YBa2Cu3O7-x Films Research Articles

Nano‐engineered pinning centres in YBCO superconducting films

For practical applications of superconducting materials in applied magnetic fields, artificial pinning centres in addition to natural ones are required to oppose the Lorentz force. These pinning centres are actually various types of defects in the superconductor matrix. The pinning centres can be categorised on their dimension (volume, surface or point) and on their character (normal cores or Δκ cores). Different samples have been produced by Pulsed Laser Deposition, with various thicknesses, temperatures and nanostructured additions to the superconducting matrix. They have been characterized by SQUID Magnetic Properties Measurement System and Physical Properties Measurement System, as well as by Transmission Electron Microscopy (TEM). Correlations between pinning architecture, TEM images, and critical currents at various fields and field orientations will be shown for a large number of YBa2Cu3Ox films with various types and architectures of artificial pinning centres.

Manipulation of YBCO film properties by the introduction of perovskite BaTiO3 nanodots as substrate decorations

Perovskite BaTiO3 (BTO) nanodots were deposited on LaAlO3 (LAO) substrates using the BTO precursor solutions with various low concentrations. Subsequently, YBa2Cu3O7-x (YBCO) superconducting films were deposited on the BTO-dots decorated LAO substrates through a low-fluorine solution method. High BTO dots (with heights of 30–50nm) and short BTO nanodots (with heights less than 30nm) were produced using a solution with a concentration of 0.025mol/L, which could not be produced using solutions of other concentrations. We found a significantly enhanced flux pining force in the YBCO film deposited on the BTO-dots decorated LAO substrate. Because of the joint pinning effect of the high BTO dots and short BTO dots, the high-performance YBCO film with a critical current density of 8.42MA/cm2 and a flux pinning force of 9.58GN/m3 was produced on the LAO substrate.

Enhancement by La-Al-O Doping in Y-Ba-Cu-O Films Both in Self-Field and Under Magnetic Field

To enhance the J c of YBa 2 Cu 3 O 7-X (YBCO) films both in self-field and under magnetic field, an effective strategy is to introduce artificial pinning centers and keep a good YBCO matrix at the same time. Here, we propose a new dopant: LaAlO 3 (LAO), based on its chemical stability and small mismatch toward YBCO. A series of YBCO films with different LAO doping contents was fabricated on LAO single-crystal substrates by metal organic deposition. We observed by X-ray diffractometer measurements and scanning electron microscopy observations that although a large amount of LAO is added, YBCO still keeps a good epitaxial growth relationship with LAO. Compared with a pure YBCO film, the J c value of a 5.0% LAO-doped sample is enhanced more than three times in self-field 77 K and seven times at 77 K and 1.5 T, respectively. These results indicate that LAO doping can effectively enhance the J c of YBCO films both in self-field and in applied magnetic fields.

Analysis of Transport Properties of MOD YBCO Films With BaZrO3as Artificial Vortex Pinning Centers

The transport properties of nanocomposite YBa 2 Cu 3 O 7-x (YBCO) films grown by metal-organic decomposition (MOD) and prepared with a low-fluorine-coating solution on SrTiO 3 single-crystal substrates were studied with dc and microwave measurement techniques. As demonstrated in previous studies, the combination of these two techniques is a powerful tool for vortex pinning investigation. The BaZrO 3 inclusions (BZO) as artificial pinning centers were introduced into the YBCO matrix by the excess of corresponding Ba and Zr precursor salts to the initial YBCO precursor solution up to 5 mol%. The effect of BZO was clearly observed in magnetic field dependence relations of critical current density (J c ) and the complex surface impedance, i.e., ΔZ, at fixed temperature and magnetic field parallel to the c-axis. The result shows that BZO particles mainly act as isotropic pinning centers. The short-range vortex dynamics was probed by the ~47.7-GHz microwave surface impedance technique. The derived vortex parameters r and k p were sensitive to the presence of nanoparticles, indicating the effect of BZO on pinning. The joint field dependence relations of J c and k p help in elucidating the pinning regime in our nanocomposite MOD YBCO/BZO films.

