Upper critical field and irreversibility line in superconducting MgB2

The upper critical field Hc2(T) of sintered pellets of the recently discovered MgB2 superconductor was investigated by transport, ac susceptibility and dc magnetization measurements in magnetic fields up to 16 T covering a temperature range between Tc∽39 K and T=3 K∽0.1Tc. The temperature dependence of the upper critical field, Hc2(T), shows a positive curvature near Tc similar to that found for the borocarbides YNi2B2C and LuNi2B2C indicating that MgB2 is in the clean limit. The irreversibility line was consistently determined from dc magnetization measurements and from the imaginary component of ac susceptibility. The irreversibility field was found to increase up to 8.5 T at 10 K.