Trapped flux, diamagnetic shielding, and Meissner effect in a disk of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$

The trapped flux, diamagnetic shielding, and Meissner effect in polycrystalline ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ disks have been investigated by the measurement of the distribution of magnetic field across the sample at 77 K. The measurements were performed for samples cooled in zero and nonzero external magnetic fields. The distribution of the magnetic field was studied both in the presence of an applied magnetic field and when this field was switched off. The results show the relationship between shielding effect, Meissner effect, and flux pinning effect. Partial creep of flux into the disk was observed to occur for zero-field-cooled samples in applied magnetic fields as low as 1 G. The time decay of the trapped flux was observed to be logarithmic in time. The logarithmic decay rate increases linearly with the magnitude of the trapped magnetic field.