Development of Biaxially Textured LZO Film on Cold-Rolled Metal Substrate

La2Zr2O7 (LZO) film grown by the chemical solution deposition (CSD) technique is a widely studied buffer layer for YBa2Cu3O7-x film growth on a rolling-assisted biaxially textured (RABiT) substrate. A biaxially textured LZO film is epitaxially grown on a cube-textured metal substrate. In a RABiT substrate, cube texture is obtained as primary recrystallization of heavily cold-rolled fcc metals and is normally developed before CSD film deposition to obtain an oriented film growth. In this paper, we show that a biaxially textured LZO film can be grown by depositing the precursor solution directly on a cold-rolled fcc metal substrate and performing a single heat treatment. In fact, we show that if the recrystallization temperature of the metallic substrate is lower than the nucleation temperature of the film, a biaxially textured film is obtained as a result of transient cube texture transfer. Biaxially textured LZO films grown on cold-rolled substrates are compared with films grown on recrystallized substrates. In particular, the growth of LZO on either pure-Cu- or Ni-based substrate was studied. Compared with the LZO film grown on a cube-textured substrate, LZO grown on a cold-rolled substrate shows similar structural properties and morphology. Moreover, films are well adherent without cracks or delamination, suggesting that grain boundary migration in the metallic substrate has no detrimental influence on the film. In some cases, secondary recrystallization of the substrate occurs, without affecting the quality of the LZO film.

Investigation of the resistive properties of HTS/manganite bilayers

Temperature dependences of the resistivity of manganite La0.7Ca0.3MnO3 (LCMO) films deposited on LaAlO3 and SrTiO3 substrates by RF magnetron sputtering were shown to be successfully simulated in the whole temperature range (covering metal, insulator and metal-insulator transition regions) using a phenomenological phase-coexistence transport model. Quantitative data on the internal parameters of these films were obtained. The possibility was also considered for investigation of individual resistive characteristics and excess conductivity of a high temperature superconducting (HTS) YBa2Cu3O7-x (YBCO) thin film in the vicinity of TC included into a YBCO/LCMO bilayer structure. It was shown that the considered YBCO film in the temperature range from 85.5 K to 114.9 K behaves as a two-dimensional system with respect to the fluctuations in the superconducting order parameter, while a three-dimensional regime is observed in a narrow range of temperatures at T < 85.5 K. Such behavior was assumed to be partly due to the FM LCMO component of the bilayer, the spin-polarized charge carriers of which enter into the YBCO film and cause a "breaking" of superconducting pairs in the superconducting gap and pseudogap regimes.

Transport properties of YBa2Cu3Ox /La0.67Sr0.33MnO3 nanostrips and YBa2Cu3Ox/La0.67Sr0.33MnO3/YBa2Cu3Ox nanojunctions

A metallic ferromagnet (F) in proximity with a superconductor (S) can transport supercurrent on a long distance through conversion of opposite-spin singlet Cooper pairs (CP) into equal-spin triplet CP (long range triplet component, LRTC), which are not broken by the exchange energy of F. The optimal conditions for the conversion are yet to be clarified; however, it is accepted that the key point to this process include high interface transparency and magnetic inhomogeneity at the SF interface. The aim of our paper is to study SF nanostrips (length of about 1500 nm and width down to 300 nm) and lateral SFS nanojunctions based on high critical temperature YBa2Cu3Ox (YBCO) and half-metallic La0.67Sr0.33MnO3 (LSMO) thin films. We applied a focused Ga+ ion beam (FIB) for patterning the SF nanostrips, as well as lateral SFS nanojunctions, by creating a slot in the nanostrip after removing the YBCO film in the slot along a length of about 200 nm. The temperature dependences of the samples resistance R(T) show critical temperature TCn ≈ 89 K of the SF nanostrips; however, the SFS nanojunctions at T < TCn show a residual resistance R < 100 Ω corresponding to a dirty LSMO (ρ≈ 10 mΩ cm) in the slot. The LRTC was not observed in our lateral SFS nanojunctions until now.

Enhanced superconducting properties in MOD-YBCO thick films with CeO2 interlayer

In YBa2Cu3O7-x (YBCO) film there exists thickness effect: the critical current density of YBCO film drops precipitously as the coating thickness increases, especially in the case that the thickness of YBCO film exceeds 1 m. In this paper, we introduce very thin layers of CeO2 into YBCO layers and successfully fabricate the structure of YBCO/YBCO/CeO2/YBCO superconducting thick film. Firstly, YBCO films with two layers are fabricated on a LaAlO3 substrate by a multiple coatings process using a trifluoroacetate metal organic deposition method. Secondly, CeO2 thin films are deposited on YBCO films by RF-sputtering. Finally, we prepare the third YBCO film on CeO2interlayer. No cracks are observed in scanning electron microscopy images of these films; further, the majority of the grains in the films are well-textured and c-axis oriented. The full-width-half-maximum of the out-of-plane texture is measured to be 1.395 for the multilayer YBCO film at a thickness of 2 m Using this multilayer technology, we achieve Jc values of up to 1.36 MA/cm2 (77 K, self-field) in films as thick as 2 m, for an extrapolated critical current of 272 A/cm. We attribute the enhanced performance of the thick YBCO film to the CeO2 interlayer which playsan important role in transmission texture and stress relaxation.

