Author: Patricia Álvarez

X. Obradors, T. Puig Molina, S. Ricart, M. Coll Bau, J. Gazquez Alabart, A. Palau Masoliver and X. Granados Superconductor Science and Technology 25, 123001 (2012) DOI: 10.1088/0953-2048/25/12/123001 Chemical solution deposition (CSD) is a very competitive technique to obtain epitaxial films and multilayers of high quality with controlled nanostructures. Based on the strong attractiveness from the cost point of view, the production of long length coated conductors based on the CSD approach is being extensively developed. The trifluoroacetate route (TFA) is the most widely used route to achieve epitaxial YBa₂Cu₃O₇ (YBCO) layers with high critical currents, however a deep understanding of all the individual consecutive processing steps, as well as their mutual influence and relationship, is required to achieve superconducting materials with high performance. In this work, we review advances in the knowledge of all the steps relevant to the preparation of YBCO thin films based on TFA precursors as a CSD methodology: solution preparation and deposition, pyrolysis processes, intermediate phase evolution, nucleation and growth phenomena, microstructural evolution and its influence on percolating supercurrents, as well as vortex pinning by natural existing defects. Finally, we discuss the open issues still existing in the TFA approach, particularly that of film nanostructuration, and we provide a future outlook for this outstanding methodology.

M. Coll Bau, J. Gazquez Alabart, A. Palau Masoliver, M Varela, X. Obradors and T. Puig Chemistry of Materials, 24, 3723 (2012) DOI: 10.1021/cm301864c Highly epitaxial and pure (001) CeO₂ ultrathin films have been prepared by atomic layer deposition (ALD) at 275 °C on Y-stabilized ZrO₂ cubic fluorite single crystal substrate using cerium β-diketonate (Ce(thd)₄) and ozone (O₃) as precursors. Substrate temperature and precursor pulses have been optimized to set the ALD window obtaining a growth per cycle of ≈0.2 Å/cycle. This extremely low growth rate has been identified as a key parameter to ensure epitaxial growth at these low temperatures. Post-thermal treatments at 900 °C in oxygen further improve ALD-CeO₂ film texture while maintaining film stoichiometry and ultrasmooth surface, rms < 0.4 nm. ALD-CeO₂ thin film growth has also been tested on perovskite single crystal substrates, SrTiO₃ and LaAlO₃, exhibiting CeO₂ epitaxial growth and thus validating ALD as an outstanding method for low temperature epitaxial growth. Furthermore, we demonstrate that by combining e-beam lithography and ALD it is feasible to obtain size-controlled CeO₂ nanostructures.

E. Solano, F. Perez-Mirabet, R. Martínez-Julián, R. Guzman, J Arbiol, T. Puig Molina, X. Obradors Berenguer, R. Yanez, A. Pomar Barbeito, S. Ricart and J. Ros Journal of Nanoparticle Research 14, 1034 (2012) DOI: 10.1007/s11051-012-1034-y Well-defined synthesis conditions of high quality MFe₂O₄ (M = Mn, Fe, Co, Ni, Zn, and Cu) spinel ferrite magnetic nanoparticles, with diameters below 10 nm, have been described based on facile and efficient one-pot solvothermal or microwave-assisted heating procedures. Both methods are reproducible and scalable and allow forming concentrated stable colloidal solutions in polar solvents, but microwave-assisted heating allows reducing 15 times the required annealing time and leads to an enhanced monodispersity of the nanoparticles. Non-agglomerated nanoparticles dispersions have been achieved using a simple one-pot approach where a single compound, triethyleneglycol, behaves at the same time as solvent and capping ligand. A narrow nanoparticle size distribution and high quality crystallinity have been achieved through selected nucleation and growth conditions. High resolution transmission electron microscopy images and electron energy loss spectroscopy analysis confirm the expected structure and composition and show that similar crystal faceting has been formed in both synthetic approaches. The spinel nanoparticles behave as ferrimagnets with a high saturation magnetization and are superparamagnetic at room temperature. The influence of synthesis route on phase purity and unconventional magnetic properties is discussed in some particular cases such as CuFe₂O₄, CoFe₂O₄, and ZnFe₂O₄.

