S Kosaka

Flux pinning centres correlated along the c-axis in PLD-YBCO films

Flux pinning centres having correlation along the c-axis in epitaxial YBCO films were investigated by measuring the magnetic-field angle ? dependence of the critical current density JC and by observing the microstructure. First, c-axis oriented YBCO films were prepared on four different substrates at three different target-to-substrate distances D by using a pulsed-laser-deposition method. Films deposited on LaAlO3 (100) and CeO2/Al2O3 with large D (112 or 142 mm) showed narrow peaks in the JC versus ? curves when the magnetic field was applied parallel to the c-axis (B//c). In these films, a high density (up to 5.5 ?m?1) of planar defects, most probably stacking faults parallel to the c-axis, were evident. The ratio of JC (B//c) to JC (B//a?b) significantly increased as the density of the planar defects increased. In contrast, films deposited on SrTiO3 (100), LaAlO3 (100) and NdGaO3 (110) with small D (50?60 mm) showed high, broad JC peaks around B//c, and had a high density (up to 430 ?m?2) of nanometre-sized precipitates that were elongated along the c-axis. The ratio of JC (B//c) to JC (B//a?b) increased as the density of these small elongated precipitates increased. In conclusion, planar defects such as stacking faults parallel to the c-axis, and the small elongated precipitates are effective as c-axis correlated flux pinning centres.

Flux pinning properties of c-axis correlated pinning centres in PLD-YBCO films

Flux pinning properties of pinning centres having correlation along the c-axis in epitaxial YBCO films were investigated by measuring the magnetic-field angle ψ-dependence of the critical current density JC and the E–J-characteristics. YBCO films were prepared by using the pulsed-laser-deposition method on four different substrates at three different target-to-substrate distances D. The ψ-dependence of JC showed large peaks when magnetic field B was applied parallel to the c-axis (), and we observed two types of JC-peak: that is, a broad peak for the films deposited at small D (50–60 mm), and a narrow peak for the films deposited at large D (112 and 142 mm). The E–J-characteristics followed the power law, , and the ψ-dependence of the n-value also showed broad peaks around for the films deposited at D = 50–60 mm, and narrow peaks for the film deposited at D = 112 mm. Based on these results and our previous microstructural observations by AFM and TEM, we confirm that the broad-angle flux pinning effect around may be attributed to a high density of elongated precipitates, and the narrow-angle pinning effect around may be attributed to dense planar defects parallel to the c-axis.

Influence of MgO substrate annealing on the microwave properties of laser ablated YBa2Cu3Oz thin films

We have systematically investigated the dependence of the crystal structure and the microwave response of laser ablated YBa2Cu3Oz (YBCO) thin films on MgO substrate microstructure formed upon thermal annealing. A dramatic improvement in both crystallinity as well as microwave surface resistance Rs (of almost an order of magnitude) is observed for the films grown on substrates annealed for the initial 2 h at 1000 ◦ Ci n fl owing O 2 ambient compared to films deposited on non-annealed substrates. However, for further annealing of the MgO substrate we found no further pronounced improvement in either Rs or the crystalline quality of YBCO. The observed improvement in Rs for the film grown on an annealed substrate can be explained by the graphoepitaxial growth of YBCO on MgO that results in very good crystallinity of YBCO film. The ratio of 0 ◦ and 45 ◦ grains, D(45 ◦ ) and the degree of in-plane alignment, �φ , have a non-monotonic relation with the substrate annealing time. At the same time, a strong correlation is observed between Rs and �φ ,a ndD(45 ◦ ), which themselves are dependent

High corrosion stability of DyBa2Cu3Oz thin films relative to YBa2Cu3Oz films: A possible relation between corrosion and c-axis correlated extended defects

An enhanced stability towards corrosion was noted for DyBa2Cu3Oz relative to YBa2Cu3Oz. Between pure and Ca doped films, Ca doped films were found to be more corrosion resistant towards aqueous environment. The observed high corrosion stability in DyBa2Cu3Oz can be explained by the improved microstructure and the absence of c-axis correlated extended defects (EDs), while the remarkable increase in corrosion stability for the Ca doped films is presumed to be due to the notable improvement in the grain boundary region and the absence of ED. It is confirmed that the films with c-axis correlated ED are highly prone to corrosion by moisture.

Microwave surface resistance of under doped Co substituted YBCO films

Abstract Laser ablated under doped YBa 2 Cu 3 O z (YBCO) films grown on LaAlO 3 substrates have been studied for its crystal structure and microwave surface resistance ( R s ). The carrier content of YBCO film was varied by site selective substitution for Cu at chain site by Co atom. It is inferred that the low laser repetition rate facilitates the alignment of grains along c -axis, while deteriorates the in-plane alignment of grains in YBa 2 Cu 2.97 Co 0.03 O z (YBCCO) films. The deterioration in in-plane alignment with laser repetition rate is attributed to the changes in Co content or the uneven distribution of Co in YBCCO films. The temperature dependent R s of YBCO film is increased with Co substitution. The observed high R s value combined with broad transition in R s ( T ) curves for the Co substituted samples is thought to be primarily due to the increase in penetration depth caused by the reduced carrier (hole) concentration in the Co substituted samples.

