X-ray spectroscopic study ofBaFeO3thin films: AnFe4+ferromagnetic insulator

Abstract

We investigated the electronic and magnetic properties of fully oxidized ${\mathrm{BaFeO}}_{3}$ thin films, which show ferromagnetic-insulating properties with cubic crystal structure, by hard x-ray photoemission spectroscopy (HAXPES), x-ray absorption spectroscopy (XAS), and soft x-ray magnetic circular dichroism (XMCD). We analyzed the results with configuration-interaction (CI) cluster-model calculations for ${\mathrm{Fe}}^{4+}$, which showed good agreement with the experimental results. We also studied ${\mathrm{SrFeO}}_{3}$ thin films, which have an ${\mathrm{Fe}}^{4+}$ ion helical magnetism in cubic crystal structure, but are metallic at all temperatures. We found that ${\mathrm{BaFeO}}_{3}$ thin films are insulating with large magnetization ( 1.$7{\ensuremath{\mu}}_{B}$/formula unit) under $\ensuremath{\sim}1$ T, using valence-band HAXPES and Fe $2p$ XMCD, which is consistent with the previously reported resistivity and magnetization measurements. Although Fe $2p$ core-level HAXPES and Fe $2p$ XAS spectra of ${\mathrm{BaFeO}}_{3}$ and ${\mathrm{SrFeO}}_{3}$ thin films are quite similar, we compared the insulating ${\mathrm{BaFeO}}_{3}$ to metallic ${\mathrm{SrFeO}}_{3}$ thin films with valence-band HAXPES. The CI cluster-model analysis indicates that the ground state of ${\mathrm{BaFeO}}_{3}$ is dominated by ${d}^{5}\underline{L}\phantom{\rule{0.222222em}{0ex}}(\underline{L}$: ligand hole) configuration due to the negative charge transfer energy, and that the band gap has significant O $2p$ character. We revealed that the differences of the electronic and magnetic properties between ${\mathrm{BaFeO}}_{3}$ and ${\mathrm{SrFeO}}_{3}$ arise from the differences in their lattice constants, through affecting the strength of hybridization and bandwidth.