New studies on ferroelectric domain walls reveal peculiar mechanical properties

by Virginia Greco

A research on the mechanical properties of ferroelectric 180° domain walls, carried out by an international team of researchers led by the Oxide Nanophysics group and the Advanced AFM Laboratory of the ICN2, has revealed that domain walls are mechanically softer than the ferroelectric domains they separate, although the domains are identical. These results and a theoretical interpretation of them have been recently published in Physical Review X.

In recent years, ferroelectric domain walls have attracted considerable attention for their functional properties, which might result useful for application in electronics and information technology. A domain wall is an interface separating domains, i.e. regions of a material having different polarity. The domain wall thickness depends on the material and type of wall, but ferroelectric 180° walls separating domains of antiparallel orientation are almost atomically thin. Despite their narrowness, they can be macroscopically extended, have functional properties different from those of the domains in the host material, and, in addition, they can be created and shaped by means of an external field. Moreover, mechanical stress can also move domain walls. Yet, very little is known about the internal mechanical properties of such two-dimensional entities.

Members of the ICN2 Oxide Nanophysics group and of the ICN2 Advanced AFM Laboratory – Christina Stefani, Dr. Neus Domingo and group leader ICREA Prof. Gustau Catalán – in collaboration with researchers of the Italian Institute of Technology, the Scuola Normale Superiore di Pisa (Italy), the Institute of Materials Science of Barcelona (ICMAB), and Cornell University, have studied the mechanical properties of ferroelectric 180° domain walls. Using scanning probe microscopy, they observed that, in the vertical direction parallel to the polar axis, the walls are mechanically softer than the domains. The results of this research have been recently published in Physical Review X, a fully open access journal of the American Physical Society.

This study confirms and extends a work previously carried out on ferroelectric lead zirconate titanate ceramics by Tsuji et al., and proposes a theoretical explanation of this phenomenon that should be universally valid for all ferroelectrics. The researchers characterized the mechanical properties of domain walls in different ferroelectric materials and morphologies (crystals of LiNbO₃ and BaTiO₃ and films of PbTiO₃), suggesting that the wall softening in the polar direction is indeed a general property of ferroelectrics, independently of their composition or morphology.

Ferroelectric materials can be used to store information, since the dipole moment – pointing up or down – can be used as an information bit. The mechanical contrast of purely ferroelectric (non-ferroelastic) domain walls can be used to “read” such information. In addition, the mechanical contrast between walls and domains provides a barrier for phonon propagation. Since the walls themselves can be created or moved with a voltage, this provides a potential basis for phononic switches, while periodically poled ferroelectric crystals may have phononic crystal properties.

Reference article:

Christina Stefani, Louis Ponet, Konstantin Shapovalov, Peng Chen, Eric Langenberg, Darrell G. Schlom, Sergey Artyukhin, Massimiliano Stengel, Neus Domingo, and Gustau Catalan, Mechanical Softness of Ferroelectric 180° Domain Walls, Phys. Rev. X 10, 041001, October 2020. DOI: 10.1103/PhysRevX.10.041001