Controllable Photovoltaic Effect of Microarray Derived from Epitaxial Tetragonal BiFeO₃ Films

Recently, the ferroelectric photovoltaic (FePV) effect has attracted great interest due to its potential in developing optoelectronic devices such as solar cell and electric–optical sensors. It is important for actual applications to realize a controllable photovoltaic process in ferroelectric-based materials. In this work, we prepared well-ordered microarrays based on epitaxially tetragonal BiFeO₃ (T-BFO) films by the pulsed laser deposition technique. The polarization-dependent photocurrent image was directly observed by a conductive atomic force microscope under ultraviolet illumination. By choosing a suitable buffer electrode layer and controlling the ferroelectric polarization in the T-BFO layer, we realized the manipulation of the photovoltaic process. Moreover, based on the analysis of the band structure, we revealed the mechanism of manipulating the photovoltaic process and attributed it to the competition between two key factors, i.e., the internal electric field caused by energy band alignments at interfaces and the depolarization field induced by the ferroelectric polarization in T-BFO. This work is very meaningful for deeply understanding the photovoltaic process of BiFeO₃-based devices at the microscale and provides us a feasible avenue for developing data storage or logic switching microdevices based on the FePV effect

Observation of Exotic Domain Structures in Ferroelectric Nanodot Arrays Fabricated via a Universal Nanopatterning Approach

We report a facile and cost-competitive nanopatterning route, using Ar ion beam etching through a monolayer polystyrene sphere (PS) array placed on a ferroelectric epitaxial thin film, to fabricate ordered ferroelectric nanodot arrays. Using this method, well-ordered BiFeO₃ epitaxial nanodots, with tunable sizes from ∼100 to ∼900 nm in diameter, have been successfully synthesized. Interestingly, a plethora of exotic nanodomain structures, e.g., stripe domains, vortex and antivortex domains, and single domains, are observed in these nanodots. Moreover, this novel technique has been extended to produce Pb­(Zr,Ti)­O₃ nanodots and multiferroic composite Co/BiFeO₃ nanodots. These observations enable the creation of exotic domain structures and provide a wide range of application potentials for future nanoelectronic devices

Electrically Driven Reversible Magnetic Rotation in Nanoscale Multiferroic Heterostructures

Electrically driven magnetic switching (EDMS) is highly demanded for next-generation advanced memories or spintronic devices. The key challenge is to achieve repeatable and reversible EDMS at sufficiently small scale. In this work, we reported an experimental realization of room-temperature, electrically driven, reversible, and robust 120° magnetic state rotation in nanoscale multiferroic heterostructures consisting of a triangular Co nanomagnet array on tetragonal BiFeO₃ films, which can be directly monitored by magnetic force microscope (MFM) imaging. The observed reversible magnetic switching in an individual nanomagnet can be triggered by a small electric pulse within 10 V with an ultrashort time of ∼10 ns, which also demonstrates sufficient switching cycling and months-long retention lifetime. A mechanism based on synergic effects of interfacial strain and exchange coupling plus shape anisotropy was also proposed, which was also verified by micromagnetic simulations. Our results create an avenue to engineer the nanoscale EDMS for low-power-consumption, high-density, nonvolatile magnetoelectric memories and beyond

Magnetoelectric Coupling in Well-Ordered Epitaxial BiFeO₃/CoFe₂O₄/SrRuO₃ Heterostructured Nanodot Array

Multiferroic magnetoelectric (ME) composites exhibit sizable ME coupling at room temperature, promising applications in a wide range of novel devices. For high density integrated devices, it is indispensable to achieve a well-ordered nanostructured array with reasonable ME coupling. For this purpose, we explored the well-ordered array of isolated epitaxial BiFeO₃/CoFe₂O₄/SrRuO₃ heterostructured nanodots fabricated by nanoporous anodic alumina (AAO) template method. The arrayed heterostructured nanodots demonstrate well-established epitaxial structures and coexistence of piezoelectric and ferromagnetic properties, as revealed by transmission electron microscopy (TEM) and peizoeresponse/magnetic force microscopy (PFM/MFM). It was found that the heterostructured nanodots yield apparent ME coupling, likely due to the effective transfer of interface couplings along with the substantial release of substrate clamping. A noticeable change in piezoelectric response of the nanodots can be triggered by magnetic field, indicating a substantial enhancement of ME coupling. Moreover, an electric field induced magnetization switching in these nanodots can be observed, showing a large reverse ME effect. These results offer good opportunities of the nanodots for applications in high-density ME devices, <i>e.g.</i>, high density recording (>100 Gbit/in.²) or logic devices

An Unusual Mechanism for Negative Differential Resistance in Ferroelectric Nanocapacitors: Polarization Switching-Induced Charge Injection Followed by Charge Trapping

Negative differential resistance (NDR) has been extensively investigated for its wide device applications. However, a major barrier ahead is the low reliability. To address the reliability issues, we consider ferroelectrics and propose an alternative mechanism for realizing the NDR with deterministic current peak positions, in which the NDR results from the polarization switching-induced charge injection and subsequent charge trapping at the metal/ferroelectric interface. In this work, ferroelectric Au/BiFe_0.6Ga_0.4O₃ (BFGO)/Ca_0.96Ce_0.04MnO₃ (CCMO) nanocapacitors are prepared, and their ferroelectricity and NDR behaviors are studied concurrently. It is observed that the NDR current peaks are located at the vicinity of coercive voltages (<i>V</i>_c) of the ferroelectric nanocapacitors, thus evidencing the proposed mechanism. In addition, the NDR effect is reproducible and robust with good endurance and long retention time. This study therefore demonstrates a ferroelectric-based NDR device, which may facilitate the development of highly reliable NDR devices

Ferroelectric Resistive Switching in High-Density Nanocapacitor Arrays Based on BiFeO₃ Ultrathin Films and Ordered Pt Nanoelectrodes

Ferroelectric resistive switching (RS), manifested as a switchable ferroelectric diode effect, was observed in well-ordered and high-density nanocapacitor arrays based on continuous BiFeO₃ (BFO) ultrathin films and isolated Pt nanonelectrodes. The thickness of BFO films and the lateral dimension of Pt electrodes were aggressively scaled down to <10 nm and ∼60 nm, respectively, representing an ultrahigh ferroelectric memory density of ∼100 Gbit/inch². Moreover, the RS behavior in those nanocapacitors showed a large ON/OFF ratio (above 10³) and a long retention time of over 6,000 s. Our results not only demonstrate for the first time that the switchable ferroelectric diode effect could be realized in BFO films down to <10 nm in thickness, but also suggest the great potentials of those nanocapacitors for applications in high-density data storage