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Perfect as the enemy of good — The effect of defects on ferroelectric switching dynamics

Published on October 8, 2024

Ferroelectrics are materials that maintain an electric dipole in the absence of an electric field. Applying an electric field to a ferroelectric can switch the direction of the electric dipole or even change their shape, and this has made them pivotal as sensors, transducers, or memories for diverse applications from ultrasound machines to video game consoles.

Conventionally, more perfect ferroelectrics have been sought for more advanced applications. But what happens when defects and impurities are intentionally introduced into a ferroelectric? Dr. Ralph Bulanadi, Dr. Kumara Cordero-Edwards, Giacomo Morpurgo, Prof. Patrycja Paruch, and their colleagues and collaborators have investigated the effects of induced defects on ferroelectric switching—and their surprising results have been featured on the cover of Physical Review Letters [1].

This work represents a culmination of a cross-continental collaboration between universities in Switzerland, Australia, and the United States. Ferroelectric lead titanate thin films grown at the University of California, Berkeley (United States) were bombarded with He2+ ions to introduce varying densities of point defects into the samples. These point defects, in combination with linear twin boundaries that form natively in lead titanate thin films, create a fascinating and complex defect landscape in which ferroelectric switching dynamics can be explored.

To investigate these thin films, researchers performed piezoresponse force microscopy measurements at both the University of Geneva (Switzerland) and the University of New South Wales (Australia). In this technique, a controlled variable voltage is applied through a small mobile probe to both switch and image the ferroelectric polarisation direction of a sample. New computational methods [2] were then used to track how different regions of the sample switched over time.

The major findings reported by the scientists are twofold. First, although the point defects tended to pin domain walls separating regions with different polarisation orientation, making it more difficult to initiate ferroelectric switching, once these domain walls get going, the global distribution of switching events was surprisingly independent of the point defect concentration. Second, the presence of the point defects appeared to “screen” the otherwise very strong pinning effects of the linear twin boundaries, leading to more spatially homogeneous and consistent switching dynamics.

These findings mark key steps forward in understanding how ferroelectric switching proceeds, and could have implications for the development of future ferroelectric devices, and research into materials and interfaces.

Read the article in PRL

Contacts:
Prof. Patrycja Paruch, Patrycja.Paruch@unige.ch
Dr Ralph Bulanadi, Ralph.Bulanadi@unige.ch

[1] Bulanadi, Ralph, et al. “Interplay between Point and Extended Defects and Their Effects on Jerky Domain-Wall Motion in Ferroelectric Thin Films.” Physical Review Letters 133.10 (2024): 106801.

[2] Bulanadi, Ralph, and Patrycja Paruch. “Identifying and analyzing power-law scaling in two-dimensional image datasets.” Physical Review E 109.6 (2024): 064135.