New Insights on the Intrinsic Anisotropy of YBCO Films

Careful investigation of the angular dependence of resistivity ρ(θ) (θ is the angle between the magnetic field and the ab-planes) and the temperature dependence of resistivity ρ(T) within the superconducting transition in an applied magnetic field B up to 1 T for a series of YBa₂Cu₃O_7−δ (YBCO) thin films revealed a large variation of intrinsic anisotropy factor γ. The series of films studied included both optimally doped and underdoped samples of different T _c , critical current density J _c , film thickness, and preparation techniques. The variation in the shape and depth of the minimum measured for ρ(θ) near θ=0° could be directly correlated to the intrinsic anisotropy of the YBCO films. The results of fitting of ρ(θ) using Bardeen–Stephen theory allowed a quantitative determination of the value of γ which varies between 7 and 230, and is independent of T _c , film thickness, or J _c . The sharper the minimum in ρ(θ) around θ=0° the larger is the anisotropy. For highly anisotropic film, ρ(θ) showed an identical behavior for B ∥ J and B ⊥ J (i.e., ρ(θ) is independent of the angle θ between B and J for this film). The large variation in γ could be attributed to the “buckling” of the CuO₂ planes.     

Influence of BaZrO<Subscript>3</Subscript> Amount on Microstructure and Properties in YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−<Emphasis Type="Italic">x</Emphasis></Subscript> Films Prepared by TFA-MOD Process

Epitaxial YBa₂Cu₃O_7−x /BaZrO₃ films were prepared by complex metal–organic solutions including Y, Ba and Cu anhydrous trifluoroacetate precursors and Zr acetylacetonate precursors. The influence of the amount of BaZrO₃ (BZO) on the structure and properties of YBa₂Cu₃O_7−x (YBCO) films was systematically investigated. The YBCO films having 7 mol % BZO have a maximum critical current density (J _c) value (77 K, 0 T) of 6.0 MA/cm². The enhanced J _c derives from a high density of BZO nanoparticles existing in the YBCO matrix. With increasing BZO amount, J _c of the YBCO films begins to decrease due to larger particles.     

Enhanced Flux Pinning Properties of BaZrO3-doped YBCO Films Grown by Pulsed Laser Deposition on LaAlO3(100) Substrates Decorated with Y2O3 Nanoislands

In this work, we intend to investigate the interaction between two types of nanoscaled artificial pinning centers and their pinning properties in YBCO thin films grown by pulsed laser deposition technique. The two types of artificial pinning centers were prepared in different processes, (1) Y₂O₃ nanoislands decorated on substrates prior to the deposition of YBCO thin film, and (2) BaZrO₃ nanoparticles self-assembled within YBCO matrix during the deposition of YBCO thin film. We compared the transport characteristics of the YBCO thin films containing these two types of artificial pinning centers with those of pure YBCO thin films grown on decorated substrates and BZO-doped YBCO thin films grown on undecorated substrates. It was found that these two types of artificial pinning centers, which are simultaneously present, acted constructively to enhance the pinning properties of YBCO thin films.     

Improvement of Pinning Force and Critical Current Density in Thick YBa 2 Cu 3 O 7−δ Films Grown on SrTiO 3 Substrates Decorated with LaNiO 3 Nanodots

We have investigated by DC magnetization measurements and frequency-dependent AC susceptibility the critical current density (J _c), pinning force (F _p) and pinning potential in thick (1.3–1.6 μm) YBa₂Cu₃O_7−δ (YBCO) films grown by Pulsed Laser Deposition on SrTiO₃ substrates decorated with LaNiO₃ nanodots deposited by a few (5–15) laser pulses, in comparison with those of a 1 μm thick YBCO reference sample. Experiments show that the highest improvement of superconducting properties was achieved for films grown on substrates decorated with 10 laser pulses on the LaNiO₃ target, which have, at 77.3 K, a J _c of 40–125% higher than in pure YBCO in fields between 1 and 2 T, and F _p increased by 40%. These results could be important for further improvement of current-carrying capability of coated conductors for in-field power applications.     

Low-temperature critical field of YBCO

We have measured the upper critical fieldH _C2(T) of YBa₂Cu₃O₇ (YBCO) thin films in magnetic fields up to 140 T forH applied parallel to thec-axis of the film. The critical field in the zero-temperature limit is 138 T, and the temperature dependence does not fit any simple model.     

