Study of the mesoscopic quantum tunneling in nanoscale molecular magnets

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We have performed sample synthesis, magnetization measurements, NMR experiments and MuSR experiments on nanoscale molecular magnets Fe8,Fe10,Fe12 and Mn6R6, and also analyzed their experimental results theoretically.1.We have succeeded the synthesis of large single crystals of Fe8, and have observed mesoscopic quantum tunneling by means of NMR from the microscopic viewpoint for the first time. The result was analyzed on a theory by Landau and Zener. The measurement of the nuclear spin relaxation rates reveals the spin dynamics of Fe8 with the discrete energy levels.2.The NMR spectra in the magnetic field parallel to the anisotropy axis of Fe8 depend strongly on the field and heat cycles below 300mK, that is caused by the initial spin states, resonant quantum tunneling and spin relaxation.3.The tunneling gap in Fe8 can be spread by the magnetic field perpendicular to the anisotropy axis. It is found that the NMR spectra on this condition can be changed by the excitation of the electron system.4.We have performed the MuSR experiments at PSI in Switzerland with Prof.A.Keren, and confirmed the resonant quantum tunneling by MuSR. We discussed the multibit magnetic memory using Fe8.5.We have analyzed the NMR spectra and relaxation rates of the antiferromagnetic ring Fe10, and clarified the discrete low-lying energy levels and the spin dynamics.6.We investigate the dependence of the spin dynamics on the number of spins and interactions for an antiferromagnetic ring Fe12 by NMR, and compare the results with those for Fe10.7.We have synthesized the ferrimagnetic ring Mn6R6, which has a possibility of quantum tunneling. The relaxation rate has been analyzed on the consideration of the discrete energy levels and powder approximation.