Jill Guyonnet

Conduction at Domain Walls in Insulating Pb(Zr0.2Ti0.8)O3 Thin Films

Domain wall conduction in insulating Pb(Zr(0.2) Ti(0.8))O(3) thin films is demonstrated. The observed electrical conduction currents can be clearly differentiated from displacement currents associated with ferroelectric polarization switching. The domain wall conduction, nonlinear and highly asymmetric due to the specific local probe measurement geometry, shows thermal activation at high temperatures, and high stability over time.

On the strain coupling across vertical interfaces of switchable BiFeO3–CoFe2O4 multiferroic nanostructures

In magnetoelectrically coupled CoFe2O4–BiFeO3 nanostructures vertical and lateral lattice parameters of both phases are determined. We find that the in-plane lattice parameter of CoFe2O4 is fully relaxed whereas it presents compressive strain along the out-of-plane direction. Although the CoFe2O4–BiFeO3 interface is semicoherent, CoFe2O4 out-of-plane lattice strain is not relaxed after selective removal of the matrix and thus it is of nonelastic origin. In spite of the absence of elastic residual strain caused by CoFe2O4–BiFeO3 interfaces, the two phases are mechanically coupled as demonstrated by the electrical switching of the magnetization.

Shear effects in lateral piezoresponse force microscopy at 180° ferroelectric domain walls

In studies using piezoresponse force microscopy, we observe a nonzero lateral piezoresponse at 180° domain walls in out-of-plane polarized, c-axis-oriented tetragonal ferroelectric Pb(Zr0.2Ti0.8)O3 epitaxial thin films. We attribute these observations to a shear strain effect linked to the sign change of the d33 piezoelectric coefficient through the domain wall, in agreement with theoretical predictions. We show that in monoclinically distorted tetragonal BiFeO3 films, this effect is superimposed on the lateral piezoresponse due to actual in-plane polarization and has to be taken into account in order to correctly interpret the ferroelectric domain configuration.

Lateral piezoelectric response across ferroelectric domain walls in thin films

In purely c-axis oriented PbZr0.2Ti0.8O3 ferroelectric thin films, a lateral piezoresponse force microscopy signal is observed at the position of 180° domain walls, where the out-of-plane oriented polarization is reversed. Using electric force microscopy measurements we exclude electrostatic effects as the origin of this signal. Moreover, our mechanical simulations of the tip/cantilever system show that the small tilt of the surface at the domain wall below the tip does not satisfactorily explain the observed signal either. We thus attribute this lateral piezoresponse at domain walls to their sideways motion (shear) under the applied electric field. From simple elastic considerations and the conservation of volume of the unit cell, we would expect a similar lateral signal more generally in other ferroelectric materials, and for all types of domain walls in which the out-of-plane component of the polarization is reversed through the domain wall. We show that in BiFeO3 thin films, with 180°, 109°, and 71° d...

Multiscaling Analysis of Ferroelectric Domain Wall Roughness

Using multiscaling analysis, we compare the characteristic roughening of ferroelectric domain walls in Pb(Zr0.2Ti0.8)O3 thin films with numerical simulations of weakly pinned one-dimensional interfaces. Although at length scales up to L(MA)≥5  μm the ferroelectric domain walls behave similarly to the numerical interfaces, showing a simple monoaffine scaling (with a well-defined roughness exponent ζ), we demonstrate more complex scaling at higher length scales, making the walls globally multiaffine (varying ζ at different observation length scales). The dominant contributions to this multiaffine scaling appear to be very localized variations in the disorder potential, possibly related to dislocation defects present in the substrate.

Non-Ising and chiral ferroelectric domain walls revealed by nonlinear optical microscopy

The properties of ferroelectric domain walls can significantly differ from those of their parent material. Elucidating their internal structure is essential for the design of advanced devices exploiting nanoscale ferroicity and such localized functional properties. Here, we probe the internal structure of 180° ferroelectric domain walls in lead zirconate titanate (PZT) thin films and lithium tantalate bulk crystals by means of second-harmonic generation microscopy. In both systems, we detect a pronounced second-harmonic signal at the walls. Local polarimetry analysis of this signal combined with numerical modelling reveals the existence of a planar polarization within the walls, with Néel and Bloch-like configurations in PZT and lithium tantalate, respectively. Moreover, we find domain wall chirality reversal at line defects crossing lithium tantalate crystals. Our results demonstrate a clear deviation from the ideal Ising configuration that is traditionally expected in uniaxial ferroelectrics, corroborating recent theoretical predictions of a more complex, often chiral structure.

Domain Walls in Ferroelectric Materials

Ferroelectric materials are characterized by a finite electric polarization in absence of an external electric field. Furthermore, this polarization must possess at least two stable states, and must have the ability to be reversibly switched from one state to another by the application of an electric field.

Lateral Piezoelectric Response Across Ferroelectric Domain Walls

With the development of PFM techniques, quantitative theoretical and simulation methods have become increasingly important in order to correctly interpret the images and understand the nature of the piezoresponse.

Roughness Analysis of 180^{\circ } Ferroelectric Domain Walls

The nanometric resolution of PFM, which allows the imaging of ferroelectric polarization with a precision of \(\approx \)10 unit cells, makes it an ideal tool to study the self-affine roughness of domain walls.

Measuring the Roughness Exponent of One-Dimensional Interfaces

Since the pioneering work of Mandelbrot et al. demonstrating the self-affine nature of cracks in metals [1], a significant number of different methods were established and used to estimate the roughness exponent of self-affine interfaces, focusing in particular on fracture surfaces [2, 3]. In all of these methods, complete knowledge of the interface position is assumed, allowing the roughness exponent to be estimated, either indirectly by determining the fractal dimension or directly through dedicated self-affine analysis.