Thomas Tybell

Ferroelectricity in thin perovskite films

We report on the investigation of ferroelectricity in thin tetragonal single-crystalline perovskite films of Pb(Zr0.2Ti0.8)O3 grown by off-axis rf magnetron sputtering. The local ferroelectric properties of atomically smooth films, with thicknesses ranging from a few unit cells to 800 A, were measured using a combination of electric force microscopy and piezoelectric microscopy. The time dependence of the measured signals reveals a stable ferroelectric polarization in films down to thicknesses of 40 A.

Domain wall creep in epitaxial ferroelectric Pb(Zr0.2Ti0.8)O3 thin films

Ferroelectric switching and nanoscale domain dynamics were investigated using atomic force microscopy on monocrystalline Pb(Zr(0.2)Ti(0.8))O(3) thin films. Measurements of domain size versus writing time reveal a two-step domain growth mechanism, in which initial nucleation is followed by radial domain wall motion perpendicular to the polarization direction. The electric field dependence of the domain wall velocity demonstrates that domain wall motion in ferroelectric thin films is a creep process, with the critical exponent mu close to 1. The dimensionality of the films suggests that disorder is at the origin of the observed creep behavior.

Nanoscale control of ferroelectric polarization and domain size in epitaxial Pb(Zr0.2Ti0.8)O3 thin films

We demonstrate that atomic force microscopy can be used to precisely manipulate individual sub-50 nm ferroelectric domains in ultrahigh density arrays on high-quality epitaxial Pb(Zr0.2Ti0.8)O3 thin films. Control of domain size was achieved by varying the strength and duration of the voltage pulses used to polarize the material. Domain size was found to depend logarithmically upon the writing time and linearly upon the writing voltage. All domains, including those written with ∼100 ns pulses, remained completely stable for the 7 day duration of the experiment.

Control and imaging of ferroelectric domains over large areas with nanometer resolution in atomically smooth epitaxial Pb(Zr0.2Ti0.8)O3 thin films

We have investigated the possibility afforded by epitaxial ferroelectric oxide thin films to control and image locally the polarization field of ferroelectrics over large areas with submicron resolution, using the metallic tip of an atomic force microscope as a mobile top electrode and local probe of the ferroelectric properties. Atomically smooth films of Pb(Zr0.2Ti0.8)O3, showing a root-mean-square roughness of typically a few angstroms, could be uniformly polarized and imaged over areas as large as 2500 μm2 without introducing any topographic disorder. Regular arrays of 100 nm wide lines and circular domains with a diameter less than 100 nm were written in arbitrary areas of the uniformly polarized regions.

Stable (110) textured Ag ribbons for biaxially aligned YBa2Cu3O7-d coated tapes

Texture development in pure Ag was investigated in order to obtain {110}uvw textured Ag ribbons that can be used as a substrate for YBa2Cu3O7 superconducting coated tapes without any buffer layer. The starting material was 99.95% pure commercial Ag foils. A 20% deformation reduction was used at each step of the cold rolling process after which an optimal annealing was achieved at 800 °C for 4 h in a primary vacuum. This process leads to large-grain ribbons with the {110}112 orientation. A sharp and pure texture is obtained with a FWHM smaller than 6° in the three x-ray pole figures that were used to calculate the orientation distribution function. Crystallographic orientation maps achieved by electron back-scattering diffraction show a misorientation from the normal direction of less than 4°. A twinning mechanism is used to explain the formation of the {110}112 texture. The stability of the {110}112 texture is confirmed by further annealings up to 900 °C, a practical temperature for coated tapes preparation. Lengths up to 25 cm with a uniform {110}112 texture have been obtained.

Preparation of {110} textured Ag ribbons for biaxially aligned superconducting tapes

The aim of our work is to produce biaxially aligned HTS conductors by epitaxial growth on textured Ag. This paper will focus on the preparation of long length textured Ag ribbons. After cold rolling and annealing in vacuum at 800/spl deg/C, different {110} directions appear in the recrystallized pure Ag ribbons. The {110} texture is obtained after cold rolling by adding a 500/spl deg/C step before the annealing at 800/spl deg/C. If the Ag ingots are annealed at 600/spl deg/C in vacuum before the cold rolling, the 800/spl deg/C annealing leads to a sharp {110} texture. The quality of the texture is studied by X-ray and electronic diffraction. Three X-ray pole figures are used to calculate the ODF function, a SEM is used to obtain orientation maps from EBDP. The surface roughness is studied by AFM. We have produced lengths of 25 cm with a sharp {110} texture (FWHM=7/spl deg/ ODF peak). The characteristics of the two {110} and {110} orientation are presented in detail. It is shown that those {110} ribbons are suitable for epitaxial growth of superconducting cuprates. The Ag textured ribbons are used as substrates for different superconducting systems such as Tl-1223 and Y123.

