Feb 24, 2010
Electrostatics versus mechanics in simple multiferroics
Ferroelectric devices are commonplace nowadays, ranging from SAW filters in mobile phones to ultrasound transducers in medical scanners and capacitor dielectrics in flash-memory cells. Many of these applications rely on the very large electromechanical coupling that occurs inside piezoelectrics. As the scale of these devices is already at the submicron scale, and in some cases at the sub 100 nm scale, the intricacies of the domain structures are becoming more relevant to untangle. Understanding the domain structures is key to enhancing device operation and may lead to novel device functionalities.
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.
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.
About the author
The work was performed at the University of Cambridge, in the nanoscience centre. Yachin Ivry is a PhD student working on nano-ferroelectrics, currently in Israel writing up his thesis. Dr Daping Chu is the chair of the centre for advanced photonics and electronics (CAPE) and Dr Colm Durkan is the head of the nanoscience research group. The work was carried out as part of the Nokia-University of Cambridge collaboration in nanotechnology.