Ultrahigh Oxygen Ion Mobility in Ferroelectric Hafnia

Depth-Resolved X-Ray Photoelectron Spectroscopy Evidence of Intrinsic Polar States in HfO2-Based Ferroelectrics

The discovery of ferroelectricity in nanoscale hafnia-based oxide films has spurred interest in understanding their emergent properties. Investigation focuses on the size-dependent polarization behavior, which is sensitive to content and movement of oxygen vacancies. Though polarization switching and electrochemical reactions is shown to co-occur, their relationship remains unclear. This study employs X-ray photoelectron spectroscopy with depth sensitivity to examine changes in electrochemical states occurring during polarization switching. Contrasting Hf0.5Zr0.5O2 (HZO) with Hf0.88La0.04Ta0.08O2 (HLTO), a composition with an equivalent structure and comparable average ionic radius, electrochemical states are directly observed for specific polarization directions. Lower-polarization films exhibit more significant electrochemical changes upon switching, suggesting an indirect relationship between polarization and electrochemical state. This research illuminates the complex interplay between polarization and electrochemical dynamics, providing evidence for intrinsic polar states in HfO2-based ferroelectrics.

Tunable Multistate Ferroelectricity of Unit-Cell-Thick BaTiO3 Revived by a Ferroelectric SnS Monolayer via Interfacial Sliding

Stabilization of multiple polarization states at the atomic scale is pivotal for realizing high-density memory devices beyond prevailing bistable ferroelectric architectures. Here, we show that two-dimensional ferroelectric SnS or GeSe is able to revive and stabilize the ferroelectric order of three-dimensional ferroelectric BaTiO3, even when the latter is thinned to one unit cell in thickness. The underlying mechanism for overcoming the conventional detrimental critical thickness effect is attributed to facile interfacial inversion symmetry breaking by robust in-plane polarization of SnS or GeSe. Furthermore, when invoking interlayer sliding, we can stabilize multiple polarization states and achieve efficient interstate switching in the heterostructures, accompanied by dynamical ferroelectric skyrmionic excitations. When invoking sliding and twisting, the moiré domains exhibit nontrivial polar vortexes, which can be laterally displaced via different sliding schemes. These findings provide an intuitive avenue for simultaneously overcoming the standing critical thickness issue in bulk ferroelectrics and weak polarization issue in sliding ferroelectricity.

Insights into Strain Engineering: From Ferroelectrics to Related Functional Materials and Beyond

Ferroelectrics have become indispensable components in various application fields, including information processing, energy harvesting, and electromechanical conversion, owing to their unique ability to exhibit electrically or mechanically switchable polarization. The distinct polar noncentrosymmetric lattices of ferroelectrics make them highly responsive to specific crystal structures. Even slight changes in the lattice can alter the polarization configuration and response to external fields. In this regard, strain engineering has emerged as a prevalent regulation approach that not only offers a versatile platform for structural and performance optimization within ferroelectrics but also unlocks boundless potential in various functional materials. In this review, we systematically summarize the breakthroughs in ferroelectric-based functional materials achieved through strain engineering and progress in method development. We cover research activities ranging from fundamental attributes to wide-ranging applications and novel functionalities ranging from electromechanical transformation in sensors and actuators to tunable dielectric materials and information technologies, such as transistors and nonvolatile memories. Building upon these achievements, we also explore the endeavors to uncover the unprecedented properties through strain engineering in related chemical functionalities, such as ferromagnetism, multiferroicity, and photoelectricity. Finally, through discussions on the prospects and challenges associated with strain engineering in the materials, this review aims to stimulate the development of new methods for strain regulation and performance boosting in functional materials, transcending the boundaries of ferroelectrics.

Granular Magnetization Switching in Pt/Co/Ti Structure with HfOx Insertion for In-Memory Computing Applications

Exploring multiple states based on the domain wall (DW) position has garnered increased attention for in-memory computing applications, particularly focusing on the utilization of spin-orbit torque (SOT) to drive DW motion. However, devices relying on the DW position require efficient DW pinning. Here, we achieve granular magnetization switching by incorporating an HfOx insertion layer between the Co/Ti interface. This corresponds to a transition in the switching model from the DW motion to DW nucleation. Compared to the conventional Pt/Co/Ti structure, incorporation of the HfOx layer results in an enhanced SOT efficiency and a lower switching current density. We also realized stable multistate storage and synaptic plasticity by applying pulse current in the Pt/Co/HfOx/Ti device. The simulation of artificial neural networks (ANN) based on the device can perform digital recognition tasks with an accuracy rate of 91%. These results identify that DW nucleation with a Pt/Co/HfOx/Ti based device has potential applications in multistate storage and ANN.

Enhanced polarization switching characteristics of HfO2 ultrathin films via acceptor-donor co-doping

In the realm of ferroelectric memories, HfO2-based ferroelectrics stand out because of their exceptional CMOS compatibility and scalability. Nevertheless, their switchable polarization and switching speed are not on par with those of perovskite ferroelectrics. It is widely acknowledged that defects play a crucial role in stabilizing the metastable polar phase of HfO2. Simultaneously, defects also pin the domain walls and impede the switching process, ultimately rendering the sluggish switching of HfO2. Herein, we present an effective strategy involving acceptor-donor co-doping to effectively tackle this dilemma. Remarkably enhanced ferroelectricity and the fastest switching process ever reported among HfO2 polar devices are observed in La3+-Ta5+ co-doped HfO2 ultrathin films. Moreover, robust macro-electrical characteristics of co-doped films persist even at a thickness as low as 3 nm, expanding potential applications of HfO2 in ultrathin devices. Our systematic investigations further demonstrate that synergistic effects of uniform microstructure and smaller switching barrier introduced by co-doping ensure the enhanced ferroelectricity and shortened switching time. The co-doping strategy offers an effective avenue to control the defect state and improve the ferroelectric properties of HfO2 films.

Fractional quantum ferroelectricity

For an ordinary ferroelectric, the magnitude of the spontaneous electric polarization is at least one order of magnitude smaller than that resulting from the ionic displacement of the lattice vectors, and the direction of the spontaneous electric polarization is determined by the point group of the ferroelectric. Here, we introduce a new class of ferroelectricity termed Fractional Quantum Ferroelectricity. Unlike ordinary ferroelectrics, the polarization of Fractional Quantum Ferroelectricity arises from substantial atomic displacements that are comparable to lattice constants. Applying group theory analysis, we identify 28 potential point groups that can realize Fractional Quantum Ferroelectricity, including both polar and non-polar groups. The direction of polarization in Fractional Quantum Ferroelectricity is found to always contradict with the symmetry of the "polar" phase, which violates Neumann's principle, challenging conventional symmetry-based knowledge. Through the Fractional Quantum Ferroelectricity theory and density functional calculations, we not only explain the puzzling experimentally observed in-plane polarization of monolayer α-In2Se3, but also predict polarization in a cubic compound of AgBr. Our findings unveil a new realm of ferroelectric behavior, expanding the understanding and application of these materials beyond the limits of traditional ferroelectrics.