The Landau-Zener Model with Decoherence: The case S=1/2
  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. 
V. G. Benza (Universita' dell'Insubria, Como, also I.N.F.M., unita' di   Como), G. Strini (Universita' di Milano)