Oxygen $K$-edge shift at the Verwey transition of magnetite

The temperature dependence of the $\mathrm{O}K$-edge threshold has been investigated in a temperature range encompassing the Verwey transition. Both above and below $T_V$ the $\mathrm{O}K$-edge threshold changes linear with temperature whereas there is discontinuous energy shift of $13\mathrm{meV}$ at $T_V$. A comparison of our results to published low and high energy photoemission experiments suggests the presence of an asymmetric gap with respect to the Fermi level. This explains present fundamental differences between photoemission and infrared derived gap energies, and supports the polaronic image of the electrical conductivity of ${\mathrm{Fe}}_3{\mathrm{O}}_4$ above the Verwey transition temperature.

Figure 1

X-ray absorption spectrum at the $\mathrm{O}K$ edge of ${\mathrm{Fe}}_3{\mathrm{O}}_4$ $(T=300\mathrm{K})$.

Figure 2

(Color online) Temperature dependence of the $\mathrm{O}K$-edge preedge in the vicinity of the Verwey transition temperature region.

Figure 3

Temperature dependency of the half height position of the $\mathrm{O}K$ edge threshold, extracted from Fig. 1b. Inset: Sample magnetization of the ${\mathrm{Fe}}_3{\mathrm{O}}_4$ single crystal. The Verwey transition is directly visible at $123.7\mathrm{K}$.

Figure 4

(Color online) Schematic of the nonspin resolved band positions and shifts as the temperature evolves through the Verwey transition. The left site is related to the more insulating behavior below $T_V$, and the right site to the situation above $T_V$. The total gap shift is $63\mathrm{meV}$. The green arrow at the left indicates the IR-derived gap of about $200\mathrm{meV}$ at $50\mathrm{K}$.