nd - m at . s up r - co n ] 1 9 A ug 2 00 1 Suppression of Superconducting Critical Current Density by Many Small Flux Jumps in MgB 2 Thin Films

By doing magnetization measurements during magnetic field sweeping on thin films of the new superconductor MgB2, it is found that in low temperature and low field region many small flux jumps are taking place. This effect strongly suppresses the central magnetization peak leading to an reduced nominal superconducting critical current density at low temperatures. A borderline for this effect to occur is determined on the field-temperature ( H-T ) phase diagram. It is suggested that these small flux jumps are due to the dense and tiny disorders and relatively easy thermal diffusion in thin films in comparison to bulk samples. 74.60.Jg, 74.76.-w, 74.70.-b, 74.40.+k, 74.60.Ge Typeset using REVTEX 1