Researchers make significant discovery with study that 'creates some new application possibilities for memory storage'

In a significant breakthrough, researchers from North Carolina State University and collaborating institutions have uncovered a size-induced phase transition in materials used in electronic devices.

“Electronic devices are getting smaller and smaller, which makes it increasingly important for us to understand how a material’s properties may change at small scales,” lead author Ruijuan Xu, assistant professor of materials science and engineering at North Carolina State University, said in a press release. “In this case, we learned that when antiferroelectric thin films get too thin, these materials go through a phase transition and become ferroelectric. That makes them less useful for energy storage but creates some new application possibilities for memory storage.”

The findings, published in the journal Advanced Materials, shed light on how the electrical properties of these materials change when reaching a critical size threshold, the release stated. Antiferroelectric materials possess a unique crystalline structure characterized by alternating dipoles throughout the material. Each unit within the structure has a positive charge paired with a negative charge, resulting in no net polarization at the macroscopic level. Ferroelectric materials, on the other hand, have dipoles that all point in the same direction and can be reversed by applying an electric field.

Researchers focused their investigation on lead-free sodium niobate (NaNbO3) membranes to explore the effects of size on the properties of antiferroelectric materials, according to the release. Previous attempts to study these effects were challenging due to strains caused by the strong connection between the thin film and the substrate layer. To overcome this challenge, the team introduced a sacrificial buffer layer between the antiferroelectric thin film and the substrate. This allowed them to selectively detach the thin film from the substrate after growing it to the desired thickness.

By analyzing the strain-free samples ranging from 9 to 164 nanometers (nm), they obtained valuable insights into the material's behavior, the release stated. Researchers discovered an unexpected phenomenon when examining the atomic-scale structure of antiferroelectric membranes. Thinner membranes, below 40 nm, underwent a complete phase transition, becoming ferroelectric. In the range of 40 nm to 164 nm, the material exhibited regions with both ferroelectric and antiferroelectric properties.

The team also found that by applying an electric field, the antiferroelectric regions could be transformed into ferroelectric regions. However, this change was irreversible. These observations highlight the influence of size on the material's properties and open new possibilities for memory storage applications. The study also provided insights into the driving forces behind these changes in antiferroelectric materials. Utilizing first principles, researchers determined that structural distortion initiated at the surface of the membrane played a key role in observed phase transitions.

By harnessing size effects, researchers may be able to control and manipulate material properties, according to the study. The techniques employed in this study could be further applied to explore similar questions regarding a range of other materials.

The study involved a collaborative effort from researchers at various institutions, including North Carolina State University, Stanford University, the Université de Toulouse, the University of Arkansas, Argonne National Laboratory, Cornell University, the University of California, Berkeley, Brown University, Lawrence Berkeley National Laboratory, and Rice University.