Topological walls separating domains of continuous polarization, magnetization, and strain in ferroic materials hold promise of novel electronic properties, that are intrinsically localized on the nanoscale and that can be patterned on demand without change of material volume or elemental composition. We have revealed that ferroelectric domain walls in multiferroic BiFeO(3) are inherently dynamic electronic conductors, closely mimicking memristive behavior and contrary to the usual assumption of rigid conductivity. Applied electric field can cause a localized transition between insulating and conducting domain walls, tune domain wall conductance by over an order of magnitude, and create a quasicontinuous spectrum of metastable conductance states. Our measurements identified that subtle and microscopically reversible distortion of the polarization structure at the domain wall is at the origin of the dynamic conductivity. The latter is therefore likely to be a universal property of topological defects in ferroelectric semiconductors.

Ferroelectric 180°domain walls are well-known to be predominantly Ising-like. Using density functional theory, and molecular dynamics simulations, the 180°domain walls in prototypical ferroelectrics lead titanate ͑PbTiO 3 ͒ and lithium niobate ͑LiNbO 3 ͒ are shown to have mixed character; while predominantly Ising-like, they also manifest some Bloch-and Néel-like character. Phase-field calculations show that such mixed wall character can be dramatically enhanced in nanoscale thin film heterostructures such as BaTiO 3 / SrTiO 3 , where the internal wall structure can form polarization vortices. Such mixed character walls can be expected to exhibit dynamical wall properties distinct from pure Ising walls.

Using a combination of piezoresponse force microscopy (PFM) and phase-field modeling, we demonstrate ubiquitous formation of center-type and possible ferroelectric closure domain arrangements during polarization switching near the ferroelastic domain walls in (100) oriented rhombohedral BiFeO(3). The formation of these topological defects is determined from the vertical and lateral PFM data and confirmed from the reversible changes in surface topography. These observations provide insight into the mechanisms of tip-induced ferroelastic domain control and suggest that formation of topological defect states under the action of local defect- and tip-induced fields is much more common than previously believed.

International audienceRelaxor ferroelectrics are a prototypical example of ferroic systems in which interplay between atomic disorder and order parameters gives rise to emergence of unusual properties, including non-exponential relaxations, memory effects, polarization rotations, and broad spectrum of bias- and emperatureinduced phase transitions. Despite more than 40 years of extensive research following the original discovery of ferroelectric relaxors by the Smolensky group, the most basic aspect of these materials - the existence and nature of order parameter - has not been understood thoroughly. Using extensive imaging and spectroscopic studies by variable-temperature and time resolved piezoresponse force microscopy, we fi nd that the observed mesoscopic behavior is consistent with the presence of two effective order parameters describing dynamic and static parts of polarization, respectively. The static component gives rise to rich spatially ordered systems on the 100 nm length scales, and are only weakly responsive to electric fi eld. The surface of relaxors undergoes a mesoscopic symmetry breaking leading to the freezing of polarization fl uctuations and shift of corresponding transition temperature

Misfit strain dependence of ferroelectric and piezoelectric properties of clamped (001) epitaxial Pb(Zr0.52,Ti0.48)O3 thin films Appl. Phys. Lett. 99, 252904 (2011) Phase transitions in ferroelectric-paraelectric superlattices J. Appl. Phys. 110, 114109 (2011) Dielectric and nonlinear current-voltage characteristics of rare-earth doped CaCu3Ti4O12 ceramics J. Appl. Phys. 110, 094101 (2011) Scanning electro-optic microscopy of ferroelectric domain structure with a near-field fiber probe J. Appl. Phys. 110, 084117 (2011) Ferroelectric domains in epitaxial PbTiO3 films on LaAlO3 substrate investigated by piezoresponse force microscopy and far-infrared reflectance A modified eighth-order Landau potential was proposed for the BaTiO 3 single crystal by taking account into the quantum mechanical effects at low temperature. While all existing thermodynamic potentials for BaTiO 3 fail to accurately describe the pressure dependence of ferroelectric transition temperatures, the temperature and hydrostatic pressure phase diagram constructed using the modified potential shows excellent agreement with experimental measurements by Ishidate, Abe, Takahashi, and Mori ͓Phys. Rev. Lett. 78, 2397 ͑1997͔͒. On the basis of the new proposed Landau potential, we calculated the dielectric coefficients, spontaneous polarizations, temperature-electric field phase diagram, and piezoelectric coefficients, all in good agreement well with existing experimental data.