Structural, 99Ru Mössbauer, dc and ac susceptibility magnetization, and magneto-transport properties of the polycrystalline Eu2Ru2O7 pyrochlore are reported in this paper. From the experimental data, we deduce that the ruthenium cations Ru4+ (S = 1) are surrounded by an unusual electronic environment, involving conduction electron polarization and extrinsic Eu3+ ions at low temperature. This situation leads to an anomalous spin-glass transition at 23 K.

Recent specific-heat data measured in Sr2YRuO6 have revealed two transition features at similar to 26 and similar to 30 K, where only one transition could be inferred before from magnetic susceptibility and transport measurements. We have investigated this temperature region using Ru-99 Mossbauer spectroscopy, magnetization, and thermodynamic measurements in order to elucidate the unusual properties in this temperature region. Below 25 K, fits to the Mossbauer spectra show that there is a unique value of the hyperfine magnetic field at each Ru site indicating static long-range magnetic order. Beyond 25 K, this static long-range magnetic order rapidly deteriorates. We have found that the temperature dependence of the Mossbauer spectra between similar to 25 and similar to 28 K can be described as due to either motional narrowing or to a temperature-dependent distribution of hyperfine magnetic fields. The Mossbauer spectra collapse to a single peak in this narrow temperature interval so that there is no evidence of static magnetic order by similar to 30 K. As a result, no evidence of the transition at 30 K is seen in the Mossbauer spectra. DOI: 10.1103/PhysRevB.87.024416

The Mossbauer spectra (MS) of powder samples of SmFe1-xCoxAsO (x = 0.0, 0.05, and 0.1) were measured in applied fields up to 9 T and at temperatures up to 298 K. SmFeAsO is magnetically ordered with T-N = 137 K and has a hyperfine magnetic field of (4.98 +/- 0.18) T at 4.2 K. In applied magnetic fields, the MS is consistent with a distribution of hyperfine magnetic fields of width H-applied + H-hyperfine. This arises because the angles between the direction of the ordered field in the crystallites making up the sample are randomly distributed about the direction of the applied field. The MS of the superconductors SmFe0.95Co0.05AsO (T-C similar or equal to 5 K) and SmFe0.9Co0.1AsO (T-C similar or equal to 17 K) are well described by a single peak from room temperature to 4.2 K indicating the absence of static magnetic order. However, the half width at half maximum, Gamma, of the peak in SmFe0.95Co0.05AsO increases with decreasing temperature from its high temperature value, 0.13 mm/s at 25 K, to 0.25 mm/s at 10 K. No such temperature dependence is seen in SmFe0.9Co0.1AsO. We analyze this temperature dependence in terms of a fluctuating hyperfine magnetic field model whose frequency at 4.2 K is found to be similar to 5-10 MHz, giving direct evidence of coexisting magnetic fluctuations and superconductivity at the interface in the phase diagram between the regions with magnetic and superconducting order. In a 5 T applied field, SmFe0.95Co0.05AsO is no longer superconducting; however, the temperature-dependent fluctuating magnetic field is still present and largely unchanged. The absence of fluctuations in superconducting SmFe0.9Co0.1AsO and their presence in superconducting SmFe0.95Co0.05AsO in zero applied field and in nonsuperconducting SmFe0.95Co0.05AsO at 5 T suggests that magnetic order is in competition with superconductivity in SmFe1-xCoxAsO.

The Moessbauer effect (ME) is frequently used to investigate magnetically ordered systems. One usually assumes that the magnetic order induces a hyperfine magnetic field, B{sub hyperfine}, at the ME active site. This is the case in the ruthenates, where the temperature dependence of B{sub hyperfine} at {sup 99}Ru sites tracks the temperature dependence of the ferromagnetic or antiferromagnetic order. However this does not happen in the rare-earth intermetallics, GdRu{sub 2} and HoRu{sub 2}. Specific heat, magnetization, magnetic susceptibility, Moessbauer effect, and neutron diffraction have been used to study the nature of the magnetic order in these materials. Both materials are found to order ferromagnetically at 83.1 and 15.3 K, respectively. Despite the ferromagnetic order of the rare-earth moments in both systems, there is no evidence of a correspondingly large B{sub hyperfine} in the Moessbauer spectrum at the Ru site. Instead the measured spectra consist of a narrow peak at all temperatures which points to the absence of magnetic order. To understand the surprising absence of a transferred hyperfine magnetic field, we carried out ab initio calculations which show that spin polarization is present only on the rare-earth site. The electron spin at the Ru sites is effectively unpolarized and, asmore » a result, B{sub hyperfine} is very small at those sites. This occurs because the 4d Ru electrons form broad conduction bands rather than localized moments. These 4d conduction bands are polarized in the region of the Fermi energy and mediate the interaction between the localized rare-earth moments.« less

Mossbauer spectra were measured from 4.2 to 145 K on a Ru-99 enriched sample of RuSr2GdCu2O8 which magnetically orders at 138 K and has a full transition to superconductivity at 8.7 K with an onset at similar to 13 K. The superconducting transition has no effect on the spectrum which is determined by the hyperfine magnetic field due to the magnetic order. At low temperatures there is a rapid decrease of this hyperfine magnetic field with increasing temperature indicating a gapless or a very low energy magnon spectrum. We use a local-moment model which includes coupling between nearest-neighbor in-plane Ru moments and between the Ru and Gd moments which are separated by a superconducting CuO layer and a SrO layer to calculate the magnon spectrum and use this to estimate the strength of the exchange interactions based on the hyperfine magnetic-field temperature dependence. The coupling strength is similar to 275 K for Ru-Ru coupling and similar to 30 K for Ru-Gd coupling.

The discovery of an orbital ordering transition in La4Ru2O10 provided an exciting link between ruthenate physics and that of the 3d transition metals. Despite clear evidence for the La4Ru2O10 orbital ordering at ∼160K (phase transition from a high temperature monoclinic structure to a low temperature triclinic structure accompanied by the opening of a spin gap), the atomic mechanism for this orbital ordering transition remains unresolved. We studied the local environment via Mössbauer effect (ME) measurements of 99Ru (97%) enriched samples over a temperature range of 4.2–196K. Fits to the spectra show that they arise solely from electric field gradients at the Ru sites without any hyperfine magnetic fields, ruling out the possibility of long range order. While the high temperature structure is accurately described by a single-site ME spectrum, the low temperature measurements can only be explained by a two-site model with significantly different symmetry at the two sites. At all temperatures the isomer shift is consistent with a +4 oxidation state.