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

2010; American Physical Society; Volume: 82; Issue: 9 Linguagem: Inglês

10.1103/physrevb.82.094413

1550-235X

Masanori Miyazaki, R. Kadono, Kohki Satoh, M. Hiraishi, Soshi Takeshita, A. Koda, Atsushi Yamamoto, H. Takagi,

Magnetic and transport properties of perovskites and related materials

The magnetism of ruthenium pyrochlore oxides ${A}_{2}{\text{Ru}}_{2}{\text{O}}_{7}$ ($A=\text{Hg}$, Cd, and Ca), whose electronic properties within a localized ion picture are characterized by nondegenerate ${t}_{2\text{g}}$ orbitals $({\text{Ru}}^{5+},4{d}^{3})$ and thereby subjected to geometrical frustration, has been investigated by the muon spin rotation/relaxation $(\ensuremath{\mu}\text{SR})$ technique. The $A$ cation (mostly divalent) was varied to examine the effect of covalency $(\text{Hg}>\text{Cd}>\text{Ca})$ on the electronic property of the oxides. In a sample with $A=\text{Hg}$, which exhibits a clear metal-insulator (MI) transition below $\ensuremath{\sim}100\text{ }\text{K}$ (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 $A=\text{Cd}$, 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 $\ensuremath{\sim}30\text{ }\text{K}$. Moreover, in ${\text{Ca}}_{2}{\text{Ru}}_{2}{\text{O}}_{7}$, which remains metallic, highly inhomogeneous local magnetism is found below $\ensuremath{\sim}25\text{ }\text{K}$; 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, ${\text{Tl}}_{2}{\text{Ru}}_{2}{\text{O}}_{7}$ $({\text{Ru}}^{4+},4{d}^{4})$. The result of $\ensuremath{\mu}\text{SR}$ indicates a nonmagnetic ground state that is consistent with the formation of the Haldane chains suggested by the neutron-diffraction experiment.