John Mangeri

Topological phase transformations and intrinsic size effects in ferroelectric nanoparticles

Composite materials comprised of ferroelectric nanoparticles in a dielectric matrix are being actively investigated for a variety of functional properties attractive for a wide range of novel electronic and energy harvesting devices. However, the dependence of these functionalities on shapes, sizes, orientation and mutual arrangement of ferroelectric particles is currently not fully understood. In this study, we utilize a time-dependent Ginzburg-Landau approach combined with coupled-physics finite-element-method based simulations to elucidate the behavior of polarization in isolated spherical PbTiO3 or BaTiO3 nanoparticles embedded in a dielectric medium, including air. The equilibrium polarization topology is strongly affected by particle diameter, as well as the choice of inclusion and matrix materials, with monodomain, vortex-like and multidomain patterns emerging for various combinations of size and materials parameters. This leads to radically different polarization vs. electric field responses, resulting in highly tunable size-dependent dielectric properties that should be possible to observe experimentally. Our calculations show that there is a critical particle size below which ferroelectricity vanishes. For the PbTiO3 particle, this size is 2 and 3.4 nm, respectively, for high- and low-permittivity media. For the BaTiO3 particle, it is ∼3.6 nm regardless of the medium dielectric strength.

Domain alignment within ferroelectric/dielectric PbTiO3/SrTiO3 superlattice nanostructures

The ferroelectric domain pattern within lithographically defined PbTiO3/SrTiO3 ferroelectric/dielectric heteroepitaxial superlattice nanostructures is strongly influenced by the edges of the structures. Synchrotron X-ray nanobeam diffraction reveals that the spontaneously formed 180° ferroelectric stripe domains exhibited by such superlattices adopt a configuration in rectangular nanostructures in which domain walls are aligned with long patterned edges. The angular distribution of X-ray diffuse scattering intensity from nanodomains indicates that domains are aligned within an angular range of approximately 20° with respect to the edges. Computational studies based on a time-dependent Landau-Ginzburg-Devonshire model show that the preferred direction of the alignment results from lowering of the bulk and electrostrictive contributions to the free energy of the system due to the release of the lateral mechanical constraint. This unexpected alignment appears to be intrinsic and not a result of distortions or defects caused by the patterning process. Our work demonstrates how nanostructuring and patterning of heteroepitaxial superlattices allow for pathways to create and control ferroelectric structures that may appear counterintuitive.

Metastable vortex-like polarization textures in ferroelectric nanoparticles of different shapes and sizes

The dependence of the polarization texture topology in ferroelectric PbTiO3 nanoparticles, embedded in a dielectric matrix, on the particle shape and size was investigated with a time-dependent Landau-Ginzburg-Devonshire approach combined with coupled-physics finite-element-method based simulations. Particle shapes belonging to the superellipsoidal family were probed, including octahedral, cubic, and intermediate geometries. For each shape, a parametric sweep of particle sizes ranging from 2 to 40 nm was conducted, revealing a general trend for the texture transformations from a monodomain, through a vortex-like, to a multidomain state, as the size increases. Critical particle sizes for the texture instabilities were found to be strongly dependent on the particle shape, with octahedral particles undergoing transitions at much larger volumes, compared to the cubic particles. Furthermore, for each of the considered non-spherical shapes of appropriate size, it was possible to obtain multiple vortex-like textures whose paraelectric cores are aligned with every rotational axis of the particle point symmetry group. The shape-dependent metastability of the vortex-like textures opens up new avenues for controlling polarization at the nanoscale in a variety of technological applications.The dependence of the polarization texture topology in ferroelectric PbTiO3 nanoparticles, embedded in a dielectric matrix, on the particle shape and size was investigated with a time-dependent Landau-Ginzburg-Devonshire approach combined with coupled-physics finite-element-method based simulations. Particle shapes belonging to the superellipsoidal family were probed, including octahedral, cubic, and intermediate geometries. For each shape, a parametric sweep of particle sizes ranging from 2 to 40 nm was conducted, revealing a general trend for the texture transformations from a monodomain, through a vortex-like, to a multidomain state, as the size increases. Critical particle sizes for the texture instabilities were found to be strongly dependent on the particle shape, with octahedral particles undergoing transitions at much larger volumes, compared to the cubic particles. Furthermore, for each of the considered non-spherical shapes of appropriate size, it was possible to obtain multiple vortex-like text...

Electromechanical control of polarization vortex ordering in an interacting ferroelectric-dielectric composite dimer

Using a free-energy based computational model, we have investigated the response of a system comprising two interacting ferroelectric nanospheres, embedded in a dielectric medium, to a static external electric field. The system response is hysteretic and tunable by changing the inter-particle distance and the orientation of the applied field, which strongly modulates the field-driven long-range elastic interactions between the particles that propagate through the dielectric matrix. At small separations, the sensitivity of the system behavior with respect to the electric field direction originates from drastically different configurations of the local vortex-like polarization states in ferroelectric particles. This suggests new routes for the design of composite metamaterials whose dielectric properties can be controlled and tuned by selecting the mutual arrangement of their ferroelectric components.Using a free-energy based computational model, we have investigated the response of a system comprising two interacting ferroelectric nanospheres, embedded in a dielectric medium, to a static external electric field. The system response is hysteretic and tunable by changing the inter-particle distance and the orientation of the applied field, which strongly modulates the field-driven long-range elastic interactions between the particles that propagate through the dielectric matrix. At small separations, the sensitivity of the system behavior with respect to the electric field direction originates from drastically different configurations of the local vortex-like polarization states in ferroelectric particles. This suggests new routes for the design of composite metamaterials whose dielectric properties can be controlled and tuned by selecting the mutual arrangement of their ferroelectric components.