Microscopic adaptation of BaHfO3 and Y2O3 artificial pinning centers for strong and isotropic pinning landscape in YBa2Cu3O7–x thin films
A study of 3 vol% Y2O3 + 2–6 vol% BaHfO3 double-doped YBa2Cu3O7–x (BHO DD) epitaxial thin films was carried out to explore the morphology adaption of c-axis aligned one-dimensional BHO artificial pinning centers (1D APCs) to secondary Y2O3 nanoparticles (3D APCs). BHO 1D APCs have been predicted to have the least rigidity in an elastic strain energy model in APC/YBa2Cu3O7–x nanocomposite films. Consequently, they could be best 'tuned' away from the c-axis alignment by local strains generated by the Y2O3 3D APCs. This provides an opportunity to generate mixed-morphology APCs, especially at high BHO concentrations. Motivated by this, we have carried out a systematic study of the transport critical current density J c(H, T, θ) on the BHO DD samples in magnetic fields (H) up to 90 kOe at different H orientations from H//c-axis (θ = 0), to θ = 45°, and to H//ab-plane (θ = 90°). Enhanced pinning at all three orientations was observed as illustrated in the comparable low alpha (α) values in the range of 0.13–0.25 at 65 K, which is consistent with the mixed 1D (in c-axis) + 2D (in ab-plane) + 3D APCs observed in transmission electron microscopy (TEM). Upon increasing BHO concentration from 2 to 4 vol%, a monotonic increase of the accommodation field H* at θ = 0°, 45° and 90° was observed, indicative of the APC concentration increase of the mixed morphologies. At 6 vol% BHO, the H* continues the increase to 85 kOe at H//c-axis (θ = 0), and >90 kOe H//ab-plane (θ = 90°), while it decreases from 80 to 85 kOe at 2–4 vol% to 60 kOe at 6 vol% at θ = 45°, which is consistent with the TEM observation of the connection of 3D APCs, appeared at lower BHO concentration into 2D ones in ab-plane at the higher BHO concentrations. These results shed light on the quantitative adaptation of APCs of mixed morphologies with increasing BHO doping in the BHO DD thin films and are important for controlling the APC pinning landscape towards minimal angular dependence.