The existence of ferroelectricity in {BiMnO3} has been a long-standing question for both experimentalists and theorists. In addition to a highly distorted bulk structure, the ionic crystal planes cause a large roughness in thin films that makes it extremely difficult to nail down the physical mechanisms underlying a possible ferroelectric-ferromagnetic phase. We approach the problem by including the substrate explicitly to study the polarization. With this model, we investigate mono-, di-, and trilayer {BiMnO3} thin films on SrTiO3 substrates. We find that thin film systems have both strong ferromagnetism and strong ferroelectricity. Substrate constraints weaken the competition between displacements induced by stereochemically active Bi-6s2 lone pairs and by Jahn-Teller distortions around Mn ions found in the bulk, such that the sum of off-center displacements of Bi ions in bulk {BiMnO3} nearly cancel. In {BiMnO3} thin films, in contrast, all Bi ions displace roughly in parallel, resulting in a strongly polar structure. We also find spontaneous charge disproportionation of Mn ion pairs in {BiMnO3} thin films.
@article{arxiv.1704.02833,
title = {Ferroelectricity in $\mathbf{BiMnO_3}$ Thin Films},
author = {Yun-Peng Wang and J. N. Fry and Hai-Ping Cheng},
journal= {arXiv preprint arXiv:1704.02833},
year = {2017}
}