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Title of Thesis

Institute/University/Department Details
Department of Physics/ Quaid-i-Azam University, Islamabad
Number of Pages
Keywords (Extracted from title, table of contents and abstract of thesis)
flux dynamics, ybco, ac fields, sweep rates, vortex dynamics, magnetization, superconductors

This thesis describes experimental investigations related to the vortex dynamics in several high Tc superconductors. YIBa2Cu307-6 (bulk granular, bulk textured, single crystals and thin films) and BSCCO (2223) (bulk) samples have been studied using dc and ac magnetization techniques to determine the effects of granularity, anisotropy, pinning strength and geometry. The samples have been characterized using electron microscopy, resistivity, ac susceptibility and dc magnetization measurements.

The hysteresis in the ac susceptibility response has been compared for granular, melt textured Y BCO and BSCCO. We identify a crossover field HCO which identifies the boundary between inter and intra-granular response being dominant. The variation of HCO with ac field amplitude, frequency, temperature and field cooling effects corresponds to the behaviour expected from the variation of penetration depth גּ. Differences in the hysteretic behaviour due to pinning are also identified.

We have studied the suppression of the dc magnetization of melt textured and single crystal Y BCD sample in crossed ac and dc fields. Magnetization has been studied as a function of ac field amplitude, frequency, dc field sweep rate, time and orientation (hacII c,hac II ab - plane) . We observe that the dc magnetization decreases linearly with ac amplitude but logarithmically with ac field frequency, in a swept field configuration. The rate of decrease (1/M0) (ˆ†M/ˆ†In f) is observed to be close to the flux creep rate and this variation has been explained quantitatively. Magnetization in crossed and uncrossed configuration obeys the logarithmic time variation in the long time domain. For short times, relaxation shows strong effects of frequency and relate the In f behavior and flux creep effects. These crossflux measurements confirm the homogenizing of the dc flux distribution due to the ac fields and the competing effect of the field sweep.

The effects of thin film geometry on the critical state magnetization in crossed and uncrossed configurations have been studied for ultra thin Y BCO films in perpendicular geometry (H II c - axis). At high temperatures we obtained an asymmetric dc field sweep rate effect, with the moment being relatively unaffected by the sweep rate in the decreasing field branch. Relaxation measurements showed the same features, confirming that it is an artifact of asymmetric flux dynamics in ultra thin films, at high temperatures. Deviations from perpendicular geometry reduce the effect of asymmetry. The dependence on temperature and orientation leads us to conclude that the asymmetry originates in the unique characteristics of the critical state for very thin films (T « A, where A is the penetration depth) in perpendicular applied fields and the effect of the field line curvature therein. We also report the effects of effectively crossed ac and dc fields on the dc magnetization of thin films in the perpendicular geometry. The data shows very strong suppression, indeed collapse of the hysteresis in small ac fields. The tanh (hac) dependence of the decrease has been explained quantitatively using the critical state model for ultra thin films.

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4196.53 KB
S. No. Chapter Title of the Chapters Page Size (KB)
1 0 Contents
318.97 KB
2 1 Introduction 21
355.52 KB
  1.1 Vortices In Superconductors 22
  1.2 Vortex Dynamics 25
  1.3 Critical State Model 29
  1.4 Barriers To Flux Motion 31
  1.5 Crossed Flux Effects 35
  1.6 Motivations 37
3 2 Sample Preparation, Characterization And Experimental Techniques 41
448.47 KB
  2.1 Introduction 41
  2.2 Magnetization Measurement Techniques 42
  2.3 Ac Susceptibility Measurements 46
  2.4 Sample Characterization Details 52
4 3 Ac Susceptibility Hysteresis In High T C Superconductors 65
622.39 KB
  3.1 Introduction 65
  3.2 Experimental Results And Discussion 71
  3.3 Conclusions 96
5 4 Effects Of Crossed Ac Magnetic Field On The Demagnetization 99
776.41 KB
  4.1 Introduction 99
  4.2 Experiments And Results 108
6 5 Flux Dynamics In Thin Films 138
915.87 KB
  5.1 Introduction 138
  5.2 Asymmetry In Sweep Rate And Relaxation Studies 153
  5.3 Experiments And Results 154
  5.4 Discussion And Conclusions 174
  5.5 Response In The Presence Of Ac And De Fields
  5.6 Experiments And Results : 179
  5.7 Discussion And Conclusions 189
7 6 Summary Of Conclusions 191
58.81 KB