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Title of Thesis

Arif Mumtaz
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)
magnetic relaxation, polycrystalline, superconductors, relaxation process, hysteresis loop, thermally activated flux creep model, thermally assisted flux flow model, poly crystalline

In this thesis extensive experimental studies have been conducted to explore the magnetic relaxation effects at low fields in some high Tc superconductors. The studies have been made on both textured (Y1Ba2Cu3O7_X) and untextured (Y1Ba2Cu3O7_X) and HgBa2Ca2Cu3OX+8) polycrystalline samples. The magnetization rotation technique has been employed to identify texturing in the samples and the existence of inter and intra grain pinning.

We report that for short time window (t‰ 1000 sec), the relaxation follows a logarithmic time dependence of the form m(t) = mo( I-Sln(t/ t)). The effects of H and H on the logarithmic relaxation rate S and on m(H) loops themselves have been studied. An increase in S with H has been observed, at least for slow sweep rates and low fields. Consistent with the literature, we found that m (H, H) α InH . Comparisons of these results. are made with the flux creep and a dynamic magnetization model and it is shown that both the anisotropy in the rate dependence of dc magnetization and the absence of history dependence in ac susceptibility (with dc sweeps) is consistent with a dynamic magnetization model which takes into account the drift of vortices. For low fields, however, the flux creep contribution to rate dependence may still be very significant, as demonstrated by comparison of normalized relaxation rate l/mo(dm/dlnt) and l/mo(dm/dlnH).

We report the effects of a small field reversal (superposed on a large fixed field) on the relaxation process. The effects have been studied over a range of fields and rates of initial field sweep, to determine the role of initial conditions, on subsequent relaxation. The relaxation data thus obtained shows a stretched exponential behavior of the form exp [ -( t / t)n / n ], where n though dependent on initial conditions, was found to be close to 0.6. The results are analyzed using thermally activated flux flow model, and a close agreement is found between experiment and theory.

The correlation between the relaxation rate S(H) and the low field hysteresis loop width ˆ†m(H) has been studied both for Y and Hg based superconductors. We report that samples having a peak or a plateau in ˆ†m(H) display nonmonotonic behavior in the relaxation rate S, as a function of field. This is interpreted in the spirit of changes in activation energy barriers as ˆ†m and Jc change with field. In addition we observed that the logarithmic decay time constant t is a macroscopic quantity that varies as the inverse of applied field, as predicted by diffusion based collective creep models.

Download Full Thesis
5453.57 KB
S. No. Chapter Title of the Chapters Page Size (KB)
1 0 Contents
324.24 KB
2 1 Introduction
600.54 KB
  1.1 Superconductors in Magnetic Fields 1
  1.2 Critical State Model 4
  1.3 Thermally Activated Flux Creep Model 7
  1.4 Thermally Assisted Flux Flow Model 11
  1.5 Collective Creep Model 14
  1.6 Overview of some Recent Works and Motivation of this Thesis 15
3 2 Sample Preparation and Characterization
853.98 KB
  2.1 Introduction 22
  2.2 Synthesis of Poly crystalline YBaCuO 23
  2.3 Synthesis of Melt Textured Samples 33
  2.4 Synthesis of HgBaCaCuO Samples 47
  2.5 dc Measurements 49
  2.6 ac Susceptometer 53
4 3 Magnetic Relaxation and Low Fields Hysteresis Anomalies
853.05 KB
  3.1 Introduction 57
  3.2 Experiments 61
5 4 Effects of Sweep Rate and Reverse Field Drift
569.54 KB
  4.1 Introduction 84
  4.2 Magnetic Relaxation at Fixed Field 85
  4.3 Relaxation in the Presence of Small Field Drift 93
  4.4 Discussion of Results 99
6 5 Field Sweep Rate Dependence of dc and QC Magnetization
2443.5 KB
  5.1 Introduction 106
  5.2 Conclusions 129
  5.3 Summary of the Results 132
  5.4 References 135
  5.5 Preprints