Angular Dependence of Vortex Pinning Properties in YBa2Cu3O7 Nanocomposite Films

Vortex pinning should be controlled to enhance critical current density (Jc) in YBa2Cu3O7 (YBCO) films for coated conductor application. Improvement of vortex pinning has been achieved by the nanocomposite structure. Nanorods are one of the most effective pinning centers that improve the Jc especially for the magnetic field parallel to the c-axis. An angular dependence of Jc has been measured in the YBCO nanocomposite films to understand the vortex pinning anisotropy, and a complicated angular dependence of Jc has been observed. The Jc anisotropy is a very important parameter, and the Jc should be improved isotropically. Incorporation of multiple types pinning centers, namely the hybrid pinning is very effective for this purpose. For the design of the hybrid pinning structure, understanding of the Jc angular dependence is needed. Also, the fabrication of the hybrid pinning is required. In this study, the angular dependence of Jc was measured in the YBCO films containing BaMO3(BMO, M=Zr, Sn, Hf) nanorods (YBCO+BMO films). The hybrid pinning structure comprising of the nanorods and the nanoparticles was formed in the YBCO films. The nanorods were self-organized in the pulsed laser deposition using the mixed target, and the nanoparticles were incorporated into the films with the artificial surface modified target method and the self-organized manner.<br/>The c-axis Jc peak and the ab-plane Jc peak were observed in the angular dependence of Jc, but the peak height and width strongly depended on magnetic field and temperature. In addition, an off-axis peak was observed in the moderate magnetic field of 4-7 T at 77 K. While the c-axis Jc peak originated from the nanorod pinning, the ab-plane Jc peak was due to the crystalline anisotropy, the intrinsic pinning, and/or the ab-plane correlated pinning such as stacking faults. When the magnetic field was lower than the matching field, the nanorods were high-density enough to pin all vortices. On the other hand, the nanorods were pinned by the vortex interaction and matric uncorrelated defects. As the magnetic field was tilted from the c-axis, the vortices were pinned by the vacant portion of nanorods, namely the vortex retrapping occurred. The vortex retrapping resulted in the off-axis peak in the moderate magnetic fields. The vortex phase diagram in the YBCO+BMO films will be discussed.<br/>The YBCO films containing BaHfO3(BHO) nanorods and Y2O3 nanoparticles (YBCO+BHO+Y2O3) were fabricated using the mixed target and the surface modified target method. Very high Jc (Fp,max=1.57TN/m3 at 4.2 K) was successfully achieved. The Jc for the magnetic field parallel to the ab-plane was improved by the Y2O3 nanoparticles, demonstrating the hybrid pinning. In the YBCO+ Ba2LuNbO6(BLNO) films, the Y2O3 nanoparticles in addition to the BLNO nanorods were formed only by the mixed YBCO+BLNO target without using the surface modified target method. Thus, the self-organized formation of the hybrid pinning was demonstrated in the YBCO+ BLNO films. The c-axis correlated pinning of the BLNO nanorods was observed in the films, and the Jc was improved also for the magnetic field parallel to the ab-plane. The hybrid pinning design will be discussed by comparing the pinning properties of the hybrid pinning structures.<br/>This work was supported by Japan Society for the Promotion of Science (JSPS) KAKENHI (Grant No. 18H01478).