Quantum coherence and macroscopic spin tunneling in a weakly ferromagnetic fine particle

The ground-state splitting due to macroscopic quantum tunneling (MQT) of the Neel vector in a weakly ferromagnetic fine particle in an external magnetic field is calculated. The fluctuation contribution to the Feynman path integral is taken into account. It is shown that the rate of MQT can oscillate due to instanton interference in the break antiferromagnetic phase up to the spin-flip transition. When a magnetic field is in the hysteresis region the field dependence of the rate of MQT is not monotonous and can be zero in the middle of hysteresis for half-integer spin sublattices.