H. Theuss

Current density and magnetic field distribution in hard thin film superconductors

Abstract In thin film superconductors with magnetic fields applied perpendicular to the film plane, demagnetization effects lead to strong deviations of the flux density distribution B ( r ) from the predictions of the conventional critical state model. In particular if the magnetization M is taken as a probe for the critical current density j c of a thin film, the radial stray field component B r parallel to the film plane has to be taken into consideration. Especially in thin circular films with thicknesses lower than the London penetration depth, λ, the circular critical current density, jc=1/μ0(∂Bz/∂B−∂Bz/∂r), is determined by the gradient ∂Br/∂z rather than by ∂Bz/∂r. Using Bz(r)-data, measured by means of the magneto-optical Faraday effect on c-axis oriented YBa2Cu3O7−δ-films, we perform a selfconsistent calculation of the radial component Br(r), of the magnetic induction as well as of the current density distribution, jc(r), and the volume pinning force, fp.

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.

The influence of a point defect structure on the magnetic properties of YBa2Cu3O7-δ polycrystals

Abstract Extensive measurements of magnetic hysteresis loops of YBa 2 Cu 3 O 7- δ polycrystals have been carried out presenting a survey of the magnetic properties of this material. The results are discussed within the framework of the critical state model in order to determine the volume pinning force F p and the critical current density j c . Two different point defect structures induced by low temperature electron irradiation and successive annealing experiments were examined. Although both treatments mainly change the oxygen arrangement, only the electron irradiation caused an enhancement of the critical current.

Flux line pinning by point defects in single crystalline high-Tc superconductors

Abstract The temperature and time dependence of the magnetization M of single crystalline high- T c YBa 2 Cu 3 O 7−δ and Bi 2 Sr 2 Ca Cu 2 O 8+δ samples is analysed in order to determine critical magnetization currents j c as well as pinning potentials U . The evaluation of the experimental data within the framework of critical state and flux creep theories is performed under the assumption of a distribution of activation energies. These energy spectra, as well as the temperature dependence of the critical current density, turn out not to depend significantly on crystal orientations relative to the applied magnetic field. Flux bundle volumes V that are coherently moved in an activation process seem to be much smaller than the volume of a single flux line. From these results we conclude that the dominant pinning process in as-grown single crystalline high- T c superconductors is given by the interaction of flux line segments with point defects, located in the Cuue5f8O planes. Furthermore, the energy spectra, resulting from evaluations considering first-order kinetics, are temperature independent and give a natural explanation for the increase of the effective potential U with temperature, if the barrier heights U are determined within a single barrier model.

Magnetic properties of Y1−xGdxBa2Cu3O7−δ polycrystals

Abstract Y 1− x Gd x Ba 2 Cu 3 O 7−δ polycrystals are investigated with the purpose to obtain information about the influence of the paramagnetic Gd moments on the superconducting properties. Gd substitution does not affect the superconducting properties seriously but the paramagnetic Gd moments are just superposed to the superconducting diamagnetic moment without significant interaction. Magnetically the substituted Gd atoms behave, as if they were free Gd 3+ ions. Relaxation experiments yield activation energies for thermally activated flux motion which are independent of the Gd content x but the critical current density, j c , decreases with increasing Gd content. Therefore the conclusion is drawn, that the pinning potential gets broader under the influence of Gd substitution but the barrier height remains constant. The maximum in the j c ( H ) curve which has become well known as the “fishtail-effect” is shifted to lower fields as the Gd content is increased. At temperatures below ≈ 10 K antiferromagnetic ordering of the Gd moments is observed, mainly for the fully substituted sample, GdBa 2 Cu 3 O 7−δ .

