Observation of long-range magnetic ordering in the proposed spin-singlet state of Hg2Ru2O7

The magnetic ground-state of the pyrochlore material Hg2Ru2O7, which undergoes a metal to Mott insulator transition (MIT) at T=107 K, was investigated using muon spin relaxation and neutron powder diffraction (both non-polarized and polarized) measurements. Unlike the expected behavior associated with a spin-singlet ground-state, these surprisingly revealed the onset of long-range magnetic ordering below the MIT.