Guo Tian

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.

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.

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.

High-density array of ferroelectric nanodots with robust and reversibly switchable topological domain states

Robust and reversible polar topological center domains were found in BiFeO3 nanodots, which are individually controllable. The exotic topological domains in ferroelectrics and multiferroics have attracted extensive interest in recent years due to their novel functionalities and potential applications in nanoelectronic devices. One of the key challenges for these applications is a realization of robust yet reversibly switchable nanoscale topological domain states with high density, wherein spontaneous topological structures can be individually addressed and controlled. This has been accomplished in our work using high-density arrays of epitaxial BiFeO3 (BFO) ferroelectric nanodots with a lateral size as small as ~60 nm. We demonstrate various types of spontaneous topological domain structures, including center-convergent domains, center-divergent domains, and double-center domains, which are stable over sufficiently long time but can be manipulated and reversibly switched by electric field. The formation mechanisms of these topological domain states, assisted by the accumulation of compensating charges on the surface, have also been revealed. These results demonstrated that these reversibly switchable topological domain arrays are promising for applications in high-density nanoferroelectric devices such as nonvolatile memories.

Resistive switching induced by charge trapping/detrapping: a unified mechanism for colossal electroresistance in certain Nb:SrTiO3-based heterojunctions

SrTiO3 remains at the core of research on oxide electronics, owing to its fascinating properties and wide applications as a commercial substrate. Heterojunctions based on Nb-doped SrTiO3 (NSTO), including both metal/NSTO Schottky junctions (MSJs) and NSTO-based ferroelectric tunnel junctions (FTJs) have received considerable attention due to the colossal electroresistance (CER) effect. However, the mechanism underpinning the CER effect is still poorly understood. Here, we conduct a comparative study on the CER effects in Au/NSTO MSJs and Au/BaTiO3/NSTO FTJs. The two types of heterojunctions show many similarities in resistive switching characteristics, including hysteretic current–voltage curves with asymmetric shapes, absence of critical switching fields, switching times on the scale of ∼1.0 μs, and resistance relaxations of the Curie–von Schweidler type. These results suggest that the CER effects in the MSJs and FTJs may have a common origin, i.e., charge trapping/detrapping, as further revealed by scanning Kelvin probe microscopy. Using temperature-dependent current–voltage, capacitance–voltage, and photo-response measurements, we demonstrate that charge trapping/detrapping could modify both the Schottky barrier profile and the tunneling process, and in turn lead to different transport mechanisms in different voltage regimes. The charge trapping/detrapping-induced CER effect can be well described by a metal–insulator–semiconductor (MIS) model, which reproduces the hysteretic current–voltage curves fairly well over a large range of voltage sweeping and thus provides a unified framework for the CER effects in certain NSTO-based heterojunctions.

Domain Evolution and Piezoelectric Response across Thermotropic Phase Boundary in (K,Na)NbO3-Based Epitaxial Thin Films

Recent research progress in (K,Na)NbO3 (KNN)-based lead-free piezoelectric ceramics has attracted increasing attention for their applications to microsystems or microelectromechanical systems (MEMS) in the form of thin films. This work demonstrates that high-quality KNN-based epitaxial films can be synthesized by a conventional sol-gel method, whose phase structure and domain characteristics have been investigated with emphasis on the temperature effect. A monoclinic MC structure is observed at room temperature in KNN-based epitaxial films, which is close to but different from the orthorhombic phase in bulk counterparts. Piezoresponse force microscopy (PFM) at elevated temperatures reveals continuous changes of ferroelectric domains in KNN films during heating and cooling cycles between room temperature and 190 °C. A distinct change in domain morphology is observed upon heating to 110 °C, accompanied by a clear variation of dielectric permittivity suggesting a thermotropic phase transition, which is revealed to belong to a MC-MA phase transition on the basis of structural and PFM analysis on local ferroelectric and piezoelectric behaviors. Enhanced piezoelectric response at the thermotropic phase boundary is observed, which is attributed to active domains and/or nanodomains formed across the boundary. Domain engineering by utilizing the phase transition should be important and effective in KNN-based films not only for property enhancement but also for its textured ceramics.

Large electroresistance and tunable photovoltaic properties of ferroelectric nanoscale capacitors based on ultrathin super-tetragonal BiFeO3 films

Ferroelectric nanocapacitors with simultaneously tunable resistance and photovoltaic effect have great potential for realizing high-density non-volatile memories and multifunctional opto-electronic nanodevices. Here, using a polystyrene sphere template method, we developed well-ordered Au nanoelectrode arrays on super-tetragonal BiFeO3 (T-BFO)/La0.7Sr0.3MnO3 (LSMO) epitaxial thin films, forming Au/T-BFO/LSMO nanocapacitors. The nanocapacitors exhibited switchable resistance states and photovoltaic responses, controllable by the ferroelectric polarization of T-BFO. Owing to the giant polarization of T-BFO, both giant electroresistance (ON/OFF current ratio >20 000) and noticeable photovoltage (∼0.4 V) were achieved in the Au/T-BFO/LSMO nanocapacitors. These results demonstrate that the T-BFO-based nanocapacitors are promising for applications in high-density memories with multiple routes for non-destructive readout, as well as other multifunctional nanodevices.

Resistive switching and photovoltaic effects in ferroelectric BaTiO3-based capacitors with Ti and Pt top electrodes

We have comparatively studied the dielectric, ferroelectric, conduction, and photovoltaic properties of Ti/BaTiO3 (BTO)/SrRuO3 (SRO) and Pt/BTO/SRO capacitors. The resistive switching (RS) is observed in the Pt/BTO/SRO capacitor while it is absent in the Ti/BTO/SRO capacitor, which may be attributed to the interfacial layer existing between Pt and BTO and the Ti/BTO Ohmic interface, respectively. Further analyses on the conduction mechanisms suggest that the RS may be caused by the opening/closing of conduction paths in the Pt/BTO interfacial layer, whereas the polarization is ruled out as the origin of RS because of the inconsistency between the RS switching voltages and coercive voltages. On the other hand, it is observed that the photovoltaic effects (PVEs) in both Ti/BTO/SRO and Pt/BTO/SRO capacitors are electrically unswitchable and the open-circuit voltages of the two capacitors are similar in magnitude, implying that the PVE is driven by an internal bias field rather than the polarization-induced f...

Unique nano-domain structures in self-assembled BiFeO3 and Pb(Zr,Ti)O3 ferroelectric nanocapacitors

In this work, self-assembled ferroelectric BiFeO3 (BFO) and Pb(Zr,Ti)O3 (PZT) nanocapacitors were fabricated by a one-step pulsed-laser deposition process. Each individual nanocapacitor consists of a SrRuO3 or LaSrMnO3 bottom electrode layer, an epitaxial ferroelectric middle layer and a self-assembled nanoisland of conductive Bi2O3 or PbO2 as the top nanoelectrode. The nanoelectrodes have a lateral size of 10-100 nm depending on various deposition equivalent thickness. The as-fabricated nanocapacitors exhibit unique so-called anti-domain structures, with opposite polarization orientation to that of the naked ferroelectric films, which can be understood by the different interface built-in-voltages between their neighboring layers. They also show apparent reduced coercive fields and enhanced piezoelectricity compared to the naked films, as revealed by the switching spectroscopy piezoresponse force microscopy (SSPFM) and band-excitation mapping. Besides that, individual addressable polarization writing and erasing properties were also observed in these nanocapacitors and the written domain can maintain stability up to 12 h, which is promising for data storage devices.