Magnetic transitions in Pr2NiO4 single crystal

The magnetic properties of a stoichiometric Pr2NiO4 single crystal have been examined by means of the temperature dependence of the complex ac susceptibility and the isothermal magnetization in fields up to 200 kOe at T=4.2 K. Three separate phases have been identified and their anisotropic character has been analyzed. A collinear antiferromagnetic phase appears first between TN = 325 K and Tc1 = 115 K, where the Pr ions are polarized by an internal magnetic field. At Tc1 a first modification of the magnetic structure occurs in parallel with a structural phase transition (Bmab to P42/ncm). This magnetic transition has a first‐order character and involves both the out‐of‐plane and the in‐plane spin components (magnetic modes gx and gxcyfz, respectively). A second magnetic transition having also a first‐order character is also clearly identified at Tc2 = 90 K which corresponds to a spin reorientation process (gxcyfz to cxgyaz magnetic modes). It should be noted as well that the out‐of‐phase component of χac shows a peak around 30 K which reflects the coexistence of both magnetic configurations in a wide temperature interval. Finally, two field‐induced transitions have been observed at 4.2 K when the field is directed along the c axis. We propose that the high‐field anomaly arises from a metamagnetic transition of the weak ferromagnetic component, similarly to La2CuO4

Crystal and magnetic structure of the oxypnictide superconductor LaO(1-x)FxFeAs: evidence for magnetoelastic coupling

High-resolution and high-flux neutron as well as X-ray powder-diffraction experiments were performed on the oxypnictide series LaO(1-x)FxFeAs with 0<x<0.15 in order to study the crystal and magnetic structure. The magnetic symmetry of the undoped compound corresponds to those reported for ReOFeAs (with Re a rare earth) and for AFe2As2 (A=Ba, Sr) materials. We find an ordered magnetic moment of 0.63(1)muB at 2 K in LaOFeAs, which is significantly larger than the values previously reported for this compound. A sizable ordered magnetic moment is observed up to a F-doping of 4.5% whereas there is no magnetic order for a sample with a F concentration of x=0.06. In the undoped sample, several interatomic distances and FeAs4 tetrahedra angles exhibit pronounced anomalies connected with the broad structural transition and with the onset of magnetism supporting the idea of strong magneto-elastic coupling in this material.Comment: 8 pages, 7 figures, regular articl

Crystal-fields in YbInNi4 determined with magnetic form factor and inelastic neutron scattering

The magnetic form factor of YbInNi4 has been determined via the flipping ratios R with polarized neutron diffraction and the scattering function S(Q,w) was measured in an inelastic neutron scattering experiment. Both experiments were performed with the aim to determine the crystal-field scheme. The magnetic form factor clearly excludes the possibility of a \Gamma7 doublet as the ground state. The inelastic neutron data exhibit two, almost equally strong peaks at 3.2 meV and 4.4 meV which points, in agreement with earlier neutron data, towards a \Gamma8 quartet ground state. Further possibilities like a quasi-quartet ground state are discussed.Comment: 7 pages, 5 figures, 2 tables, submitted to PR

Cambrian ensialic rift-related magmatism in the Ossa-Morena Zone

The Late Ediacaran (c. 560–550 Ma) Série Negra sediments of the Évora–Aracena metamorphic belt, Ossa-Morena Zone, SW Iberian Massif, preserve a record of the erosion of an Avalonian–Cadomian magmatic arc and subsequent related turbiditic sedimentation. Detrital zircon from the Série Negra is characterized by predominantly Ediacaran and Cryogenian ages, with few Paleoproterozoic and Archean cores, and a marked lack of Grenvillian ages. These features, when combined with the metasediments' enrichment in LREE (La/Yb=14), negative Eu-anomalies, low 147Sm/144Nd values (0.121) and negative εNd550=−5.5, indicate that the protolith Série Negra sediments were derived from a continental magmatic arc. A period of Late Cadomian (ca. 560–540 Ma) tectonism was followed by an extended episode of widespread bimodal magmatism related to Cambrian (ca. 540–500 Ma) rifting. This tectonic inversion is expressed in the geological record by a regional Early Cambrian unconformity. SHRIMP zircon U–Th–Pb ages from four felsic orthogneisses from the Évora Massif record Cambrian (527±10 Ma, 522±5 Ma, 517±6 Ma and 505±5 Ma) crystallization ages for their igneous protoliths. This confirms the existence of widespread Lower Paleozoic igneous activity in the Ossa-Morena Zone: (i) a Lower Cambrian (ca. 535–515 Ma) igneous–felsic dominated–sedimentary complex (with calc-alkaline signature and associated carbonate and siliciclastic deposition), and (ii) a Middle Cambrian–?Ordovician (ca. 515–490 Ma) igneous–bimodal–sedimentary complex (with calc-alkaline and tholeiitic signatures and associated dominant siliciclastic deposition, but also carbonate sediments). The Cambrian felsic magmatism was characterized by negative Eu-anomalies, (La/Lu)N=0.8–11, 147Sm/144Nd=0.1289–0.1447 and εNd500 ranging from −1.5 to −0.8. A tendency towards peraluminous compositions suggests late fractionation, low degrees of partial melting, or the mixing of crustal and mantle-derived material in the magma source region. Some felsic rocks possibly represent the last residual melts of hightemperature, zircon-undersaturated mafic magmas later affected by crustal contamination, while others indicate partial melting of crustal metasediments variably contaminated by basaltic liquids. The transition from early felsic dominated to later more mafic magmatism suggests the gradual opening of the system to tholeiitic N–E-MORB products (ThN/TaNb1.0). The as yet undated (Cambrian–?Ordovician) E-MORB amphibolites have 147Sm/144Nd=0.1478–0.1797 and εNd500 values ranging from +6.4 to +7.3, while the N-MORB amphibolites have 147Sm/144Nd=0.1818–0.1979 and εNd500 values of +5.8 and +7.0, reaching a maximum of +9.1. In contrast, other amphibolites have a negative Ta-anomaly (1.35bThN/TaNb2.41) reminiscent of lavas from “orogenic” settings or alternatively, typical of crustally-contaminated within-plate magmas. These “VAB-like” amphibolites have 147Sm/144Nd values ranging from 0.1639 to 0.1946 and εNd500 values of +3.5 to +5.2, suggesting derivation by crustal assimilation processes. The subalkaline igneous precursors of the amphibolites were most likely generated in a rift setting by asthenospheric upwelling. These results strengthen the proposed geodynamic scenarios for the SW Iberian Massif by which Cadomian accretion gave rise to an ensialic rift that developed into a proto-oceanic basin and incipient spreading (opening of the Rheic Ocean?). A similar transition from a convergent to a divergent plate boundary during the Ediacaran to Cambrian–?Ordovician has also been reported in other segments of the northern Gondwana margin

