Fluorination Effects on the Ferroelectric and Magnetic Properties of Ba-Substituted BiFeO ₃ Thin Films Studied by Electron Spectroscopy

Ba- and F-codoped bismuth ferrite stabilizes a P 4 mm structure suitable for ferroelectric switching and exhibits magnetic order, making it a promising candidate for room-temperature multiferroic applications. In this study, we investigated the influence of fluorination on the electronic, magnetic, and ferroelectric properties of Bi 0.8 Ba 0.2 FeO 2.9 thin films using synchrotron-based X-ray photoemission spectroscopy (XPS) and X-ray magnetic circular dichroism (XMCD). Resonant photoemission spectroscopy spectra revealed that Fe-derived valence band states remain essentially unchanged upon fluorination, whereas antiresonant spectra showed clear modifications in the O 2p and Bi 6s states, along with the emergence of F 2p-derived states. XMCD measurements at the Fe L-edge confirmed room-temperature ferromagnetism in both films, originating from Fe 3+ ions at octahedral sites. Ferroelectric measurements demonstrated that Bi 0.8 Ba 0.2 FeO 2.8 F 0.2 exhibits a polarization–electric-field hysteresis loop, whereas Bi 0.8 Ba 0.2 FeO 2.9 remains nonferroelectric. These contrasting behaviors are attributed to the localization of the Bi 3+ lone pair at randomly distributed oxygen vacancies in Bi 0.8 Ba 0.2 FeO 2.9, which suppresses its stereochemical activity. In contrast, fluorination restores lone pair activity by filling these vacancies.