  We study the dynamics of a spin coupled to an oscillating magnetic field, in
the presence of decoherence and dissipation. In this context we solve the
master equation for the Landau-Zener problem, both in the unitary and in the
irreversible case. We show that a single spin can be "magnetized" in the
direction parallel to the oscillating bias. When decay from upper to lower
level is taken into account, hysteretic behavior is obtained.
