Until recently, little was understood about the interplay between ferroelectricity and ferroelasticity, mostly due to the fact that there was no experimental tool capable of probing both phenomena simultaneously. In a study published in the journal Nanotechnology, this barrier has been overcome.

High-resolution mapping

Researchers from the University of Cambridge, UK, have used an atomic force microscopy (AFM) technique to simultaneously resolve topography, ferroelectric and ferroelastic domain structures with a resolution of almost 1 nm. The recently developed technique is an extension of the well known piezoresponse force microscopy, but utilizes the AFM cantilever dynamics to maximum advantage to pick out the differences in contact stiffness between in-plane (a-axis) and out-of-plane (c-axis) oriented crystallographic regions.

Ferroelastic domains arise to locally release shear strain and consist of alternating a and c oriented crystallographic regions. The periodicity of the resulting stripe pattern, known as a polytwin, directly reflects the shear strain, following an inverse square root law. The periodicity of ferroelastic domains observed here was in the range of a few tens of nanometres to several hundred nanometres – right at the same length scale as the size of typical devices.

The study also points out that the local geometry plays a crucial role in determining the orientation of these polytwins. The most intriguing result is that these polytwins bunch together in bundles. This study has shown that these bundles in themselves act as domains and mediate the process of ferroelectric domain reversal.

In short, the operation of many ferroelectric-based devices is far more complex than many people thought, and a window has just been opened showing us yet another level of co-operative behaviour at the nanoscale that has ramifications at the macroscale.