Perpendicular switching of polarization in layered ferroelectrics

Ferroelectric materials exhibit a spontaneous electric polarization that can be reversed by an electric field¹, a property central to non-volatile memories^2,3, sensors⁴ and actuators^5,6. In most conventional ferroelectrics, the polarization originates from a softening of a polar mode⁷. The amplitude of the mode along a given direction couples with the applied field along the same direction⁸. Ferroelectrics with a predominant in-plane polarization are harder to use in standard device geometries and therefore the field has mostly focused on out-of-plane ferroelectrics. Developing approaches that enable manipulation of the in-plane polarization component with an out-of-plane field would therefore provide new opportunities for device design and functionality. Here we have discovered that a trilinear coupling between the in-plane and out-of-plane polarization, mediated by means of the octahedral tilts and rotations in the layered ferroelectric Bi₄Ti₃O₁₂, naturally fulfils this challenge and enables perpendicular switching of the polarization state. In its bulk monoclinic phase, this material hosts a large in-plane polarization (about 50 μC cm^-2) driven by a proper ferroelectric instability, together with smaller out-of-plane polarization (about 5 μC cm^-2) of improper origin, induced by oxygen octahedral distortions. We demonstrate that, in c-axis-oriented epitaxial films, the in-plane polarization switches deterministically under an out-of-plane electric field. This cross-coupling between orthogonal polarization components provides a route to transverse manipulation of ferroic order parameters and establishes layered ferroelectrics as a platform for capacitive computing concepts.