Charged Ferroelectric Domain Walls for Deterministic ac Signal Control at the Nanoscale.

The direct current (dc) conductivity and emergent functionalities at ferroelectric domain walls are closely linked to the local polarization charges. Depending on the charge state, the walls can exhibit unusual dc conduction ranging from insulating to metallic-like, which is leveraged in domain-wall-based memory, multilevel data storage, and synaptic devices. In contrast to the functional dc behaviors at charged walls, their response to alternating currents (ac) remains to be resolved. Here, we reveal ac characteristics at positively and negatively charged walls in ErMnO3, distinctly different from the response of the surrounding domains. By combining voltage-dependent spectroscopic measurements on macroscopic and local scales, we demonstrate a pronounced nonlinear response at the electrode-wall junction, which correlates with the domain-wall charge state. The dependence on the ac drive voltage enables reversible switching between uni- and bipolar output signals, providing conceptually new opportunities for the application of charged walls as functional nanoelements in ac circuitry.

Observation of Uncompensated Bound Charges at Improper Ferroelectric Domain Walls.

Low-temperature electrostatic force microscopy (EFM) is used to probe unconventional domain walls in the improper ferroelectric semiconductor Er0.99Ca0.01MnO3 down to cryogenic temperatures. The low-temperature EFM maps reveal pronounced electric far fields generated by partially uncompensated domain-wall bound charges. Positively and negatively charged walls display qualitatively different fields as a function of temperature, which we explain based on different screening mechanisms and the corresponding relaxation time of the mobile carriers. Our results demonstrate domain walls in improper ferroelectrics as a unique example of natural interfaces that are stable against the emergence of electrically uncompensated bound charges. The outstanding robustness of improper ferroelectric domain walls in conjunction with their electronic versatility brings us an important step closer to the development of durable and ultrasmall electronic components for next-generation nanotechnology.

Large Carrier Mobilities in ErMnO3 Conducting Domain Walls Revealed by Quantitative Hall-Effect Measurements.

Kelvin probe force microscopy (KPFM) has been used to directly and quantitatively measure Hall voltages, developed at conducting tail-to-tail domain walls in ErMnO3 single crystals, when current is driven in the presence of an approximately perpendicular magnetic field. Measurements across a number of walls, taken using two different atomic force microscope platforms, consistently suggest that the active p-type carriers have unusually large room temperature mobilities of the order of hundreds of square centimeters per volt second. Associated carrier densities were estimated to be of the order of 1013 cm-3. Such mobilities, at room temperature, are high in comparison with both bulk oxide conductors and LaAlO3-SrTiO3 sheet conductors. High carrier mobilities are encouraging for the future of domain-wall nanoelectronics and, significantly, also suggest the feasibility of meaningful investigations into dimensional confinement effects in these novel domain-wall systems.

Antireflective nanocoatings for UV-sensation: the case of predatory owlfly insects.

Moth-eye nanostructures, discovered to coat corneae of certain nocturnal insects, have inspired numerous technological applications to reduce light reflectance from solar cells, light-emitting diodes, and optical detectors. Technological developments require such nanocoatings to possess broadband antireflective properties, transcending the visual light spectrum, in which animals typically operate. Here we describe the corneal nanostructures of the visual organ exclusive in UV sensation of the hunting insect Libelloides macaronius and report their supreme anti-light-reflectance capacity.

Alternative moth-eye nanostructures: antireflective properties and composition of dimpled corneal nanocoatings in silk-moth ancestors.

Moth-eye nanostructures are a well-known example of biological antireflective surfaces formed by pseudoregular arrays of nipples and are often used as a template for biomimetic materials. Here, we provide morphological characterization of corneal nanostructures of moths from the Bombycidae family, including strains of domesticated Bombyx mori silk-moth, its wild ancestor Bombyx mandarina, and a more distantly related Apatelodes torrefacta. We find high diversification of the nanostructures and strong antireflective properties they provide. Curiously, the nano-dimple pattern of B. mandarina is found to reduce reflectance as efficiently as the nanopillars of A. torrefacta. Access to genome sequence of Bombyx further permitted us to pinpoint corneal proteins, likely contributing to formation of the antireflective nanocoatings. These findings open the door to bioengineering of nanostructures with novel properties, as well as invite industry to expand traditional moth-eye nanocoatings with the alternative ones described here.

