Experimental Research on Roles of Quadrupolar and Octupolar Interactions in Magnetism of Rare Earth Compounds.

本工作的目的是阐明多极相互作用在稀土化合物磁性中的作用。结果表明：（1）发现了DyB_2C_2的一个新的相变，完成了我们所发现的反铁四极（AFQ）和反铁磁（AFM）连续相变的复杂相图;（2）用中子衍射方法对DyB_2C_2单晶进行了研究，发现DyB_2C_2中的四极有序是由准一阶有序驱动的。三维四极相互作用，其由沿着四极c轴的极强AFQ相互作用和基底c平面中的弱铁四极（FQ）相互作用组成。此外，在~ <161>Dy穆斯堡尔谱实验中，<161>由于原子力显微镜与原子力显微镜相互作用之间的阻挫效应的变化，相邻Y离子的取代位置不同，~ Dy的磁超精细场也不同程度地减小或增大; 3. Gd取代20%Dy，在原子力显微镜的转变温度以下出现了新的原子力显微镜转变序列;已经阐明，在40 K时，4f基态和第一激发态都是单态，AFM跃迁和场-诱导的AFQ跃迁与包括150 K附近的较高激发态的4f能级有关，即，5.对Gd稀释的TbB_2C_2的精确测量表明，TbB_2C_2的基态为交换诱导的货车Vleck磁转变。2是AFM有序和反铁八极（AFO）有序共存的相。基于TbB_2C_2的（AFQ+AFO）基态，我们对TbB_2C_2及其稀释体系的反常磁行为有了满意的理解

The present works have been performed with the aim of clarifying the roles of multipolar interactions in the magnetism of rare earth compounds. The results are summarized as follows.1.We found out a new phase transition and completed the complicated phase diagrams of DyB_2C_2 that had been found by us to show the successive antiferroquadrupolar (AFQ) and antiferromagnetic (AFM) transitions.2.Neutron diffraction experiments on the Y-diluted DyB_2C_2 single-crystals have revealed that the quadrupolar ordering in DyB_2C_2 is driven by quasi-one-dimensional quadrupolar interactions which consist of extremely strong AFQ interactions along the tetragonal c-axis and weak ferroquadrupolar (FQ) interactions in the basal c-plane. Moreover, the ^<161>Dy Mossbauer experiments, the neighboring Y ion decreases or increases the ^<161>Dy magnetic hyperfine fields depending on the substituted site because of the change of frustration between the AFQ and AFM interactions.3.The substitution of 20%Dy by Gd brought forth a new transition sequence in which the AFQ transition occurs below the AFM transition temperature.4.By the Y-dilution of TbB_2C_2, it has been clarified that both the 4f ground state and the first excited state at 40 K are singlets and that the AFM transition and the field-induced AFQ transition concern with the 4f levels including the higher excited states around 150 K, that is, the phase transition is a exchange-induced Van Vleck magnetic transition.5.The precise examination on the Gd-diluted TbB_2C_2 has revealed that the ground state of TbB_2C_2 is a coexistent phase of AFM order and antiferro-octupolar (AFO) order. The anomalous magnetic behaviors of TbB_2C_2, and the diluted system have been understood satisfactorily based on the (AFQ+AFO) ground state of TbB_2C_2