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

Abstract

We report results of NMR experiments on the ruthenium oxide Hg2Ru2O7 with the pyrochlore structure, which exhibits a metal-insulator transition at TMI = 107 K. In the metallic phase above TMI, the nuclear spin-lattice relaxation rate 1/T1 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, 99,101 Ru-NMR signals are observed at zero magnetic field, providing evidence for a commensurate antiferromagnetic order. The estimated ordered moment is about 1 µB per Ru, much smaller than 3 µB expected for the ionic (4d) 3 configuration of Ru 5+ . Thus, the localized spin models are not appropriate for the insulating phase of Hg2Ru2O7. We also discuss possible antiferromagnetic spin structures.

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

“…A recent NMR study using Hg 199 and enriched 99 Ru nuclei suggests a considerably complicated magnetic structure consisting of four nonequivalent Ru sites. 23 In this study, it was considered that ͑i͒ the internal field is canceled for a quarter of Hg nuclear sites and that ͑ii͒ the angle between the primary axis of the electric field gradient and Ru magnetic moment is 90°, and hence, a magnetic multiple-q ͑ 0͒ structure was suggested, in which Ru spins are antiparallel between those mutually at the diagonal sites of Ru hexamers on the Kagomé lattice ͑perpendicular to the ͓111͔ axis͒. We additionally assume that the triangular lattice layers have an antiferromagnetic spin structure and simulate the internal field at the presumed muon site.…”

“…Recent nuclear magnetic resonance (NMR) study using Hg 199 and enriched 99 Ru nuclei suggests a rather complicated magnetic structure consisting of four non-equivalent Ru sites. 21 Considering that i) the internal field is canceled at a quarter of Hg nuclear sites, and that ii) the angle between the primary axis of the electric field gradient and Ru magnetic moment is 90 • , they suggest a magnetic structure of multiple-q ( = 0) in which Ru spins are antiparallel between those mutually at the diagonal sites of Ru-hexamers on the Kagomé lattice (perpendicular to the [111] axis). Our simulation of internal field at the presumed muon site with further assumption of antiferromagnetic spin structure for triangular lattice layers reproduces the two-band structure of muon precession frequency with the local fields ∼ 0.2 and ∼ 0.4 T/µ B .…”

Tetrahedral Magnetic Order and the Metal-Insulator Transition in the Pyrochlore Lattice ofCd2Os2O7

“…Two classes of pyrochlore oxides with MI transitions have been identified: one class exhibits transitions with a large structural distortion and the other class superficially shows no distortion. The former class includes Tl 2 Ru 2 O 7 [4,5], and Hg 2 Ru 2 O 7 [6][7][8], in which the singlet state or magnetic long-range order appears with the relaxation of frustrations owing to the large lattice distortions. The latter class includes Cd 2 Os 2 O 7 [9,10] and R 2 Ir 2 O 7 (R = Nd, Sm, Eu) [11], in which certain magnetic orders seem to occur simultaneously at the MI transition; they have not yet been determined experimentally.…”

“…The synchrotron radiation experiments were performed at beamlines BL-02B1 and BL-19XU at SPring-8 with the approval of RIKEN (Proposal Nos. 2009B1200 and 1 ( 3 ) 2 ( 5 ) 3 ( 5 ) 4 ( 7 ) 8 TABLE I. Intensity of the 006 magnetic reflection, fmag 2 , in the π-σ′ channel at ψ= -45° (experimentally observed) and in the π-π′ channel at ψ= 0° (unobserved).…”

“…For example, the spinel oxide ZnCr 2 O 4 shows a spin-driven Jahn-Teller distortion [28]. Further, the pyrochlore oxides Tl 2 Ru 2 O 7 and Hg 2 Ru 2 O 7 also exhibit large lattice distortions with the MI transition [4][5][6][7][8]. Thus, the magnetic symmetry is expected to be associated with the lattice symmetry in Cd 2 Os 2 O 7 as well.…”