Influence of nano-particle diameter on superconducting properties in BaMO₃(M = Sn, Hf)/YBa₂Cu₃O_y quasi-multilayered films

Highlights: • BSO nanoparticles which grown at higher temperature have larger diameter. • BSO nanoparticles which have large diameter make a broad peak of J_c around B || c. • BSO/YBCO film which grown at 770 °C shows the improvement of J_c at high temperature. • BHO doped YBCO multilayered film does not show the improvement of J_c. - Abstract: In order to investigate the influence of diameter and spatial distribution of three-dimensional (3D) pinning centers on critical current density J_c, BMO (BaSnO₃ (BSO) or BaHfO₃ (BHO)) doped YBa₂Cu₃O_y (YBCO) thin films were fabricated by a quasi-multilayering process using a pulsed laser deposition method. The prepared films are referred as BMO(m,n)T_s, where m and n denotes the number of laser pulse on the BMO target and the total number of BMO/YBCO bilayers, respectively and T_s is the growth temperature. BSO(1,100)750 and BSO(1,100)770 show the improvement of J_c in comparison with the pure YBCO sample in wide range of magnetic field directions at 65 K. However, at 77.3 K, improvement of J_c was seen in only BSO(1,100)770. The BSO nano-particles within BSO(1,100)770 are considered to have larger diameter, so that BSO nano-particles can immobilize the flux lines in the high temperature region. In addition, BSO(1,100)770 shows the high peak of J_c centered at θ = 0° in the angular dependence of J_c. On the other hand, the J_c of BHO(1,100)770 falls below that of pure YBCO samples in all magnetic field orientation. In addition, at 65 K, there is no peak of J_c at any angles except for θ = 90°. These results indicates that the diameter of BHO nano-particles in BHO(1,100)770 might be much smaller than that of BSO nano-particle and BHO nanoparticles cannot work as effective pinning center.