Topological defect engineering in ferroelectrics: from domain wall memory to emergent functionalities.

The word ‘defect’ conjures up images of classical point and line defects to a typical materials scientist. However a concept that is even more pervasive is the phenomenon of a topological defect. Particularly in ferroic materials topological defects are inherent. Due to degeneracy in possible orientations of the order parameter upon cooling below the ferroic phase transition temperature, ferroic phases tend towards the formation of discrete domain structures. Adjacent domains are separated by naturally occurring planar topological defects called domain walls.

 

The first part of talk will focus on “ferroelectric domain wall memory device”. The discovery of electrical conductivity in specific types of ferroelectric domain walls gave rise to “domain wall nanoelectronics”, a technology in which the wall (rather than the domain) stores information. Here, we have recently demonstrated a prototype non-volatile ferroelectric domain wall memory, scalable to below 100 nm, whose binary state is defined by the existence or absence of conductive walls. The device can be read-out nondestructively at moderate voltages (< 3 V), exhibits relatively high OFF-ON ratios (~10³) with excellent endurance and retention characteristics, and possesses multilevel data storage capacity. Our work[1] thus constitutes an important step toward integrated nanoscale ferroelectric domain wall memory devices.

 

In the second half of the talk, nanoscale bubble domains in ultrathin ferroelectric films will be introduced. This new type of nanoscale ferroelectric domains, has been observed in ultrathin epitaxial PbZr_0.2Ti_0.8O₃/SrTiO₃/PbZr_0.2Ti_0.8O₃ sandwich structures.[2] Using piezoresponse force microscopy and aberration-corrected atomic-resolution scanning transmission electron microscopy mapping techniques, it is confirmed that the bubble domain are laterally confined spheroids of sub-10 nm size with local dipoles opposite to the macroscopic polarization of their surrounding ferroelectric matrix. An incommensurate phase and symmetry breaking is found within these domains, which result in local polarization rotation and hence a mixed Néel-Bloch-like character to the bubble domain walls.

 

These findings highlight the richness of polar topologies that may develop in ultrathin ferroelectric structures and bring forward the prospect of emergent electronic functionalities due to topological transitions.

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Bio: Nagarajan (Nagy) Valanoor received his B.Eng. in Metallurgy from the University of Pune (1997) and Ph.D. from the University of Maryland (2001) under supervision of Prof. Ramesh in Materials Science and Engineering, respectively. Following his Ph.D. he continued as a research associate at Maryland until 2003. He followed this with an Alexander von Humboldt Fellowship with Prof. Rainer Waser at Forschungszentrum Juliech. In 2005 he was offered a lectureship at the School of Materials Science and Engineering at UNSW, where he is currently Professor and Research Director.

 

He has built, from scratch, a world-class suite of facilities at UNSW in the areas of nanofabrication, thin film synthesis and atomic scale measurements. He is currently a PI (along with 18 others) on ARC Centre of Excellence for Future Low Energy Electronics Technologies (FLEET), a $33M, 7 years grant to investigate topological phenomena in electronic materials. His publications (>170) include publications in Science, Science Advances, Nature materials, Advanced Materials, Nano Letters, Nature Communications and ACS Nano.  His h-index is 44 and career citations exceed 8000. Notable awards include: 2006, 2010 (Back-to back) Australian Research Fellowship, 2009 Edgeworth David Medal, 2010 International Symposium on Integrated Ferroelectrics Young Investigator award, 2012 UNSW VC Research Supervision award, 2014 IEEE Ultrasonics, Ferroelectrics, and Frequency Control Society, Ferroelectrics Young Investigator Award and inaugural University of Maryland Young Alumnus award.