Th. Schuster

EuSe as magneto-optical active coating for use with the high resolution Faraday effect

Abstract The high resolution Faraday effect requires a magneto-optical active thin film coating for observations of penetrated flux in superconductors. The use of EuSe for this coating offers many advantages, such as better handling during the evaporation process and a higher achievable contrast compared to the ‘classical’ mixture of EuS and EuF 2 . In this paper the apparatus is described and the conditions discussed for achieving optimal thickness of the magneto-optical layer. This technique was used for observations of penetrating flux in niobium and in the high T c superconductors YBa 2 Cu 3 O 7−δ (single crystals, sintered specimens and epitaxial thin films) and Bi 2 Sr 2 CaCu 2 O 8−δ single crystals.

Current capability of filaments depending on their position in (Bi,Pb)2Sr2Ca2Cu3O10+δ ‐multifilament tapes

The microstructures of single filaments of silver‐sheathed (Bi,Pb)2Sr2Ca2Cu3O10+δ‐multifilament tapes are investigated by polarized light and scanning electron microscopy. Large differences in the microstructure of filaments depending on their position in the cross section of the tape are found. The influence of this microstructure on the critical current density is investigated by magneto‐optics. A drastic reduction of the critical current density by a factor of about 10 of the outer, compared to the filaments in the center of the tape is observed.

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.

Critical current limiting structural defects in melt‐textured YBa2Cu3O7−δ

Structural analyses and investigations of flux penetration of typically melt‐textured YBa2Cu3O7−δ samples were performed. This allows regions of suppressed critical current density to be directly related to microstructural defects. At small‐angle grain boundaries the critical current density was found to be reduced by a factor of up to 20.

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.

Flux penetration in granular YBa2Cu3O7−δ samples: A magneto-optical study

Abstract Using the high-resolution Faraday effect (HRF) technique, flux distributions of sintered, granular YBa 2 Cu 3 O 7−δ samples are obtained. The flux penetration is found to occur stepwise and only the last step of the penetration of Abrikosov vortices into individual grains can be observed directly. The high spatial resolution of the HRF technique enables one to determine the flux-density profiles inside individual grains. From these profiles, the intragranular critical current densities and acting local pinning forces are obtained. The intragranular critical current densities and the corresponding volume pinning forces are found to be reciprocally dependent on the grain size. This behavior could be explained by pinning at the grain boundaries between superconducting YBa 2 Cu 3 O 7−δ grains and non-superconducting phases like BaCuO 2 assuming a disturbed layer Δ x with a thickness of approximately 0.1 μm. Furthermore, the influence of melt processing on the weak-link behavior can be directly observed in the flux distributions. Melt-processed samples are found to show an intermediate behavior of flux penetration between sintered, polycrystalline samples and single-crystalline materials like single crystals and epitaxial thin films. It is shown that the parameter γ = H g cl / H J cl , linking the lower critical field of the grains, H g cl , to the lower critical field of the matrix, H J cl , has a significant influence on the flux-penetration behavior in type-II superconductors.

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.

Influence of low magnetic fields on the transport properties of sintered YBa2Cu3O7- delta with different grain sizes

Using a four-point terminal method, measurements of critical current densities, Jcj(T, B), have been performed at magnetic fields mu 0Hext<or=30 mT in the temperature range 5 K<or=T<or=91 K on sintered YBa2Cu3O7- delta samples. The mean grain sizes of these samples range from 4 mu m up to 14 mu m. Determinations of the lower critical field, Hc1J (T), of the matrix phase and the corresponding Josephson penetration depth, lambda J(T), are presented. The dependence of Hc1J(T) and lambda J(T) on the mean grain size of the samples is shown. The grain size of sintered YBaCuO and the Josephson penetration depth turn out to be very important parameters for the attainable critical current density. A maximum current carrying capacity is reached if the mean grain size, (rG), and the Josephson penetration depth, lambda J(T) are similar in size. The results are discussed in terms of a weakly-coupled network of superconducting grains.