  We address the problem of spin dynamics in the presence of a thermal bath, by
solving exactly the appropriate quantum master equations with
continued-fraction methods. The crossover region between the quantum and
classical domains is studied by increasing the spin value S, and the asymptote
for the classical absorption spectra is eventually recovered. Along with the
recognized relevance of the coupling strength, we show the critical role played
by the structure of the system-environment interaction in the emergence of
classical phenomenology.
