Local electronic structure ofFe2+impurities in MgO thin films: Temperature-dependent soft x-ray absorption spectroscopy study

We report on the local electronic structure of Fe impurities in MgO thin films. Using soft x-ray absorption spectroscopy, we verified that the Fe impurities are all in the $2+$ valence state. The fine details in the line shape of the $\text{Fe}\text{ }{L}_{2,3}$ 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 $\text{Fe}\text{ }3d$. Using this ${\text{Fe}}^{2+}$ impurity system as an example, we show that an accurate measurement of the orbital moment in ${\text{Fe}}_{3}{\text{O}}_{4}$ will provide a direct estimate for the effective local low-symmetry crystal fields on the ${\text{Fe}}^{2+}$ sites, important for the theoretical modeling of the formation of orbital ordering.