Magnetic properties of the Mott insulator with the pyrochlore structure Y 2 Ru 2 O 7 were investigated using 99 Ru Mössbauer spectroscopy. Bulk magnetization measurements confirmed appearance of the spin-glass-like antiferromagnetic order with very large differences between zero-field-cooled and field-cooled magnetizations below the Néel temperature T N =76͑2͒ K. The Mössbauer spectra were obtained above and below T N . Analysis of the spectrum recorded at 4.2 K suggested a noncollinear local ordering of ruthenium magnetic moments. The electronic structure of the individual ruthenium ion, responsible for magnitudes of hyperfine parameters and the ruthenium magnetic moment, was discussed within the framework of the crystalline electric-field model, including the spin-orbit coupling and the exchange interaction in the molecular-field approximation. Reasonably good agreement between calculated and experimentally determined parameters was obtained. Simultaneously, the experimentally observed easy plane of ruthenium magnetic moments was explained. A. Sample preparation and characterizationPolycrystalline samples of Y 2 Ru 2 O 7 and Y 2 Ru 1.99 Sn 0.01 O 7 were prepared by the solid-state reaction from weighted in proper molar ratios mixtures of powders: Y 2 O 3 , RuO 2 , and enriched 119 SnO 2 . The mixtures were prereacted at 800°C for 12 h in air. After grinding they were pressed into pellets and sintered at 1150°C for 16 h in air. The grinding and sintering cycle was repeated several times. The x-ray-diffraction measurements were performed using Cu K␣ radiation in D501 Siemens powder diffractometer.
Bulk magnetic measurements and 99 Ru and 155 Gd Mössbauer effect investigations were performed to examine magnetic properties of the gadolinium pyrochlore ruthenate Gd 2 Ru 2 O 7 . A magnetic transition to the antiferromagnetic state at T N = 114͑1͒ K is accompanied by a small difference between field-cooled and zerofield-cooled magnetic susceptibilities. 99 Ru and 155 Gd Mössbauer effect investigations at 4.2 K confirmed that both ruthenium and gadolinium sublattices are magnetically ordered. Successful analysis of each spectrum with one set of hyperfine parameters proves that all ruthenium moments have the same magnitude and form the same angle with the local three-fold symmetry axes. The same conclusion concerns Gd magnetic moments. The measured temperature dependence of the hyperfine field at Gd site, together with the molecular-field model, points to the fact that Gd moments also order at T N , with tiny magnetic moments due to Gd-Ru exchange interaction.
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