Magnetic ground state of pyrochlore oxides close to metal-insulator boundary probed by muon spin rotation

The magnetism of ruthenium pyrochlore oxides A₂Ru ₂O₇ (A=Hg, Cd, and Ca), whose electronic properties within a localized ion picture are characterized by nondegenerate t2g orbitals (Ru⁵⁺, 4 d³) and thereby subjected to geometrical frustration, has been investigated by the muon spin rotation/relaxation (μSR) technique. The A cation (mostly divalent) was varied to examine the effect of covalency (Hg>Cd>Ca) on the electronic property of the oxides. In a sample with A=Hg, which exhibits a clear metal-insulator (MI) transition below ∼100K (associated with a weak structural transition), a nearly commensurate magnetic order is observed to develop in accordance with MI transition. Meanwhile, in the case of A=Cd, where the MI transition is suppressed to the level of small anomaly in the resistivity, the local-field distribution probed by muons indicates emergence of a certain magnetic inhomogeneity below ∼30K. Moreover, in Ca₂Ru₂O₇, which remains metallic, highly inhomogeneous local magnetism is found below ∼25K; this magnetism arises from randomly oriented Ru moments and thus is described as a "frozen spin-liquid" state. The systematic trend of increasing randomness and itinerant character with decreasing covalency suggests a close relationship between the two characters. To understand the effect of orbital degeneracy and associated Jahn-Teller instability, we examine a tetravalent ruthenium pyrochlore, Tl₂Ru₂O₇ (Ru⁴⁺, 4 d ⁴). The result of μSR indicates a nonmagnetic ground state that is consistent with the formation of the Haldane chains suggested by the neutron-diffraction experiment.