Competitive Antiferroquadrupolar and Antiferromagnetic Interactions in Tetragonal Rare-earth Compounds RB_2C_2.

用单晶生长、晶体结构鉴定和磁化率、磁化强度、比热、电阻率、穆斯堡尔效应和中子散射等测试手段，研究了四方Rb2C2化合物中的反铁四极(AFQ)相互作用和有序性。得到了反铁四极(AFQ)有序温度为T_q=24.3K的DYB_2C_2和反铁磁(AFM)温度为T_N=15.3K和HoB_2C_2的反铁磁(AFM)相图，其逆转变顺序为T_N=5.8K和T_q=5.0K。用磁场作用下单晶的中子衍射法测定了AFq或其在各相中的共存结构。此外，还揭示了T_N=21.7K的原子力显微镜TbB_2C_2的反常相图起源于场诱导AFQ有序化转变的发生，并阐明了其场诱导相的原子力显微镜和AFQ结构。T_q≦T≦T_N在HoB_2C_2中具有…(AFM+AFQ)相与立方(Ce，La)B6中的(相IV)有一些相似之处，这一特征一直是神秘和有争议的。研究还发现，TbB_2C_2的AFM相与(Dy，Gd)B_2C_2中的IV相非常相似，其中IV相的转变顺序与HoB_2C_2中的相同。由于其神秘的性质，可能受到八极相互作用的强烈影响，因此是该领域未来研究的重要目标。在四个化合物中发现了四个相，其中三个是在Rb_2C_2化合物中发现的。结合中子衍射和^&lt；161&gt；Dy穆斯堡尔实验给出了重要的信息，即强的AFq相互作用在四方c轴上以伪一维相互作用的形式进行，而AEM相互作用在C面上起很强的作用。这些AFQ和AFM相互作用的竞争导致了Rb_2C_2中各种反常的磁性。较少

Growth of single crystals, identification of crystal structure and various measurements such an magnetic susceptibility, magnetization, specific heat, electric resistivity, Mossbauer effect and neutron scattering have been performed is order to clarify antiferroquadrupolar (AFQ) interaction and ordering in tetragonal RB_2C_2 compounds.Precise magnetic phase diagrams were determined no DyB_2C_2 with an AFQ ordering temperature of T_Q=24.3 K and an antiferromagnetic (AFM) one of T_N=l5.3 K and HoB_2C_2 with the reverse transition sequence of T_N=5.8 K and T_Q=5.0 K. The AFM, AFQ or their coexistent structure in every phase was also determined by neutron diffraction on single crystals in magnetic fields. Moreover, it was revealed that anomalous phase diagrams of the AFM TbB_2C_2 with T_N=21.7 K originates in occurrence of field-induced AFQ ordering transitions, and the AFM and AFQ structures of their field-induced phases were also clarified.The magnetic phase of T_Q≦T≦T_N in HoB_2C_2 whic … More h is adjacent to (AFM+AFQ) phase has some resemblance to that (phase IV) in cubic (Ce,La)B_6 that the character has been mysterious and controversial. It has been also found that the AFM phase of TbB_2C_2 is very similar to phase IV in the both compounds and that phase IV appears in (Dy,Gd)B_2C_2 where the transition sequence is the same to that in HoB_2C_2. Phase IV strongly attract us and is the most important target in future research in this field because of its mysterious character which might be affected strongly by octupolar interactions. Phase IV has been found in four compounds, and we found three of them in the RB_2C_2 compounds.Combination of neutron diffraction and ^<161>Dy Mossbauer experiments gave the important information that the strong AFQ interaction mediates along the tetragonal c-axis like a pseude-one-dimmensional interaction while the AEM interactions act strongly in the C-plane. The competition of these AFQ and AFM interactions causes various and anomalous magnetic properties in RB_2C_2. Less