Neutron scattering studies on the actinide system

The results of our recent neutron scattering studies on actinide '115' compounds are reviewed. Our motivation is to study which kind of ground states appear in many body f electron systems, where the itinerant system is only realized in actinide compounds. For this purpose the '115' family is interesting, because high-quality single crystalline samples with different number and character of f electrons and valence d electrons can be grown. The heavy fermion superconductors CeTIn5 (T: Co, Rh, Ir) and PuCoGa5 with Tsc=18.5K belong to this family. The variety of the magnetic ordering with A-, C-, G-type, ferromagnetic, and canted structures in U-'115' and Np-'115' is reminiscent of the orbital degree-of-freedom which may play an important role for the ground state properties. In contrast, localized rare earth compounds, RRhIn5 show an identical antiferromagnetic order with q=[1/201/2]. We show that the double transition in NpRhGa5 and NpNiGa5 is the 5f electronic transition between the low- and high-moment state. Surprisingly, a recent localized f electron model can systematically explain the magnetic structures, the phase diagrams, and the double transition in NpTGa5 (T: Co, Rh, Ni). Up to now there is no clear explanation for the magnetic structures with band theory. Resonant X-ray scattering studies on NpTGa5 in order to detect the magnetic and quadrupole order are now in progress.