Investigation of upper critical fields and irreversibility line in superconducting MgB 2

Abstract

The upper critical field H c2( T) of sintered pellets of MgB 2 superconductor was investigated by transport, AC susceptibility and DC magnetization measurements in magnetic fields up to 16 T. The highest H c2 values obtained from magnetization and resistance measurements ( H c2 mag≈ H c2 90) can be attributed to the grains oriented with their ab-planes parallel to the applied field. H c2( T) shows a positive curvature near T c similar to that found for the borocarbides YNi 2B 2C and LuNi 2B 2C indicating that MgB 2 is in the clean limit. The irreversibility field consistently determined from DC magnetization and from the imaginary component of AC susceptibility was found to increase up to 8.5 T at 10 K.