Scanning Hall Micro-Probe Measurements of Yttrium BARIUM(2) COPPER(3) OXYGEN(7-Y)

A new scanning Hall micro-probe technique was developed to measure the magnetic field around superconductors. This was used to make precision measurements of the 'critical state' profiles in single crystals of YBa_2 Cu_3O_{7 -y}.. With this technique we discovered an unexpected 'field induced rotation' of the magnetization profile in single crystal YBa_2Cu_3 O_{7-y}. The remanent (trapped flux) state was studied and for crystals with a rectangular aspect ratio, the remanent ridge was found to be rotated. This rotation takes on an opposite sense when the field is reversed, leading to a field induced asymmetry. An explanation using flux flow arguments containing a giant Hall angle may be relevant, however this argument predicts the wrong sign for the rotation in the remanent state. This unexpected phenomemon disappears above 25K, and also coincides with a collapse of the remanent peak in an applied field. In addition, for zero field cooled samples, there are positions on the superconductor where the measured fields actually oppose the applied fields. These three effects were not expected and are not understood at the present time. The time dependent behaviour of these magnetic fields was also studied. Precision measurements of the relaxation of the remanent magnetic fields in single-crystal YBa_2Cu_3O _{7-y} have been made and compared with more conventional magnetometer measurements. The vortex glass theory of Fisher, Fisher, and Huse that predicts a slightly modified Anderson-Kim logarithmic decay was compared to the results and found to be inadequate. We find that, for all temperatures from 4.2 to 85K, the relaxation follows a power law decay B(t)~(1+t/tau) ^{-1/sigma} more convincingly than a logarithmic decay. The detailed temperature dependence of the exponent (1/sigma) is compared with a recent calculation by Vinokur, Feigel'man, and Geshkenbein predicting such a power law decay. The field around a superconducting crystal is calculated, enabling a quantitative analysis of the critical state profiles and a novel method to determine J _{C}. The invasion of magnetic flux above H_{C1} was examined and this provides direct evidence for the existance of a surface barrier inhibiting the entry of magnetic flux lines.