Macroscopic quantum coherence in ferrimagnets

We study macroscopic quantum coherence (MQC) in small magnetic particles where the magnetization (in ferromagnets) or the Néel vector (in antiferromagnets) can tunnel between energy minima. We consider here the more general case of MQC in ferrimagnets by studying a model for a mesoscopic antiferromagnet with an uncompensated magnetic moment. Through semiclassical calculations we show that even a small moment has a drastic effect on MQC. In particular, there is a rapid crossover to a regime where the MQC tunnel splitting is equal to that obtained for a ferromagnet, even though the system is still an antiferromagnet for all other aspects. We calculate this tunnel splitting via instanton methods and compare it with numerical evaluations. As an application we reexamine the experimental evidence for MQC in ferritin and show that even though the uncompensated moment of ferritin is small it greatly modifies the MQC behavior. The excess spin allows us to extract values for experimental parameters without making any assumption about the classical attempt frequency, in contrast to previous fits. Finally, we also discuss the implications of our results for MQC in molecular magnets.