Local electronic structure of Fe$^{2+}$ impurities in MgO thin films: Temperature-dependent soft x-ray absorption spectroscopy study

TL;DR

This study investigates the local electronic structure of Fe$^{2+}$ impurities in MgO thin films using temperature-dependent soft x-ray absorption spectroscopy, revealing valence state, Jahn-Teller distortions, and crystal field effects.

Contribution

It provides detailed insights into the electronic structure and temperature-dependent behavior of Fe$^{2+}$ impurities in MgO, highlighting the role of spin-orbit coupling and crystal field effects.

Findings

Fe impurities are in the 2+ valence state

Presence of dynamical Jahn-Teller distortion

Temperature dependence due to thermal population of excited states

Abstract

We report on the local electronic structure of Fe impurities in MgO thin films. Using soft x-ray absorption spectroscopy (XAS) we verified that the Fe impurities are all in the 2+ valence state. The fine details in the line shape of the Fe $L_{2,3}$ edges provide direct evidence for the presence of a dynamical Jahn-Teller distortion. We are able to determine the magnitude of the effective $D_{4h}$ crystal field energies. We also observed a strong temperature dependence in the spectra which we can attribute to the thermal population of low-lying excited states that are present due to the spin-orbit coupling in the Fe 3d. Using this Fe$^{2+}$ impurity system as an example, we show that an accurate measurement of the orbital moment in Fe$_3$O$_4$ will provide a direct estimate for the effective local low-symmetry crystal fields on the Fe$^{2+}$ sites, important for the theoretical modeling of the formation of orbital ordering.