Effects of Sm-doping on Structures and Properties of YBCO Coated Conductors Fabricated by TFA-MOD Process

The Sm-doping YBCO films, Y_1−x Sm _x Ba₂Cu₃O _y and Y₁Sm _x Ba_2−x Cu₃O _y (x=0.1, 0.3), were fabricated on LAO (00l) single-crystal substrates by TFA-MOD process in order to improve the superconducting properties of YBCO films. The XRD analyses indicated that these samples have a strong c-axis texture, but the secondary phase of BaCuO₂ are found from Y₁Sm _x Ba_2−x Cu₃O _y films with Sm³⁺ substitution on Ba²⁺ sites. High quality surface, better in-plane and out-of-plane texture of the Sm-doping films have been obtained compared with that of the undoped YBCO film. The Y_0.9Sm_0.1Ba₂Cu₃O _y films showed the highest value of T _c and J _c in self-field. Superconducting properties of all Sm-doping films are better than those of the pure film. These results strongly suggest that Sm-substitution on Y or Ba sites of YBCO phase is a powerful method to efficiently improve J _c and introduce artificial pinning centers in YBCO films.     

Combination of Ag Substrate Decoration with Introduction of?BaZrO3 Nano-Inclusions for?Enhancing Critical Current Density of?YBa2Cu3O7 Films

The combination of two methods: Ag substrate decoration and introduction of BZO nano-inclusions has been used in a pulsed laser deposition (PLD) method to increase the critical density (J _c ) of YBCO films. The films were deposited on single crystal SrTiO₃ (STO) substrates decorated with various architecture of Ag nano-dots. We have studied the diameter and density of Ag nano-dots and their influence on J _c of BZO-added YBCO films. We found that 15 laser pulses on the Ag target gives an optimum result in increasing J _c in comparison with BZO-doped YBCO films of the same thickness in self-field and low applied magnetic fields. A higher number of laser pulses on the Ag target led to increasing critical current density in high applied magnetic fields only (above 2 T). We have studied films of the thickness from 0.4 ??m to 3.8 ??m and found that the highest J _c at all applied fields investigated is achieved for a 1.2 ??m thick film. The transmission electron microscopy clearly shows BZO nano-rods that provide strong c-axis pinning centres in the films.     

High Vortex Depinning Temperatures in YBCO Films with BZO Nanorods

The Bose glass theory for the vortex matter in superconductors with correlated disorder predicts the depinning of vortices due to the renormalization of the vortex pinning barriers by thermal fluctuations. For YB₂Cu₃O₇ (YBCO) in external magnetic fields H oriented along the columnar pins generated by various techniques theoretical estimates give a depinning temperature T _dp very close to the critical temperature T _c (T _dp～0.95T _c), whereas the results of standard magnetization relaxation experiments are repeatedly interpreted in terms of a much lower T _dp (～0.5T _c). We investigated the temperature T variation of the normalized magnetization relaxation rate S for YBCO thin films containing BaZrO₃ (BZO) nanorods preferentially oriented along the c axis, with H along the nanorods. The nonmonotonous S(T) variation below the matching field observed up to close to T _c does not support a low T _dp. The often considered S(T) maximum occurring at relatively low T (which was connected to a disappointing T _dp) is related to the occurrence of thermomagnetic instabilities. We show that the accommodation of vortices to the columnar pins in the presence of the T dependent macroscopic currents induced in the sample is signaled by a pronounced S(T) deep located at high T, in agreement with a T _dp close to T _c. By increasing the film thickness and using the substrate decoration the BZO nanorods splay out, leading to the inhibition of (detrimental) vortex excitations involving double vortex kink or superkink formation, characteristic for high-quality thin films and single crystals with columnar pins along the c axis.     

