Probing local structure and dynamics of ferroelectric domain walls using nonlinear optics and scanning probe microscopy

Open Access
Author:
Aravind, Vasudeva Rao
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
June 30, 2009
Committee Members:
  • Venkatraman Gopalan, Dissertation Advisor
  • Venkatraman Gopalan, Committee Chair
  • Darrell G Schlom, Committee Member
  • Long Qing Chen, Committee Member
  • Xiaoxing Xi, Committee Member
Keywords:
  • piezoresponse force microscopy
  • scanning probe microscopy
  • optical second harmonic generation
  • multiferroic
  • ferroelectric
  • strontium titanate
  • lithium niobate
Abstract:
Domains and domain walls are a fundamental property of interest in ferroelectrics, magnetism, ferroelastics and superconductors. Unlike magnetic Bloch walls, ideal ferroelectric domain walls are well accepted to be one to two lattice units wide, over which polarization and strain change across the wall. However, walls in real ferroelectrics appear to show unexpected property variations over micrometer length scales across the wall. Diverse functionalities in materials can be created simply by patterning of ferroelectric domains. The "up" and "down" polarization states created in a ferroelectric material can act as data storage bits. Domain shaping into diverse shapes and length scales is also critical to THz surface accoustic wave devices. Domain wall width also directly influences domain wall motion at various length scales. The central focus of this thesis work is to develop a fundamental understanding of local structure and dynamics of 180 degree domain walls in ferroelectrics. For this study we utilize two techniques: nonlinear optics and scanning probe microscopy. Nonlinear optics is demonstrated to be an excellent tool to probe the onset of ferroelectric and ferroelastic order parameter, as well as their orientation in ferroelectrics. Scanning probe microscopy is demonstrated to be a quantitative technique to study domain walls in ferroelectric materials with a resolution of about 10nm. This technique is then used to demonstrate that real 180 degree domain walls in ferroelectric materials can be as broad as 10-100nm. Finally, a recently developed technique called Switching Spectroscopy Piezoresponse Force Microscopy (SS-PFM) is utilized to demonstrate for the first time that 180 degree domain walls can act as local nucleation centers in ferroelectric materials reducing coercive fields in their vicinity.