Effects of small magnetic fields on the critical current of thin films

Abstract

Magnetic fields applied perpendicularly to superconducting thin films may produce dendritic patterns, where penetrated and Meissner regions coexist, as observed in Nb, YBCO and MgB/sub 2/. Due to large demagnetizing factors in the perpendicular geometry, small fields may be enough to drive portions of the sample into the mixed state. Lack of symmetry and local defects might then permeate the dendritic mode. This mode, in turn, can be detected by AC-susceptibility measurements, since penetrated fingers act as intergranular material, which reveals itself in a characteristic manner. Films of a few hundred nanometers thick and millimeter lateral sizes, develop dendrites even when submitted to Earth's field, what drastically limits the critical current, J/sub c/. This contribution studies how this field-induced granularity is further affected by the action of excitation AC-fields of small magnitudes and audio-frequencies.