A Study of magnetic grand states in rare earth-transition metal compounds by neutron diffraction technique

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The magnetism in rare earth and transition metal compounds is quite complicated and characteristic because of the coexistence and/or competition of RKKY interaction between 4f electrons and molecular fields by 3d electrons. In particular, recently, Onodera and his coworkers found evidence of the quadrapoler ordering in rare earth compounds with tetragonal crystal structures. It is, therefore, very interesting to make clear the complicated magnetism in rare earth and transition metal compounds. To understand the magnetic properties in rare earth compounds, it is quite important to decide the crystal and magnetic structures in details. Neutron diffraction technique is one of the most effective methods to make clear the static behavior of magnetic materials. We have constructed the new neutron powder diffractometer, HERMES, at the JRR-3M reactor in Japan Atomic Energy Research Institute. In this project, we have successively improved the experimental circumstance of HERMES and the experimental efficiency by the Grant-in-Aid for Scientific Research. Using HERMES, we were able to determine crystal and magnetic structures phase diagrams in many magnetic compounds. One of the most important results in this project is that we have found that the magnetic structure in the tetragonal compound DyB_2C_2 is characteristic one : the magnetic moments are in the c-plane, and are perpendicular between adjacent ones along c-axis. This characteristic magnetic structure indicates that the RKKY interaction competes with the qoadrapoler interaction. This result is very important in the study of the magnetism in rare earth compounds because this is the evidence that DyB_2C_2 is the first tetragonal compounds which undergoes the quadrapoler ordering.