QUANTUM DYNAMICAL CALCULATIONS ON THE MAGNETIZATION REVERSAL IN CLUSTERS OF SPIN-1 2 PARTICLES - RESONANT COHERENT QUANTUM TUNNELING/

In the present work the reversal of magnetization and the coherence of tunneling when an external magnetic field is rotated instantaneously are studied in systems of a few spin-1/2 particles described by an ani sotropic Heisenberg Hamiltonian at T=0. Our calculations demonstrate t hat this model for small magnetic particles exhibits collective tunnel ing of the magnetization only for some specific resonant values of the applied magnetic field. These resonant effects occur at fields much l ower than the values corresponding to the vanishing of the barrier in the Stoner-Wohlfarth model. The former model is at variance with the e xact calculations presented in this paper.