Bubbles and Anti-Hedgehogs: Studying the Nanostructures That Could Lead to the Future of Data Storage

Image of some of the nanostructures that physics researchers say could be used to store data.
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Image of some of the nanostructures that physics researchers say could be used to store data.

Electronic devices are getting smaller, faster and more efficient, but these improvements are limited by the size of their components — the transistors, resistors, capacitors and other components that are crammed onto smaller and smaller integrated circuits.

In order to overcome this challenge, physicists are exploring a new method of storing and transmitting electrical current and information: using the complex structures of certain crystalline materials, referred to as "ferroelectric" or "ferromagnetic." At the nanoscale, "ferroelectric" materials exhibit ordered patterns of electric dipoles. Electric dipoles are pairs of positive and negative charges that are caused by atomic displacement — atoms that have moved from the "ideal" position dictated by the crystalline lattice. Ferromagnetic materials exhibit similar patterns caused by a characteristic called spin.

These patterns can sometimes feature complex objects. Certain types of objects, called topological defects, are stable enough to withstand the wear and tear faced by electronic devices, and researchers can move them or alter their properties by applying an electric field. What if, physicists ask, these patterns could store and transmit information as reconfigurable elements of an integrated circuit?

Researchers in Laurent Bellaiche's physics group at the University of Arkansas work with colleagues around the globe to study these materials, using state-of-the-art experimental techniques and computer simulations. In a previous study, they used computer modeling to demonstrate that domain walls, or boundaries that separate different perfectly ordered regions in ferroic materials, could transfer electric charge through a type of electrical current called displacement current.

Now, Bellaiche and colleagues Yousra Nahas and Sergei Prokhorenko are working with a team that includes physicists from China and Germany.  With grants from Defense Advanced Research Projects Agency (DARPA) and the Army Research Office, this team is the first to observe certain topological defects in ferroelectric materials. Theoretically, these structures could be configured and manipulated individually by an electric field, or by using changes in temperature and pressure, which means they could be used to encode data.

In a paper published in Physical Review Letters, the researchers described the structures, which they observed through high-resolution scanning transmission electron microscopy and  simulated at the U of A High Performance Computing Center.

As Nahas and Prokhorenko explained, having both experimental observations and computer simulations is important because these two methods not only validate but also complement each other. "The observation proves the model is accurate," said Nahas, "but also, the model can provide microscopic understanding to reality itself."

Prokhorenko added, "Sometimes it's hard to identify the main factors leading to a certain observation. Simulations allow us to construct a simplified model of reality, and eliminate factors one after the other to understand why it happens. They can also help prove that the obtained result is robust and reproducible."

The researchers observed several different kinds of structures, which they then simulated to study and characterize further. The structures are formed by variations in the orientation of the dipoles, which are illustrated with arrows in the images below. The simulations were performed at the Arkansas High Performance Computing Center, which is funded through multiple National Science Foundation grants and the Arkansas Economic Development Commission.

Microscopy Image and Data

This image was taken with a scanning electron microscope. It shows layers of lead titanate crystal (marked PTO in the image) on a strontium titanate substrate (STO).
This image is a visual representation of the data the researchers gathered using the microscope. The different colors mark the patterns the researchers observed, formed by the orientation of the dipoles.


This is a magnified image of the section marked A in the image above. The pink region shows a domain wall, or boundary between two regions. The red arrows show swirl-like shapes called vortices.


The researchers observed dipolar disclinations, or mushroom-shaped patterns, in several regions of the material. The “stem” of the mushroom is a domain wall. On either side of the stem, the dipoles are facing away from each other, while the dipoles in the stem are oriented at right angles to these. The “cap” of the mushroom is formed by dipoles that radiate out from the stem. The image on the left is created from experimental data; the image on the right is a computer simulation of the same structure.

Waves and Bubbles

Experimental and simulated images of dipole waves (shown by arrows) and bubbles (shown by circles). In the experiment, the data is limited to two dimensions, the x and y-axes. Researchers theorized that the center of the bubble included a “significant component along the out-of-plane direction,” in other words, dipoles oriented along the z-axis, which can’t be seen in the experimental data. However, using simulations, they were able to predict that these components did exist.

Hedgehogs and Anti-hedgehogs

“Hedgehog” dipolar patterns have been predicted in ferroelectric materials. These are spherical regions characterized by dipoles that point outward, surrounding a central point called a singularity. Anti-hedgehog patterns are the same thing, except with inward-pointing dipoles. The pattern shown in the experimental data bears resemblance to this predicted pattern.

Camilla Shumaker, director of science and research communication
University Relations
479-575-7422, camillas@uark.edu


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