Inhomogeneous Magnetic Field Penetration in Superconducting Niobium Films

The peculiarities of magnetic field penetration in the form of fingering or dendritic instabilities are studied by magneto-optical technique in the niobium films of different thickness. It is observed that the reduction of Nb film thickness reduces the threshold field for instabilities, in agreement with the theoretical predictions. In addition, it is shown that the silver overlayer deposited on the top of Nb film leads also to the enhancement of instabilities, in marked contrast to the effect of gold or aluminum overlayers, which are known to suppress instabilities.