Controllable Ferroelastic Switching in Epitaxial Self-Assembled Aurivillius Nanobricks

Significantly Enhanced Electrostrain in Oriented Epitaxial Self-Assembled Aurivillius-Type Piezoelectric Films via Regulating Polarization Vectors

High-temperature piezoelectric films with excellent piezoelectric and ferroelectric properties lay the foundation for the development of high-temperature piezo-MEMS devices. However, due to the poor piezoelectricity and strong anisotropy, it remains a challenge to obtain high quality Aurivillius-type high-temperature piezoelectric films with high performance, which impedes their practical implements. Here, a feasible polarization vector regulation strategy associated with oriented epitaxial self-assembled nanostructures for enhancing electrostrain is proposed. Guided by lattice matching relation, non-c-axis oriented epitaxial self-assembled Aurivillius-type calcium bismuth niobate (CaBi₂Nb₂O₉, CBN) high-temperature piezoelectric films were successfully prepared on different oriented Nb-STO substrates. By the lattice matching relationship, hysteresis measurement, and piezoresponse force microscopy analysis, it is confirmed that the polarization vectors transform from a two-dimensional plane to a three-dimensional space, and the out-of-plane polarization switching is enhanced. A platform for more possible polarization vectors is provided in the self-assembled (013)CBN film. More importantly, enhanced ferroelectric (P_r ∼ 13.4 μC/cm²) and large strain (∼0.24%) were obtained in the (013)CBN film, which promotes the great application prospect of CBN piezoelectric films in high-temperature MEMS devices.

Deterministic reversal of single magnetic vortex circulation by an electric field

The ability to control magnetic vortex is critical for their potential applications in spintronic devices. Traditional methods including magnetic field, spin-polarized current etc. have been used to flip the core and/or reverse circulation of vortex. However, it is challenging for deterministic electric-field control of the single magnetic vortex textures with time-reversal broken symmetry and no planar magnetic anisotropy. Here it is reported that a deterministic reversal of single magnetic vortex circulation can be driven back and forth by a space-varying strain in multiferroic heterostructures, which is controlled by using a bi-axial pulsed electric field. Phase-field simulation reveals the mechanism of the emerging magnetoelastic energy with the space variation and visualizes the reversal pathway of the vortex. This deterministic electric-field control of the single magnetic vortex textures demonstrates a new approach to integrate the low-dimensional spin texture into the magnetoelectric thin film devices with low energy consumption.

Strain-Engineered Ferroelastic Structures in PbTiO3 Films and Their Control by Electric Fields

We study the interplay between epitaxial strain, film thickness, and electric field in the creation, modification, and design of distinct ferroelastic structures in PbTiO₃ thin films. Strain and thickness greatly affect the structures formed, providing a two-variable parameterization of the resulting self-assembly. Under applied electric fields, these strain-engineered ferroelastic structures are highly malleable, especially when a/c and a₁/a₂ superdomains coexist. To reconfigure the ferroelastic structures and achieve self-assembled nanoscale-ordered morphologies, pure ferroelectric switching of individual c-domains within the a/c superdomains is essential. The stability, however, of the electrically written ferroelastic structures is in most cases ephemeral; the speed of the relaxation process depends sensitively on strain and thickness. Only under low tensile strain-as is the case for PbTiO₃ on GdScO₃-and below a critical thickness do the electrically created a/c superdomain structures become stable for days or longer, making them relevant for reconfigurable nanoscale electronics or nonvolatile electromechanical applications.

Atomically Resolved Edge States on a Layered Ferroelectric Oxide

The emerging surface/edge electronic phases driven by broken symmetry effects have attracted great attention in low-dimensional electronic systems. However, experimental proof on their existence in ferroelectric oxides at the atomic scale is still missing. In this work, metallic surface states are observed on layered Bi₂WO₆ by scanning tunneling microscopy/spectroscopy. Differential conductance is remarkably enhanced near the step edge compared with that on the terrace, forming a one-dimensional edge state. Density functional theory calculations verify that symmetry breaking at the surface determines the electronic structures and O 2p orbitals contribute the most to the density of states around the Fermi level. Our discovery provides a new strategy toward the hidden phases on other correlated oxide surfaces.