Influence of Interfacial LSMO Nanoparticles/Layer on the Vortex Pinning Properties of YBCO Thin Film

YBa₂Cu₃O_7?δ (YBCO) thin films have been deposited on bare and La_0.67Sr_0.33MnO₃ (LSMO) modified single crystal SrTiO₃ (STO) substrates. The effect of randomly distributed ferromagnetic LSMO nanoparticles and a complete LSMO layer, present at STO/YBCO interface, on the superconducting properties of YBCO thin films has been investigated by temperature dependent magnetization studies. The YBCO thin film on LSMO nanoparticles decorated STO substrate shows significant improvement in the critical current density and pinning force density as compared to the YBCO thin film deposited on bare STO substrate and this improvement is more significant at higher applied magnetic field. However, the LSMO/YBCO bilayer showed the improved flux pinning properties only up to a magnetic field of 1.5 T above which it deteriorates. In the case of LSMO/YBCO bilayer, the underlying LSMO layer gives rise to magnetic inhomogeneities due to domain structure, which leads to improved flux pinning properties limited to lower field. However, in the case of LSMO nanoparticles decorated substrate, the presence of LSMO nanoparticles at YBCO/STO interface seems to introduce magnetic inhomogeneities as well as structural defects, which might be acting as correlated pinning sites leading to improved flux pinning properties of the YBCO thin film over a wide range of applied magnetic field.     

Microstructure and electrical properties of YBCO thin films

Microstructures of c-axis oriented YBCO thin films made by high-pressure d.c. sputtering on LaAlO₃ and MgO substrates were examined by TEM. The a-axis oriented grains, second phases and micro-twins were frequently observed in the film. The a-axis oriented grains expanded along their c-axis directions during film growth. The a- and b-axis misorientations were observed in the film on MgO due to serious lattice mis-match between YBCO and MgO. The second phases were often accompanied with a-axis oriented grains suggesting they act as nuclei. These observed results were correlated with the measured T _c and J _c of the films.     

Growth and Characterization of YBCO Thin Films by Sequential Deposition and Annealing

A novel thin film growth procedure, sequential deposition and annealing (SDA), which contains the advantages of both in situ and ex situ procedures, was proposed. Y₁Ba₂Cu₃O_{7 – x} (YBCO) high temperature superconducting thin films were grown and characterized by the SDA procedure. Purely c-axis-oriented YBCO thin films with no foreign phases and other oriented grains were successfully prepared. The superconducting transition properties of SDA-grown YBCO thin films were measured by measurement of inductance and resistance. The inductance measurements gave a T _c ^onset of 85 K and a T _c of 5 K. The resistance measurements gave a T _c ^onset of 90 K and a T _c of 5 K. Atomic force microscopy studies showed that SDA-grown YBCO thin films had micrometer-size grains surrounded by many nanometer-size grains. The nanometer-size grains in SDA-grown YBCO thin films are responsible for degradation of superconducting transition properties.     

Flux dynamics of YBCO/Ag composite

DC magnetization measurements have been carried out on bulk YBCO/Ag composites with silver content up to 20wt per cent. DC fields in the range 0·5 mT to 200 mT have been used to investigate the inter- and intragranular properties at 77K. The AC susceptibility as a function of temperature at different AC fields (0·026–0·30 mT) has also been studied. Under small DC fields (≈ 4 mT), depending on the Ag content andH _max, the M-H loop shows a complicated behaviour. This behaviour can be explained on the basis of effect of strong field dependence of transport critical current, grain size and intragrain critical current densityJ _cgm on low-field M-H loop. The estimation of intergranular critical current densityJ _cjm from these loops does not remain a simple function of ΔM/d. The AC susceptibility measurements show a small increase inJ _c(T) with silver content under low AC fields only, consistent with the transportJ _c data; beyond thatJ _c(T) decreases. This improvement inJ _c(T) and transportJ _c with silver can be ascribed to the improved coupling between grains but not to the pinning. Also at higher field (H _max>20 mT) the addition of Ag decreases the intragrain critical current density. The upper critical field of intergranular regionH _c2j and lower critical field of intragrain regionH _c1g also decrease with silver content.     