YBa2Cu3Ox-SrTiO3 multilayer approach on textured Ni-W tapes: Morphology, structure and critical current density improvement

Multilayer epitaxial structure YBa2Cu3Ox–interlayer–YBa2Cu3Ox is a promising architecture for YBa2Cu3Ox (YBCO) superconductors. We formed single YBCO layers and multilayer epitaxial structures YBCO–interlayer–YBCO by pulsed laser deposition method on the cube textured Ni-W tapes. The use of SrTiO3 interlayers enabled to stop YBCO film morphology and microstructure degradation and maintain high critical current density (Jc). Jc in 1μm thick multilayer film composed of four 250nm thick YBCO layers is comparable to Jc in single-layer 250nm thick YBCO film. Application of this method allows overcoming such fundamental problem as degradation of Jc in thick films and multiplying critical current with number of YBCO layers.

Effect of the firing process on the critical current density of YBa2Cu3O7-x films derived from the sol-gel method

The firing process plays a crucial role on the microstructures of sol-derived YBa2Cu3O7-x (YBCO) films; thus, it affects the critical current density, Jc, of the films. In this paper, the effect of the oxygen partial pressure–temperature (PO2–T) combination during the firing process on the microstructure and Jc of an YBCO film derived from a low-fluorine sol-gel method was investigated. The results indicate that the optimal PO2–T combination for a high-Jc YBCO film is different from that derived from an in-situ deposition reported previously. When the optimal PO2–T combination is adopted, the resulting YBCO films exhibit good biaxial texture characteristics and dense surfaces over the entire firing temperature range of 700–775 °C. In addition, the YBCO films exhibit a Jc of above 2 MA/cm2, even though they are fired at 700 °C. Moreover, Jc is near 10 MA/cm2 for the YBCO film fired at 775 °C.

High- &lt;inline-formula&gt; &lt;tex-math notation="TeX"&gt;$T_{\rm c}$&lt;/tex-math&gt;&lt;/inline-formula&gt; SQUID vs. Low- &lt;inline-formula&gt; &lt;tex-math notation="TeX"&gt;$T_{\rm c}$&lt;/tex-math&gt;&lt;/inline-formula&gt; SQUID-Based Recordings on a Head Phantom: Benchmarking for Magnetoencephalography

We explore the potential that high critical-temperature (high-Tc) superconducting quantum interference device (SQUID) technology has for magnetic recordings of brain activity, i.e., magnetoencephalography (MEG). To this end, we performed a series of benchmarking experiments to directly compare recordings with a commercial (low-Tc SQUID-based) 306-channel MEG system (Elekta Neuromag TRIUX, courtesy of NatMEG) and a single channel high-Tc SQUID system. The source on which we recorded is a head phantom including 32 artificial current dipoles housed inside a half-spherical shell (courtesy Elekta Oy) for calibrating MEG systems. The high-Tc SQUID magnetometer consisted of a single layer YBa2Cu3O7-x (YBCO) film on a 10 mm × 10 mm bicrystal substrate with a magnetic field sensitivity of ~40 fT/Hz down to 10 Hz. We recorded serial activations of eight tangential current dipoles located at different depths from the surface of the head phantom. Results indicate that our individual high-Tc SQUID demonstrated signal-to-noise ratios (SNRs) about 7-14 times lower than that of similarly-positioned low-Tc SQUIDs in a commercial MEG system. Only considering single-channel SNR, high-Tc SQUIDs with resolution better than 18 fT/Hz would be required to outperform the low-Tc system for shallow dipole sources. This work demonstrates a proof of principle study for future multichannel high-Tc MEG system development.

