Vortex domain patterns in low-dimensional ferroelectrics and multiferroics have been extensively studied with the aim of developing nanoscale functional devices. However, control of the vortex domain structure has not been investigated systematically. Taking into account effects of inhomogeneous electromechanical fields, ambient temperature, surface and size, we demonstrate significant influence of mechanical load on the vortex domain structure in ferroelectric nanoplatelets. Our analysis shows that the size and number of dipole vortices can be controlled by mechanical load, and yields rich temperature-stress (T-S) phase diagrams. Simulations also reveal that transformations between “vortex states” induced by the mechanical load are possible, which is totally different from the conventional way controlled on the vortex domain by the electric field. These results are relevant to application of vortex domain structures in ferroelectric nanodevices, and suggest a novel route to applications including memories, mechanical sensors and transducers.

The collective dipole behaviors in (BaTiO3)m/(SrTiO3)n composite nanowires are investigated based on the first-principles-derived simulations. It demonstrates that such nanowire systems exhibit intriguing dipole orders, due to the combining effect of the anisotropic electrostatic interaction of the nanowire, the SrTiO3-layer-modified electrostatic interaction and the multiphase ground state of BaTiO3 layer. Particularly, a strong polar-toroidal coupling that is tunable by the SrTiO3-layer thickness, temperature, external strains and electric fields is found to exist in the nanowires, with the appearance of fruitful dipole states (including those being purely polar, purely toroidal, both polar and toroidal, or distorted toroidal) and phase boundaries. As a consequence, an efficient cross control of the toroidal (polar) order by static (curled) electric field, and superior piezoelectric and piezotoroidal responses, can be achieved in the nanowires. The result provides new insights into the collective dipole behaviors in nanowire systems.

No abstract

Phase field simulations were conducted to investigate the effect of misfit strain on the vortex domain structure (VDS) in a BaTiO3 nanodot. Taking into account the effect of inhomogeneous eletromechanical fields, ambient temperature and surface effects, our calculations demonstrate a strong effect of misfit strain on the orientation and magnitude of the polarization dipoles. As a consequence, fruitful equilibrium vortex domain patterns can be obtained by adjusting the epitaxial misfit strain between the substrate and the nanodot. While the nanodot exhibits a single transition from a paraelectric to a near-rhombohedral vortex state at zero misfit strain with the decrease of temperature, complicated transformations of vortex domain patterns are found under nonzero misfit strain. Typically, orthorhombic, tetragonal and several unreported vortex domain patterns (e.g., with zero toroidal moment) are found. Moreover, misfit strain-induced transformations into these domain patterns are also predicted for a ferroelectric nanodot with initial near-rhombohedral vortex state. Combining effects of the ambient temperature and misfit strain, a "temperature-misfit strain" phase-diagram depicting the fruitful vortex domain patterns of the nanodot was obtained. Our simulations indicate promising application of strain engineering in controlling the VDS in ferroelectric nanostructures.

Ferroelectric vortex domain structure which exists in low-dimensional ferroelectrics is being intensively researched for future applications in functional nanodevices. Here we demonstrate that adjusting surface charge screening in combination with temperature can provide an efficient way to gain control of vortex domain structure in ferroelectric nanodot. Systematical simulating experiments have been conducted to reveal the stability and evolution mechanisms of domain structure in ferroelectric nanodot under various conditions, including processes of cooling-down/heating-up under different surface charge screening conditions, and increasing/decreasing surface charge screening at different temperatures. Fruitful phase diagrams as functions of surface screening and temperature are presented, together with evolution paths of various domain patterns. Calculations discover up to 25 different kinds of domain patterns and 22 typical evolution paths of phase transitions. The fruitful controllability of vortex domain structure by surface charge screening in combination with temperature should shed light on prospective nanodevice applications of low-dimensional ferroelectric nanostructures.