Year

2005

Degree Name

Doctor of Philosophy (PhD)

Department

Institute for Superconducting and Electronic Materials - Faculty of Engineering

Abstract

Magnetic moment measurements performed on a single crystal sample of (Tl,Pb)(Sr,Ba)[subscript 2]Ca[subscript 2]Cu[subscript 3]Oy reveal an increase in magnetic moment at intermediate and high fields over a wide temperature range below H[subscript c2] Magnetic hysteresis (m-H) loops were taken at different constant temperatures using either a MPMS-5T SQUID magnetometer or a PPMS-9T magnetometer and within the temperature range 0.1 < T/Tc < 0.85. The main objective of this thesis was to study the mechanisms involved in the emergence of the peak in the magnetic hysteresis loops in the two samples (the so-called peak effect). It is argued that point disorder in the form of oxygen vacancies or substitution is responsible for pinning at intermediate and high field. Vortex dynamics at fields below and above the peak in hysteresis loops for the sample are shown to be well explained by a quasi-lattice Bragg Glass state and elastic Collective and Plastic Vortex Creep, respectively. For this purpose, relaxation in magnetic moment was measured at different fields and temperatures. Following that, the field and temperature dependence of the normalized relaxation rate, S, the effective activation energy, U, and the critical exponent were calculated. Analysis of all these quantities in the material led to the conclusion that the transition from a highly ordered vortex lattice (Abrikosov lattice) to an amorphous vortex lattice is responsible for the rise in magnetic moment at low fields, and subsequently the crossover from this amorphous state to a plastically deformed vortex lattice, which is assisted by dislocations, is responsible for the suppression of the peak effect in m-H loops. The investigation of flux pinning by twin planes has attracted considerable attention, since naturally-formed twin planes occur regularly in Y123 crystals. One of the characteristic properties of vortex pinning by twin planes is a cusp-like dependence of the irreversibility field (H[subscript irr]) on the angle (?) between the twin plane and the magnetic field. The cusp appears around (?) = (?), within the angular range (?). The angle (?) is called the lock-in angle. The measurement of the angular dependence of H[subscript irr] has been the principal method used to study the locking of magnetic vortices by twin planes and the determination of (?). However, due to the high irreversibility field of 123 systems, such measurements could be conducted only at high temperatures, close to T[subscript c], at which the irreversibility field is subject to a high degree of uncertainty. Further, H [subscript irr] is defined by an arbitrary value of magnetization, at which the ascending and descending branches of the magnetic hysteresis loop, or the zero-field cooled and field-cooled branches of the temperature dependence of the magnetization, are seen to merge. Because of this, it is highly desirable to develop an alternative method for obtaining (?), which relies on universal physical principles instead of arbitrary definitions. Such a method is presented in this work. This method was developed for an off-stoichiometric Sm123 single crystal. Due to the global behaviour of the hysteresis loop at high temperatures close to T[subscript c] for twinned superconductors this method is not restricted to this compound and can be used for all twinned superconductors irrespective of their microstructures or oxygen contents. This method for determining the lock-in angle (?) for pinning of the vortices on extended defects, i.e. twin planes here, was employed without any pre-assumed criterion for defining (?). Appropriate scaling of the hysteresis loops for different angles close to the c-axis led to a clear discrimination between the two vortex dynamics regimes. From this scaling the lock-in angle was determined to be 6 degrees � 0.1 degrees, which means that the uncertainty in determining the lock-in angle using this method is considerably less than with other existing methods.[Note: this abtract contained scientific formulae that would not come across onthis form. Please see the 01Front files for full details].

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.