  The transport properties of Nb/CuMn multilayers with a regular array of
electron beam lithography obtained antidots have been measured at different
temperatures in the presence of external perpendicular magnetic fields.
Hysteretical I-V characteristics have been observed which disappear when
approaching the upper critical magnetic field curve H_{c2}(T). Comparing these
data with other results (Arrhenius plots of resistive transition curves,
logV-logI characteristics) we have been able to relate the onset of the
hysteresis to the presence of an irreversibility line. We discuss several
possible mechanisms to clarify the nature of this line. Among them the most
plausible seems to be the vortex melting mainly induced by quantum
fluctuations.
