M.R. Koblischka

Determination of flux-density gradients in YBa2Cu3O7−δ superconductors using the high-resolution Faraday effect

Abstract Domain patterns of magnetic flux penetrated in YBa2Cu3O7−δ superconductors have been observed by means of a high-resolution Faraday technique. The experimental conditions for observing domain patterns with the high-resolution Faraday effect are discussed. Flux-density profiles and the related flux-density gradients were determined from an analysis of the domain patterns. An analysis of the measured flux-density gradients in single-crystals shows that twin-boundaries are not the strongest acting pinning mechanism at low temperatures. Our results support the assumption that intrinsic pinning determines the flux-density gradients and governs the critical currents. It is shown that the flux-density gradients in grains of the sintered specimens are larger than in single-crystals. The resulting thirty times greater pinning forces are measured both magneto-optically and with a SQUID-magnetometer. This behaviour may be explained by superimposed extrinsic pinning effects such as grain-boundaries.

Observation of flux penetration in YBa2Cu3O7−δ superconductors by means of the magneto-optical Faraday effect

Abstract The penetration of the Shubnikov phase into both single-crystal and sintered YBaCuO specimens has been observed by means of the magneto-optical Faraday effect using the high Verdet constant in thin evaporated films of a mixture of EuS and EuF 2 . This method allows the direct observation of the flux motion. After cycling magnetic field from zero, the trapped flux structure in the single crystal consists of large domains which are related to the sample shape, whereas in sintered specimens only a part of the total number of the grains transforms into the Shubnikov phase.

Study of flux distributions in high-Tc single crystals and thin films using magneto-optic techniques

Abstract The current progress of magneto-optical techniques for direct investigations of magnetic flux structures in type-II superconductors with high spatial resolution is presented. Wide possibilities are offered by the high-resolution Faraday effect technique requiring a thin layer of EuSe as an indicator and the method using ferrimagnetic garnet films with in-plane anisotropy. The advantages of both methods are combined to investigate flux structures of an YBa 2 Cu 3 O 7 (YBCO) thin film partly irradiated with 25 MeV oxygen ions and for inhomogeneously twinned YBCO single crystals.

Observation of nucleation and annihilation of flux-lines with opposite sign in high-Tc superconductors

Abstract Using the high-resolution Faraday effect (HRF), we observed nucleation of domains in high- T c single crystals consisting of flux-lines with opposite signs while reducing the applied external magnetic field to zero. This nucleation of domains with reversed flux-lines is found in YBa 2 Cu 3 O 7−δ single crystals as well as in Bi 2 Sr 2 CaCu 2 O 7−δ single crystals. The resulting domain pattern in the remanent state is then composed of domains with opposite polarity which are seperated by an annihilation zone. An experimental proof for the existence of opposite signs of the vortices has been given by observing the behaviour of the domains on the direction of the applied external magnetic field. We also show the dependence of the width of the annihilation zone on the external magnetic field, indicating the role of the pinning forces on this behaviour.

Flux-lines of inversed sign in YBa2Cu3O7−δ thin films

Abstract Using the high-resolution Faraday technique (HRF), we observed directly the nucleation of flux-lines of opposite sign in the remanent state of YBaCuO thin films. These negative flux-lines are nucleated by the stray fields caused by the pinned vortices. Due to the large demagnetizing factor of the thin film, this flux is able to penetrate the sample. Such a coupled behaviour can be observed directly. Experimental evidence for the existence of vortices of opposite sign is given. The influence of various polarizer/ analyzer settings on these observations is discussed. Furthermore, we show effects of flux-creep in thin films and the role of the negative flux-lines in this behavior is analyzed.

Magneto-optical study of flux penetration in heavy-ion irradiated high-Tc single crystals

Abstract Single crystals of Bi2Sr2CaCu2O8+δ (Bi-2212) and of DyBa2Cu3O7−δ have been exposed to high-energy heavy-ion irradiation (230 MeV 58Ni-ions, 0.5 GeV 127I-ions and 1.0 GeV 208Pb-ions) with various fluences perpendicular to the sample surface and hence parallel to the c-axis of the crystals. The irradiation with Ni-ions produces spherical regions of amorphized material, whereas the iodine and lead ions create columnar defects along the paths of the projectiles in the superconductors. Using the high-resolution Faraday effect (HRF) technique, domain patterns of the Shubnikov phase are investigated. From the measured flux-density profiles, the acting local pinning forces and critical current densities are determined. To allow direct comparison of the irradiation effects under the same experimental conditions, one half of each crystal was covered by an absorber during the irradiation. Fluence-dependent measurements of the pinning forces are performed and the influence of irradiation on flux-creep effects is also investigated. All heavy-ion irradiated samples show an enhancement of the measured critical current densities up to a factor of 33 as compared to the unirradiated region of the same crystal. The effective activation energies for flux creep remain unchanged after each kind of irradiation in both types of single crystals.

Observation of inverse domains in high Tc superconductors

Nucleation of domains of inverse polarity is observed in the remanent state of high Tc superconductors using the high‐resolution Faraday effect (HRF). The remanent state consists of domains with trapped flux and of domains with opposite polarity, separated from each other by an annihilation zone. It is shown that the stray fields of microstructural defects contribute to the nucleation process. Using the capability of the HRF technique to observe dynamic processes, experimental evidence for the different signs of the vortices is given. The influence of the pinning forces on this behavior is also shown.

Flux penetration of melt‐processed YBa2Cu3O7−δ : Direct observation of anisotropy

Flux distributions observed on melt‐processed YBa2Cu3O7−δ samples are presented with the external magnetic field oriented parallel and perpendicular to the c axis using the high‐resolution Faraday effect technique. In this way, flux penetration along and across the Cu–O planes is observed directly. The measured flux density profiles allow a determination of the different critical current densities and show the influence of intrinsic pinning.

Direct observation of flux-creep in high-Tc superconductors using the high-resolution Faraday effect

Abstract Using the capability of the high-resolution Faraday effect (HRF) to allow dynamic observations of the Shubnikov phase, we directly observed effects of flux-creep in high - T c superconductors. In this paper we show measurements on (RE) Ba 2 Cu 3 O 7-δ with RE=Y, Dy, Gd and on Bi 2 Sr 2 CaCu 2 O 8−δ single crystals and also on YBaCuO thin films and sintered materials. Furthermore, this technique enables us also to measure the flux creep locally. From the obtained photographs we determine the time dependence of the total flux in the samples. Using these data and the model of Hagen et al. the effective activation energies have been determined for these materials.

Study of pinning behaviour in YBaCuO using the high-resolution faraday effect

The magnetic flux-penetration into single- and polycrystals has been studied by means of the magneto-optical Faraday effect. The flux density profile in YBaCuO single- and polycrystals reveals a hyperbolic behaviour in contrast to the parabolic decrease as observed in Nb. The volume pinning forces fp in grains of sintered specimens determined according to Friedel et al.1, show a reciprocal dependence on the grain size of sintered samples. This points to surface pinning effects.