Observation of Transient Overcritical Currents in YBCO Thin Films using High-Speed Magneto-Optical Imaging and Dynamic Current Mapping

Abstract

The dynamics of transient current distributions in superconducting YBa2Cu3O7−δ thin films were investigated during and immediately following an external field ramp, using high-speed (real-time) Magneto-Optical Imaging and calculation of dynamic current profiles. A number of qualitatively unique and previously unobserved features are seen in this novel analysis of the evolution of supercurrent during penetration. As magnetic field ramps up from zero, the dynamic current profile is characterized by strong peaks, the magnitude of which exceed the conventional critical current density (as determined from static current profiles). These peaks develop close to the sample edges, initially resembling screening currents but quickly growing in intensity as the external field increases. A discontinuity in field and current behaviour is newly observed, indicating a novel transition from increasing peak current toward relaxation behaviour. After this transition, the current peaks move toward the centre of the sample while reducing in intensity as magnetic vortices penetrate inward. This motion slows exponentially with time, with the current distribution in the long-time limit reducing to the expected Kim-model profile.

3D magnetization currents, magnetization loop, and saturation field in superconducting rectangular prisms

“…Current loops close to the prism side surfaces are still nearly square ( figure 3). Non-square current paths in thin films with regions with |J| < J c were found in [14,15] from numerical modeling and in [27][28][29] from inversion of measured magnetic field maps. Although thin film models assume uniform J along the film thickness, their predictions are valid for the thicknessaveraged J of samples with small but finite thickness.…”

A new approach to the inverse problem for current mapping in thin-film superconductors

“…In detecting the magnetic field around superconducting samples, the magneto-optical imaging technique measures the local z-component of the magnetic field at each point in a Faraday-active indicator film, 9 which is placed at a finite distance h above the sample. In our experimental setup, h is taken to be the distance between the top of the sample and the bottom of the indicator film ( Fig.…”

“…Magnetic imaging techniques are increasingly prevalent in many fields of science and medicine, as they can be used to determine the properties of materials and provide powerful medical diagnostic tools through non-contact, non-destructive measurements. Such techniques in physics and materials science research include but are not limited to Magnetic force microscopy; 1 SQUID (Superconducting Quantum Interference Device) microscopy, 2,3 as well as the more recent invention of SQUID-on-tip microscopy, whereby a nanoSQUID is fabricated on a quartz tip; 4,5 Magneto-optical imaging (MOI) for ferromagnetic and superconducting materials, 6,7 and its dynamic version for rapid imaging of transient current effects; 8,9 lHall-probe microscopy; 6,[11][12][13][14] bitter decoration; 15 scanning magnetoresistive microscopy; 16 scanning electron microscopy with polarization analysis (SEMPA); 17 and electron holography. 18 In medical and biological applications, such techniques include optical magnetic imaging of living cells, 19 magnetoencephalography (MEG) of brain activity, [20][21][22] magnetocardiography (MCG) for magnetic heart signals, 20,23 and Magnetic Resonance Imaging (MRI) for producing images of soft tissues, organs, body, and brain.…”

“…20 The similar inverse problem of the Biot-Savart law also allows the calculation of plane current components from magnetic field measurements of a sample. This is particularly useful for superconductors and thin films, [7][8][9][10]28 ferromagnetic and superconducting hybrids, 29,30 and even magnetic flux quanta (Abrikosov vortices). 3 In practice, methods of magnetic field detection via magnetometers often measure only the out-of-plane z-component of the magnetic field extending from a sample.…”

Advanced magneto-optical Kerr effect measurements of superconductors at low temperatures

“…1,2 Prominent results can be extracted from magneto-optical Faraday microscopy based on visible light [3][4][5][6][7] or scanning x-ray microscopy (SXM) respectively photoemission electron microscopy (PEEM) based on polarized x-rays featuring higher spatial resolution. [8][9][10] In this paper we show that a magneto-optical Kerr effect (MOKE) set-up in combination with a soft-magnetic sensor layer and an elaborate measurement scheme allows direct access to supercurrents present in thin superconducting films.…”