Flux creep after field trapping in YBa₂Cu₃O_x foams

The time-dependence of the field distribution on the surface of YBa₂Cu₃O_x (YBCO) foam samples after field trapping is analysed. The foam samples were magnetised using a bulk permanent magnet at 77 K, and the trapped fields (TFs) were recorded with a scanning Hall probe 1 mm above the sample surface. Besides a large TF peak, several small peaks are observed. The time dependence of the local fields of these peaks and of the large peak are clearly different, which points to a different origin. In this way, the time-dependent TF measurements reveal important information about the current flow in the foam samples. A non-logarithmic relaxation process takes place in the foam samples. Furthermore, we compare these results with classic creep measurements performed on an individual foam strut removed from the bulk. The creep rate for the TF distribution is found to be ∼8%, whereas the creep rate of the foam strut is about 4% in a large temperature and field range (20-60 K, 0-2 T).