Imaging Ferroelectric Domains and Domain Walls Using Charge Gradient Microscopy: Role of Screening Charges

Advanced scanning probe microscopies (SPMs) open up the possibilities of the next-generation ferroic devices that utilize both domains and domain walls as active elements. However, current SPMs lack the capability of dynamically monitoring the motion of domains and domain walls in conjunction with the transport of the screening charges that lower the total electrostatic energy of both domains and domain walls. Charge gradient microscopy (CGM) is a strong candidate to overcome these shortcomings because it can map domains and domain walls at high speed and mechanically remove the screening charges. Yet the underlying mechanism of the CGM signals is not fully understood due to the complexity of the electrostatic interactions. Here, we designed a semiconductor–metal CGM tip, which can separate and quantify the ferroelectric domain and domain wall signals by simply changing its scanning direction. Our investigation reveals that the domain wall signals are due to the spatial change of polarization charges, while the domain signals are due to continuous removal and supply of screening charges at the CGM tip. In addition, we observed asymmetric CGM domain currents from the up and down domains, which are originated from the different debonding energies and the amount of the screening charges on positive and negative bound charges. We believe that our findings can help design CGM with high spatial resolution and lead to breakthroughs in information storage and energy-harvesting devices

Enhancement of Local Piezoresponse in Polymer Ferroelectrics <i>via</i> Nanoscale Control of Microstructure

Polymer ferroelectrics are flexible and lightweight electromechanical materials that are widely studied due to their potential application as sensors, actuators, and energy harvesters. However, one of the biggest challenges is their low piezoelectric coefficient. Here, we report a mechanical annealing effect based on local pressure induced by a nanoscale tip that enhances the local piezoresponse. This process can control the nanoscale material properties over a microscale area at room temperature. We attribute this improvement to the formation and growth of β-phase extended chain crystals <i>via</i> sliding diffusion and crystal alignment along the scan axis under high mechanical stress. We believe that this technique can be useful for local enhancement of piezoresponse in ferroelectric polymer thin films

Coupled Lattice Polarization and Ferromagnetism in Multiferroic NiTiO₃ Thin Films

Polarization-induced weak ferromagnetism (WFM) was demonstrated a few years back in LiNbO₃-type compounds, MTiO₃ (M = Fe, Mn, Ni). Although the coexistence of ferroelectric polarization and ferromagnetism has been demonstrated in this rare multiferroic family before, first in bulk FeTiO₃, then in thin-film NiTiO₃, the coupling of the two order parameters has not been confirmed. Here, we report the stabilization of polar, ferromagnetic NiTiO₃ by oxide epitaxy on a LiNbO₃ substrate utilizing tensile strain and demonstrate the theoretically predicted coupling between its polarization and ferromagnetism by X-ray magnetic circular dichroism under applied fields. The experimentally observed direction of ferroic ordering in the film is supported by simulations using the phase-field approach. Our work validates symmetry-based criteria and first-principles calculations of the coexistence of ferroelectricity and WFM in MTiO₃ transition metal titanates crystallizing in the LiNbO₃ structure. It also demonstrates the applicability of epitaxial strain as a viable alternative to high-pressure crystal growth to stabilize metastable materials and a valuable tuning parameter to simultaneously control two ferroic order parameters to create a multiferroic. Multiferroic NiTiO₃ has potential applications in spintronics where ferroic switching is used, such as new four-stage memories and electromagnetic switches

Hierarchically Self-Assembled Block Copolymer Blends for Templating Hollow Phase-Change Nanostructures with an Extremely Low Switching Current

Phase change memory (PCM) is one of the most promising candidates for next-generation nonvolatile memory devices because of its high speed, excellent reliability, and outstanding scalability. However, the high switching current of PCM devices has been a critical hurdle to realize low-power operation. Although one solution is to reduce the switching volume of the memory, the resolution limit of photolithography hinders further miniaturization of device dimensions. In this study, we employed unconventional self-assembly geometries obtained from blends of block copolymers (BCPs) to form ring-shaped hollow PCM nanostructures with an ultrasmall contact area between a phase-change material (Ge₂Sb₂Te₅) and a heater (TiN) electrode. The high-density (approximately 0.1 terabits per square inch) PCM nanoring arrays showed extremely small switching current of 2–3 μA. Furthermore, the relatively small reset current of the ring-shaped PCM compared to the pillar-shaped devices is attributed to smaller switching volume, which is well supported by electro-thermal simulation results. This approach may also be extended to other nonvolatile memory device applications such as resistive switching memory and magnetic storage devices, where the control of nanoscale geometry can significantly affect device performances

X‑ray Irradiation Induced Reversible Resistance Change in Pt/TiO₂/Pt Cells

The interaction between X-rays and matter is an intriguing topic for both fundamental science and possible applications. In particular, synchrotron-based brilliant X-ray beams have been used as a powerful diagnostic tool to unveil nanoscale phenomena in functional materials. However, it has not been widely investigated how functional materials respond to the brilliant X-rays. Here, we report the X-ray-induced reversible resistance change in 40-nm-thick TiO₂ films sandwiched by Pt top and bottom electrodes, and propose the physical mechanism behind the emergent phenomenon. Our findings indicate that there exists a photovoltaic-like effect, which modulates the resistance reversibly by a few orders of magnitude, depending on the intensity of impinging X-rays. We found that this effect, combined with the X-ray irradiation induced phase transition confirmed by transmission electron microscopy, triggers a nonvolatile reversible resistance change. Understanding X-ray-controlled reversible resistance changes can provide possibilities to control initial resistance states of functional materials, which could be useful for future information and energy storage devices

Quantitative Observation of Threshold Defect Behavior in Memristive Devices with <i>Operando</i> X‑ray Microscopy

Memristive devices are an emerging technology that enables both rich interdisciplinary science and novel device functionalities, such as nonvolatile memories and nanoionics-based synaptic electronics. Recent work has shown that the reproducibility and variability of the devices depend sensitively on the defect structures created during electroforming as well as their continued evolution under dynamic electric fields. However, a fundamental principle guiding the material design of defect structures is still lacking due to the difficulty in understanding dynamic defect behavior under different resistance states. Here, we unravel the existence of threshold behavior by studying model, single-crystal devices: resistive switching requires that the pristine oxygen vacancy concentration reside near a critical value. Theoretical calculations show that the threshold oxygen vacancy concentration lies at the boundary for both electronic and atomic phase transitions. Through <i>operando</i>, multimodal X-ray imaging, we show that field tuning of the local oxygen vacancy concentration below or above the threshold value is responsible for switching between different electrical states. These results provide a general strategy for designing functional defect structures around threshold concentrations to create dynamic, field-controlled phases for memristive devices