The Vortex Path Model Analysis of the Field Angle Dependence of the Critical Current Density in Nanocomposite YBa2Cu3 O 7−x – BaZrO3 Films Obtained by Low Fluorine Chemical Solution Deposition

In the present paper, we analyze the role of in situ grown BaZrO₃ (BZO) inclusions in YBa₂Cu₃O_7?x (YBCO) thin films prepared by chemical solution deposition using a low fluorine coating solution, on the field angle dependence of the critical current density, J _c (??), data using the vortex path model. In order to form a coherent picture on the BZO doping influence on the pinning properties of the YBCO matrix, detailed structural analyses performed by X-ray diffraction techniques and microstructural evaluation by transmission electron microscopy are also presented. The evaluation of different contributions to the overall, J _c , permitted us to prove the effectiveness of the BZO inclusions acting as isotropic pinning centers, reflected in a uniform component of high relative value with respect to other components. For the studied 10 mol % BZO doping concentration, a threefold increase in the critical current density, J _c , of the YBCO host is measured, in self-field at 77 K, corresponding to a value of J _c =2.9MA/cm², whereas a factor 10 is measured at 1 T (J _c =0.35 MA/cm²).     

Effect of post-annealing on thin YBCO films deposited from a nanocrystalline target

Pulsed laser deposition is used to ablate thin superconducting YBCO films on SrTiO ₃ substrates. The most important parameters of thin superconducting films are high critical current density, ability to stand magnetic fields and smoothness of surfaces. Smoothness is important in fabrication of layered structures and for research of basic properties of thin superconducting structures. The target sintered from YBCO nanopowder is a promising material for making films which meet most of the requirements above. Investigations by AFM show that our target has grains about one order of magnitude smaller than usual grain size of commercial targets. At optimal deposition parameters, the oxygen pressure of 0.4 torr in the chamber and the substrate temperature 725°C, films with T _c = 90 K, J _c =8 × 10⁶ A/cm ² (77 K) and RMS surface roughness = 1.5 nm are obtained. Thermal annealing of the deposited films for 18 h at 900°C further increases the value of J _c .     

Influence of Heat Treatment on Structure and Superconducting Properties of YBCO Film by Chemical Solution Deposition Method

YBCO films have been fabricated on a (00l) LaAlO₃ single-crystal substrate via self-developed fluorine-free polymer-assisted metal organic deposition (PA-MOD) method. The influence of heat treatment on texture, microstructure and superconducting properties of YBCO films has been investigated. After a pyrolysis process ranging from 145 °C to 500 °C with different heating rates, the samples were fired at 760–780 °C in Ar and O₂ mixture gas followed by annealing at 450 °C in pure O₂. The results indicate the film fired at 770 °C after decomposition at the rate of 0.5 °C/min showed the highest T _c of 90.4 K and J _c (77 K, 0 T) over 2 MA/cm². According to the XRD patterns, phi-scan and omega-scan curves as well as SEM images, the good properties may be attributed to better biaxial texture and purer YBCO phase as well as better grain connectivity.     

A Comparative Study of YBCO Thin Film Growth by Inverted Cylindrical Magnetron Sputtering and Inverted Cylindrical Sputtering

A comparative study of Y₁Ba₂Cu₃O_7–x (YBCOrpar; thin film growth by inverted cylindrical magnetron sputtering and non-magnetron inverted cylindrical sputtering was done. At the total pressure of 40 Pa epitaxial YBCO thin films with good superconducting transition properties and smooth surface were prepared by the magnetron source. Inductive measurements gave = 89.5 K and T _C = 0.5 K. The full width at half maximum (FWHM) value of the rocking curve was 0.18°. The average surface smoothness of the film was 250 Å. With the non-magnetron source at the same total pressure, the superconducting transition properties of the YBCO thin films were not good because of intense bombardment by energetic ions. However, by raising the total pressure to 60 Pa, epitaxial YBCO thin films with good superconducting transition properties and smooth surface could be prepared by non-magnetron inverted cylindrical sputtering— = 90.5 K, T _C = 0.5 K. The FWHM value of the rocking curve was 0.23° and the average surface smoothness of the film was 900 Å.     

