The time delayed reversal of the magnetization of magnetic system with
distributions of energy barriers, is in general strongly dependent on
temperature at high temperature. At low temperature it becomes indepe
ndent of the temperature. This result is interpreted in terms of a cro
ssover between a thermal regime, in which the irreversible rotation of
the magnetization is thermally activated, and a quantum regime in whi
ch it proceeds by quantum tunnelling. In this paper we show some resul
ts obtained in nanoparticles, layers and multilayers. In particular an
analogy with Josephson junction will be made for the case of the sand
wiches (domain wall junction). The role of dissipation will be discuss
ed.