Towards Isotropic Vortex Pinning in YBCO Films with Double-doping BHO-Y₂O₃ and BZO-Y₂O₃ Artificial Pining Centers

Strong and isotropic vortex pinning landscape is demanded for high field applications of YaBa₂Cu₃O_7-x (YBCO) epitaxial thin films. Double-doping (DD) of artificial pinning centers (APCs) of mixed morphologies has been identified as a viable approach for this purpose. This work presents a comparative study on the transport critical current density J _c (H, θ) of 3.0 vol.%Y₂O₃+2.0 (or 6.0) vol.% BaZrO₃ (BZO DD) and 3.0 vol.%Y₂O₃+ 2.0 (or 6.0) vol.% BaHfO₃ (BHO DD) films. Based on the elastic strain model, BaHfO₃ (BHO) nanorods have lower rigidity than their BaZrO₃ (BZO) counterparts, which means their c-axis alignment is more susceptible to the local strain generated by the secondary dopant of Y₂O₃. Considering the increasing strain field with higher BZO (or BHO doping), the higher susceptibility may result in a large portion of the BHO APCs moving away from perfect c-axis alignment and enhancing isotropic pinning with respect to the H orientation. This is confirmed since the BHO DD films illustrate a less pronounced J _c peak at H//c-axis and hence more isotropic J _c(θ) than their BZO DD counterparts. At 9.0 T, the variation of the J _c across the entire θ range (0-90 degree) is less than 18% for the BHO DD film, in contrast to about 100% for the 2.0 vol.% BZO DD counterpart. At the higher BHO concentration of 6.0 vol.%, this higher tunability of the Y₂O₃ leads to increased ab-plane aligned BHO APCs and hence enhanced J _c at H//ab-plane. (paper)