Collapse of the hyperfine magnetic field at the Ru site in ferromagnetic rare earth intermetallics

The M\"{o}ssbauer Effect(ME) is frequently used to investigate magnetically ordered systems. One usually assumes that the magnetic order induces a hyperfine magnetic field, $B_{hyperfine}$, at the ME active site. This is the case in the ruthenates, where the temperature dependence of $B_{hyperfine}$ at $^{99}$Ru sites tracks the temperature dependence of the ferromagnetic or antiferromagnetic order. However this does not happen in the rare-earth intermetallics, GdRu$_2$ and HoRu$_2$. Specific heat, magnetization, magnetic susceptibility, M\"{o}ssbauer 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 82.3 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 $B_{hyperfine}$ in the M\"{o}ssbauer 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 {\it 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, as a result, $B_{hyperfine}$ is very small at those sites. This occurs because the 4$d$ Ru electrons form broad conduction bands rather than localized moments. These 4$d$ conduction bands are polarized in the region of the Fermi energy and mediate the interaction between the localized rare earth moments.Comment: 34 pages -Revtex + 17 ps figure

Mental Stress Pressor Response and Post-Stress Aortic Wave Reflection

Reactivity to mental stress has been linked to cardiovascular risk, and is shown to negatively influence aortic wave reflection for up to an hour after acute mental arithmetic. It has been postulated that sympathetic catecholamine release during the stressful task drives the sustained elevation in wave reflection. In the present study we sought to determine how muscle sympathetic nerve activity (MSNA) and mean arterial pressure (MAP) reactivity influence post mental stress aortic augmentation index (AIx). Twenty-seven volunteers (25±1 years) with resting blood pressure ≥120/80 mmHg participated in the present study. Baseline pulse wave analysis and pulse wave velocity recordings were assessed on participants after 10 minutes of supine rest. Participants were then instrumented for measurements of MSNA (microneurography), beat-to-beat blood pressure (finger plethysmography), and heart rate (3-lead ECG). Subsequently MSNA, blood pressure, and heart rate were measured during a 5-minute baseline, 5-minute mental stress task (serial subtraction), and 5-minute recovery. Finally, aortic wave reflection (i.e. AIx) was reassessed 10 minutes after completion of mental stress. Statistical analyses included paired t-tests to compare MSNA, MAP, heart rate (HR), and perceived stress during baseline and mental stress. We used standard multiple regression with change in AIx as the dependent variable and changes in MSNA, MAP, and perceived stress as the independent variables. Means were considered significantly different when p\u3c0.05. The mental stress task significantly increased HR (Δ15±2 beats/minute), MAP (Δ14±1 mmHg), and perceived stress (Δ1.9±0.1 a.u.), while MSNA was not significantly increased. There was however a range of changes in MSNA from -13 to +20 bursts/minute. The change in MAP during mental stress was a significant predictor (ꞵ=0.47; p=0.03) of the change in AIx (post-stress vs. baseline). Changes in MSNA and perceived stress were not predictors of mental stress-related changes in AIx. Our results indicate that the aortic wave reflection responses to mental stress appear to be linked to the pressor response, but not sympathetic or perceived stress responses. This is novel preliminary data that suggests there may not be a direct link between sympathetic activation and aortic wave reflection following cognitive stress

Magnetism and superconductivity in Sr

We report magnetization, surface resistance ($R_{\rm s}(T,H)$), and electron spin resonance (ESR) for non-superconducting Ba2GdRu2CuuO6, and find that all three magnetic ions (Gd, Ru, and Cu) are ordered at low temperatures. Both ESR (Gd sublattice) and weak ferromagnetic resonance (dopant Cu) are observed, while no magnetic resonance due to either paramagnetic or ordered Ru is detected. In addition, for superconducting ($T_{\rm c}\sim45$ K) Sr2YRu1-uCuuO6, resistivity, muon spin rotation (µ+SR), and 99Ru Mössbauer absorption are reported. None of the O6 materials (e.g., Sr2YRu1-uCuuO6) have cuprate planes, although Cu is employed as a dopant. In Sr2YRu1-uCuuO6, the Ru moments order at a temperature (∼23 K) below that for the resistive onset of superconductivity, while the Cu orders at a higher temperature, ∼86 K. Therefore at low temperatures, this material exhibits magnetic order, coexisting with diamagnetism. The only non-magnetic layers in the superconducting O6 structure, the SrO layers, carry holes and exhibit diamagnetic screening characteristic of type-II superconductivity