Improper Ferroelectrics at the Ultrathin Limit

Recently, improper ferroelectrics –materials in which electric polarization arises as a secondary effect of a leading order parameter– are receiving increasing attention. The family of hexagonal manganites (<i>h</i>-RMnO₃, R = rare earth) is a model system for improper ferroelectricity, where the electric polarization emerges as a byproduct of a primary lattice-trimerizing structural distortion, a combination of tilt and rotation of the MnO₅ bipyramids around the R³⁺ ion. The trimerization is described by a two-component order parameter <b>Q </b>consisting of the tilt amplitude Q and the angle φ of the MnO₅ bipyramids, but can also be related to the amplitude and phase of the corrugation of the R layer, which is easily visible and quantifiable by HAADF-STEM imaging [1]. However, while bulk <i>h</i>-RMnO bulk crystals have been intensively studied in the last decade, not as much of research at the atomic scale has been conducted on the improper ferroelectric behavior of technologically relevant ultrathin films.<br/>Rather than the depolarizing field being the main challenge to achieving polar thin films, as in proper ferroelectrics, improper ferroelectricity in ultrathin films is determined by the thin-film specific behavior of its driving non-polar order parameter. In YMnO₃ thin films, substrate clamping and critical fluctuations of the order parameter lead to a lowering of the temperature of the phase transition, T_C^film. Thus, T_C^film decreases smoothly with YMnO₃ thickness and, inexplicably, at room temperature, the spontaneous polarization reaches zero for the two-unit-cell film [2].<br/>Our current work focuses on the investigation of nanoscale confinement effects on the lattice trimerization of ultrathin YMnO₃ films. The structure and properties of the YMnO₃ films, with a special focus on the interfaces, are investigated by using a combination of quantitative (in-situ) HAADF-STEM and STEM-EELS, and density functional calculations. Our results advance the understanding of the evolution of improper ferroelectricity within the confinement of ultrathin films, which is essential for their successful implementation in nanoscale devices, such as ferroelectric tunnel junctions and resistive memories [3].<br/>[1] E. Holtz <i>et al. Nano Lett. </i><b>17</b>, 5883 (2017).<br/>[2] J. Nordlander <i>et al.</i> <i>Nat. Commun. </i><b>10</b>, 5591 (2019).<br/>[3] A. Vogel <i>et al.</i> <i>In preparation</i> (2022).