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A study of 3 vol% Y 2 O 3 +2-6 vol% BaHfO 3 double-doped YBa 2 Cu 3 O 7-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 Y 2 O 3 nanoparticles (3D APCs). BHO 1D APCs have been predicted to have the least rigidity in an elastic strain energy model in APC/YBa 2 Cu 3 O 7-x nanocomposite films. Consequently, they could be best 'tuned' away from the c-axis alignment by local strains generated by the Y 2 O 3 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.

C-axis-aligned one-dimensional artificial pinning centers (1D-APCs) provide strong correlated pinning of magnetic vortices and enhance critical current density Jc in superconducting YBa2Cu3O7-x (YBCO) films and coated conductors. A fundamental question arises on what determines the pinning efficiency of different 1D-APCs. To shed light on this question, this work investigates the correlation between the 1D-APC/YBCO interface and the pinning efficiency of 1D-APCs of BaHfO3 (BHO) and BaZrO3 (BZO) of comparable diameters of 5–6 nm. Intriguingly, a highly coherent BHO 1D-APC/YBCO interface is revealed even at a high BHO doping level of 6 vol. %, in contrast to a semi-coherent BZO 1D-APC/YBCO interface with a large number of dislocations. This leads to a profound effect on the pinning efficiency of these 1D-APCs. Specifically, a record high pinning force density peak Fp,max ∼ 182.0 GN m−3 at Hmax > 9.0 T and 65 K is obtained in the former, which is 2.5 times of the best reported in the latter. Moreover, a ratio of Hmax to accommodation field H* determined from the 1D-APC areal concentration is found up to 3.5 in the former in contrast to 0.7 in the latter, demonstrating the critical impact of the APC/YBCO interface on the pinning efficiency of 1D-APCs.

BaZrO3 (BZO) one-dimensional artificial pinning centers (1D-APCs) aligned along the c-axis of the YBa2Cu3O7 (YBCO) have been adopted to enhance the magnetic vortex pinning in BZO/YBCO nanocomposite films. However, the pinning force density F p of the BZO 1D-APCs remains moderate at temperatures near 77 K. A hypothesis of the major limiting factor is the defective BZO 1D-APCs/YBCO interface as a direct consequence of the large interfacial strain originated from the BZO/YBCO lattice mismatch of ∼7.7%. Herein, we explore enlarging the c-axis of the YBCO dynamically to reduce the lattice mismatch and hence to prevent formation of the defective BZO 1D-APCs/YBCO interface. Specifically, the c-axis enlargement was achieved by partial replacement of Cu with Ca on the YBCO lattice using strain-directed Ca diffusion into YBCO from two Ca0.3Y0.7Ba2Cu3O7− x (CaY-123) spacers of only 10 nm in thickness inserted into the 2 vol% BZO 1D-APC/YBCO nanocomposite thin films of ∼150 nm in total thickness. The achieved elongated c-axis is attributed to the formation of stacking faults induced by Ca-replacement of Cu on YBCO lattice. The reduced BZO/YBCO lattice mismatch allows formation of a coherent BZO 1D-APC/YBCO interface with negligible defects. This leads to an enhanced F p value up to 98 GN m−3 at 65 K, which is 70% higher than that of the reference 2 vol% BZO 1D-APC/YBCO sample. Furthermore, the benefit of the enhanced pinning of the BZO 1D-APCs with a coherent interface with YBCO can be extended to a large angular range of the magnetic field orientation. This study reveals the significant effect of the BZO/YBCO interface on the pinning efficiency of BZO 1D-APCs and provides a promising approach to achieve a coherent interface in BZO/YBCO nanocomposite films.

An elastic strain model was applied to evaluate the rigidity of the c-axis aligned one-dimensional artificial pinning centers (1D-APCs) in YBa2Cu3O7-δ matrix films. Higher rigidity was predicted for BaZrO3 1D-APCs than that of the BaHfO3 1D-APCs. This suggests a secondary APC doping of Y2O3 in the 1D-APC/YBa2Cu3O7-δ nanocomposite films would generate a stronger perturbation to the c-axis alignment of the BaHfO3 1D-APCs and therefore a more isotropic magnetic vortex pinning landscape. In order to experimentally confirm this, we have made a comparative study of the critical current density Jc (H, θ, T) of 2 vol.% BaZrO3 + 3 vol.%Y2O3 and 2 vol.%BaHfO3 + 3 vol.%Y2O3 double-doped (DD) YBa2Cu3O7-δ films deposited at their optimal growth conditions. A much enhanced isotropic pinning was observed in the BaHfO3 DD samples. For example, at 65 K and 9.0 T, the variation of the Jc across the entire θ range from θ=0 (H//c) to θ=90 degree (H//ab) is less than 18% for BaHfO3 DD films, in contrast to about 100% for the BaZrO3 DD counterpart. In addition, lower α values from the Jc(H) ∼ H-α fitting were observed in the BaHfO3 DD films in a large θ range away from the H//c-axis. Since the two samples have comparable Jc values at H//c-axis, the improved isotropic pinning in BaHfO3 DD films confirms the theoretically predicted higher tunability of the BaHfO3 1D-APCs in APC/YBa2Cu3O7-δ nanocomposite films.