We study macroscopic quantum coherence in antiferromagnetic molecular magnets in the presence of magnetic fields. Such fields generate artificial tunnel barriers with externally tunable strength. We give detailed semi-classical predictions for the tunnel splitting in various regimes for low and high magnetic fields. We show that the tunneling dynamics of the Néel vector can be directly measured via the static magnetization and the specific heat. We also report on a new quantum phase arising from fluctuations. The analytic results are complemented by numerical simulations.PACS numbers: 73.40. Gk, 75.60.Jp, 75.10.Jm, 03.65.Sq, 75.30.Gw Quantum spin dynamics in mesoscopic magnets has received much attention over the recent years, both from experiment and from theory [1]. A number of nanosized particles in the superparamagnetic regime have been identified as promising candidates for the observation of macroscopic quantum phenomena (MQP) such as the tunneling of the magnetization out of a metastable potential minimum, or, more strikingly, macroscopic quantum coherence (MQC), where the magnetization (or the Néel vector) tunnels coherently between classically degenerate directions over many periods. On one hand, these phenomena are interesting from a fundamental point of view as they extend our understanding of the transition from quantum to classical behavior. On the other hand, the measurement of MQP quantities such as the tunnel splitting provides independent information about microscopic parameters such as anisotropies and exchange constants.A prominent example of such MQC behavior that has attracted wide attention (but also scrutiny) is the antiferromagnetic ferritin [2]. More recently, molecular magnets [3] such as the ferric wheel or Mn 12 have emerged as promising candidates for the experimental observation of MQP [4,5] mainly for three reasons. First, molecular magnets have well-defined structures and magnetic properties. Thus, precise values for the tunneling rates can be calculated. Second, molecular magnets can be produced as single crystals that contain a macroscopic number of identical magnetic subunits, which leads to a natural amplification of the single-unit signal. Third, the typically high symmetry of these magnets reduces the number of independent parameters.In this letter we discuss novel tunneling scenarios in antiferromagnetic (AFM) molecular magnets. A key feature of our discussion is to exploit the well-known fact [6] that an effective anisotropy can be generated in an AFM by applying a magnetic field. Thus it is possible to create tunnel barriers that are tunable by an external parameter. Evidently, such control parameters are highly desirable as they open the door to systematic tests of MQC. We concentrate on ring-like structures such as Fe 6 , Fe 10 ,, where the spins interact with their nearest neighbors via exchange coupling. In particular, we show that the tunneling rates become field dependent and thus can be measured via the static magnetization and (less surprisingly) also via the S...

Temperature-dependent magnetization measurements on a series of synthetic ferritin proteins containing from 100 to 3000 Fe(III) ions are used to determine the uncompensated moment of these antiferromagnetic particles. The results are compared with recent theories of macroscopic quantum coherence which explicitly include the effect of this excess moment. The scaling of the excess moment with protein size is consistent with a simple model of finite size effects and sublattice noncompensation.

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 semi-classical 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 re-examine 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.

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