Ferroelectricity and magnetic order at room temperature and electronic states of bismuth iron oxyfluoride thin films

Iron oxyfluorides with large tetragonal polar distortions are expected to exhibit multiferroic properties that coexist with their ferroelectric and magnetic orders. However, oxyfluoride multiferroicity has not been experimentally confirmed in polycrystals, owing to their high leakage currents and the presence of impurity phases. In this study, we fabricated single-crystalline Ba-doped bismuth iron oxyfluoride thin films by combining pulsed laser deposition with topochemical fluorination and investigated their room-temperature ferroelectric and magnetic properties. The substitution of Ba at the Bi site in the precursor films was found to play an important role in the introduction of fluorine into the precursor. The Bi_1−xBa_xFeO_3−xF_x (x = 0.2, 0.3) films exhibited ferroelectricity and a small magnetization at 300 K. Moreover, electronic structures due to the F 2p states were formed in the valence and conduction bands of the Bi_1−xBa_xFeO_3−xF_x (x = 0.2, 0.3) films. The findings demonstrate the potential of oxyfluoride compounds, thereby broadening the range of materials that can be used as room-temperature multiferroics.