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" Vortex flux pinning in type-II superconductors "
M.-K. A. M. Hasan
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Latin Dissertation
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| Language of Document
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English
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| Record Number
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1113082
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| Doc. No
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TLpq304279673
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| Main Entry
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M.-K. A. M. Hasan
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| Title & Author
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Vortex flux pinning in type-II superconductors\ M.-K. A. M. Hasan
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| College
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University of Illinois at Chicago
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| Date
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1995
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| student score
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1995
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| Degree
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Ph.D.
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| Page No
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75
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| Abstract
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Rotational magnetization vector measurements on polycrystalline samples of usd\rm YBa\sb2Cu\sb3O\sb7usd (YBCO) and (Ba, K)BiO3 at various fixed fields (H) and temperatures (T) reveal that the vortex flux density (B) in a rotational state consists of a component B{\rm R}, which rotates rigidly with sample rotation, and a BF component, which stays at a fixed frictional angle usd(\theta\sb{\rm F})usd relative to H. Also, BR decreases and ultimately vanishes with increasing H, while BF grows monotonically, implying that the vortex pinning strength have a broad distribution. This has been confirmed by the measurements on YBCO of the remanent flux density Brm which can be decomposed analogously into Busd\rm\sb{R}\sp{rm}usd and Busd\rm\sb{F}\sp{rm}usd at angle usd\rm\theta\sb{F}\sp{rm}usd relative to H. The quantity Hsinusd\theta\sb{\rm F}usd,, which at equilibrium equals usd\tau\sb{\rm p}/\muusd (the average pinning torque per vortex of moment mu) decreases with increasing high H. This result and the distribution in the strength of the pinning are shown to be consistent with the collective pinning process of vortex bundling. At fixed H, usd\tau\sb{\rm p}usd decreases rapidly with increasing T, varying approximately as T for both samples. For polycrystalline YBCO at 4.2 K, BR and BF are found to relax differently with time. The negative creep sign of BR indicates that the number of rotational vortices decreases with time, whereas BF shows a positive creep with a negative change in usd\theta\sb{\rm F},usd which indicates that more frictional vortices enter the sample with a tendency of alignment in the direction of H. For grain-oriented YBCO at 4.2 K, the vortex creep measurements of B along the c-axis at different fields showed that: whenever the hysteretic changes of H are reversed in sign, the vortex flux creep (dB/dlogt) decreases very rapidly to zero, where it lingers before changing sign. At the same turning values of H, (dB/dH) also goes to zero. These properties are attributable to the reversals of the vortex motion which occur at the turning values of H and cause a reversal of frictional pinning forces.
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Condensation
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Pure sciences
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yttrium barium copper oxide
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