[en] The critical current density in YBa₂Cu₃O_7-x (YBCO) thin films, which limits their application in high magnetic fields, can be enhanced by the introduction of artificial pinning centers as, e.g., provided by nanoparticles. An inert gas phase condensation process was used to prepare Y₂O₃ nanoparticles from an yttrium target by DC magnetron sputtering. With this method, both the size distribution and the areal density of the particles as determined from TEM investigations are independently controlled during deposition. Particles with a mean diameter of about 8 nm were deposited on SrTiO₃ substrates, which are terminated by TiO₂ through etching with BHF solution and subsequent annealing in 1 atm O₂ at 900 C. The behavior of the particles on the substrate at varying temperatures in an O₂ atmosphere of 0,7 mbar (YBCO deposition conditions) was studied by AFM and ICP-MS. A 300 nm thin YBa₂Cu₃O_7-x layer was deposited onto the likewise pre-coated substrates by off-axis pulsed laser deposition. Both inductive and resistive transport measurements at 77 K in magnetic fields of up to 9 T reveal the influence of the particles on the superconducting properties of YBa₂Cu₃O_7-x. (orig.)

[en] The direct microscopic imaging of flux lines by means of low-temperature MFM was applied to study the vortex pinning mechanism at natural and artificial defects in high temperature superconducting films. A low-temperature scanning probe microscope (Omicron Cryogenic SFM) allows measurements in UHV combined with magnetic fields of 7 T (vertical) and 3 T (transversal). Flux lines have been successfully imaged on a pure YBCO film as well as on a YBCO film grown on a template with gas phase prepared Y₂O₃ nanoparticles. The investigated films with a mean roughness less than 10 nm were deposited by off-axis PLD and cooled down in the microscope to 7.7 K in a magnetic field prior to imaging. The number of vortices observed corresponds to the theoretically expected one. The vortex distribution was compared with the topography. The in situ transport measurements allow an estimation of the pinning strength at the different defects present in the sample. For future studies on chemical deposited films a mechanical polishing procedure is developed to obtain sufficiently smooth sample surfaces. (orig.)

[en] The critical current density $J_{\mathrm{c}}$ in ${\mathrm{Y}\mathrm{B}\mathrm{a}}_2{\mathrm{C}\mathrm{u}}_3{\mathrm{O}}_{7-<!--\; ???\; -->\mathrm{\delta <!--\; ??\; -->}}$ thin films, which limits their application in external magnetic fields, can be enhanced by the introduction of artificial pinning centers such as non-superconducting nanoparticles inducing additional defects and local strain in the superconducting matrix. To understand the correlation between superconductivity, defect structures and particles, a controlled integration of particles with adjustable properties is essential. A powerful technique for the growth of isolated nanoparticles in the range of 10 nm is dc-magnetron sputtering in an inert gas flow. The inert gas condensation (IGC) of particles allows for an independent control of both the particle diameter distribution and the areal density. We report on the integration of such gas-phase-condensed ${\mathrm{H}\mathrm{f}\mathrm{O}}_2$ nanoparticles into pulsed laser deposited (PLD) ${\mathrm{Y}\mathrm{B}\mathrm{a}}_2{\mathrm{C}\mathrm{u}}_3{\mathrm{O}}_{7-<!--\; ???\; -->\mathrm{\delta <!--\; ??\; -->}}$ thin film multilayers with a combined PLD-IGC system. The particles and the structure of the multilayers are analyzed by transmission electron microscopy on cross-sectional FIB lamellae. As a result of the IGC particle implementation, randomly as well as biaxially oriented ${\mathrm{B}\mathrm{a}\mathrm{H}\mathrm{f}\mathrm{O}}_3$ precipitates are formed in the ${\mathrm{Y}\mathrm{B}\mathrm{a}}_2{\mathrm{C}\mathrm{u}}_3{\mathrm{O}}_{7-<!--\; ???\; -->\mathrm{\delta <!--\; ??\; -->}}$ thin films. With as few as three interlayers of nanoparticles, the pinning force density is enhanced in the low-field region. (paper)

[en] The magnitude and anisotropy of the critical current density J_c as well as the irreversibility field H_irr of BaZrO₃ (BZO)-doped YBa₂Cu₃O_7-x (YBCO) thin films are determined by both the amount of BZO and the deposition temperature. The influence of these two parameters was investigated on BZO/YBCO quasi-multilayers in two temperature series with different doping amounts (4 and 9 vol% BZO). The deposition temperature was varied between 790 and 850 deg. C. The samples show an increase in H_irr, logarithmic peaks in J_c(B) and strong c-axis peaks in J_c(θ). Nevertheless, the two series reveal opposite temperature trends for J_c: whereas J_c increases with temperature for 4 vol% BZO, it decreases for 9 vol%. This is explained by a diffusion model for the formation and growth of BZO nanocolumns and an optimum balance between correlated and random pinning centres for high J_c. TEM images support this model.

[en] Y₂O₃ nanoparticles prepared by an inert gas phase condensation process were used to introduce artificial pinning centres in YBa₂Cu₃O_7-δ thin films. The areal density of the particles was varied between 120 and 4200 particles μm^-2 without changing the mean particle diameter of approximately 9 nm. Y₂O₃ particles were deposited on TiO₂ terminated SrTiO₃ (100) single-crystal substrates with areal densities up to 1654 particles μm^-2 and subsequently covered with YBa₂Cu₃O_7-δ by off-axis pulsed laser deposition (PLD). The areal density of the room temperature deposited particles is not changed by the substrate heating during PLD although their height on the substrate decreases. An influence on J_c is demonstrated for particle densities above 1588 particles μm^-2, indicating that the substrate decoration with Y₂O₃ nanoparticles from the gas phase affects the formation of artificial pinning centres in the YBa₂Cu₃O_7-δ films and can be applied to further study of the effect of particle size and areal density on defect formation in YBa₂Cu₃O_7-δ