Fig. 7-3 The crystal structure of U3Pd20Si6 U atoms occupy two different crystallo-graphical sites, 4a and 8c, which form an fcc and a simple cubic lattice with half periodicity, respectively.

Fig. 7-4 The magnetic structure and phase diagram in the magnetic field vs. temperature plane Arrows indicate the magnetic moments of the U ions. In the 'AFM' phase, the U moments on the 8c sites order antiferromagnetically. This antiferromagnetic order coexists with ferromagnetic order on the 4a sites in the 'FM+AFM' phase. A new type of spin-flop transition was observed under magnetic field in the 'SF' phase.

The actinides are the radioactive elements from Ac (atomic number: Z=89) to Lr (Z=103), including U (Z=92). So far, actinides have been studied extensively from the point of view of applications for nuclear reactor fuel and fundamental aspects of nuclear and atomic physics. On the other hand, recent studies have revealed that actinide compounds show interesting and highly unusual electronic and magnetic properties. The 5f electrons occupying the outermost shell change character from localized to itinerant, depending on the type of neighboring ions, applied pressure and magnetic field, etc. Therefore, we expect there to be a large variety of interesting and useful actinide compounds, many of which remain unknown. One example is the discovery of U3Pd20Si6, which is a magnetic compound having U 5f electrons of localized character. It can be understood that the large inter-atomic distance between U atoms isolated by Si and Pd cages gives rise to the localized character of the 5f electrons. In order to determine the magnetic structure of U3Pd20Si6, neutron scattering experiments have been carried out on triple-axis-spectrometers installed at JRR-3 in JAERI. Low temperatures below 1K and high magnetic field up to 10 T (Tesla) were required for the present study. For this purpose we have developed a liquid-He cryogen-free superconducting magnet and a 3He-4He dilution refrigerator for neutron scattering experiments. Our recent studies have revealed the coexistence of ferromagnetic ordering of U moments on the 4a sites and antiferro-magnetic ordering on the 8c sites in U3Pd20Si6. Higher-order exchange interactions play a significant role in the collinear magnetic structure and for the new type of spin-flop transition which occurs with application of a magnetic field. We also point out that the electronic orbital degrees of freedom may be important for the electronic and magnetic properties of 5f electron systems as well. A new type of heavy-fermion superconductivity based on quadrupole interactions might also be expected to occur in actinide compounds. The nature of 5f electrons is the one of the most interesting subjects for the physics of strongly-correlated electron systems. Our study contributes to the progress of condensed matter physics and presents useful results to understand and control the nature of 5f electrons, with potentially useful applications for mankind.

Reference Y. Koike et al., Magnetic Structure, Phase Diagramm, and a New Type of Spin-Flop Transition Dominated by Higher Order Interaction in a Localized 5f System U3Pd20Si6, Phys. Rev. Lett., 89, 077202 (2002).

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