Electronic structure of the two-dimensional negative charge-transfer materialSr3FeMO7(M=Fe, Co)

Abstract

We studied the relation between the electronic structure and the physical properties of ${\mathrm{Sr}}_3\mathrm{Fe}M{\mathrm{O}}_7$ ($M=\mathrm{Fe}$, Co). The main technique used in the study was Fe $2p$, Co $2p$, and O $1s$ x-ray-absorption spectroscopy. The experimental spectra were analyzed in terms of cluster model and band-structure calculations. The analysis of the spectra shows that both ${\mathrm{Sr}}_3{\mathrm{FeFeO}}_7$ and ${\mathrm{Sr}}_3{\mathrm{FeCoO}}_7$ are in the negative-charge-transfer regime (this means that their ground states are highly covalent and contain considerable O $2p$ hole character). The Fe ions are in a high-spin state and the Co ions in an intermediate-spin state stabilized by strong hybridization. The electronic structure helps to explain the trends in the electrical conductivity and the observed magnetic moments. Further, the relatively large O $2p$ hole weight in the ground state explains also the absence of Jahn-Teller distortions. ${\mathrm{Sr}}_3{\mathrm{FeCoO}}_7$ illustrates ferromagnetism and magnetoresistance in a two-dimensional negative-charge-transfer material.