Progress in Nanoengineered Microstructures for Tunable High‐Current,High‐Temperature Superconducting Wires

High critical current densities (J_c) in thick films of the Y₁Ba₂Cu₃O_7–δ (YBCO, T_c ≈ 92 K) superconductor directly depend upon the types of nanoscale defects and their densities within the films. A major challenge for developing a viable wire technology is to introduce nanoscale defect structures into the YBCO grains of the thick film suitable for flux pinning and the tailoring of the superconducting properties to specific, application‐dependent, temperature and magnetic field conditions. Concurrently, the YBCO film needs to be integrated into a macroscopically defect‐free conductor in which the grain‐to‐grain connectivity maintains levels of inter‐grain J_c that are comparable to the intra‐grain J_c. That is, high critical current (I_c) YBCO coated conductors must contain engineered inhomogeneities on the nanoscale, while being homogeneous on the macroscale. An analysis is presented of the advances in high‐performance YBCO coated‐conductors using chemical solution deposition (CSD) based on metal trifluoroacetates and the subsequent processing to nano‐engineer the microstructure for tuneable superconducting wires. Multi‐scale structural, chemical, and electrical investigations of the CSD film processes, thick film development, key microstructural features, and wire properties are presented. Prospects for further development of much higher I_c wires for large‐scale, commercial application are discussed within the context of these recent advances.     

Nanostructural characterization of YBCO films on metal tape with textured buffer layer fabricated by pulsed-laser deposition

Thick YBa₂Cu₃O_7-x (YBCO) films with high critical current density (J_c) values were deposited by pulsed-laser deposition (PLD) on Hastelloy with a textured CeO₂/Gd₂Zr₂O₇ buffer layer. Both cross-sectional and plan-view TEM specimens of the YBCO films were prepared, and then the nanostructural characterization of the films was performed by transmission electron microscopy (TEM). The YBCO films less than 1 μm thick were predominantly composed of c-axis-oriented grains, however, many a-axis-oriented grains, which grew larger with the increase of the thickness of the YBCO film, were formed beyond about 1 μm from the CeO₂ interface. We found Y₂O₃ and copper oxides between a- and c-axes-oriented grains. In particular, Y₂O₃ grains were formed between the {001} plane of an a-axis-oriented grain and the {100} or {010} plane of a c-axis-oriented grain. The orientation relationships between Y₂O₃ and YBCO are found to be; (001)YBCO//(001)Y₂O₃ and (100)YBCO//(110)Y₂O₃. In addition, we also found gaps between YBCO grains. Since a-axis-oriented grain growth and the formation of Y₂O₃, copper oxides and the gaps are considered to reduce the J_c values of the YBCO film, it is important to determine the optimum process conditions to suppress the nucleation of a-axis-oriented grains, impurity oxides and gaps.     

Superconducting behavior of high-Tc YBa2Cu3O7 thin films with BaO impurity produced by pulsed laser deposition

We have investigated the superconducting behavior of high-T _c YBa₂Cu₃O₇ (YBCO) thin films containing BaO impure phase produced by pulsed laser deposition. The thin films were characterized by the standard four-probe method, X-ray diffraction (XRD), and scanning electron microscopy (SEM). XRD showed that all these thin films contained BaO impurity, with thec-axis normal to the surface of the substrates. The presence of impurity existed from substrate temperatureT _s of 727 to 796°C. When these thin films with BaO impurity were measured under the magnetic fields, it was found that the critical current densityJ _c increased slightly with increase in magnetic fieldB within the range ofB500 G, in the case ofB perpendicular to thec-axis of the film.     

Influence of CeO2-Cap Layer on the Texture and Critical Current Density of YBCO Film

YBa₂Cu₃O₇ (YBCO) thin films have been fabricated on different textured CeO₂-cap layers by pulsed laser deposition (PLD). The texture and critical current density J _c of YBCO thin films have been systematically investigated. Both in-plane and out-of-plane textures of YBCO films and CeO₂-cap films were characterized by X-ray diffraction (XRD). And the critical currents of YBCO films were measured by the conventional four-probe method. It was found that the texture and J _c of YBCO films were largely dependent on the texture of CeO₂-cap layers under the optimized deposition conditions. With increasing the degree of in-plane and out-of-plane texture of CeO₂-cap layers, J _c of YBCO films decreased from 4.23×10⁶ A/cm² to 0.47×10⁶ A/cm². The FWHM values of the omega scan rocking curves of YBCO films decreased from 3.71 to 1.84° and the phi scan rocking curves from 6.68 to 4.91° with improvement of CeO₂-cap layer texture. Our results indicated that the fabrication of high texture quality of CeO₂-cap layer was necessary for the epitaxial growth of high-J _c YBCO films. The high-quality YBCO films which are comparable with those grown on single crystal substrates could be achieved on high textured CeO₂-cap layers buffered metal substrates.