Recently, few-layer suspended stoichiometric ${\mathrm{BiFeO}}_3$ flakes were synthesized and found to display large tetragonality and giant dipole moments per unit surface. Little is known, however, about the ground state properties of this compound in the two-dimensional (2D) limit. By performing first-principles electronic structure calculations, we determine the ground state structural, magnetic, and electronic properties of suspended stoichiometric ${\mathrm{BiFeO}}_3$ flakes with number of layers ($n$) ranging from 1 to 4. We show that, even if the orthorhombic flakes with G-type antiferromagnetic order are the most stable ones for $n\le 4$, the metastable cubic ${\mathrm{BiFeO}}_3$ multilayers have physical properties in excellent agreement with experimental data, including enhanced tetragonality and large dipole moments per unit surface in the 2D limit. In these cubic multilayers, the broken inversion symmetry, determined by the different top and bottom terminations, results in a strong Fe offset along the $z$ direction coexisting with metallicity. Our work shows that the cubic phase, nonmagnetic and stable only above $927\mathrm{K}$ in bulk and in thin films, is a “polar” magnetic metal and a type I “multiferroic” in the 2D limit at room temperature.

• Received 2 July 2021
• Accepted 28 October 2021

DOI:https://doi.org/10.1103/PhysRevB.104.174111