A study of the critical magnetic fields and currents of thin niobium films

Temperature dependences of critical magnetic fields and critical currents of superconducting niobium films of different thickness are investigated. Experimental data are compared with the results of theoretical models. It is stated experimentally, that absolute value of critical magnetic fields Hsub(c perpendicular), Hsub(c parallel) drops with the increase of niobium film thickness and their relation Hsub(c perpendicular)/Hsub(c parallel) increases. Temperature dependences Hsub(c perpendicular), Hsub(c parallel) are linear and their slope changes with the change of film thickness. Temperature dependence of critical current density of niobium films is proportional (1-tsup(2))sup(n), where n changes from 2/3 to approximately one with decrease of film thickness. Type of pinning of vortex filaments changes in niobium films sputtered on hot substrate (tsub(n)=850 deg C) with the change of thickness. Phenomenon of cyclic instability of critical current in thin films analogous to ''train'' effect owserved in superconducting solenoids is found. It is supposed that this phenomenon is caused, first of all, by freezing of magnetic fluxes in the sample volume