Neutron Scattering Study on the Crystal Structure, Magnetic Ordering, and Crystalline Electric Field Excitations in an Uranium Metallic Compound U₃Pd₂₀Si₆

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Systematic neutron scattering experiments were carried out in order to reveal the crystal and magnetic structure as well as the localized character of the uranium 5f electrons in a ternary metallic compound U₃Pd₂₀Si₆. High-resolution neutron powder patterns were well described by the model calculation assuming the R₃Pd₂₀X₆ structure. We found the successive magnetic sublattice ordering at the Néel temperature T_N =19 K and the Curie temperature T_C = 2 K, respectively. Below T_N = 19 K the uranium spins on 8c site showed an antiferromagnetic order with the propagation vector Q = [111]. Meanwhile uranium spins on 4a site remained paramagnetic down to T_C = 2 K and ordered ferromagnetically below T_C. Inelastic scattering experiments revealed clear evidence for the crystalline electric field excitations at the excitation energy of about 13 and 23 meV. We also found a strong magnetic excitation at low energy about 3 meV. A quasi-elastic response was observed above T_N, while the inelastic excitation energy showed remarkable temperature dependence below T_N. These behaviors involve that the observed low energy response would be the antiferromagnetic spin wave excitation, consistent with the localized character of the 5f electrons. The existence of crystalline electric field and spin wave excitations indicates that U₃Pd₂₀Si₆ is one of a few example of the uranium intermetallics in which uranium 5f electrons have a localized character.