Enhancement Of Magnetic Vortex Pinning by APCs with Coherent BZO 1D-APC/YBCO Interface
Strong magnetic vortex pinning is crucial for in-field high critical current density (Jc) sustaining applications. This pinning force can be realized by inserting artificial pinning centers (APCs) in high temperature superconductors (HTS) such as YBa2Cu3O7-x (YBCO). Unfortunately, a degraded APC/YBCO interface can limit the pinning force making high field/high Jc applications unfeasible. The effect of APC/YBCO interface on pinning is best exemplified by the difference in pinning force generated by c-axis (direction of film growth) aligned one-dimensional (1D) APC fabricated with BaZrO3 (BZO) on one hand and BaHfO3 (BHO) on the other. A considerably higher pinning force density (Fp) was observed on the BHO 1D-APCs, which was attributed to a less defective BHO/YBCO interface than the BZO/YBCO case. This defect-ridden BZO 1D-APC/YBCO interface has been blamed for the reduced pinning efficiency of BZO 1D-APCs and is consistent with a microstructure analysis that revealed a defective, oxygen-deficient YBCO column around the BZO 1D-APCs due to the large lattice mismatch of ~7.7% between the BZO (3a=1.26 nm) and YBCO (c=1.17 nm). In this thesis, I report a dynamic lattice enlargement approach on the tensile strained YBCO lattice during the BZO 1D-APCs growth to induce c-axis elongation of the YBCO lattice up to 1.26 nm near the BZO 1D-APC/YBCO interface via Ca/Cu substitution on single Cu-O planes of YBCO, which prevents the interfacial defect formation by reducing the BZO/YBCO lattice mismatch to ~1.4%. Specifically, this is achieved by inserting thin Ca reservoir spacers in BZO/YBCO nanocomposite films. A defect-free, coherent BZO 1D-APC/YBCO interface is confirmed in transmission electron microscopy and elemental distribution analyses. This has led to a record high pinning force density Fp illustrating the critical importance of a coherent BZO 1D-APC/YBCO interface in the pinning efficiency. The question of how the APC/YBCO interface affects angular range of the pinning effectiveness for a given 1D APC is important for applications that require Jc to be maintained at all orientation in the presence of magnetic field. Although, as mentioned above, differences are present at the interfaces of BZO and BHO 1D APCs with YBCO, both materials have comparable diameters in the range of 5-6 nm. Therefore, they provide ideal systems for investigation of the angular range of pinning effectiveness by 1D APCs. By evaluating the nanocomposites’ maximum pinning force density (Fp, max) and its location Bmax, normalized to that of the reference YBCO film as functions of magnetic field (B) orientation at temperatures of 65–77 K, a quantitative correlation between the pinning efficiency of the BZO 1D-APCs and their effective angular range was obtained. Our results indicate that all 1D APCs can provide enhanced Bmax over certain angular ranges away from the c-axis. However, 1D APCs with higher pinning efficiency, such as BHO 1D APCs can have enhanced Fp, max over the entire angular range of B-orientations at temperatures of 65-77 K with respect to that for the reference YBCO sample. A microstructure analysis has revealed a defective, oxygen-deficient YBCO column around the BZO 1D-APCs due to the large lattice mismatch of ~7.7% between the BZO (3a=1.26 nm) and YBCO (c=1.17 nm), which has been blamed for the reduced pinning efficiency of BZO 1D-APCs. This thesis reports a dynamic lattice enlargement approach on the tensile strained YBCO lattice during the BZO 1D-APCs growth to induce c-axis elongation of the YBCO lattice up to 1.26 nm near the BZO 1D-APC/YBCO interface via Ca/Cu substitution on single Cu-O planes of YBCO, which prevents the interfacial defect formation by reducing the BZO/YBCO lattice mismatch to ~1.4%. Specifically, this is achieved by inserting thin Ca0.3Y0.7Ba2Cu3O7-x (CaY-123) spacers as the Ca reservoir in 2-6 vol.% BZO/YBCO nanocomposite multilayer (ML) films. A defect-free, coherent BZO 1D-APC/YBCO interface is confirmed in transmission electron microscopy and elemental distribution analyses. Excitingly, up to five-fold enhancement of Jc (B) at magnetic field B=9.0 T//c-axis and 65-77 K was obtained in the ML samples as compared to their BZO/YBCO single-layer (SL) counterpart’s. This has led to a record high pinning force density Fp together with significantly enhanced Bmax at which Fp reaches its maximum value Fp,max for BZO 1D-APCs at B//c-axis. At 65 K, the Fp,max ~158 GN/m3 and Bmax ~ 8.0 T for the 6% BZO/YBCO ML samples represent a significant enhancement over Fp,max ~36.1 GN/m3 and Bmax ~ 5.0 T for the 6% BZO/YBCO SL counterparts. This result not only illustrates the critical importance of a coherent BZO 1D-APC/YBCO interface in the pinning efficiency, but also provides a facile scheme to achieve such an interface to restore the pristine pinning efficiency of the BZO 1D-APCs. The hypothesis states that the highly strained YBCO columns around the BZO 1D-APCs due to the 7.7% larger lattice constant of BZO than YBCO’s in the c-axis provide channels to facilitate Ca diffusion, more preferably near the BZO/YBCO interface, that lead to an enlarged YBCO’s c-axis lattice constant as a consequence of the Ca/Cu substitution on the YBCO’s Cu-O planes. Confirmation of this hypothesis is hence important to understand the mechanism of Ca diffusion in BZO/YBCO ML films for further enhancement of pinning efficiency of the BZO 1D-APCs. A comparative study of two sets of BZO/YBCO ML films: singly-doped (SD-ML, BZO only) and the doubly-doped (DD-ML, BZO + Y2O3). The modulated strain field in the SD-ML films is in contrast to the non-modulated strain in the DD-ML samples as confirmed by transmission electron microscopy. Interestingly, improved pinning by four times of the reference SD single-layer (SD-SL) film’ was observed in the SD-ML samples even when the constituent BZO/YBCO layer thickness was varied between 50-100 nm, indicating that the Ca diffusion along the strained BZO/YBCO interface and the Ca/Cu substitution on YBCO’s Cu-O planes indeed correlate with the modulated strain field. In contrast, pinning degrades by twice in DD-ML samples with respect to their DD-SL counterpart’s. This result therefore suggests that strained BZO/YBCO interfaces serve as channels for Ca diffusion in SD-ML, which leads to the coherent BZO/YBCO interfaces and hence improved pinning of BZO 1D-APCs. This effect diminishes when such a modulated strain field is removed in the DD-ML samples.
Condensed matter physics, Applied physics, Artificial Pinning Center, High Temperature Superconductors, Multilayer Structure, Thin Films, Vortex Pinning, YBCO
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