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

Abstract

In studies using piezoresponse force microscopy, we observe a nonzero lateral piezoresponse at 180°d omain walls in out-of-plane polarized, c-axis-oriented tetragonal ferroelectric Pb͑Zr 0.2 Ti 0.8 ͒O 3 epitaxial thin films. We attribute these observations to a shear strain effect linked to the sign change of the d 33 piezoelectric coefficient through the domain wall, in agreement with theoretical predictions. We show that in monoclinically distorted tetragonal BiFeO 3 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. © 2009 American Institute of Physics. ͓doi:10.1063/1.3226654͔ Ferroelectric materials, characterized by their reversible spontaneous electric polarization, show great potential for multifunctional applications ranging from nonvolatile memories 1,2 to nanoscale sensors and actuators. 3 Controlling the structure and stability of ferroelectric domains in these materials is a key requirement for device implementation. In particular, the dynamics of domain walls, the interfaces separating regions with differently oriented ferroelectric polarization in the films, can significantly affect performance. 4 Understanding domain wall behavior at the nanoscopic scales of current and future devices, however, requires techniques with the requisite nanoscale resolution.One such technique is piezoresponse force microscopy ͑PFM͒, 5 in which a metallic atomic force microscope ͑AFM͒ tip is used to apply an alternating voltage across the ferroelectric material, resulting in a local mechanical response at the film surface due to the inverse piezoelectric effect. This piezoelectric response can be detected from the induced displacement of the AFM cantilever, recorded by the position of a laser beam reflected onto a quadrant-split photodetector. The vertical deflection and angular torsion of the tip are referred to as vertical and lateral PFM, respectively. The response phase provides information on the polarization, while its amplitude is related to the polarization magnitude. 6 Depending on the piezoelectric coefficient tensor d ij , linked to the crystal symmetry, a combination of these two measurements allows access to both out-of-plane and in-plane components of polarization. Although quantitative measurements of piezoelectric coefficients via PFM are challenging, the technique has provided valuable information about the behavior of domain walls and switching dynamics both in thin films 7,8 and in device structures. [9][10][11] In this context, understanding the origins of the PFM signal observed at ferroelectric domain walls is an important issue.Considering only piezoelectric effects, in a c-axis-oriented tetragonal ferroelectric film with an electric field applied along the polarization axis, the piezoelectric response is determined by the d 33 coefficient, leading to a purely vertical PFM signal. However, in such films, a nonzero lateral PFM response has been ...

Real space mapping of polarization dynamics and hysteresis loop formation in relaxor-ferroelectric PbMg1/3Nb2/3O3–PbTiO3 solid solutions

“…However, in both PMN-32%PT and PMN-35%PT crystals one can observe a clear correspondence between the vertical and lateral domains so that the domain walls in both images coincide. This behavior is consistent with LPFM behavior at 180°-domain walls [28][29][30] and thus can be ascribed to a crosscoupling with the vertical signal. Based on the examination of the images, we conclude that the in-plane component contribution to the LPFM signal is below the detection limit ͑less than ϳ20% of vertical signal͒.…”

Ferroelectric or non-ferroelectric: Why so many materials exhibit “ferroelectricity” on the nanoscale

“…[109][110][111][112][113][114][115] Similar approaches were pursued by other groups. [116][117][118][119][120] These analyses were extended to quantitatively describe a ferroelectric domain wall profile in vertical and lateral PFM 115,121,122 , orientational dependence of the PFM signal, 112,123 and to quantify piezoresponse spectroscopy 124 , as will be discussed below. Perhaps even more importantly, the PFM signal (more rigorously, electromechanical surface displacement) was shown to be independent of contact area, ensuring that PFM, unlike force based SPMs, can be an intrinsically quantitative technique.…”

Nanoscale studies of ferroelectric domain walls as pinned elastic interfaces

“…PFM thus allows the position of individual ferroelectric domain walls to be mapped with sub-10 nm resolution over areas essentially limited only by the scan range (10s-100s µm). Moreover, novel domain-wall-specific functional properties can be readily identified, including characteristic lateral piezoresponse features in out-of-plane polarised samples [69,70,71], and, with additional conductive tip current measurements, domain wall electrical conduction in these otherwise insulating materials [72,73,74,75,76].…”