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

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We report results of NMR experiments on the ruthenium oxide Hg2Ru2O7 with the pyrochlore structure, which exhibits a metal--insulator transition at T_{\text{MI}} = 107 K. In the metallic phase above T_{\text{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, 99,101Ru-NMR signals are observed at zero magnetic field, providing evidence for a commensurate antiferromagnetic order. The estimated ordered moment is about 1 \mu_{\text{B}} per Ru, much smaller than 3 \mu_{\text{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.
2011-03-15