* Corresponding author.Tel.: #55-16-260-8228; fax: #55- 16-261-4835. E-mail address: faraujo@power.ufscar.br (F.M. Araujo- Moreira). Journal of Magnetism and Magnetic Materials 226}230 (2001) 290}292 Magnetic properties of SIS and SNS Josephson junction arrays W. Maluf Jr., G.M. Cecato, P. Barbara, C.J. Lobb, R.S. Newrock, F.M. Araujo-Moreira* Group de Supercond. e Magnetismo-GSM, Departamento de Fisica, Universidade Federal de Sao Carlo, Caixa Postal 676, Sao Carlos SP, 13565-905, Brazil CSR, Department of Physics, University of Maryland at College Park MD, 20742-4111, USA Department of Physics, University of Cincinnati, Cincinnati, OH 45221-0011, USA Abstract Josephson junction arrays (JJA) are powerful arti"cial structures equivalent in many aspects to granular superconduct- ing systems. In this work we show some new and striking results obtained from the magnetic properties of JJA. They associate both Wollheben e!ect (WE) and vortex avalanche phenomena (VA) to geometrical parameters of shunted and unshunted samples, and to the SIS (superconductor}insulator}superconductor) and SNS (superconductor} normal}superconductor) structures. We also show the size e!ects on the appearance of the Kosterlitz}Touless (KT) transition observed in SNS samples, determined by the mutual-inductance technique.  2001 Elsevier Science B.V. All rights reserved. Keywords: Superconductivity; Josephson junction arrays; Wollheben e!ect; Vortex avalanche It is still unclear which are the parameters controlling the occurrence of some remarkable properties observed in granular superconducting systems. That is the case of vortex avalanche (VA) phenomena, "rst reported by Herke et al. [1], and the Wohlleben E!ect (WE) "rst detected by Braunisch et al. [2]. VA has been reported as a self-criticality mechanism associated with magnetic #ux penetrating the sample. On the other hand, WE has been attributed to the presence of pinning centers between the grains with su$ciently high values of critical current [3]. However, the parameters to control the appearance of both WE and VA in granular superconductors are still to be determined. In this paper we show results obtained from the magnetic properties of Josephson junction ar- rays (JJA) related to both VA and WE. These results were obtained from the magnetic properties of shunted and unshunted SIS samples. As we can observe (Fig. 1), shunted samples have molybdenum resistors in parallel with the junction, while the unshunted do not. We also show the size e!ects on the appearance of the Koster- litz}Touless (KT) transition observed in SNS samples, determined by the mutual-inductance technique. In 1979, Beasley et al. [4] suggested that the transition of Kosterlitz-Thouless (KT) could be observed in two- dimensional superconductors systems. They showed that the penetration depth   could reach dimensions compa- rable with the size of the sample. This behavior would allow that vortex lines had logarithmical interactions along the whole sample, making possible the observation of a KT transition. We have characterized several SIS (150100 junc- tions) and SNS (300300, 300200, 300100, 30075, and 30050 junctions) samples with di!erent sizes and structures (shunted and unshunted). We have used a very sensitive home-made susceptometer, based on the screen- ing method. It involves the use of primary and secondary 0304-8853/01/$ - see front matter  2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 1 3 4 5 - 7