Domain wall roughness in stripe phase BiFeO3 thin films

Ferroelectric Size Effects on Statics and Dynamics of Domain Wall

Domain walls separating differently oriented polarization regions of ferroelectric materials are known to greatly impact nanoscale materials and device functionalities. Though the understanding of size effects in ferroelectric nanostructures has progressed, the effect of thickness downsizing on domain wall scaling behavior has remained unexplored. Using piezoresponse force microscopy, epitaxial BaTiO3 film thickness size (2-90 nm) effects on the critical scaling universality of the domain wall dynamical creep and static roughness exponents including dimensionality is demonstrated. Independently estimated static roughness exponents ranging between 0.34 and 0.28 and dynamical creep exponents transition from 0.54 to 0.22 elucidate the domain wall dimensionality transition from two- to quasi-one-dimension in the thickness range of 10-25 nm, which is later validated by evaluating effective dimensionality within the paradigm of random-bond universality. The observed interdimensional transition is further credenced to the compressive strain and long-range strain-dipolar interactions, as revealed by the structural analyses and additional measurements with modified substrate-induced strain. These results provide new insights into the understanding of size effects in nanoscale ferroelectricity, paving the way toward future nanodevices.

Defining ferroelectric characteristics with reversible piezoresponse: PUND switching spectroscopy PFM characterization

Detecting ferroelectricity at micro- and nanoscales is crucial for advanced nanomaterials and materials with complicated topography. Switching spectroscopy piezoresponse force microscopy (SSPFM), which involves measuring piezoelectric hysteresis loops via a scanning probe microscopy tip, is a widely accepted approach to characterize polarization reversal at the local scale and confirm ferroelectricity. However, the local hysteresis loops acquired through this method often exhibit unpredictable shapes, a phenomenon often attributed to the influence of parasitic factors such as electrostatic forces and current flow. Our research has uncovered that the deviation in hysteresis loop shapes can be caused by spontaneous backswitching occurring after polarization reversal. Moreover, we've determined that the extent of this effect can be exacerbated when employing inappropriate SSPFM waveform parameters, including duration, frequency, and AC voltage amplitude. Notably, the conventional 'pulse-mode' SSPFM method has been found to intensify spontaneous backswitching. In response to these challenges, we have redesigned SSPFM approach by introducing the positive up-negative down (PUND) method within the 'step-mode' SSPFM. This modification allows for effective probing of local piezoelectric hysteresis loops in ferroelectrics with reversible piezoresponse while removing undesirable electrostatic contribution. This advancement extends the applicability of the technique to a diverse range of ferroelectrics, including semiconductor ferroelectrics and relaxors, promising a more reliable and accurate characterization of their properties.

Individual Barkhausen Pulses of Ferroelastic Nanodomains

Ferroelectric materials, upon electric field biasing, display polarization discontinuities known as Barkhausen jumps, a subclass of a more general phenomenon known as crackling noise. Herein, we follow and visualize in real time the motion of single 90° needle domains induced by an electric field applied in the polarization direction of the prototypical ferroelectric BaTiO_{3}, inside a transmission electron microscope. The nature of motion and periodicity of the Barkhausen pulses leads to distinctive interactions between domains forming a herringbone pattern. Remarkably, the tips of the domains do not come into contact with the body of the perpendicular domain, suggesting the presence of strong electromechanical fields around the tips of the needle domains. Additionally, interactions of the domains with the lattice result in relatively free movement of the domain walls through the dielectric medium, indicating that their motion-related activation energy depends only on weak Peierls-like potentials. Control over the kinetics of ferroelastic domain wall motion can lead to novel nanoelectronic devices pertinent to computing and data storage applications.

Dynamic Tilting of Ferroelectric Domain Walls Caused by Optically Induced Electronic Screening

Optical excitation perturbs the balance of phenomena selecting the tilt orientation of domain walls within ferroelectric thin films. The high carrier density induced in a low-strain BaTiO_{3} thin film by an above-band-gap ultrafast optical pulse changes the tilt angle that 90° a/c domain walls form with respect to the substrate-film interface. The dynamics of the changes are apparent in time-resolved synchrotron x-ray scattering studies of the domain diffuse scattering. Tilting occurs at 298 K, a temperature at which the a/b and a/c domain phases coexist but is absent at 343 K in the better ordered single-phase a/c regime. Phase coexistence at 298 K leads to increased domain-wall charge density, and thus a larger screening effect than in the single-phase regime. The screening mechanism points to new directions for the manipulation of nanoscale ferroelectricity.

Universality classes of the domain-wall creep motion driven by spin-transfer torques

With the stochastic Landau-Lifshitz-Gilbert equation, we numerically simulate the creep motion of a magnetic domain wall driven by the adiabatic and nonadiabatic spin-transfer torques induced by the electric current. The creep exponent μ and the roughness exponent ζ are accurately determined from the scaling behaviors. The creep motions driven by the adiabatic and nonadiabatic spin-transfer torques belong to different universality classes. The scaling relation between μ and ζ based on certain simplified assumptions is valid for the nonadiabatic spin-transfer torque, while invalid for the adiabatic one. Our results are compatible with the experimental ones, but go beyond the existing theoretical prediction. Our investigation reveals that the disorder-induced pinning effect on the domain-wall rotation alters the universality class of the creep motion.

