Dynamical calculations on the reversal of single quantum spins: Quantum coherence

Quantum coherence in small single-domain ferromagnetic particles is studied using dynamical calculations of the reversal of a single quantum spin resulting from a rotation of an external magnetic field, for 1/2 less than or equal to S less than or equal to 75. The analysis of the time evolution of the spin, the temporal correlation, and the spectrum leads to the conclusion that, for small spin S less than or equal to 15, coherent tunneling back and forth between the easy directions occurs at some specific resonant fields only. Away from resonance, quantum coherence in the spin temporal evolution is absent. These results are in qualitative accordance with those obtained for a Heisenberg Hamiltonian with exact quantum calculations and are at variance with semiclassical approximations. However, as the spin becomes larger, the number of resonant fields increases, the resonances become very small, and coherence in time evolution is lost for any value of the magnetic field.