X-ray spectroscopic study of BaFeO$_{3}$ thin films; an Fe$ ^{4+}$ ferromagnetic insulator

TL;DR

This study uses advanced spectroscopic techniques to analyze BaFeO3 thin films, revealing their insulating ferromagnetic nature and the electronic structure differences from SrFeO3, driven by lattice effects.

Contribution

It provides a detailed spectroscopic and theoretical analysis of BaFeO3 thin films, highlighting the role of lattice constants in their electronic and magnetic properties, which was not previously understood.

Findings

BaFeO3 thin films are insulating with significant magnetization.

The ground state of BaFeO3 is dominated by d5L configuration due to negative charge transfer energy.

Differences between BaFeO3 and SrFeO3 arise from lattice constant effects on hybridization.

Abstract

We investigated the electronic and magnetic properties of fully oxidized BaFeO3 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 Fe4+, which showed good agreement with the experimental results. We also studied SrFeO3 thin films, which have an Fe4+ ion helical magnetism in cubic crystal structure, but are metallic at all temperatures. We found that BaFeO3 thin films are insulating with large magnetization (2.1muB/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 2p core-level HAXPES and Fe 2p XAS spectra of BaFeO3 and SrFeO3 thin films are quite similar, we compared the insulating BaFeO3 to metallic SrFeO3 thin films with valence-band HAXPES. The CI cluster-model analysis indicates that the ground state of BaFeO3 is dominated by d5L (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 BaFeO3 and SrFeO3 arise from the differences in their lattice constants, through affecting the strength of hybridization and bandwidth.