Current-driven dynamics and ratchet effect of skyrmion bubbles in a ferrimagnetic insulator.

Magnetic skyrmions are compact chiral spin textures that exhibit a rich variety of topological phenomena and hold potential for the development of high-density memory devices and novel computing schemes driven by spin currents. Here, we demonstrate the room-temperature interfacial stabilization and current-driven control of skyrmion bubbles in the ferrimagnetic insulator Tm3Fe5O12 coupled to Pt, showing the current-induced motion of individual skyrmion bubbles. The ferrimagnetic order of the crystal together with the interplay of spin-orbit torques and pinning determine the skyrmion dynamics in Tm3Fe5O12 and result in a strong skyrmion Hall effect characterized by a negative deflection angle and hopping motion. Further, we show that the velocity and depinning threshold of the skyrmion bubbles can be modified by exchange coupling Tm3Fe5O12 to an in-plane magnetized Y3Fe5O12 layer, which distorts the spin texture of the skyrmions and leads to directional-dependent rectification of their dynamics. This effect, which is equivalent to a magnetic ratchet, is exploited to control the skyrmion flow in a racetrack-like device.

High-speed domain wall racetracks in a magnetic insulator.

Recent reports of current-induced switching of ferrimagnetic oxides coupled to heavy metals have opened prospects for implementing magnetic insulators into electrically addressable devices. However, the configuration and dynamics of magnetic domain walls driven by electrical currents in insulating oxides remain unexplored. Here we investigate the internal structure of the domain walls in Tm3Fe5O12 (TmIG) and TmIG/Pt bilayers, and demonstrate their efficient manipulation by spin-orbit torques with velocities of up to 400 ms-1 and minimal current threshold for domain wall flow of 5 × 106 A cm-2. Domain wall racetracks are defined by Pt current lines on continuous TmIG films, which allows for patterning the magnetic landscape of TmIG in a fast and reversible way. Scanning nitrogen-vacancy magnetometry reveals that the domain walls of TmIG thin films grown on Gd3Sc2Ga3O12 exhibit left-handed Néel chirality, changing to an intermediate Néel-Bloch configuration upon Pt deposition. These results indicate the presence of interfacial Dzyaloshinskii-Moriya interaction in magnetic garnets, opening the possibility to stabilize chiral spin textures in centrosymmetric magnetic insulators.

Electrical half-wave rectification at ferroelectric domain walls.

Domain walls in ferroelectric semiconductors show promise as multifunctional two-dimensional elements for next-generation nanotechnology. Electric fields, for example, can control the direct-current resistance and reversibly switch between insulating and conductive domain-wall states, enabling elementary electronic devices such as gates and transistors. To facilitate electrical signal processing and transformation at the domain-wall level, however, an expansion into the realm of alternating-current technology is required. Here, we demonstrate diode-like alternating-to-direct current conversion based on neutral ferroelectric domain walls in ErMnO3. By combining scanning probe and dielectric spectroscopy, we show that the rectification occurs at the tip-wall contact for frequencies at which the walls are effectively pinned. Using density functional theory, we attribute the responsible transport behaviour at the neutral walls to an accumulation of oxygen defects. The practical frequency regime and magnitude of the direct current output are controlled by the bulk conductivity, establishing electrode-wall junctions as versatile atomic-scale diodes.

Domain Wall Architecture in Tetragonal Ferroelectric Thin Films.

Non-Ising-like 180° ferroelectric domain wall architecture and domain distribution in tetragonal PbZrx Ti1-x O3 thin films are probed using a combination of optical second harmonic generation and scanning transmission electron microscopy. In the remnant state, a specific nonlinear optical signature of tilted 180° domain walls corresponding to a mixed Ising-Néel-type rotation of polarization across the wall is shown.