Magnetization studies of non-collinear flux systems in high temperature superconductors

This thesis describes experimental investigations into the magnetic response of high T /sub c/ superconductors in the presence of two or more non collinear flux densities. Single crystals and melt textured samples of YBa/sub 2/Cu/sub 3/O/sub 7-x/ have been studies and characterised using electron microscopy, resistivity and ac susceptibility measurements and magnetization hysteresis. We report that the presence of a remnant flux density along an axis of the sample leads to enhanced flux entry in a transverse direction i.e. the c-axis. In the experiments in which the transverse flux density has been obtained in the form of a field cooled remanence, it is seen to decrease as a longitudinal field is applied. This expulsion of the transverse flux across the longitudinal flux lines could be made possible by flux cutting processes. We also report on relaxation measurements in the crossed flux configuration. The relaxation data have been it to a logarithmic time dependence M(t). The relaxation rate S increases in a strongly non linear manner with increasing transverse flux. The relaxation time constant has been found to be a macroscopic quantity, its variation with transverse flux being very similar to that of the relaxation rate S. The effect of varying sweep rates on the magnetization hysteresis response has also been investigated in the crossed flux configuration. The magnetization is found to be logarithmically dependent upon the sweep rate H. The data has been used to extract the sweep creep rate C and using the sweep creep data. Studies of flux systems with a flux orientation gradient have also been made suing the magnetization rotation technique for both the field cooled and the zero field cooled magnetic states. The data show significant shifts of the maxima and minima from their expected positions in M(0), both in increasing angles as well as upon reversal of the sense of rotation. The results have been explained using the concepts of the anisotropy of the flux line energy and the intrinsic torque within the framework of the generalized critical state model, which invokes the ideas of a flux orientation gradient in addition to a gradient of flux density