Condensed Matter Seminar

In Situ X-ray Studies of Ferroelectricity in Ultrathin Perovskite Films

Prof. Carol Thompson
Northern Illinois University

Friday, October 6, 2006 - 4:00 P.M., NSH 184

The paraelectric-to-ferroelectric phase transition in ultrathin films of perovskites displays complex behavior driven by a fascinating competition between polarization, strain, electric field, domain wall energy, and surface chemistry. We have been using 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, and electrical and chemical boundary conditions. The ability to perform x-ray scattering in the film growth chamber under in-situ conditions allows us to determine optimum growth conditions, to control the thickness of the films to sub-unit-cell accuracy, and to preserve film stoichiometry during high temperature study. When films are grown on insulating SrTiO₃, we find that the ferroelectric phase forms as nanoscale stripe domains. Such equilibrium stripe domains form to minimize the depolarizing field. When films are grown on conducting SrRuO₃ layers on SrTiO₃, the polar phase forms in a single domain. Although we observe a thickness-dependent transition temperature, in both cases the polar phase is stable at room temperature in films with thicknesses as small as three unit cells. We also observe that the orientation of the polarization in single-domain films is determined by the chemistry of the environment. Real-time x-ray observations of the film and surface structure provide direct evidence for the nature of this chemically induced polarization switching.