Study of the mesoscopic quantum tunneling in nanoscale molecular magnets

我们对Fe8、Fe10、Fe12和Mn6R6四种纳米级分子磁体进行了样品合成、磁化强度测量、核磁共振实验和MuSR实验，并对实验结果进行了理论分析。1.成功合成了Fe8大单晶，首次从微观角度用核磁共振观察到介观量子隧穿。根据朗道和齐纳的理论对结果进行了分析。自旋弛豫速率的测量揭示了Fe_8的自旋动力学过程，其能级是离散的。2.平行于Fe_8各向异性轴的磁场中的NMR谱强烈地依赖于低于300mK的场循环和热循环，这是由初始自旋态引起的，共振量子隧穿和自旋弛豫。3.垂直于各向异性轴的磁场可以扩展Fe_8中的隧穿带隙。发现在此条件下，电子系统的激发可以改变NMR谱。4.我们与A.Keren教授在瑞士PSI进行了MuSR实验，证实了MuSR的共振量子隧穿。讨论了Fe_（8.5）的多比特磁存储器。分析了反铁磁环Fe_（10）的NMR谱和弛豫速率，阐明了其分立的低能级和自旋动力学。6.利用NMR研究了反铁磁环Fe_（12）的自旋动力学与自旋数和相互作用的关系。并与Fe_（10.7）的结果进行了比较。我们合成了具有量子隧穿可能性的亚铁磁环Mn_6R_6。在考虑离散能级和粉末近似的情况下，对弛豫速率进行了分析。

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.