| Summary: | An efficient non-destructive magneto-optic imaging system was developed that employs
special iron-garnet indicator films to image magnetic fields in high temperature
superconductors. The flexibility of the system allows the investigation of many different
types of samples having different sizes and characteristics.
Two very high quality, low pinning, thin, flat YBa₂Cu₃O₆.₉₅ single crystals
are imaged using this system to observe field penetration dynamics. Slightly
different pinning characteristics of both samples enable a comparison to be made
between the different flux penetration dynamics observed. In one ultra low pinning
crystal, penetrating flux is found to accumulate in the sample's central regions
while avoiding the inner edges and in the other higher pinning crystal flux tends
to initially avoid both the central regions and sample's inner edges. These results
compare favourably with a theoretical model of Zeldov et al.[21] that characterizes
penetration phenomenon in low pinning superconductors.
Field penetration of a Tl₂Ba₂CaCu₂ thin film is also observed for different
sample temperatures up to the sample's superconducting transition temperature.
A new computational technique is employed to calculate the critical current of the
film using imaged field strengths. A comparison of the imaged fields and calculated
currents is made with a theoretical model devised to characterize field penetration
in superconductors having a thin circular geometry. From these results the critical
current temperature dependence is established and is fitted to a simple phenomenon
logical model. A small discontinuity in the data 10 K below the sample's transition
temperature is also noted and is examined. === Science, Faculty of === Physics and Astronomy, Department of === Graduate
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