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Event Scheduled for Apr 19, 2018

Event: MSE PhD Dissertation Proposal - James Steffes

Location: IMS-159

Time: 01:00 pm

Details of Event:
PhD Dissertation Proposal

Presenter: James Steffes
Major Advisor: Dr. Bryan Huey
Associate Advisors: Dr. George Rossetti, Dr. C. Barry Carter, Dr. Michael Pettes

Date: Thursday, April 19th, 2018
Time: 1:00 PM
Place: IMS 159

Title: Thickness Scaling of Ferroelectricity in BiFeO3 by Tomographic Atomic Force Microscopy

Intrinsic and extrinsic properties of ferroelectric materials are known to have strong dependencies on electrical and mechanical boundary conditions, resulting in finite-size effects at length scales below several hundred nanometers. In ferroelectric thin films, equilibrium domain size is proportional to the square root of film thickness, which precludes the ability for current tomographic microscopies to accurately resolve complex domain morphologies in sub-micron films. Nanometer-scale three-dimensional imaging of spontaneous polarization is critical for understanding equilibrium states in polar materials, as well as for engineering devices based on such phenomena, however such capabilities remain a substantial experimental challenge. Computed tomography atomic force microscopy (CT-AFM) is proposed as a novel experimental modality for three-dimensional ferroelectric property measurements with volumetric resolution below 20 nm3.
This dissertation proposal outlines the use of CT-AFM to investigate the size-dependence of ferroelectricity in the room temperature multiferroic BiFeO3 across two decades of thickness to below 5 nm. Multiferroic BiFeO3 was chosen as a model system for illustrating the capabilities of CT-AFM due to its technological relevance in low-power, electrically-switchable magnetic logic. CT-AFM provides unprecedented tomographic imaging capabilities of ferroelectric domains in BiFeO3 with a significant improvement in spatial resolution compared to existing domain tomography techniques. In addition to tomographic imaging, CT-AFM is employed for direct, thickness-dependent measurements of the local spontaneous polarization and ferroelectric coercive field in BiFeO3. The thickness-resolved ferroelectric properties of BiFeO3 strongly correlate with cross-sectional TEM, Landau-Ginzburg-Devonshire phenomenological theory, and the semi-empirical Kay-Dunn scaling law for ferroelectric coercive fields. These results provide an unambiguous determination of a stable and switchable polar state in BiFeO3 to thicknesses below 5 nm. Three-dimensionally resolved conductive filaments are found to exist at defects in the ferroelectric domain structure of BiFeO3, and according to CT-AFM are shown to be localized to such defects throughout the entire thickness of the film, again to below 5 nm. Such findings demonstrate the accuracy and utility of CT-AFM for nanoscale three-dimensional property measurements, thereby providing novel insight into finite-size effects in ferroelectric and multiferroic materials.

Target Audience: Not Available

Sponsored By: Materials Science and Engineering Department

Pamphlet/Flyer: No Pamphlet/Flyer Available

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