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.
|ジャーナル||Physical Review B - Condensed Matter and Materials Physics|
|出版ステータス||Published - 2010 9月 8|
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