Uniaxial Strain-Controlled Ferroelastic Domain Evolution in BiFeO3

We investigate the effect of variable uniaxial tensile strain on the evolution of 71° ferroelastic domains in (001)-oriented epitaxial BiFeO₃ (BFO) thin films using piezoresponse force microscopy (PFM). For this purpose, a newly designed bending stage has been employed, which allows tensile bending as wells as in situ PFM characterization. In situ PFM imaging reveals polarization-strain correlations at the nanoscale. Specifically, ferroelastic domains with in-plane polarization along the direction of applied tensile strain expand, whereas the adjoining domains with orthogonal in-plane polarization contract. The switching is mediated by significant domain wall roughening and opposite displacement of the successive walls. Further, the domains with long-range order are more susceptible to an applied external mechanical stimulus compared to the domains, which exhibit short-range periodicity. In addition, the imprint state of film reverses direction under applied tensile strain. Finally, the strain-induced changes in the domain structure and wall motion are fully reversible and revert to their as-grown state upon release of the applied stress. The strain-induced non-180° polarization rotation constitutes a route to control connected functionalities, such as magnetism, via coupled in-plane rotation of the magnetic plane in multiferroic BFO thin films.

Terahertz-Range Polar Modes in Domain-Engineered BiFeO_{3}

The dielectric permittivity and properties of electrically active lattice resonances in nanotwinned BiFeO_{3} crystals have been studied theoretically using an earlier established interatomic potential. The results suggest that an array of 71° domain walls with about 2-5 nm spacing enhances the static permittivity of BiFeO_{3} by more than an order of magnitude. This enhancement is associated with an electrically active excitation, corresponding to a collective vibration of pinned domain walls at a remarkably high frequency of about 0.3 THz.

Microwave a.c. conductivity of domain walls in ferroelectric thin films

Ferroelectric domain walls are of great interest as elementary building blocks for future electronic devices due to their intrinsic few-nanometre width, multifunctional properties and field-controlled topology. To realize the electronic functions, domain walls are required to be electrically conducting and addressable non-destructively. However, these properties have been elusive because conducting walls have to be electrically charged, which makes them unstable and uncommon in ferroelectric materials. Here we reveal that spontaneous and recorded domain walls in thin films of lead zirconate and bismuth ferrite exhibit large conductance at microwave frequencies despite being insulating at d.c. We explain this effect by morphological roughening of the walls and local charges induced by disorder with the overall charge neutrality. a.c. conduction is immune to large contact resistance enabling completely non-destructive walls read-out. This demonstrates a technological potential for harnessing a.c. conduction for oxide electronics and other materials with poor d.c. conduction, particularly at the nanoscale.

Energy landscape scheme for an intuitive understanding of complex domain dynamics in ferroelectric thin films

Fundamental understanding of domain dynamics in ferroic materials has been a longstanding issue because of its relevance to many systems and to the design of nanoscale domain-wall devices. Despite many theoretical and experimental studies, a full understanding of domain dynamics still remains incomplete, partly due to complex interactions between domain-walls and disorder. We report domain-shape-preserving deterministic domain-wall motion, which directly confirms microscopic return point memory, by observing domain-wall breathing motion in ferroelectric BiFeO₃ thin film using stroboscopic piezoresponse force microscopy. Spatial energy landscape that provides new insights into domain dynamics is also mapped based on the breathing motion of domain walls. The evolution of complex domain structure can be understood by the process of occupying the lowest available energy states of polarization in the energy landscape which is determined by defect-induced internal fields. Our result highlights a pathway for the novel design of ferroelectric domain-wall devices through the engineering of energy landscape using defect-induced internal fields such as flexoelectric fields.

BiFeO3 epitaxial thin films and devices: past, present and future

The celebrated renaissance of the multiferroics family over the past ten years has also been that of its most paradigmatic member, bismuth ferrite (BiFeO3). Known since the 1960s to be a high temperature antiferromagnet and since the 1970s to be ferroelectric, BiFeO3 only had its bulk ferroic properties clarified in the mid-2000s. It is however the fabrication of BiFeO3 thin films and their integration into epitaxial oxide heterostructures that have fully revealed its extraordinarily broad palette of functionalities. Here we review the first decade of research on BiFeO3 films, restricting ourselves to epitaxial structures. We discuss how thickness and epitaxial strain influence not only the unit cell parameters, but also the crystal structure, illustrated for instance by the discovery of the so-called T-like phase of BiFeO3. We then present its ferroelectric and piezoelectric properties and their evolution near morphotropic phase boundaries. Magnetic properties and their modification by thickness and strain effects, as well as optical parameters, are covered. Finally, we highlight various types of devices based on BiFeO3 in electronics, spintronics, and optics, and provide perspectives for the development of further multifunctional devices for information technology and energy harvesting.