Unconventional Multiferroic Phenomena in Ultrathin BiFeO₃

BiFeO₃ (BFO) is a room temperature magnetoelectric material demonstrating direct coupling between ferroelectric and magnetic order parameters. While studied extensively in film form, scaling to thinner thickness is required for applications in low power computing. However, scaling ferroelectric and antiferromagnetic order to ultrathin thickness poses significant challenges. Polar order vanishes in nearly all known perovskite ferroelectric materials at thin thickness due to uncompensated bound charges at the ferroelectric surface, causing a “dead layer” formation of a few nanometers. Moreover, dead layers also commonly form in ultrathin oxide antiferromagnets (including BFO) due to off-stoichiometry or reduction in magnetic anisotropy. To overcome these limitations, we explore the polar and magnetic states of ultrathin, sub-5 nm thick BFO under various boundary conditions and epitaxial constraints. First, by engineering open circuit boundary conditions and octahedral tilt distortions, we uncover mixed-phase polar and antipolar regions of BFO . We demonstrate that electric (polar) fields can be used to both erase and introduce centrosymmetry, effectively erasing and writing multiferoic order via a first-order phase transition¹. Second, we demonstrate that epitaxial strain and short circuit boundary conditions can be used to establish polar and magnetic order in ultrathin BFO via a second order phase transition accompanied by the observation of topological magnetoelectric defects. This work highlights new strategies for designing magnetoelectric materials at the thinnest length scales.