Deyang Chen

Observation of polar vortices in oxide superlattices

The complex interplay of spin, charge, orbital and lattice degrees of freedom provides a plethora of exotic phases and physical phenomena. In recent years, complex spin topologies have emerged as a consequence of the electronic band structure and the interplay between spin and spin–orbit coupling in materials. Here we produce complex topologies of electrical polarization—namely, nanometre-scale vortex–antivortex (that is, clockwise–anticlockwise) arrays that are reminiscent of rotational spin topologies—by making use of the competition between charge, orbital and lattice degrees of freedom in superlattices of alternating lead titanate and strontium titanate layers. Atomic-scale mapping of the polar atomic displacements by scanning transmission electron microscopy reveals the presence of long-range ordered vortex–antivortex arrays that exhibit nearly continuous polarization rotation. Phase-field modelling confirms that the vortex array is the low-energy state for a range of superlattice periods. Within this range, the large gradient energy from the vortex structure is counterbalanced by the corresponding large reduction in overall electrostatic energy (which would otherwise arise from polar discontinuities at the lead titanate/strontium titanate interfaces) and the elastic energy associated with epitaxial constraints and domain formation. These observations have implications for the creation of new states of matter (such as dipolar skyrmions, hedgehog states) and associated phenomena in ferroic materials, such as electrically controllable chirality.

180° Ferroelectric Stripe Nanodomains in BiFeO3 Thin Films.

There is growing evidence that domain walls in ferroics can possess emergent properties that are absent in the bulk. For example, 180° ferroelectric domain walls in the ferroelectric-antiferromagnetic BiFeO3 are particularly interesting because they have been predicted to possess a range of intriguing behaviors, including electronic conduction and enhanced magnetization. To date, however, ordered arrays of such domain structures have not been reported. Here, we report the observation of 180° stripe nanodomains in (110)-oriented BiFeO3 thin films grown on orthorhombic GdScO3 (010)O substrates and their impact on exchange coupling to metallic ferromagnets. Nanoscale ferroelectric 180° stripe domains with {112̅} domain walls were observed in films <32 nm thick. With increasing film thickness, we observed a domain structure crossover from the depolarization field-driven 180° stripe nanodomains to 71° ferroelastic domains determined by the elastic energy. These 180° domain walls (which are typically cylindrical or meandering in nature due to a lack of strong anisotropy associated with the energy of such walls) are found to be highly ordered. Additional studies of Co0.9Fe0.1/BiFeO3 heterostructures reveal exchange bias and exchange enhancement in heterostructures based on BiFeO3 with 180° domain walls and an absence of exchange bias in heterostructures based on BiFeO3 with 71° domain walls; suggesting that the 180° domain walls could be the possible source for pinned uncompensated spins that give rise to exchange bias. This is further confirmed by X-ray circular magnetic dichroism studies, which demonstrate that films with predominantly 180° domain walls have larger magnetization than those with primarily 71° domain walls. Our results could be useful to extract the structure of domain walls and to explore domain wall functionalities in BiFeO3.

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.

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.

A Strain-Driven Antiferroelectric-to-Ferroelectric Phase Transition in La-Doped BiFeO3 Thin Films on Si

A strain-driven orthorhombic (O) to rhombohedral (R) phase transition is reported in La-doped BiFeO3 thin films on silicon substrates. Biaxial compressive epitaxial strain is found to stabilize the rhombohedral phase at La concentrations beyond the morphotropic phase boundary (MPB). By tailoring the residual strain with film thickness, we demonstrate a mixed O/R phase structure consisting of O phase domains measuring tens of nanometers wide within a predominant R phase matrix. A combination of piezoresponse force microscopy (PFM), transmission electron microscopy (TEM), polarization-electric field hysteresis loop (P-E loop), and polarization maps reveal that the O-R structural change is an antiferroelectric to ferroelectric (AFE-FE) phase transition. Using scanning transmission electron microscopy (STEM), an atomically sharp O/R MPB is observed. Moreover, X-ray absorption spectra (XAS) and X-ray linear dichroism (XLD) measurements reveal a change in the antiferromagnetic axis orientation from out of plane (R-phase) to in plane (O-phase). These findings provide direct evidence of spin-charge-lattice coupling in La-doped BiFeO3 thin films. Furthermore, this study opens a new pathway to drive the AFE-FE O-R phase transition and provides a route to study the O/R MPB in these films.

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...

Fabrication of high-density BiFeO3 nanodot and anti-nanodot arrays by anodic alumina template-assisted ion beam etching.

Efficient and cost-competitive fabrication of high-quality ferroelectric and multiferroic nanostructures is of general interest. In this work, a top-down nano-patterning technique is developed by the Ar+ ion beam etching in combination with the sacrificed ultrathin anodic alumina (AAO) mask. This technique is demonstrated by preparation of the epitaxial BiFeO3 (BFO) nanostructures of various geometries, including nanodot and anti-nanodot arrays. The lateral dot size is as small as ∼60 nm and an ultrahigh dot density of ∼60 Gbit/inch2 is achieved. It is revealed that the etching process involves sequential shape evolution of both the AAO mask and the underlying BFO film, resulting in the nanodots and anti-nanodots arrays of various geometries. The as-etched BFO nanodots array exhibits well-established ferroelectric domain structures and reversible polarization switching, as examined by piezoresponse force microscopy (PFM). It is suggested that this technique is extendable to fabrication of a wide range of functional oxide nanostructures for potential nanoelectronic applications.