Temperature Dependence of Magnetically Active Charge Excitations in Magnetite across the Verwey Transition

We study the electronic structure of bulk single crystals and epitaxial films of ${\mathrm{Fe}}_3{\mathrm{O}}_4$. Fe $2p$ core level spectra show clear differences between hard x-ray (HAX) and soft x-ray photoemission spectroscopy (PES). The bulk-sensitive spectra exhibit temperature ($T$) dependence across the Verwey transition, which is missing in the surface-sensitive spectra. By using an extended impurity Anderson full-multiplet model—and in contrast to an earlier peak assignment—we show that the two distinct Fe species ($A$ and $B$ site) and the charge modulation at the $B$ site are responsible for the newly found double peaks in the main peak above $T_V$ and its $T$-dependent evolution. The Fe $2p$ HAXPES spectra show a clear magnetic circular dichroism (MCD) in the metallic phase of magnetized 100-nm-thick films. The model calculations also reproduce the MCD and identify the contributions from magnetically distinct $A$ and $B$ sites. Valence band HAXPES shows a finite density of states at $E_F$ for the polaronic half metal with a remnant order above $T_V$ and a clear gap formation below $T_V$. The results indicate that the Verwey transition is driven by changes in the strongly correlated and magnetically active $B$-site electronic states, consistent with resistivity and optical spectra.

Figure 1

Measured Fe $2p$ PES spectra of single crystal ${\mathrm{Fe}}_3{\mathrm{O}}_4$. (a) Comparison between Fe $2p$ HAXPES and SX-PES. The spectra were corrected by subtracting a Shirley-type background. (b) Enlargements of the energy range corresponding to Fe $2p_{2/3}$ line. (c) $T$-dependent Fe $2p$ HAXPES spectra across the Verwey transition. (Bottom panel) Difference of 300 and 150 K spectra to the 110 K spectrum.

Figure 2

Calculated $T$-dependent Fe $2p$ spectra of ${\mathrm{Fe}}_3{\mathrm{O}}_4$, together with the experimental HAXPES spectra.

Figure 3

(a) Fe $2p$ HAXPES spectra of 100-nm-thick films of ${\mathrm{Fe}}_3{\mathrm{O}}_4$ for the circularly polarized light. (b) Polarization components of the $A$ and $B$ sites are displayed.

Figure 4

(a) Measured $T$ dependence of the VB spectra of single crystal ${\mathrm{Fe}}_3{\mathrm{O}}_4$ across the Verwey transition, using a hard x ray ($h\nu =7.94\mathrm{keV}$). (b) Solid, dashed, and dot-dashed curces represent the fitted curves, $(a_1{E}_B+a_0)f(E_B,0)$, and $a_0(E_B-E_0)\theta (E_B-E_0)$, respectively.