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

We report results of NMR experiments on the ruthenium oxide Hg 2 Ru 2 O 7 with the pyrochlore structure, which exhibits a metal–insulator transition at T MI = 107 K. In the metallic phase above T MI , 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, 99,101 Ru-NMR signals are observed at zero magnetic field, providing evidence for a commensurate antiferromagnetic order. The estimated ordered moment is about 1 µ B per Ru, much smaller than 3 µ B expected for the ionic (4 d ) 3 configuration of Ru 5+ . Thus, the localized spin models are not appropriate for the insulating phase of Hg 2 Ru 2 O 7 . We also discuss possible antiferromagnetic spin structures.