Study of quantum magnetism in nano spin polyhedrons

自旋多面体是高对称性的反铁磁性自旋团簇，没有无序。各种磁网络是可能的，因为在这样的团簇中不需要平移对称。我们研究了两种类型的团簇Mo72Fe30和Fe9。前者称为三角形角点共享网络，后者称为边共享网络。Mo72Fe30的自旋受挫较强，能态高度简并。Mo72Fe30在全磁化强度的1/3处表现出导数磁化强度的下降，并用无序有序机制解释了下降的原因。在低温下，系统的比热和ESR谱线宽度出现较大范围的异常。这种现象被解释为由团内磁关联饱和引起的“准长程有序”。对于尺寸效应，在Mo72Fe30中证实了磁化台阶的涂抹，其中离散能级之间的能量间隔小于交错场和反对称交换相互作用等混合项。另一方面，在Fe9中发现了明显的台阶，台阶磁化强度的降低归因于自旋手性引起的不对称混合。研究了自旋多面体的重要组成部分--三角形自旋环的动力学。发现了明显的绝热磁化和半步磁化现象。比较了双核络合物和稀土基团簇等不同体系的磁化过程。自旋手性的重要性在各种系统中都得到了检验，例如自旋管。

Spin polyhedrons are antiferromagnetic spin clusters of high symmetry without disorders. Variety of magnetic networks is possible because a translational symmetry in not needed in such clusters. We have investigated two types of clusters Mo72Fe30 and Fe9. The former cluster is so called corner-sharing network of triangles and latter is the edge-sharing network. The spin frustration is strong and the energy state is highly degenerated.The Mo72Fe30 shows a dip of derivative magnetization at the 1/3 of full magnetization and the origin of dip has been interpreted by the "order by disorder" mechanism. At low temperature, the system shows a broad anomaly in specific heat and in ESR line width. This phenomenon is interpreted as "quasi long range order" caused by the saturation of intra-cluster magnetic correlation. As for the size effect, the smearing of magnetization step is confirmed in Mo72Fe30, where the energy separation among the discrete levels is smaller than the mixing term such as staggered fields and anti-symmetric exchange interaction. On the other hand, clear steps are found in Fe9 and the reduction of the step magnetization is attributed to the asymmetric mixing caused by the spin chirality.The dynamics of the triangle spin ring, which is an important component of spin polyhedrons, has been investigated. A clear adiabatic magnetization and half-step magnetization phenomenon is found. The comparison of magnetization process is made with different systems such as two-nuclear complex and rare earth based cluster. The importance of spin chirality is examined in various system such as spin tube.