Probing ferroelectrics: Domain wall effects

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  • Published: Jun 15, 2014
  • Author: David Bradley
  • Channels: X-ray Spectrometry
thumbnail image: Probing ferroelectrics: Domain wall effects

Ironically ferro

Kinetic energy distribution of photoelectrons under X-PEEM conditions reveals domain wall details. Credit: Applied Physics Letters/Jakob Schaab et al.

The secret of the domain wall phenomenon in ferroelectric materials has been probed with X-ray photoemission electron microscopy (X-PEEM), which was used to investigate the ferroelectric erbium manganese oxide.

Ferroelectricity is a property displayed by some materials that exhibit spontaneous, but reversible, electrical polarization. The term is analogous to ferromagnetism wherein a material exhibits a permanent magnetic moment. Ferromagnetism was already known when ferroelectricity was discovered in 1920 and so the prefix "ferro" was used to describe the property despite the fact that, ironically, most ferroelectric materials do not contain iron.

Technological capacity

Ferroelectric materials are used in tuneable capacitors for medical ultrasound machines (because they are piezoelectric they can be used to generate and then detect the ultrasound "ping" used in this type of imaging. They are also used as the sensors in high-quality infrared cameras (these materials are also pyroelectric and respond to temperature changes as small as a few millionths of a degree Celsius). They also have applications in fire sensors, sonar, vibration sensors, and even fuel injectors for diesel engines.

The so-called domain walls in ferroelectric materials, such as the capacitor ceramic barium titanate, are critical to their behaviour. The domain walls separate regions of material with different electric polarization orientations and give rise to lines of electrical conduction within the material that are very different from that of the surrounding bulk. Scaling the domain walls of ferroelectrics has been attempted with scanning probe microscopy and the technique has offered many insights at the nanoscopic level. However, obtaining high-resolution images is slow and painstaking as well as generating experimental artefacts because of contact resistance or inhomogeneous probe fields and so interpreting the data is difficult and often gives rise to ambiguities.

Now, an international research collaboration has shown that X-PEEM can reveal important characteristics of the domain walls in a second ferroelectric, erbium manganese oxide, and so help explain its behaviour. Details are reported in the journal Applied Physics Letters and hold the promise of exploiting such materials for improved solar panels, electronic sensors, computer memory, and other applications.

Extreme X-PEEM

"X-PEEM is a particularly interesting tool because it allows for studying properties such as the chemistry, electronic structure, and symmetry of a material," explains Dennis Meier of the Swiss Federal Institute of Technology, ETH Zurich. Meier and Ingo Krug, an instrument and beamline scientist at Technische Universität Berlin, devised the X-PEEM experiments at the synchrotron BESSY II in Berlin, Germany. X-PEEM involves bathing the sample in X-rays and observing the variations in the electrons emitted to create image contrast. "In general, it is very difficult to directly image ferroelectric domain walls," Meier says.

The electrically conducting tail-to-tail domain walls in erbium manganese oxide are very thin, just a few atoms thick. Moreover, they can be built and demolished with relative ease electrically. "In our X-PEEM data, however, the tail-to-tail domain walls can easily be identified and are clearly separate from the surrounding bulk. We did not expect this remarkable sensitivity," Meier adds. As well as producing high contrast images, X-PEEM can also discern the behaviour of specific elements in the vicinity of the domain walls.

The collaboration includes members from ETH Zurich, Institut für Optik und Atomare Physik, the University of Stuttgart and the Peter Grünberg Institute in Germany, the University of Bordeaux, France, and Lawrence Berkeley National Laboratory; University of California Berkeley. The members hope to facilitate important new experiments that will lead to the development and design of novel electronic devices.

Related Links

Appl Phys Lett 2014, online: "Imaging and characterization of conducting ferroelectric domain walls by photoemission electron microscopy"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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