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

通过单晶的生长、晶体结构的鉴定以及磁化率、磁化率、比热、电阻率、穆斯堡尔效应和中子散射等测量，阐明了四边形RB_2C_2化合物中反四极铁（AFQ）的相互作用和有序性。DyB_2C_2的AFQ有序温度为T_Q=24.3 K，反铁磁相图为T_N=l5.3 K和HoB_2C_2的反铁磁相图，相反的转变顺序为T_N=5.8 K和T_Q=5.0 K。用中子衍射法测定了单晶在磁场中的AFM、AFQ或它们在各相的共存结构。结果表明，T_N=21.7 K的AFM TbB_2C_2的异常相图源于场致AFQ有序转变的发生，并明确了其场致相的AFM和AFQ结构。HoB_2C_2中T_Q≦T≦T_N的磁相与（AFM+AFQ）相相邻，与立方（Ce,La）B_6中的（IV相）有一定的相似之处，其性质一直是神秘而有争议的。还发现TbB_2C_2的AFM相与两种化合物中的IV相非常相似，IV相出现在（Dy,Gd）B_2C_2中，其转变顺序与HoB_2C_2相同。由于其神秘的特性，可能会受到八极相互作用的强烈影响，因此IV相具有很强的吸引力，是该领域未来研究的重要目标。在四个化合物中发现了IV期，我们在RB_2C_2化合物中发现了三个。中子衍射和^<161>Dy穆斯堡尔实验的结合给出了强AFQ相互作用像伪一维相互作用一样沿四方c轴进行的重要信息，而AEM相互作用则在c平面上强烈作用。这些AFQ和AFM相互作用的竞争导致RB_2C_2的各种异常磁性能。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