Xingsen Gao

Enhanced dielectric and multiferroic properties of single-phase Y and Zr co-doped BiFeO3 ceramics

We report the structural, dielectric, and multiferroic properties of single-phase Y and Zr co-doped BiFeO3 (BFO) ceramics. Enhancement in dielectric properties, frequency independence, and ferroelectric properties are observed in co-doped BFO due to the reduction of oxygen vacancies by co-doping. Co-doped samples exhibited double hysteresis loop-like magnetization-magnetic field curves with a significantly enhanced remnant magnetization (∼0.41496 emu/g when Y and Zr concentration is 0.15 and 0.05). The possible reasons may be the co-doping induced collapse of cycloidal spin structure, internal structural distortion, and reduction of average particle size to be less than period of the spin cycloid. The improved dielectric and multiferroic properties obtained by co-doping demonstrate the possibility of bulk BFO to practical applications.

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, e.g., high density recording (>100 Gbit/in.(2)) or logic devices.

Current rectifying and resistive switching in high density BiFeO3 nanocapacitor arrays on Nb-SrTiO3 substrates.

Ultrahigh density well-registered oxide nanocapacitors are very essential for large scale integrated microelectronic devices. We report the fabrication of well-ordered multiferroic BiFeO3 nanocapacitor arrays by a combination of pulsed laser deposition (PLD) method and anodic aluminum oxide (AAO) template method. The capacitor cells consist of BiFeO3/SrRuO3 (BFO/SRO) heterostructural nanodots on conductive Nb-doped SrTiO3 (Nb-STO) substrates with a lateral size of ~60 nm. These capacitors also show reversible polarization domain structures, and well-established piezoresponse hysteresis loops. Moreover, apparent current-rectification and resistive switching behaviors were identified in these nanocapacitor cells using conductive-AFM technique, which are attributed to the polarization modulated p-n junctions. These make it possible to utilize these nanocapacitors in high-density (>100 Gbit/inch2) nonvolatile memories and other oxide nanoelectronic devices.

High Efficiency Solar Cells As Fabricated by Sb2S3-Modified TiO2 Nanofibrous Networks

High-efficiency hybrid solar cells (HSCs) based on electrospun titanium dioxide (TiO2) nanofibers plus poly(3-hexylthiophene) (P3HT) are fabricated by means of both the pretreatment using tetrahydrofuran (THF) vapor and the surface modification using n-type antimony chalcogenide (Sb2S3) on the TiO2 nanofibrous networks. It is revealed that the THF pretreatment not only reinforces the interfacial physical contact but also suppresses the interfacial recombination. The Sb2S3 modification improves the light absorption and charge transfer. Given that the active layer of the HSCs is as thin as 300 nm, it is demonstrated that the power conversion efficiency (PCE) is enhanced over 175%, exhibiting a PCE of 2.32%.

Monte Carlo simulation on the size effect in ferroelectric nanostructures

The ferroelectric domain structures in a two-dimensional square lattice with different lattice sizes under a set of finite boundary conditions (zero dipole and clamped strain on lattice boundaries) are investigated using Monte Carlo simulation, based on the Landau phenomenological model. Given the finite boundary conditions, the ferroelectric domain structure evolves gradually from the 90°-striped pattern into the single-vortex pattern with reducing lattice size. When the finite boundary conditions apply only onto one-dimensional boundaries, as an approach to the case of thin films, the single-domain pattern is favored with reducing lattice size. The physics underlying the evolution of domain structures with varying lattice size is discussed.

Multi-step magnetization of the Ising model on a Shastry–Sutherland lattice: a Monte Carlo simulation

The magnetization behaviors and spin configurations of the classical Ising model on a Shastry-Sutherland lattice are investigated using Monte Carlo simulations, in order to understand the fascinating magnetization plateaus observed in TmB(4) and other rare-earth tetraborides. The simulations reproduce the 1/2 magnetization plateau by taking into account the dipole-dipole interaction. In addition, a narrow 2/3 magnetization step at low temperature is predicted in our simulation. The multi-step magnetization can be understood as the consequence of the competitions among the spin-exchange interaction, the dipole-dipole interaction, and the static magnetic energy.

SrTiO3 modified TiO2 electrodes and improved dye-sensitized TiO2 solar cells

The SrTiO(3)-coated TiO(2) (TiO(2)/SrTiO(3)) electrodes prepared by radio frequency magnetron sputtering are used to improve the performance of dye-sensitized TiO(2) solar cells by means of surface modification. The structural and performance characterizations reveal that the TiO(2)/SrTiO(3) electrodes, in comparison with fresh TiO(2) electrodes, have low density of oxygen vacancies, passivated surface states, and suppressed interfacial recombination effect, thus resulting in improved performance parameters of the cells. An optimized coating of SrTiO(3) layer on the TiO(2) film surface allows an enhancement of the power conversion efficiency from 4.78% to 5.91%

First-principles study on the magnetic properties in Mg doped BiFeO3 with and without oxygen vacancies

The magnetic properties of Mg-doped BiFeO3 (BFO) with and without oxygen vacancies are studied through first-principles calculations. The Mg-doping prefers to occupy the ferromagnetic planes and produces an obvious improved magnetization, and the magnetization is linearly enhanced with increasing Mg-doped content, which is consistent with the trend reported in experiment. However, our calculated result is significantly larger than the experimental one, and the reason is revealed that the relative energy differences of various spin-ordering configurations are small. Furthermore, oxygen vacancy in Mg-doped BFO can further enhance the magnetization, while keeping the insulating band gap character. The calculated results imply that the oxygen vacancy in Mg-doped BFO would be an effective way to improve the multiferroicity of BFO.

Interface Engineering of Domain Structures in BiFeO3 Thin Films

A wealth of fascinating phenomena have been discovered at the BiFeO3 domain walls, examples such as domain wall conductivity, photovoltaic effects, and magnetoelectric coupling. Thus, the ability to precisely control the domain structures and accurately study their switching behaviors is critical to realize the next generation of novel devices based on domain wall functionalities. In this work, the introduction of a dielectric layer leads to the tunability of the depolarization field both in the multilayers and superlattices, which provides a novel approach to control the domain patterns of BiFeO3 films. Moreover, we are able to study the switching behavior of the first time obtained periodic 109° stripe domains with a thick bottom electrode. Besides, the precise controlling of pure 71° and 109° periodic stripe domain walls enable us to make a clear demonstration that the exchange bias in the ferromagnet/BiFeO3 system originates from 109° domain walls. Our findings provide future directions to study the room temperature electric field control of exchange bias and open a new pathway to explore the room temperature multiferroic vortices in the BiFeO3 system.

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/inch(2). Moreover, the RS behavior in those nanocapacitors showed a large ON/OFF ratio (above 10(3)) 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.