DISSIPATION AND INTERFERENCE EFFECTS IN MACROSCOPIC MAGNETIZATION TUNNELING AND COHERENCE

Macroscopic quantum coherence (MQC) and tunneling (MQT) of the total moment of a ferromagnetic particle are considered in the presence of a magnetic field, and the topological quenching, or spin-parity effect in the tunneling rate that has been found in recent papers is shown to follow from a selection rule due to an underlying rotational symmetry in both cases. The oscillation in the tunneling rate with magnetic field is considered more carefully for the MQT porblem. In addition to the rotational symmetry, this oscillation is shown to require that the intermediate state obtained immediately after the magnetization has tunneled out of its metastable orientation have a narrow decay width. The tolerances on the decay width and misalignment of the magnetic field are derived, and the decay of the intermediate state is qualitatively discussed, along with its implications for the free induction decay of the moment in a small particle. © 1995 The American Physical Society.