impact of femtosecond laser pulses provide versatile opport unities for switching solids between distinctly-different crystalline, electronic, and magnetic states and thus modify their functional properties in a significant way. In this paper, we report on the laser-induced spin reorientation and Verwey phase transitions in a single crystalline ferrimagnetic magnetite Fe$_3$O$_4$. Using femtosecond optical and magneto-optical pump-probe techniques, we define the range of the initial sample temperatures and laser fluences when partialor complete photo-induced phase transitions occur from a monoclinic insulating to a cubic metallic state with concomitant switching of magnetic anisotropy from the uniaxial to the cubic one. We thus reveal a connection between these phase transitions when driven by femtosecond laser pulses.Using transient linear and quadratic magneto-optical effects, we examine magnetization dynamics launched the switching of the magnetic anisotropy axis. We unveil the presence of the domains under going the laser-induced phase transitions even below the established threshold fluence for the transitions, as well as when the material is initially in the cubic phase. This is the manifestation of the first-order of these both laser-induced phase transitions beyond the range of thermodynamic equilibrium.