Upper critical field and irreversibility line in superconducting MgB2

The upper critical field H-c2(T) of sintered pellets of the recently discov ered MgB2 superconductor was investigated by transport, ac susceptibility a nd de magnetization measurements in magnetic fields up to 16 T covering a t emperature range between T-c similar to 39 K and T = 3 K similar to 0.1T(c) . The temperature dependence of the upper critical field, H-c2(T), shows a positive curvature near T-c similar to that found for the borocarbides YNi2 B2C and LuNi2B2C indicating that MgB2 is in the clean limit. The irreversib ility line was consistently determined from de magnetization measurements a nd from the imaginary component of ac susceptibility. The irreversibility h eld was found to increase up to 8.5 T at 10 K. (C) 2001 Elsevier Science Lt d. All rights reserved.