Magnetooptic techniques coupled with high-speed cinephotography have been used to study flux jumps in a Nb-25 % Zr superconducting disk triggered by the application of an external magnetic field. It is shown that the exponential time constant z u which characterizes the penetration of the magnetic flux and the.flux jumpfield HIj are independent of the sweep rate of the applied magnetic field over three orders of magnitude. It is also demonstrated that magnetic coupling between the disk-shaped superconductor and a high-purity normal metal beneath it can significantly damp the motion of magnetic flux into the superconductor, thus enhancing its stability against flux jumps.
Quantitative imaging of stray fields and magnetization distributions in hard magnetic element arraysA magneto-optically active glass was used to image the magnetic field distribution in superconductors using the Faraday effect. Polarized white light illumination of the glass resulted in. various colors depending on the setting of the analyzing polaroid. These colors are shown to be consistent with the known dependence of the Faraday rotation angle on the applied magnetic field, the temperature of the glass, and the wavelength of the light. This technique was used to observe field distributions in poly crystalline and single-crystal YBa Z Cu 3 0 7 samples. In the ceramic sample, the field was uniform within the resolution (50 Itm) of this technique and field magnitudes were measured with a 10% accuracy. In the single crystal, the magnetic field distribution was not uniform showing field gradients imaged as color gradients on the pictures of the glass. Contours of constant magnetic field were drawn from these photographs and from these, a critical current density of 10 9 A/m 2 was deduced in an external field of 136 mT.
Using a magneto-optical technique, we have nondestructively imaged macroscopic variations in magnetic flux that penetrate laser-ablated, c-axis oriented, epitaxial YBa2Cu3O7 superconducting films deposited on crystalline (100)SrTiO3 substrates. Flux penetration into YBa2Cu3O7 films in an externally applied magnetic field is enhanced at crystalline substrate defect sites. A relationship between flux penetration and substrate defects is experimentally demonstrated with a YBa2Cu3O7 film deposited on an intentionally degraded crystalline SrTiO3 surface.
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