Mapping the flux penetration profile in a 2G-HTS tape at the microscopic scale: deviations from a classical critical state model

Abstract

Understanding vortex behaviour at microscopic scales is of extreme importance for the development of higher performance coated conductors with larger critical currents.Here, we study and map the critical state in a YBCO-based coated conductor at different temperatures using two distinct operation modes of scanning Hall microscopy. An analytical Bean critical state model for long superconducting strips is compared with our measurements and used to estimate the critical current density. We find several striking deviations from the model; pronounced flux front roughening is observed as the temperature is reduced below 83 K due to vortex-bundle formation when strong broadening of the flux front profile is also seen. In higher magnetic fields at the lower temperature of 65 K, fishtail-like magnetization peaks observed in local magnetization measurements are attributed to flux-locking due to an increase in the critical current density near the edges of the tape, which we tentatively link to vortex pinning matching effects. Our measurements provide valuable insights into the rich vortex phenomena present in coated conductor tapes at the microscopic scale.

Imaging of Strong Nanoscale Vortex Pinning in GdBaCuO High-Temperature Superconducting Tapes

“…In fact, the profile presented by these lines much more closely mirrors the classical 3D Bean critical state behavior whereby d B z /d y is a constant for a given sign of the critical current density [ 27 ]. A similar behavior has been observed in local magnetisation data measured on other types of 2G-HTS tapes [ 24 ], though not to the same extent as that seen here, indicating a significantly faster relaxation rate in our samples. This in turn reflects the type, structure and topology of the specific pinning centres employed which lead to distinctly different creep barriers for different configurations.…”

“…In contrast, scanning probe microscopy techniques enable one to build a microscopic picture of the superconducting properties and represent ideal tools to study vortex behavior at discrete pinning centres [21]. Scanning Hall probe microscopy (SHPM) is a quantitative and non-invasive magnetic imaging technique that has frequently been used to study supercurrent transport [22], ac losses [23] and flux penetration in cuprate superconductors at the microscale [24]. In this paper, we use SHPM, supported by magnetisation and electronic transport measurements, to obtain a microscopic picture of the vortex pinning landscape at very low applied fields across a wide temperate range from 10 to 89 K. We couple this with scanning electron microscopy (SEM) imaging to attempt to develop a detailed understanding of the role second-phase inclusions play in pinning vortices and enhancing J c .…”