The influence of geometry on the magnetic properties of YBCO
Abstract
The purpose of the present study was to investigate the influence of geometry on the magnetic properties of the high temperature superconductor YBa$\sb2$Cu$\sb3$O$\sb{7-\delta}$. In particular, this study was interested in addressing what effects a flat geometry has on the magnetic properties of a superconductor when a magnetic field is applied perpendicular to the flat plane. Much of the past data concerning magnetic hysteresis in these materials has been collected using bulk magnetic measurement techniques. Unfortunately, bulk measurements must be interpreted using models which describe physical behavior apparent only at smaller length scales. To avoid this interpreted step, and to test some traditionally accepted ideas regarding the spatial dependence of current density in these samples, the present study uses a novel approach to observing the local magnetic behavior at the surface of these superconductors. The local magnetic flux density was directly monitored using micro Hall sensors placed at the surface of the sample. It was found that demagnetization effects are extremely important for large aspect ratio samples and that the observed magnetic behavior cannot be described by the standard elliptical approximation. A modified critical state model valid for thin samples was developed which incorporates demagnetization effects for the perpendicular geometry. This model can be used to explain the observed magnetic hysteresis, magnetic relaxation, and the effects transport current has on the critical state. Novel evidence for surface barriers in YBCO thin films is briefly presented. A geometrical barrier to flux penetration is found to exist in a detwinned YBa$\sb2$CU$\sb3$O$\sb{7-\delta}$ single crystal and also in a YBa$\sb2$CU$\sb3$O$\sb{7-\delta}$ polycrystalline system. This barrier cannot exist in samples which have elliptical cross-sections and is the result of the flat geometry.
Degree
Ph.D.
Advisors
Crooker, Purdue University.
Subject Area
Condensation|Electromagnetism
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