Nanoscale Ferroelectricity
G. B. Stephenson*
Materials Science Division and Center for Nanoscale Materials, Argonne National Laboratory
One important scientific area for the Cornell ERL will be performing in situ studies of materials synthesis and processing. In this talk I will use examples from our current research at APS to highlight areas where the properties of the proposed ERL x-ray source would have a strong scientific impact.
Ferroelectricity is an example of a cooperative phenomenon that shows strong size effects. The paraelectric-to-ferroelectric phase transition in ultrathin films displays complex behavior driven by a fascinating competition between polarization, strain, electric field, domain wall energy, and surface chemistry. For decades, researchers have found that ferroelectric behavior is typically suppressed in films that are sufficiently thin. Various explanations have been put forward: intrinsic suppression of polarization at surfaces, the effect of depolarizing electric fields, or extrinsic effects of composition or strain. As a result, the factors responsible for the size dependence of the paraelectric-to-ferroelectric phase transition remain unresolved, in particular for the technologically important perovskites. We have been using in situ synchrotron x-ray scattering to investigate the ferroelectric properties of ultra-thin, coherently strained epitaxial films of PbTiO3 as a function of film thickness, temperature, vapor ambient, and electrical boundary conditions. The ability to perform x-ray scattering in the film growth chamber allows us to determine optimum growth conditions, to control the thickness of the films to sub-unit-cell accuracy, and to control surface and film stoichiometry during high temperature study. When films are grown on insulating SrTiO3, we find that the ferroelectric phase forms as nanoscale 180o stripe domains. When films are grown on conducting SrRuO3 layers on SrTiO3, the polar phase forms in a single domain. Although we observe a thickness-dependent TC, in both cases the polar phase is stable at room temperature in films with thicknesses as small as three unit cells.
*Collaborators: R.-V. Wang, F. Jiang, D. D. Fong, S. K. Streiffer, C. Thompson, J. A. Eastman, P. H. Fuoss, A. M. Kolpak, A. M. Rappe, and K. R. Elder.
