Quantum tunnelling in magnetic particles, layers and multilayers

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 independent of the temperature. This result is interpreted in terms of a crossover between a thermal regime, in which the irreversible rotation of the magnetization is thermally activated, and a quantum regime in which it proceeds by quantum tunnelling. In this paper we show some results obtained in nanoparticles, layers and multilayers. In particular an analogy with Josephson junction will be made for the case of the sandwiches (domain wall junction). The role of dissipation will be discussed.