Specific features of penetration and trapping of a magnetic flux in film and bulk YBCO high-temperature superconductors

Abstract

An oscillatory differential method of local diagnostics of superconductors is applied to the analysis of the trapped magnetic flux and the effective demagnetization factor in YBCO samples. Regular steps over certain intervals of the external field are observed on the magnetic-field dependence of these parameters. It is demonstrated that, as the external field increases, crystallites in a sample are decomposed into sub- and nanocrystallites with a size much less than the penetration depth [Formula: see text] of the magnetic field. The first critical thermodynamic magnetic fields of sub- and nanocrystallites are determined. These results allow one to reveal the interaction mechanism between magnetic and crystalline microstructures of superconductors and provide a deeper insight into the physical processes that occur in high-temperature superconductors (HTSCs) in a magnetic field.

Local internal demagnetizing fields as one of the main fundamental reasons for the low density of critical current in twinned YBa2Cu3O7 − x HTSCs

“…By local approximation to the spatial region of the magnetic response and differential method of signal detection, one can experimentally distinguish the interface between the vortex and Meissner regions (IVMRs) and investigate the distribution of internal local demagnetizing fields in YBCO HTSCs near and on the TBs. The oscillatory differential method of local approximation [3,4] is based on the detection of the fields H D and H Dtr induced by the trapped magnetic flux solely due to the front of the DOLMF. To this end, a capacitor of C ≈ 2.5 × 10 −8 F is connected parallel to a solenoid (with inductance of L ≈ 4.22 H and active resistance of R ≈ 92.6 Ω at temperature of 77 K) creating an external field.…”

“…The ZFC mode provides a direct interaction of the magnetic field with a sample in the Meissner state, thus allowing the investigation of the macroscopic properties of the sample. Since in the ZFCMFA mode the magnetic flux of the preceding step is already frozen in the sample, a step-by-step destruction of weak links occurs by the stray fields of the previously trapped vortices, and an interaction is guaranteed between the external field and individual twins [3,4]. Thus, in contrast to the ZFC mode, the ZFCMFA mode allows one to study, in addition to the macroscopic properties of HTSCs, the physical processes inside a sample.…”

“…This leads to the strong dependence of magnetic transport properties of HTSCs on the density of the net of weak links, in particular, on the density and spatial distribution of TBs and the presence of other defects. In [3,4], the author demonstrated that, as the external magnetic field increases, an HTSC sample is stepwise decomposed into groups of twins with quantized spatial scales. This scenario leads to the fact that the groups of twins resulting from the decomposition of the sample are sorted according to their demagnetizing factors (n).…”

“…Due to the high level of intratwin currents, the magnetic response signal increases because the operating area of the sensor covers a large number of twins with close values of n that are contained in each group of twins. Moreover, the decomposition of the sample into twins is responsible for the fact that the linear size of twins decreases step by step and becomes less than λ, the penetration depth of the magnetic field into the sample and its twins [3,4]. In this situation, questions arise about the legitimacy of applying corrected analytical [5] and generalized [6] models of the Bean critical state to hard type II superconductors, which allow one to adequately describe the distribution of the flux and current density in a sample and unambiguously determine J c from experimental results.…”

“…Thus, in view of the critical role of H D and H Dtr in the formation of macroscopic parameters of polycrystalline HTSCs, one should correct and numerically simulate the equations when constructing a model of the critical state. When information on J c (H D ) and J c (H Dtr ) and their dependence on the external field, temperature, size and spatial distribution of twins is available, one can establish the value, spatial distribution, and the field and temperature dependence of the thermodynamic field H i [3,4]. This, in turn, will help to construct a generalized analytical model of a uniformly magnetized sample (MUMS), that allows one to adequately describe the spatial distribution of the flux and current density in the HTSC sample.…”