X-ray spectroscopic study of BaFeO_3 thin films: An Fe~(4+) ferromagnetic insulator

摘要

We investigated the electronic and magnetic properties of fully oxidized 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 Fe~(4+), which showed good agreement with the experimental results. We also studied SrFeO_3 thin films, which have an Fe~(4+) ion helical magnetism in cubic crystal structure, but are metallic at all temperatures. We found that BaFeO_3 thin films are insulating with large magnetization (1.7 μ_B/formula unit) under ~1 T, using valence-band HAXPES and Fe 2p XMCD, which is consistent with the previously reported resistivity and magnetization measurements. Although Fe 1p core-level HAXPES and Fe 2p XAS spectra of BaFeO_3 and SrFeO_3 thin films are quite similar, we compared the insulating BaFeO_3 to metallic SrFeO_3 thin films with valence-band HAXPES. The CI cluster-model analysis indicates that the ground state of BaFeO_3 is dominated by d~5L (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 BaFeO_3 and SrFeO_3 arise from the differences in their lattice constants, through affecting the strength of hybridization and bandwidth.