Scanning probe microscopy for the imaging and control of ferroelectric oxides

Abstract Using a combination of scanning probe techniques, including contact mode atomic force microscopy, electric force microscopy, piezoelectric microscopy, and scanning tunneling microscopy, the ferroelectric properties of ferroelectric/metallic oxide heterostructures (Pb(Zr 0.52 Ti 0.48 )O 3 /SrRuO 3 and Sr(Ru 0.37 Ti 0.63 )O 3 /Pb(Zr 0.2 Ti 0.8 )O 3 ) and atomically smooth epitaxial ferroelectric oxides (Pb(Zr 0.2 Ti 0.8 )O 3 and Pb(Zr 0.52 Ti 0.48 )O 3 ) have been studied. Ferroelectric domains in as-grown films were imaged with nanometer resolution, and the domain structure could be modified locally and reversibly. Using the local polarization field of the ferroelectric, nonvolatile, reversible field effects were induced in SrRuO 3 at the submicron level, changing its sheet resistance by up to 300 ohms per square in a fashion that does not require any permanent electrical contacts or associated lithographic processing.

The microwave surface impedance of DyBa/sub 2/Cu/sub 3/O/sub 7-x/ very thin films

Measurements of the surface impedance of DyBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (DyBCO) thin films have been made around 6 GHz by using the parallel plate resonator method with sapphire dielectrics. The films were deposited by RF magnetron sputtering on LaAlO/sub 3/ substrates to thicknesses between 20 and 320 nm. The change of the London penetration depth as a function of temperature is deduced from resonance frequency shifts. A generalized two fluid model is used to fit the data in terms of an effective penetration depth /spl lambda//sub eff/(0) at 0 K, taking into account the effect of grain boundary weak links. This analysis give an interpretation of the changes in /spl lambda//sub eff/(0) as a function of the film thickness and yields values for /spl lambda//sub eff/(0) around 160 nm for films thinner than 120 nm.

Electronic Doping in Epitaxial Pb(Zr0>52Ti0.48)03/SrRuO3 Heterostructures using a Ferroelectric Field Effect

We report on ferroelectric field effect experiments in ultrathin layers of the metallic perovskite SrRuC 0.52 Ti 0.48 )O 3 /SrRuO 3 epitaxial heterostructures. Switching the ferroelectric polarization of the Pb(Zr 0.52 Ti 0.48 )O 3 layer induces a ∼ 10% change in the sheet resistance of the SrRuO 3 layer that is nonvolatile and also reversible. Hall effect measurements that take into account the anomalous Hall effect reveal a carrier concentration of n ∼ 2 × 10 22 electrons/cm 3 and allow us to understand quantitatively the sign and magnitude of the observed resistance change. Of key importance for these experiments is the crystalline and surface quality of the SrRuO 3 and Pb(Zr 0.52 Ti 0.48 )O 3 layers. We also discuss how this general approach of nonvolatile doping using ferroelectrics opens new possibilities of directly creating small electronic structures without using traditional lithographic techniques.

Nonvolatile, Reversible Writing of Electronic Nanostructures in Epitaxial Ferroelectric / Metallic Oxide Heterostructures using a Field Effect

Using scanning probe microscopy, we have written nonvolatile electronic nanofeatures in the metallic perovskite oxide SrRuO 3 . The structures were written in epitaxial thin film Pb(Zr 0.52 Ti 0.48) O 3 (PZT) / SrRuO 3 heterostructures by locally switching the polarization field of the ferroelectric PZT layer with an atomic force microscope (AFM). The resulting field effect changes the sheet resistance of the SrRuO 3 layer by up to 300 ohms per square. Using the AFM as an electric field microscope, it is also possible to visualize the charge distribution of the written areas on the PZT surface. Large areas of up to 100 μm 2 have been polarized and imaged with submicrometer resolution, with the smallest features having linewidths of 170 nm. This approach to local electronic doping is reversible and allows one to write nonvolatile submicron electronic features in two dimensions without lithographic steps or permanent electrical contacts required.