A finite size effect on vortex configuration is investigated by the measurements of current-voltage (I-V) characteristics on amorphous-W/Si multilayers as a function of the number of the bilayers N in the absence of field. Critical scaling analysis of the data for the samples with N ≥ 10 shows that the vortex loops are the favored excitation with the dynamic critical exponent z ∼ 3.32, while for 10 > N ≥ 3 it reveals to deviate from the scaling function corresponding with a finite size effect on vortex loops. At a small current density, the vortex loop is probed at a length scale larger than the total sample thickness. A remarkable change in the I-V characteristics is observed between N = 3 and 2. This indicates the vortex configuration crossover to vortex-antivortex string pairs piercing all layers at N = 2. In sample with N = 1, the typical KT transition is observed.
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