Huge compact flux avalanches in superconducting Nb thin films

Using a magneto-optical technique we perform a quantitative analysis of magnetic flux penetration in superconducting niobium thin films on A-plane (1120) sapphire substrates as a function of temperature (1.5 K less than or equal to T less than or equal to T-c = 9.2 K). In these samples we observe huge compact avalanches (HCAs for brevity), very much like some snow-avalanches and unlike the rough dendritic flux penetration observed on R-plane (1102) sapphire. The behavior observed is consistent with the occurrence of thermo-magnetic avalanches as proposed by Aranson et al. [Phys. Rev. Lett. 87 (2001) 067003]. For increasing temperature, we find: (1) an increased branching of the HCA. (2) An increased applied field necessary for the first HCA to occur. (3) A decrease in the number of HCAs, accompanied by more 'regular' flux penetration (above 6.2 K avalanches are completely absent). (4) An increase of the total amount of flux as well as the area of the HCA. (C) 2004 Elsevier B.V. All rights reserved.