A. Palau, C. Montón, V. Rouco, X. Obradors and T. Puig Phys. Rev. B 85, 012502 (2012) DOI: http://dx.doi.org/10.1103/PhysRevB.85.012502 We investigate guided vortex motion in high-temperature YBa2Cu3O7 thin films patterned with an array of asymmetric blind antidots. A preferential vortex motion along spatial asymmetric pinning potentials has been directly observed by changing the driving current direction. Transport measurements reveal that effective ratchet potentials are created by fixed vortices strongly pinned within the antidots while the spatial asymmetry is transferred to interstitial vortices. We study a novel ratchet system that requires vortex-vortex interactions to work, in contrast with the individual vortex effects studied in conventional ratchet systems. By tuning the magnetic field and temperature, we are able to control the transition from a single vortex pinning regime to a region where collective effects become important and determine the range where the rectification effect is activated.

A. Llordes, A. Palau, J. Gazquez, M. Coll, R. Vlad, A. Pomar, J. Arbiol, R. Guzman, S. Ye, V. Rouco, F. Sandiumenge, S. Ricart, T. Puig, M. Varela, D. Chateigner, J. Vanacken, J. Gutierrez, V. Moshchalkov, G. Deutscher, C. Magen and X. Obradors Nature Materials, 11, 329-336 (2012) DOI: 10.1038/nmat3247 Boosting large-scale superconductor applications require nanostructured conductors with artificial pinning centres immobilizing quantized vortices at high temperature and magnetic fields. Here we demonstrate a highly effective mechanism of artificial pinning centres in solution-derived high-temperature superconductor nanocomposites through generation of nanostrained regions where Cooper pair formation is suppressed. The nanostrained regions identified from transmission electron microscopy devise a very high concentration of partial dislocations associated with intergrowths generated between the randomly oriented nanodots and the epitaxial YBa₂Cu₃O₇ matrix. Consequently, an outstanding vortex-pinning enhancement correlated to the nanostrain is demonstrated for four types of randomly oriented nanodot, and a unique evolution towards an isotropic vortex-pinning behaviour, even in the effective anisotropy, is achieved as the nanostrain turns isotropic. We suggest a new vortex-pinning mechanism based on the bond-contraction pairing model, where pair formation is quenched under tensile strain, forming new and effective core-pinning regions.

A. Llordes, K. Zalamova, S. Ricart, A. Palau, A. Pomar, T. Puig, A. Hardy, M. K. Van Bael and X. Obradors Chem. Mat. 22, 1686-1694 (2010) DOI: 10.1021/cm903080k A thorough analytical study on the thermal decomposition evolution of the metal-trifluoroacetate precursor toward high-performance YBa₂Cu₃O₇ superconducting films is presented. Evolved gas analysis (EGA), using Fourier transform infrared spectroscopy (FTIR) and mass spectrometry (MS), as well as X-ray diffraction (XRD), was performed to determine the complete chemical decomposition reaction of the metal-trifluoroacetate precursors. It is noteworthy that, contrary to what had been previously described, HF was not detected in the released gas. Moreover, we present new processing conditions that successfully reduced and even eliminated the undesirable porosity of the pyrolyzed films. Focused-ion-beam (FIB) studies demonstrated that the formation of pores was related to a fast escape of the released gas during precursor decomposition. The oxygen partial pressure was determined to be a key parameter to control both the kinetics and thermodynamics of the decomposition reaction and, hence, the porosity. This is of great importance because dense films are required to achieve high critical current densities in YBa₂Cu₃O₇ superconducting films.  

M. Gibert, T. Puig, X. Obradors, A. Benedetti, F. Sandiumenge and R. Hühne Adv. Mater. 19, 3937-3942 (2007) DOI: 10.1002/adma.200700361 Self-organized rows of CeO₂ nanodots (see image) have been achieved through centering of islands within the terraces of vicinal perovskite substrates. The extraordinary effectiveness for lateral confinement, with islands’ heights ∼ 20 times larger than the lattice steps, results from the non-coherent vertical interfaces at the substrate steps between dissimilar crystallographic structures.

