Recent findings suggest early proof of principle for a new way to read, write and erase information in much smaller areas than conventional memory. The method entails ferroelectric thin films, which store information using the domains’ polarization directions like the 1s and 0s in a conventional transistor.
Ferroelectric materials possess a peculiar property: Electric charges naturally align in small regions, called domains, endowing the material with built-in electric dipoles. When exposed to a sufficiently strong external electric field, the dipoles line up—a property that makes ferroelectrics appealing for a variety of applications, including actuators and sensors.
Researchers from the University of Sydney, in Australia found a way to manipulate ferroelectric domains on thin films of hexagonal yttrium manganite. They used an electron gun—in the form of a transmission electron microscope—to send a beam of negatively charged particles through the thin film; they were then able to orient and control the polarization of ferroelectric domains (FDs). The scientists say their method represents a new way to write and erase information on these materials.
“We want to provide a roadmap for engineering things that use this property,” says Zibin Chen, a PhD student at the university who led the study. In the paper, published in Physical Review Letters on July 8, he and his coauthors wrote that their method could be “ideal for manufacturing miniature and integrated electronic devices.” The ferroelectric domains created with this technique are approximately 0.2 nm in diameter, nearly 100 times smaller than bits in commercial memory devices.
Researchers have long been interested in using ferroelectrics to store data—already, Sony’s Playstation 2 and some debit cards have had ferroelectric memory devices. Ferroelectric materials are also piezoelectric and pyroelectric, which means, respectively, they generate charge when under mechanical stress or during temperature changes. Previous efforts to control ferroelectric domains for the purpose of storing information have taken advantage of these properties, whether by heating the material or changing its shape. Other researchers have used scanning probe microscopy to switch domains, but that approach requires contact between the microscope tip and the film—which can damage both.
Chen and his collaborators used a transmission electron microscope to send a beam of electrons through the sample without touching it. The researchers reported that the stream of negative particles from the electron gun interacted with the material to create an electric field, which then flipped the orientation of domains polarized in the opposite direction. The researchers liken that process to recording information on the film. Along the same lines, Chen and his colleagues reported that when they moved the beam, they could “erase” that information. They used the beam to write the letters U, S, Y, and D on the film (an abbreviation for the University of Sydney).
Materials scientist Dennis Meier, at the Norwegian University of Science and Technology, in Trondheim, who was not involved in the research, thinks the approach by Chen and his team might be useful in a burgeoning area—engineering specific domain and domain-wall patterns, which could be used to produce small circuits.
At the same time, he points out that he would like to see more extended writing and erasing cycles in future experiments, in order to fully establish the approach for reversible domain control—which might work for data recording and erasing.
“In my opinion, it is crucial to switch the same area between positive and negative orientations several times,” Meier says. “That is one of the key aspects for controlling ferroelectrics on demand.”
Chen says in future work, he plans to investigate ways to combine the TEM approach with other previous efforts to deform or heat the material, for example. “Maybe by applying or combining the other external stimuli, we can control the shrinkage of the domain size or make it more effective at producing the new domains,” he says.