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

We investigate the reversal of magnetization and the coherence of tunn eling when an external magnetic field is rotated instantaneously in sy stems of a few (N) spin 1/2 particles described by an anisotropic Heis enberg Hamiltonian at T=0. The temporal evolution is calculated by a n umerically exact solution of the time-dependent Schrodinger equation, and the mean value in time of each spin component is computed as a fun ction of the magnetic field. The correlation function and the spectrum are analyzed in terms of the macroscopic quantum coherence. Our calcu lations demonstrate that this model for small (N<11) magnetic particle s exhibit collective tunneling of the magnetization only for some spec ific resonant values of the applied magnetic field, at variance with t he Stoner-Wohlfarth model. (C) 1996 American Institute of Physics.