Crystal structure and phonon softening in Ca3Ir4Sn13

We investigated the crystal structure and lattice excitations of the ternary intermetallic stannide Ca3Ir4Sn13 using neutron and x-ray scattering techniques. For T > T* ~ 38 K the x-ray diffraction data can be satisfactorily refined using the space group Pm-3n. Below T* the crystal structure is modulated with a propagation vector of q = (1/2, 1/2, 0). This may arise from a merohedral twinning in which three tetragonal domains overlap to mimic a higher symmetry, or from a doubling of the cubic unit cell. Neutron diffraction and neutron spectroscopy results show that the structural transition at T* is of a second-order, and that it is well described by mean-field theory. Inelastic neutron scattering data point towards a displacive structural transition at T* arising from the softening of a low-energy phonon mode with an energy gap of Delta(120 K) = 1.05 meV. Using density functional theory the soft phonon mode is identified as a 'breathing' mode of the Sn12 icosahedra and is consistent with the thermal ellipsoids of the Sn2 atoms found by single crystal diffraction data

Entropy calculation for a toy black hole

In this note we carry out the counting of states for a black hole in loop quantum gravity, however assuming an equidistant area spectrum. We find that this toy-model is exactly solvable, and we show that its behavior is very similar to that of the correct model. Thus this toy-model can be used as a nice and simplifying `laboratory' for questions about the full theory.Comment: 18 pages, 4 figures. v2: Corrected mistake in bibliography, added appendix with further result

Aceites vegetales como fluidos hidráulicos en maquinaria agrícola

The formulation of environmentally friendly lubricants following the criterion of the European EcoLabel is expensive owing to the lack of technological development in this area. The present work deals with the development of lubricant formulations from vegetable oils, in particular using high oleic sunflower oil as base fluid. These new biolubricants have to perform as good as the reference lubricants used in the real application (an agricultural tractor) but with the additional condition and value of their biodegradability without toxicity. Formulation development has been performed by Verkol Lubricantes, involving the selection of the base oil and the design of the additive package. The investigation performed by Tekniker in the laboratory has covered different aspects, characterizing the most important physicochemical properties of the lubricants, including their behavior at low temperatures and their resistance to oxidation. The tribological properties of the new biolubricants have also been studied, analyzing their ability to protect the interacting surface from wear, as well as the level of friction generated during sliding. Moreover, the compatibility of the new formulated oil with all the seals present in the real application has been taken into consideration. The selected lubricant is now being tested in agricultural machinery from AGRIA.La formulación de lubricantes amigables con el medioambiente siguiendo los criterios Europeos de la EcoLabel resulta cara debido a la falta de desarrollo tecnológico en esta área. En el presente trabajo se han desarrollado formulaciones de lubricantes a partir de aceites de origen vegetal, en particular empleando como aceite base el GAO (Girasol de Alto Oleico). Estos nuevos lubricantes deben presentar un comportamiento tan bueno como el de los lubricantes de referencia empleados en la aplicación real (un tractor agrícola), pero con la condición y valor añadido de ser biodegradables y no tóxicos. Las formulaciones han sido desarrolladas por Verkol lubricantes, comenzando por la selección del aceite base hasta el diseño del paquete de aditivos. La investigación llevada a cabo por Tekniker en el laboratorio ha cubierto diferentes aspectos. Se han caracterizado las propiedades físico-químicas más importantes, incluyendo su comportamiento a bajas temperaturas y su resistencia a la oxidación. También se han estudiado las propiedades tribológicas de los nuevos biolubricantes, analizando su habilidad para proteger frente al desgaste las superficies interactuantes, así como el nivel de fricción generado durante el deslizamiento. Paralelamente, se ha considerado la compatibilidad de las nuevas formulaciones de aceite con todas las juntas presentes en la aplicación real. El lubricante seleccionado está siendo ensayado en un tractor agrícola de AGRIA