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

TL;DR

This study investigates the temperature-dependent electronic and magnetic structure of magnetite across the Verwey transition, revealing changes in charge excitations, magnetic dichroism, and gap formation linked to correlated Fe states.

Contribution

The paper provides a comprehensive analysis of bulk and surface electronic structures of magnetite across the Verwey transition using advanced spectroscopy and modeling, highlighting the role of specific Fe states.

Findings

Charge excitations vary with temperature and are surface-sensitive.

Magnetic circular dichroism is observed in the metallic phase.

A gap opens below the Verwey transition, indicating a metal-insulator change.

Abstract

We have studied the electronic structure of bulk single crystals and epitaxial films of magnetite Fe$_3$O$_4$. Fe $2p$ core-level spectra show clear differences between hard x-ray (HAX-) and soft x-ray (SX-) photoemission spectroscopy (PES), indicative of surface effects. The bulk-sensitive spectra exhibit temperature ($T$)-dependent charge excitations across the Verwey transition at $T_V$=122 K, which is missing in the surface-sensitive spectra. An extended impurity Anderson model full-multiplet analysis reveals roles of the three distinct Fe-species (A-Fe$^{3+}$, B-Fe$^{2+}$, B-Fe$^{3+}$) below $T_V$ for the Fe $2p$ spectra, 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 magnetically distinct sites associated with the charge excitations. Valence band HAXPES shows finite density of states at $E_F$ for the polaronic metal with 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-Fe$^{2+}$ and B-Fe$^{3+}$ electronic states, consistent with resistivity and bulk-sensitive optical spectra.