Showing papers by "Tom H. Johansen published in 2001"
TL;DR: In this paper, magneto-opitcal studies of a c-oriented epitaxial MgB2 film show that below 10 K the global penetration of vortices is dominated by complex dendritic structures abruptly entering the film.
Abstract: Magneto-opitcal studies of a c-oriented epitaxial MgB2 film show that below 10 K the global penetration of vortices is dominated by complex dendritic structures abruptly entering the film. We suggest that the observed behavior is due to a thermo-magnetic instability which is supported by vortex dynamics simulations. The instability is also responsible for large fluctuations in the magnetization curves in MgB2 at low temperatures.
148 citations
TL;DR: In this article, a simple model based on the existence of two optical transitions of diamagnetic character, one tetrahedral and one octahedral, was proposed to interpret the Faraday rotation spectra of single crystal films of bismuth-substituted ferrite garnets.
Abstract: Single crystal films of bismuth-substituted ferrite garnets have been synthesized by the liquid phase epitaxy method where gadolinium gallium garnet substrates are dipped into the flux. The growth parameters are controlled to obtain films with in-plane magnetization and virtually no domain activity, which makes them excellently suited for magnetooptic imaging. The Faraday rotation spectra were measured across the visible range of wavelengths. To interpret the spectra we present a simple model based on the existence of two optical transitions of diamagnetic character, one tetrahedral and one octahedral. We find excellent agreement between the model and our experimental results for photon energies between 1.77 and 2.53 eV, corresponding to wavelengths between 700 and 490 nm. It is shown that the Faraday rotation changes significantly with the amount of substituted gallium and bismuth. Furthermore, the experimental results confirm that the magnetooptic response changes linearly with the bismuth substitution.
126 citations
TL;DR: In this paper, a real-time imaging of individual vortices in a NbSe2 single crystal using polarized light microscopy is presented, which enables observation of the static vortex lattice as well as single vortex motion at low flux densities.
Abstract: We demonstrate here real-time imaging of individual vortices in a NbSe2 single crystal using polarized light microscopy. A new high-sensitivity magneto-optical (MO) imaging system enables observation of the static vortex lattice as well as single vortex motion at low flux densities.
113 citations
TL;DR: In this paper, the magneto-optical imaging technique has been improved to enable single vortex observation at low flux densities, which is the main advantage of the new method is its high temporal resolution combined with the applicability to any superconducting sample with a flat surface.
Abstract: We present here a new experimental tool for the direct observation of magnetic vortices in type-II superconductors. The magneto-optical imaging technique has been improved to enable single vortex observation at low flux densities. The main advantage of the new method is its high temporal resolution combined with the applicability to any superconducting sample with a flat surface. We give a short description of the experimental set-up and show examples of results obtained for a NbSe2 single crystal at 4.0 K.
104 citations
TL;DR: In this paper, a c-oriented MgB2 film with a critical current density of 107 A cm-2 was analyzed at temperatures below 10 K. The static and dynamic properties of the dendrites were discussed.
Abstract: Magneto-optical studies of a c-oriented MgB2 film with a critical current density of 107 A cm-2 demonstrate a breakdown of the critical state at temperatures below 10 K. Instead of conventional uniform and gradual flux penetration in an applied magnetic field, we observe an abrupt invasion of complex dendritic structures. When the applied field subsequently decreases, similar dendritic structures of the return flux penetrate the film. The static and dynamic properties of the dendrites are discussed.
86 citations
TL;DR: Magneto-opitcal studies of a c-oriented epitaxial MgB2 film with critical current density 10^7 A/cm^2 demonstrate a breakdown of the critical state at temperatures below 10 K as discussed by the authors.
Abstract: Magneto-opitcal studies of a c-oriented epitaxial MgB2 film with critical current density 10^7 A/cm^2 demonstrate a breakdown of the critical state at temperatures below 10 K [cond-mat/0104113]. Instead of conventional uniform and gradual flux penetration in an applied magnetic field, we observe an abrupt invasion of complex dendritic structures. When the applied field subsequently decreases, similar dendritic structures of the return flux penetrate the film. The static and dynamic properties of the dendrites are discussed.
68 citations
TL;DR: In this paper, the dynamics of magnetic flux distributions across a YBaCuO strip carrying transport current is measured using magneto-optical imaging at 20 K. The current is applied in pulses of 40-5000 ms duration and magnitude close to the critical one, 5.5 A.
Abstract: The dynamics of magnetic flux distributions across a YBaCuO strip carrying transport current is measured using magneto-optical imaging at 20 K. The current is applied in pulses of 40-5000 ms duration and magnitude close to the critical one, 5.5 A. During the pulse some extra flux usually penetrates the strip, so the local field increases in magnitude. When the strip is initially penetrated by flux, the local field either increases or decreases depending both on the spatial coordinate and the current magnitude. Meanwhile, the current density always tends to redistribute more uniformly. Despite the relaxation, all distributions remain qualitatively similar to the Bean model predictions.
