Cooperative response of Pb(ZrTi)O$_3$ nanoparticles to curled electric fields

Using first-principles based effective Hamiltonian and finite temperature Monte Carlo simulations we investigate cooperative responses, as well as microscopic mechanism for vortex switching, in zero-dimensional Pb(Zr$_{0.5}$Ti$_{0.5}$)O$_3$ nanoparticles under curled electric fields. We find that the generally accepted domain coexistence mechanism is not valid for toroid switching. Instead dipoles are shown to display unusual collective behaviors by forming a new vortex with perpendicular (but not opposite) toroid moment. The strong correlation between the new and original vortices is revealed to be critical for reversing toroid moment. Microscopic insight for the puzzling collective response is discussed. Based on our finding, we further describe a technological approach that is able to drastically reduce the magnitude of the curled electric field needed for vortex switching.

Unusual polarization patterns in flat epitaxial ferroelectric nanoparticles

Interest in epitaxial ferroelectric nanoislands was growing rapidly in recent years driven by their potential for devices, especially ultradense memories. Recent advances in the bottom- up (self-assembly) nanometer scale techniques have opened up the opportunities of fabricating high-quality epitaxial ferroelectric nanoislands with extremely small thickness and lateral size on the order of 1 nm and 20 nm, respectively. On the other hand, recent emergence of powerful probes, such as piezoresponse force microscopy (PFM), has enabled imaging of a local domain structure with sub-10 nm resolution. In spite of those developments, a clear understanding of the polarization patterns in epitaxial ferroelectric nanoislands is lacking, and some important characteristics, like a critical lateral size for ferroelectricity, are not yet established. Here, we perform ab-initio studies of non-electroded epitaxial Pb(Zr0.5Ti0.5)O3 and BaTiO3 nanoislands and show the existence of novel polarization patterns driven by the misfit strains and/or anisotropy energy. The results allow interpretation of the data and design of the ferroelectric nanostructures with tailored response to external field.