Zijian Hong

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.

Room temperature multiferroic Ba4Bi2Fe2Nb8O30: Structural, dielectric, and magnetic properties

A room temperature multiferroic compound Ba4Bi2Fe2Nb8O30 was synthesized using a solid state reaction technique. Rietveld analysis of x-ray diffraction data shows that Ba4Bi2Fe2Nb8O30 has a tetragonal (space group P4bm) tungsten bronze structure. In this structure, the Fe3+ and Nb5+ statistically occupy the octahedral center while the Ba2+ ions and the Bi3+ ions occupy the pentagonal channels and the square channels, respectively. Diffuse dielectric peaks with strong frequency dispersion in the temperature range from 150 to 300 K can be attributed to the random distribution of Fe3+ and Nb5+ at B sites. Magnetic hysteresis loop at room temperature is also obtained, which suggests that Ba4Bi2Fe2Nb8O30 is a room temperature multiferroic compound. The coupling between the relaxor behavior and magnetic ordering is verified by observing an increase of magnetization near the maximum dielectric constant temperature.

Nanoscale mechanical switching of ferroelectric polarization via flexoelectricity

Flexoelectric coefficient is a fourth-rank tensor arising from the coupling between strain gradient and electric polarization and thus exists in all crystals. It is generally ignored for macroscopic crystals due to its small magnitude. However, at the nanoscale, flexoelectric contributions may become significant and can potentially be utilized for device applications. Using the phase-field method, we study the mechanical switching of electric polarization in ferroelectric thin films by a strain gradient created via an atomic force microscope tip. Our simulation results show good agreement with existing experimental observations. We examine the competition between the piezoelectric and flexoelectric effects and provide an understanding of the role of flexoelectricity in the polarization switching. Also, by changing the pressure and film thickness, we reveal that the flexoelectric field at the film bottom can be used as a criterion to determine whether domain switching may happen under a mechanical force.

Stability of Polar Vortex Lattice in Ferroelectric Superlattices

A novel mesoscale state comprising of an ordered polar vortex lattice has been demonstrated in ferroelectric superlattices of PbTiO3/SrTiO3. Here, we employ phase-field simulations, analytical theory, and experimental observations to evaluate thermodynamic conditions and geometric length scales that are critical for the formation of such exotic vortex states. We show that the stability of these vortex lattices involves an intimate competition between long-range electrostatic, long-range elastic, and short-range polarization gradient-related interactions leading to both an upper and a lower bound to the length scale at which these states can be observed. We found that the critical length is related to the intrinsic domain wall width, which could serve as a simple intuitive design rule for the discovery of novel ultrafine topological structures in ferroic systems.

Giant Ferroelectric Polarization in Ultrathin Ferroelectrics via Boundary‐Condition Engineering

Tailoring and enhancing the functional properties of materials at reduced dimension is critical for continuous advancement of modern electronic devices. Here, the discovery of local surface induced giant spontaneous polarization in ultrathin BiFeO3 ferroelectric films is reported. Using aberration-corrected scanning transmission electron microscopy, it is found that the spontaneous polarization in a 2 nm-thick ultrathin BiFeO3 film is abnormally increased up to ≈90-100 µC cm-2 in the out-of-plane direction and a peculiar rumpled nanodomain structure with very large variation in c/a ratios, which is analogous to morphotropic phase boundaries (MPBs), is formed. By a combination of density functional theory and phase-field calculations, it is shown that it is the unique single atomic Bi2 O3-x layer at the surface that leads to the enhanced polarization and appearance of the MPB-like nanodomain structure. This finding clearly demonstrates a novel route to the enhanced functional properties in the material system with reduced dimension via engineering the surface boundary conditions.

Light-activated Gigahertz Ferroelectric Domain Dynamics

Using time- and spatially resolved hard x-ray diffraction microscopy, the striking structural and electrical dynamics upon optical excitation of a single crystal of BaTiO_{3} are simultaneously captured on subnanoseconds and nanoscale within individual ferroelectric domains and across walls. A large emergent photoinduced electric field of up to 20×10^{6}  V/m is discovered in a surface layer of the crystal, which then drives polarization and lattice dynamics that are dramatically distinct in a surface layer versus bulk regions. A dynamical phase-field modeling method is developed that reveals the microscopic origin of these dynamics, leading to gigahertz polarization and elastic waves traveling in the crystal with sonic speeds and spatially varying frequencies. The advances in spatiotemporal imaging and dynamical modeling tools open up opportunities for disentangling ultrafast processes in complex mesoscale structures such as ferroelectric domains.

Blowing polar skyrmion bubbles in oxide superlattices

Abstract Particle-like topological structures such as skyrmions and vortices have garnered ever-increasing interests due to their rich physical insights and potential broad applications in spintronics. Here we discover the reversible switching between polar skyrmion bubbles and ordered vortex arrays in ferroelectric superlattices under an electric field, reminiscent of the Plateau-Raleigh instability in fluid mechanics. An electric field phase diagram is constructed, showing a wide stability window for the observed polar skyrmions. A “volcano”-like pontryagin density distribution is formed, indicating the formation of a smooth circular skyrmion. The topological charge Q at different applied field is calculated, verifying the field-driven topological transition between Q = 0 and Q =  ±1 states. This study is a demonstration for the computational design of field-induced topological phase transitions, giving promise for the design of next-generation nanoelectronic devices.

Interaction between Ferroelectric Polarization and Defects in BiFeO3 Thin Films

Nanoscale impurity defects, with structures different from host materials, are known to commonly exist in functional complex oxides as a result of slight stoichiometry fluctuations that occur during material growth. Local perturbations induced by these defects, such as charge, strain, and atomic interaction, could have a profound effect on the physical properties of oxide nanomaterials. A direct correlation of the defects to the material functionalities, however, are often hampered by the lack of a fundamental understanding of the microscopic mechanisms underlying the coupling between the defects and the host lattice. Here, with a combination of atomic-scale STEM and in situ TEM, we perform a systematic study of atomic-scale polarization structures and microscopic domain-switching processes in the prototypical multiferroic BiFeO3 thin films to explore the interaction between ferroelectric polarization and defects.

Corrigendum: Observation of polar vortices in oxide superlattices

Nature 530, 198–201 (2016); doi: 10.1038/nature16463 In this Letter, the surname of author Christian M. Schleputz was incorrectly spelled “Schlepuetz”. This has been corrected in the online versions of the paper.

Erratum: Corrigendum: Observation of polar vortices in oxide superlattices

Nature 530, 198–201 (2016); doi: 10.1038/nature16463 In this Letter, the surname of author Christian M. Schleputz was incorrectly spelled “Schlepuetz”. This has been corrected in the online versions of the paper.