Microwave Losses in YBCO Coplanar Waveguide Resonators at Low Power and Millikelvin Range

We have investigated the temperature dependence of microwave losses in the low-power limit and millikelvin temperature range of a thin-film YBa2Cu3O7-x (YBCO) λ/2 coplanar waveguide resonator patterned on a (La0.3Sr0.7) (Al0.65Ta0.35)O3 (LSAT) substrate covered by a CeO2 seed layer. The unloaded quality factor is mainly governed by the surface resistance of the YBCO film and the dielectric losses caused by resonant absorption due to a bath of two-level fluctuators in the dielectrics. The value of the unloaded quality factor at 20 mK and low power, i.e., Q0∼2000, allows for the realization of YBCO microwave quantum circuits implementing biepitaxial grain-boundary Josephson junctions.

High pinning performance of YBa2Cu3O7−x films added with Y2O3 nanoparticulate defects

We report the epitaxial growth and superconducting properties of Y2O3-added YBa2Cu3Ox (YBCO) films grown on SrTiO3-buffered MgO substrates by pulsed-laser deposition using surface-modified YBCO targets. Areas of Y2O3 sectors on the YBCO target were increased to 5.44% and 9.22% of the total YBCO pellet in order to find a correlation between the Y2O3 content, morphology, and the pinning properties of YBCO + Y2O3 mixed films. The maximum global pinning forces, FP, at 77 K were 14.3 GN m−3 and 1.15 GN m−3 for the Y2O3 5.44A% and 9.22A%, respectively. The 5.44A% Y2O3-added sample presents a very high value of pinning force at 77 K, approaching the value obtained in YBCO films with added BaZrO3 nanorods, but with less depression in the superconducting critical temperature, Tc. In accordance with scanning transmission electron microscopy (STEM) observations, both films present nanoparticulate Y2O3 dispersed in a YBCO matrix where Y2Ba4Cu8O16 (Y248) intergrowths were also observed. Consistent with the strong pinning theory, the size and distribution of randomly dispersed Y2O3 particles are optimal for the flux pinning of a 5.44A% Y2O3-YBCO film, while in the case of a 9.22A% film, the YBCO matrix is degraded by jam-packed Y248 intergrowth, which leads to a comparatively poor pinning performance. We further used the single-vortex dynamics model to account for vortex pinning in the samples. The 5.44A% Y2O3-YBCO film result shows good agreement with the model fit up to 4 T of the applied magnetic field.

Effects of Cu/Ba Ratio in Precursor on MOCVD-Deposited YBa&lt;sub&gt;2&lt;/sub&gt;Cu&lt;sub&gt;3&lt;/sub&gt;O&lt;sub&gt;7-x&lt;/sub&gt; Films on YYC Buffered Ni-W Alloy Tape

To study the effects of Cu/Ba ratio of precursor on YBa2Cu3O7-x (YBCO) film, we have employed the technique of metal-organic chemical vapor deposition to prepare 500 nm thick YBCO films on CeO2/YSZ/Y2O3 (YYC) buffered Ni-W alloy tapes at series of Cu/Ba ratios of precursor. The analysis obtained from X-ray diffraction and scanning electron microscope revealed that the YBCO films crystallized better and became more continuous and denser as Cu/Ba ratio increased from 0.81 to 1.00, yielding that the critical current density (Jc) of YBCO films at 77K and 0T rose from 1.0 MA/cm2 to 1.4 MA/cm2. Moreover, the energy dispersive spectroscopy indicated that the increase in Cu/Ba ratio of precursor made the Cu/Ba ratio of the YBCO film matrix closer to the theoretical value of 1.5. However, for the Cu/Ba ratio of precursor in the range of 1.00~1.21, the crystallization and texture deteriorated severely and many unexpected precipitates of Ba-Cu-O and Cu-O arose, resulting in the dramatic drop of Jc from 1.4 MA/cm2 to 0.1 MA/cm2.

Preparation of YBCO Thin Films by Metal Organic Deposition Method

Due to its high in-field superconducting performance, high irreversible magnetic field, low AC loss and high transition temperature, YBa2Cu3O7-x(YBCO) has been taken as a hot research object. Well-oriented La0.4Sr0.6TiO3(LSTO) thin films were prepared on the biaxially textured Ni-W alloy substrates by metal-organic deposition (MOD) method which could used for YBCO coated conductor. The homogeneous and stable precursor solution was prepared by Y, Ba and Cu acetates and deposited on LSTO/Ni-W composite structure by spin-coating method. YBCO films were obtained after heat treatment. Effect of thermal decomposition time on YBCO films was discussed,and the results showed that the proper time of organic decomposition was 3h at 400°C.

