Multilevel spin dynamics in time-dependent external magnetic field

We investigate the dynamics of a ‘giant spin’ with 21 levels (S=10) in time dependent external magnetic field. The model can describe the time evolution of the spin degree of freedom in molecular nanomagnets, and our ‘exact numerical’ treatment of the problem reflects the staircase-like behavior of the experimentally observed magnetization curves. This effect is explained in terms of the level structure, which, at certain values of the external magnetic field, exhibits avoided crossings where the probability of the transitions increases. We show that the multilevel nature of the problem causes these transition probabilities to deviate significantly from the predictions of the traditional Landau-Zener-Stückelberg model.