Analysis of magnetic relaxation in type-II superconductors

Abstract Three different methods for the analysis of magnetic-relaxation data, M ( t, T ), are presented: In one model a spectrum, p ( U ), of activation energies U is considered, whereas the other model is based on the existence of a single barrier, permitting arbitrary dependences U ( j ∼ M ), where j denotes the current density. These two models are applied to experimental relaxation data, carried out on Bi 2 Sr 2 CaCu 2 O 8+δ single crystals. The results are not in contradiction with each other but probe different aspects of the creep process. In consequence the time decay of the magnetization, M ( t, T ), can be interpreted either by the existence of a spectrum p ( U ) of activation energies or as being due to a single activation energy, U ( M ( t )), that increases with time. In a third, unique model, the combination of a spectrum p ( U ) assuming a linear relation U ∼ j is considered. From theoretical investigations of demagnetization effects the conclusion is drawn, that magnetic relaxation of samples with extreme geometries, i.e. thin films, has to be considered carefully, because there are stray-field effects contributing to the relaxation function, that do not necessarily follow the same time law.

Influence of geometry on angle-dependent critical current density measurements in thin-film superconductors

Abstract A simple model is presented describing the angular dependence of the critical current density jc in thin isotropic superconducting films resulting from demagnetization effects. In this model the angular dependence jc(θ) arises from the field dependence jc(B(θ)), if θ is the angle between the film normal and the homogeneous external magnetic field. The calculated jc(θ) curves are qualitatively similar to measurements on highly anisotropic high-Tc superconducting films, which are usually interpreted as resulting from intrinsic pinning of flux lines between the Cu-O planes. In conclusion, the analysis of the experimental jc(θ) data carried out on anisotropic thin films should take into account the significant shape effects that lead to a behavior which is similar to the behavior expected for anisotropic thin films due to intrinsic pinning.

Direct observation of thermally activated flux motion in high-Tc superconductors

Abstract The time dependence of the local magnetic flux density B(r, t) and of the magnetic flux Φ(t) is observed directly and dynamically in epitaxially grown high temperature superconducting thin films using the high resolution magneto-optical Faraday effect. After zero field cooling and application of an external magnetic field, the magnetic flux penetrates spontaneously into the sample. Subsequently, at constant applied field the flux front further penetrates towards the center of the Y1Ba2Cu3O7−x films due to thermal activation. The radial flux density distribution shows a characteristic time dependence. The magnetic flux obeys Φ(t) = ∫ S B ( r , t) d f = n(t) × Φ 0 , and accordingly the number n(t) of flux lines each carrying a flux quantum Φ0 into the sample increases following a typical ln (t/τ)-law.

Paramagnetic properties of oxygen deficient YBa2Cu3O7−δ polycrystals

Abstract At a series of oxygen deficient YBa2Cu3O7−δ polycrystals with 0≲δ≲0.6 the temperature dependent magnetic susceptibility was investigated at temperatures above the critical temperature Tc. Fitting these data to a Curie-Weiss-law χ(T)=C/(T−Θ)+χ0 provides a method to derive the number of Bohr magnetons μB per Cu2+ ion in the Cu-O chains. Considering nearly full stoichiometric samples with δ≈0, this quantity is in accordance with the value normally found in Cu2+ compounds, but it decreases with increasing δ. The increase of the temperature independent part χ0 of the susceptibility with the oxygen stoichiometry 7−δ is correlated with an increase of the density of states N(EF) at the Fermi level EF and provides a presupposition for a rising occupation of conducting charge carriers with the oxygen content.

Magnetic relaxation and activation energies of Xe irradiated DyBa2Cu3O7-δ single crystals

Abstract Magnetic-relaxation measurements M(t, T) have been carried out on DyBa 2 Cu 3 O 7−δ single crystals before and after irradiation with 340 MeV Xe ions. The data are analyzed within the framework of first-order kinetics assuming a distribution of activation energies. The results are in good agreement with the assumption, that the most important pinning mechanism is given by the core interaction of pancake vortices or short sections of a flux line with defects. The irradiation-induced enhancement of the average activation energies of just about 70% can be explained by the broadening of the flux-line core if the flux line is located in the amorphous channels with radius R > ξ caused by irradiation damage.