J. Gutiérrez, A. Llordés, J. Gazquez, M. Gibert, N. Romá, S. Ricart, A. Pomar, F. Sandiumenge, N. Mestres, T. Puig and X. Obradors Nat. Mater. 6, 367-373 (2007) DOI:10.1038/nmat1893 Power applications of superconductors will be tremendously boosted if an effective method for magnetic flux immobilization is discovered. Here, we report the most efficient vortex-pinning mechanism reported so far which, in addition, is based on a low-cost chemical solution deposition technique. A dense array of defects in the superconducting matrix is induced in YBa₂Cu₃O_7-x–BaZrO₃ nanocomposites where BaZrO₃ nanodots are randomly oriented. Non-coherent interfaces are the driving force for generating a new type of nanostructured superconductor. Angle-dependent critical-current measurements demonstrate that a strong and isotropic flux-pinning mechanism is extremely effective at high temperatures and high magnetic fields leading to high-temperature superconductors with record values of pinning force. The maximum vortex-pinning force achieved at 65 K, 78 GN m^-3, is 500% higher than that of the best low-temperature NbTi superconductors at 4.2 K and so a great wealth of high-field applications will be possible at high temperatures.

J. Gazquez, F. Sandiumenge, M. Coll, A. Pomar, N. Mestres, T. Puig, X. Obradors, Y. Kihn, M. J. Casanove and C. Ballesteros Chem. Mat. 18, 6211-6219 (2006) DOI: 10.1021/cm0617891 We describe the conversion of yttrium, barium, and copper trifluoracetate-derived solid precursors to epitaxial YBa₂Cu₃O_x superconducting ceramics on (001)-oriented LaAlO₃substrates. Transmission electron microscopy, electron energy loss spectroscopy, energy-dispersive X-ray analysis, and X-ray diffraction are used to characterize the reaction path and nucleation mechanism yielding high critical current YBa₂Cu₃O₇. Our results show that the pyrolysis of the trifluoracetate solutions yields a nanostructured, partially amorphous Ba_1-xY_xF_2+x matrix having a Ba/Y ratio close to 2, with homogeneously dispersed CuO nanoparticles. Upon heating, the chemical trajectory of the fluoride matrix and the overall microstructural evolution of the ceramic precursor prior to YBa₂Cu₃O₇ nucleation is driven by the decomposition and oxidation of this solid solution. The Y solid solubility decreases with temperature yielding Y₂O₃ which reacts with the CuO particles forming Y₂Cu₂O₅ at about 700 °C. In addition, electron energy loss spectroscopy reveals a high oxygen concentration and almost no Y in the matrix quenched from 795 °C, at a stage where the YBa₂Cu₃O_x phase still forms disconnected 50−100 nm thick islands spaced by 1−2 μm. The observed evolution from Ba_1-xY_xF_2+x to a barium oxyfluoride mostly occurs prior to the heteroepitaxial nucleation of YBa₂Cu₃O_x at about 700 °C. Hence, a microstructural scenario is defined which favors competitive nucleation growth between heteroepitaxial YBa₂Cu₃O_x and bulk Y₂Cu₂O₅. X-ray diffraction pole-figure analysis reveals that the oxyfluoride phase is heavily textured, exhibiting two epitaxial relationships with the (001)-LaAlO₃ substrate:  (001)OF//(001)LaAlO₃, [110]OF//[100]LaAlO₃ and (111)OF//(001)LaAlO₃, [110]OF//[100]LaAlO₃ (OF stands for oxyfluoride). High-resolution observations of the growth front support that (111)-oriented oxyfluoride regions provide low-barrier nucleation sites for c-axis-oriented YBa₂Cu₃O_x on the buried (001)-LaAlO₃ substrate. However, owing to its high mismatch, this orientation only represents roughly 15% percent of the total OF volume. Considering that the nucleation of YBa₂Cu₃O_x is confined to those regions, this would lead to an anomalously large internuclei spacing, as is indeed observed, favoring the formation of large YBa₂Cu₃O_x grains and films with a low mosaic spread.