Metal-Insulator Transition and Magnetic Order in the Pyrochlore Oxide Hg2Ru2O7

We report results of NMR experiments on the ruthenium oxide Hg2Ru2O7 with the pyrochlore structure, which exhibits a metal-insulator transition at T-MI = 107 K. In the metallic phase above TMI, the nuclear spin-lattice relaxation rate 1/T-1 and the Knight shift at the Hg sites follow the Korringa relation, indicating the absence of substantial spatial spin correlation. At low temperatures in the insulating phase, Ru-99,Ru-101-NMR signals are observed at zero magnetic field, providing evidence for a commensurate antiferromagnetic order. The estimated ordered moment is about 1 mu(B) per Ru, much smaller than 3 mu(B) expected for the ionic (4d)(3) configuration of Ru5+. Thus, the localized spin models are not appropriate for the insulating phase of Hg2Ru2O7. We also discuss possible antiferromagnetic spin structures.