J. Zabaleta, S. Valencia, F. Kronast, C. Moreno, Patricia Abellan, J. Gazquez Alabart, H. Sepehri-Amin, F. Sandiumenge Ortiz, T. Puig Molina, N. Mestres Andreu and X. Obradors Berenguer
Nanoscale 5, 2990-2998 (2013)
The chemical composition and the magnetic structure of individual La0.7Sr0.3MnO3 (LSMO) ferromagnetic manganite epitaxial nanostructures less than 200 nm in width are explored using Photoemission Electron Microscopy (PEEM). X-ray absorption spectra (XAS) provide separate information on the surface and the bulk composition of the nanoislands and give evidence of Mn2+ present on the surface of otherwise stoichiometric nanostructures. Ferromagnetic domains less than 70 nm are resolved using X-ray magnetic circular dichroism (XMCD), which allows for the detection of magnetic vortex states in both (001)LSMO square and (111)LSMO triangular manganite nanoislands. The evolution of single nanostructures under an in-plane magnetic field is seen to depend on the specific nanoisland size and geometry. In particular, PEEM XMCD imaging allows detecting opposite chiralities as well as a variety of magnetization behaviors for different nanoislands.
R. Guzman, J. Gazquez, V. Rouco, A. Palau, C. Magen, M. Varela, J. Arbiol, X Obradors and T. Puig
Appl. Phys. Lett. 102, 081906 (2013)
In this letter we use high resolution scanning transmission electron microscopy to study epitaxial YBa2Cu3O7−δ(YBCO)nanocomposite thin films. We find that twin boundaries (TB) in YBCOnanocomposite thin films are disturbed by the presence of secondary phase nanoparticles as well as by intergrowths. Secondary phases promote the nucleation of TBs and, at the same time, result in bending, decreasing and changing the TB’s spacing. On the other hand, the local strain ensuing from the partial dislocation associated to Y248 and Y125 intergrowths break the verticalcoherence of TBs. This interaction results in a complex domain structure where twin boundarycoherence is no longer satisfied and twin spacing is reduced down to a few nanometers precluding vortex channeling at low temperatures.
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)
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)
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)
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)
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)
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)
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.
T. Puig, J. Gutierrez, A. Pomar, A. Llordes, J. Gazquez, S. Ricart, F. Sandiumenge and X. Obradors
Supercond. Sci. Technol. 21, 034008 (2008)
A methodology of general validity to study vortex pinning in YBa2Cu3O7−δ (YBCO) is described. It is based on measuring the angular-dependent critical current density in a large range of temperatures and magnetic fields and it permits one to identify, separate and quantify the three basic vortex pinning contributions associated to anisotropic–strong, isotropic–strong and isotropic–weak pinning centers. The corresponding vortex pinning phase diagrams for these three contributions are built up. This methodology is applied to the recently discovered solution-derived YBCO–BaZrO3 nanocomposites which display a maximum vortex pinning force of 78 GN m−3 at 65 K, being five times larger than that of NbTi at 4.2 K. These nanocomposites show an effective anisotropy tending to unity and the best superconducting properties observed so far in YBCO. We present the richness and complexity of their microstructure and the correlation with the vortex pinning properties. The application of the vortex pinning methodology and comparison with a standard solution-derived YBCO film enable us to identify the isotropic–strong defect contribution being at the origin of their unique properties.
M. Gibert, T. Puig, X. Obradors, A. Benedetti, F. Sandiumenge and R. Hühne
Adv. Mater. 19, 3937-3942 (2007)
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.