Irreversibility fields and critical current densities in strongly pinned YBa2Cu3O7-x films with BaSnO3 nanorods: The influence of segmented BaSnO3 nanorods

We report irreversibility fields Birr and critical current densities Jc in YBa2Cu3O7-x thin films containing segmented BaSnO3 nanorods. Birr and Jc are systematically changed by selection of nanorod length and its spacing, and the variations are explained by the “harmonic oscillator” approach based on the Bose glass state. Analysis of pinning energies and Jc indicates that segmented nanorods behave as single columnar pin for B || c, in contrast, as nanoparticle pins for B || ab. The present results may help to further investigate anisotropy tuning of pinning structures.

Phase Evolution of YBa2Cu3O7-x films by all-chemical solution deposition route for coated conductors

In order to understand the all-chemical-solution-deposition (CSD) processes for manufacturing coated conductors, we investigated the phase evolution of YBa2Cu3O7 (YBCO) films deposited by a low-fluorine metal-organic solution deposition (LF-MOD) method on CSD derived Ce0.9La0.1O2/Gd2Zr2O7/NiW. It is shown that the phase transition from the pyrolyzed film to fully converted YBCO film in the LF-MOD process is similar to that in typical trifluoroacetates-metal organic deposition (TFA-MOD) processes even though the amount of TFA in the solution is reduced by almost one half compared with typical TFA-MOD cases. Moreover, we found that the formation of impurities (mainly BaCeO3, NiWO4 and NiO) is strongly related to the annealing temperature, i.e., the diffusion controlled reactions become intensive from 760 oC, which might be connected with the poor structural and superconducting properties of the films deposited at high sintering temperatures. Based on these results, the optimized growth conditions of YBCO films were established, and a high critical current density (Jc) of about 2 MA/cm2 (77 K, self field) is achieved in a 200 nm thick YBCO film in the architecture made by our all CSD route.

Pulse magnetization and voltage-current characteristic of YBa2Cu3O7-x film

The measurements of YBa2Cu3O7-X films magnetization are made at ramp rate of the external magnetic field up to 105 T/sec at temperature of liquid nitrogen in fields up to 4 T. The linear dependence of the magnetic moment of films on rate of an external magnetic confirms, that in the large electrical fields down to 1 V/cm the voltage-current characteristic can be described by the flux flow resistance Rflow and the critical current Jflow, similar to traditional low temperature superconductors: E = Rflow·(J − Jflow)·Θ(J − Jflow), where θ is the Heaviside function. The experimental data of pulse magnetization of films of thickness 0,2 microns on substrates Al2O3 of size 10 × 10 mm with critical currents higher 106 A/cm2 have allowed to determine field dependence of the critical current and the flux flow resistance. In particular the resistance Rflow is increased approximately by 3 times for increase of an external field from 0,5 up to 4 T, and the Jflow values approach the critical currents determined from quasi-stationary magnetization.

Nonuniform magnetic stresses in high temperature superconducting thin films

We provide an experimental measurement of the magnetic stresses in high temperature superconducting thin films based on the Coherent Gradient Sensor which has many merits such as real time, non-destructive, full field, vibration insensitivity, etc. As an example, the radial stress, hoop stress of the YBa2Cu3O7-x (YBCO) film, and shear stress between the film and the (00l) SrTiO3 substrate subjected to various magnetic fields at 40 K are obtained. For the interfacial shear stress in radial direction between the film and substrate, it is found that the magnitude order of which is at GPa at somewhere under a small magnetic field.

The Effect of Polyethyleneimine (PEI) on the Texture and Morphology of YBCO Films

The effect of polyethyleneimine (PEI) in precursor solution on the morphology and superconducting properties of YBa2Cu3O7-x (YBCO) films grown on LaAlO3 (LAO) substrates by metal-organic deposition (MOD) method was investigated. The morphologies of the YBCO films were different based on the different amount of PEI addition from 0.25 g to 0.9 g in 10 mL precursor solution. Furthermore, research results indicate that the PEI helps to improve the critical current density (Jc), the uniformity of surface morphology and c-axis orientation of thin film. The highest critical current density (Jc) of 2.17 MA/cm2 at 77 K is obtained in YBCO films fabricated on LAO substrates when the amount of PEI is 0.5 g in 10 mL precursor solution. YBCO layers with about 400 nm in thickness can be processed without cracks and holes during the whole heat treatment, which takes only 7.5 h.