Reduction of microwave surface resistance of YBa2Cu3Oy films by overdoping

Abstract We successfully reduced the microwave surface resistance R s of YBa 2 Cu 3 O y (YBCO) deposited on MgO substrates by covering the films with an overdoped Y 1− x Ca x Ba 2 Cu 3 O y (Ca-doped YBCO) layer. The YBCO films deposited by laser ablation on MgO substrates had in-plane misaligned c -axis-oriented. These misaligned grains cause large angle grain boundaries, which would increase R s . For the R s reduction due to Ca-doping, the optimum thickness of the Ca-doped YBCO layer was about 50 nm, and we did not observe the reduction of R s for films on LaAlO 3 substrates with perfectly in-plane aligned grains. From these experimental results, we concluded that the selective diffusion of Ca ions to the grain boundaries of the YBCO layer is the origin of the low R s . Moreover, films that were annealed in oxygen for 60 h showed lower R s than that of short-time-annealed (for 1 h after deposition) films. These simple methods to reduce R s of YBCO films would be applicable to the fabrication of microwave passive devices.

The influence of external factors on the corrosion resistance of high temperature superconductor thin films against moisture

High temperature superconductor (HTS) thin films have been systematically investigated for their corrosion resistance against moisture by studying the role of external factors such as temperature (T), relative humidity (RH), and the type of substrates in the corrosion. In general, (i) the corrosion is progressed monotonously with increasing T as well as RH, (ii) a threshold level of water vapor is needed to cause degradation, and (iii) between T and RH, the influence of T is more dominant. HTS films on SrTiO3 and CeO2 buffered sapphire (cbs) substrates showed better corrosion stability and a low rate of degradation in the critical current density as compared to that of the film grown on MgO substrate. Between DyBa2Cu3Oz (DBCO) and YBa2Cu3Oz, the former is reproducibly found to have many fold higher corrosion resistance against moisture. This observed enhancement in the corrosion resistance in DBCO could be explained by the improved microstructure in the films and the better lattice matching with the subst...

Effect of underdoping on the microwave surface resistance of YBa2Cu3Oz thin films

The effect of underdoping on the microwave surface resistance (Rs) and the Hall number (nH) in the Co-substituted YBa2Cu3Oz (YBCO) films grown on LaAlO3 substrates has been investigated, and the results are compared with oxygen-deficient YBCO films. From the analysis of Hall data, it is inferred that the nH is suppressed upon the Co substitution. As the Co concentration increases, the in-plane resistivity (ρ) increases and ρ(T) develops a downward curvature below room temperature, whereas the nH decays exponentially. Microwave studies on both the Co-substituted YBCO as well as the oxygen-deficient YBCO reveal that the reduction in carrier concentration of YBCO, by any means, leads to an undesirable effect on Rs. The Rs(T) value of Co-substituted films increases as the Co concentration is increased. Near the superconducting transition temperature, Tc, the observed high Rs value and a broad transition in the Rs(T) curve in the Co-substituted films are presumed to be due to an increase in the penetration dep...

Correlative study of Rs and Jc for 123 superconducting thin films

We have studied the relationship between the microwave surface resistance, Rs (measured by a dielectric resonator technique at 22?GHz), and the critical current density, Jc (obtained via a transport method), for various superconducting thin films belonging to the 123 family. It is found that Rs = 4.29 ? 107Jc?0.98 for YBa2Cu3Oz (YBCO) films on MgO, which is in good agreement with the reported value (Rs,22?GHz = 1.8 ? 107Jc?1) for YBCO on BaSnO3 buffered MgO. On the other hand, we observed Rs = 6.8 ? 1023Jc?2.55 and Rs = 2.2 ? 105Jc?0.76, respectively, for Y0.90Ca0.10Ba2Cu3Oz on MgO and Dy0.40Ho0.60Ba2Cu3Oz ((D, H)BCO) on LaAlO3 films, where Rs is in ? and Jc is in A?m?2. The observed deviation in the Rs?Jc relations (from the simple relation, which holds good for pure YBCO films) for (D, H)BCO films may be attributed to the existing different temperature dependence of Rs for these films. On the other hand, in the case of Y1?xCaxBa2Cu3Oz (YCBCO) films, the very low Jc and the microstructural imperfections are considered a plausible cause for the observed deviation from the simple relation. Though the pre-exponential factor, m, in Rs = mJc?n is believed to be highly sample dependent, it is inferred from the correlative results (obtained from eight?films, i.e., YBCO (two?films); (D, H)BCO (three?films); YCBCO (three?films with x = 0.02, 0.05, 0.10)) that m and n in the Rs = mJc?n correlation are related to each other in such a way that log(m) varies linearly with n. An m value of the order of 107 is observed typically when the n value is 1.

Transport properties of pulsed laser deposition grown Dy0.4Ho0.6Ba2Cu3Oz thin films

Pulsed laser deposition (PLD) grown Dy 0 . 4 Ho 0 . 6 Ba 2 Cu 3 O z [(D, H)BCO] superconducting films on LaAlO 3 substrates have been studied for their critical current density (J c ) and microwave surface resistance (R s ) in comparison with YBCO films. It is inferred that the best films (grown at 800 °C) with transition temperature (T c ) 92 K showed a higher J c value above 80 K compared to that of YBCO films. Also, at 77 K, the field dependent J c value of (D, H)BCO films (5 kA cm - 2 at 8 T) is 2.5 times higher than that of YBCO films (2 kA cm - 2 at 8 T). The best (D, H)BCO films showed lower R s in the liquid nitrogen (N 2 ) temperature region (4.2 mΩ at 77 K) than YBCO films (7.65 mΩ at 77 K) while in the low temperature region (below 60 K) YBCO films exhibit better microwave performance irrespective of their deposition temperature. These significantly improved values (both the J c as well as the R s values above 77 K) reveal that a rare earth (RE) mixed 123 thin film is a rather better candidate than YBCO thin film for use in fabricating microwave passive devices operating at high fields and at liquid N 2 temperature.