Flux pinning properties of BaMO3-doped YBa2Cu3O7-x thin films （M＝ Zr， Sn）
MELE Paolo, 松本 要 （九工大）； 堀出 朋哉 （京大）； 一瀬 中 （電中研）； 向田 昌志 （九大）； 吉田 豊 （名大）； 堀井 滋 （東大）； 喜多 隆介 （静岡大）
pmele*post.matsc.kyutech.ac.jp
Abstract:One of the most fascinating challenges of the materials science is to disclose the YBa2Cu3O7-x (YBCO) and REBa2Cu3O7-x (REBCO: RE=Er, Nd, Gd, Sm) superconducting coated conductors to the practical applications. Especially, drastic improvements of the critical current densities (Jc) and the global pinning force (Fp) at various temperatures and under magnetic field B are required. At this purpose, the introduction of Artificial Pinning Centers (APCs) in high-temperature superconducting epitaxial films was recognized as highly effective for the pinning of vortices. Several nano-engineering techniques were recently developed to insert and control APCs, and the transport performances of YBCO and REBCO films were dramatically enhanced, by insertion of nanosized dopants, especially belonging to perovskite phase BaMO3 (BMO, M = Zr, Hf, In, Sn). In this work, we present BaSnO3 (BSO) embedded YBCO films with outstanding pinning performances in comparison with optimized BaZrO3 (BZO) doped films. YBCO + 4 wt% BZO and YBCO + 4 wt% BSO mixed targets were used in PLD system to form BZO and BSO doped YBCO films respectively.The dopings of BZO and BSO in YBCO films resulted in the incorporation of extended nanorods in both cases. However, even if the concentration of dopant and the deposition conditions (T = 800 °C. pO2 = 200 mTorr, E = 340 mJ/pulse) were the same, TEM analysis reveals that the morphology of nanorods is different. The BZO nanorods are more dense and their diameter is thinner if compared with BSO ones. Furthermore, the BZO nanorods are curved, with a strong deviation from the c-axis crystallographic direction of the film, while the BSO nanorods are almost straight and parallel to c-axis. Since we expect that the nanorods work as c-axis correlated pinning centres, the extension of straight portion is very important to get large pinning force when B is applied parallel to c-axis. The performance of the BSO doped sample is dramatically superior not only to the reference YBCO sample, but also to the BZO doped sample, in the whole range of temperatures and magnetic field up to 9 T. The BZO doped YBCO shows a remarkable performance with FpMAX = 16.3 GN/m3 (77 K, B//c), almost the same as the typical values of NbTi practical wires at 4.2 K, and FpMAX = 52.7 GN/m3 (65 K, B//c) is achieved. Moreover, the BSO doped sample presents the terrific values of 28.3 GN/m3 (77 K, B//c), and 103 GN/m3 (65 K, B//c), almost twice as those of BZO. Within our knowledge these values represent the record performance for short samples. In the angular dependences of Jc, both BZO and BSO doped samples display a sharp Jc peak for the fields aligned with the ab plane (Theta=90°, B//ab), which can be attributed to the intrinsic pinning originating from the layered structure of YBCO or stacking faults and additional broad peaks near Theta=180° (B//c), typical of strongly c-axis correlated pinning. This is consistent with the presence of nanorods as revealed by TEM. In the case of BSO doping the c-axis correlated peak is much more pronounced than in the case of BZO.