38 citations
TL;DR: It is shown that when a region with flux is invaded by antiflux the interface can become unstable if there is a relative tangential flux motion, similar to the instability of the tangential discontinuity in classical hydrodynamics.
Abstract: A possible mechanism of the macroturbulence instability observed in fluxline systems during remagnetization of superconductors is proposed. It is shown that when a region with flux is invaded by antiflux the interface can become unstable if there is a relative tangential flux motion. This condition occurs at the interface owing to the anisotropy of the viscous motion of vortices. The phenomenon is similar to the instability of the tangential discontinuity in classical hydrodynamics. The obtained results are supported by magneto-optical observations of flux distribution on the surface of a YBCO single crystal with twins.
27 citations
TL;DR: In this paper, a cylindrical clamp is used to prevent radial expansion in high-T c superconductors, which is necessary to achieve the highest trapped magnetic fields in today's bulk HTSs, and an exact solution is obtained for the isotropic magnetoelastic problem with the superconductor behaving magnetically according to the critical state model.
Abstract: External enforcement using, e.g., a stainless steel hoop, is necessary to achieve the highest trapped magnetic fields in today's bulk high- T c superconductors (HTSs). Presented here are calculations of the pinning-induced stress during magnetic activation of HTS bulks enforced by a cylindrical clamp which prevents radial expansion. An exact solution is obtained for the isotropic magnetoelastic problem with the superconductor behaving magnetically according to the critical-state model. For the activation process where the applied field is removed after field cooling in the full penetration field B p , it is found that the ideal clamp reduces the overall highest tensile stress by 22%. For an activation applying a field pulse (half-cycle) up to 2 B p , the maximum local tension is reduced by 40%. Significant differences are found also in the spatial distributions of the stress.
16 citations
TL;DR: In this article, an electrodynamic model that describes the dispersion of magnetostatic surface waves in ferrite/superconductor structures is suggested and a new approach to determining the microwave sheet resistance RS of superconducting films in a magnetic field is elaborated.
Abstract: An electrodynamic model that describes the dispersion of magnetostatic surface waves in ferrite/superconductor structures is suggested. On its basis, a new approach to determining the microwave sheet resistance RS of superconducting films in a magnetic field is elaborated. The values calculated (RS=0.20–0.96 mΩ) agree with results obtained by the Tauber method. For YIG/YBCO structures, the controllable phase shift is about 1.5π when the depth of magnetostatic wave penetration into the YBCO film varies from 2.0 to 0.8 µm.
11 citations
TL;DR: In this paper, the authors used magneto-optical imaging to verify the applicability of the critical-state model and found deviations from this symmetry at large temperatures, which demonstrates a departure from the critical state behavior.
Abstract: The critical-state in a thin strip of ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ is studied by magneto-optical imaging. The distribution of magnetic flux density is shown to have a specific symmetry in the remanent state after a large applied field. The symmetry was predicted [Phys. Rev. Lett. 82, 2947 (1999)] for any ${j}_{c}(B),$ and is therefore suggested as a simple tool to verify the applicability of the critical-state model. At large temperatures we find deviations from this symmetry, which demonstrates a departure from the critical-state behavior. The observed deviations can be attributed to an explicit coordinate dependence of ${j}_{c}$ since both a surface barrier and flux creep would break the symmetry in a different way.
TL;DR: In this article, the relationship between the residual loss and the deviations of the magnetic field from perfect axisymmetry in a superconductor magnetic bearing was investigated, where the drag torque acting on a high temperature supercondonductor slowly rotating in a nonaxisymmetric magnetic field was studied.
Abstract: To further understand the relationship between the residual loss and the deviations of the magnetic field from perfect axisymmetry in a superconductor magnetic bearing, the drag torque acting on a high temperature superconductor slowly rotating in a nonaxisymmetric magnetic field was studied. A circular YBaCuO disk is rotated about its central axis, in the presence of a nonaxisymmetric magnetic field created from a pair of coils placed in its vicinity. This nonaxisymmetric field is either a uniform, or a uniform gradient magnetic field perpendicular to the axis of rotation of the YBaCuO disk. The torque exerted by the coils on the YBaCuO disk was monitored as a function time as the rotation proceeds, for different values of the current being passed through the coil. This torque exhibits periodic variations with a nonzero mean drag component which can be related to power loss, while the periodic variations were found to be connected to initial conditions and/or nonuniformity of the YBaCuO disk itself.