Interface Structure and Dynamics in Multiferroic Phase Transformations

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NON-TECHNICAL DESCRIPTION: The functional properties of emerging electronic ceramics based on structurally and chemically complex metal oxides depend very strongly on the crystal structures of these compounds. A series of recently discovered materials have competing stable phases with different crystallographic structures and can be reproducibly and rapidly transformed between phases. These structural phases differ significantly in their properties; new functionality in electronic and optical devices is enabled by switching between phases. At present, the detailed structural mechanism of these phase transformations, including the motion of the boundaries between phases, is not clear. This project comprises a series of experimental studies designed to probe the dynamics of this phase transformation, to answer key questions about the physical mechanism involved, and ultimately to provide feedback to the design of improved electroceramics.TECHNICAL DETAILS: The single-phase complex oxide bismuth ferrite, a prototypical multiferroic complex oxide, can be reproducibly transformed between structural phases with different symmetries, optical properties, and magnetic structures. Prof. Evans focuses on this model system because bismuth ferrite can be reliably synthesized using epitaxial growth techniques (e.g., ion sputtering, pulsed laser deposition, and chemical vapor deposition) such that it is near a phase boundary where phase transitions can be utilized in electromechanical, optoelectronic, and magnetic devices. Evans's initial results show that bismuth ferrite can be reproducibly transformed between structural phases on timescales as short as tens of nanoseconds, but these studies do not have sufficient time resolution to probe the fundamental timescale of the transformation. Through this project, Evans will (1) determine the short-time dynamics of rhombohedral behaviour (R-like) and tetragonal behaviour (T-like) phase populations in bismuth ferrite using picosecond-scale optical excitation and few-nanosecond-resolution electric field pulses, (2) probe the strain distribution and structure of individual R-like/T-like phase interfaces using X-ray nanobeam diffraction, (3) study the structure and dynamics of individual R-like/T-like interfaces in applied electric fields to determine the difference in dynamics arising from differing interface structures, and (4) explore the extension of these concepts to the interface dynamics in related phase-transforming complex oxides. Evans's work uses emerging synchrotron X-ray nanodiffraction methods, including recent advances in the analysis of diffraction patterns acquired with highly convergent coherent X-ray beams. This project supports the professional preparation of graduate students, participation of undergraduate students in research, targeted outreach through the development and demonstration of educational activities, and the dissemination of materials characterization and X-ray nanobeam diffraction methods to the broader research community.