Abstract
In this paper we report on a comprehensive study of the low temperature relaxation in several magnetic materials: ferromagnetic clusters, random magnets, antiferromagnetic particles and DCC magnetic tapes. As we discuss in the paper, our only requirement to observe quantum tunneling of magnetization is to deal with magnetic systems having a lot of metastable states. The magnetic viscosity, S ≡ (1/ Mo)∂M / ∂ln(t) is independent of temperature in the quantum regime and the transition between the thermal and quantum regime is rather sharp as expected from a system with weak damping.