A Phase-field Investigation of Domain Structures in Ferroelectric Bismuth Ferrite Thin Films

Open Access
Author:
Winchester, Benjamin Michael
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
June 25, 2012
Committee Members:
  • Long Qing Chen, Dissertation Advisor
  • Long Qing Chen, Committee Chair
  • Venkatraman Gopalan, Committee Member
  • Roman Engel Herbert, Committee Member
  • Qiang Du, Committee Member
  • Qiang Du, Special Member
Keywords:
  • ferroelectric
  • diffuse interface
  • phase-field
  • bismuth ferrite
Abstract:
The ferroelectric domain structure of multiferroic BiFeO3 thin films are strongly affected by the electrical boundary conditions. We employ a Ginzburg-Landau-Devonshire phase-field model to investigate the effects of the electrical boundary conditions on domain structure in BiFeO3 thin films. We examine the domain structure under short-circuit and under open-circuit boundary conditions with varying levels of compensation. As the degree of compensation changes, we find a smooth transition between the two types of domain structure. In the open-circuit case, we note small triangular nanodomains at the surface/wall interfaces that may be useful in nanoelectronic applications. The ferroelectric domain structures of epitaxial BiFeO3 thin films on miscut substrates were studied using a phase-field model. The effects of substrate vicinality towards (100) are considered by assuming charge-compensated surface and film/substrate interface. The predicted domain structures show remarkable agreement with existing experimental observations, including domain wall orientations and local topological domain configurations. The roles of elastic, electric, and gradient energies on the domain structures were analyzed. It is shown that the substrate strain anisotropy due to the miscut largely determines the domain variant selection and domain configurations. We employ phase-field modeling to explore the elastic properties of artificially-created 1-D domain walls in (001)p-oriented BiFeO3 thin films. The walls are composed of a junction of the four polarization variants, all with the same out-of-plane polarization: a “vortex” is comprised of polarization variants rotating around the junction, and an “anti-vortex” is comprised of two polarization variants pointing towards the junction and two pointing away. It was found that these junctions exhibit peculiarly high electroelastic fields induced by the neighboring ferroelastic/ferroelectric domains. These fields may contribute to the segregation of charged defects and charge carriers to the vortex and anti-vortex cores. We use a Ginzburg-Landau-Devonshire model to explore localized switching in epitaxial BiFeO3 thin films. We considered switching under a PFM tip for a range of misfit strains as well as in the proximity of a 71° domain wall. We find a strong correlation between the mismatch strain and coercive bias, in good agreement with previous theoretical and experimental results. We also note a large, non-linear change in the piezoresponse near the domain wall.