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。结果总结如下：1。我们发现了DyB_2C_2的一个新的相变，并完成了我们发现的DyB_2C_2复杂的相图，显示了连续的反铁四极（AFQ）和反铁磁（AFM）相变。对y稀释后的DyB_2C_2单晶进行的中子衍射实验表明，DyB_2C_2的四极有序是由准一维四极相互作用驱动的，即沿四方c轴方向的极强AFQ相互作用和基底c平面的弱铁四极相互作用。此外，在^<161>Dy穆斯堡尔实验中，由于AFQ和AFM相互作用之间的干扰变化，相邻的Y离子随取代位的不同而减小或增加^<161>Dy的超精细磁场。Gd取代20%的dy产生了一个新的转变序列，其中AFQ转变发生在AFM转变温度以下。通过对TbB_2C_2的y稀释，澄清了40k时4f基态和第一激发态都是单重态，而AFM跃迁和场致AFQ跃迁与4f能级有关，包括150k左右的高激发态，即相变是交换诱导的Van Vleck磁跃迁。对gd稀释后的TbB_2C_2进行了精确的检测，发现TbB_2C_2的基态是AFM阶和反八极性铁（AFO）阶的共存相。基于TbB_2C_2的（AFQ+AFO）基态，很好地理解了TbB_2C_2及其稀释体系的异常磁性行为

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