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Cobalt ferrite ultrathin films with inverse spinel structure are among the best candidates for spin-filtering at room temperature. We have fabricated high-quality epitaxial ultrathin CoFe2O4 layers on Ag(001) following a three-step method: an ultrathin metallic CoFe2 alloy was first grown in coherent epitaxy on the substrate, and then treated twice with O2, first at RT and then during annealing. The epitaxial orientation, the surface, interface and film structure were resolved combining LEED, STM, Auger and in situ GIXRD. A slight tetragonal distortion was observed, that should drive the easy magnetization axis in plane due to the large magneto-elastic coupling of such a material. The so-called inversion parameter, i.e. the Co fraction occupying octahedral sites in the ferrite spinel structure, is a key element for its spin-dependent electronic gap. It was obtained through in-situ x-ray resonant diffraction measurements collected at both the Co and Fe K edges. The data analysis was performed using the FDMNES code and showed that Co ions are predominantly located at octahedral sites with an inversion parameter of 0.88 +- 0.05. Ex-situ XPS gave an estimation in accordance with the values obtained through diffraction analysis., Comment: 17 pages, 9 figures, 3 tables

Chiral multiferroic langasites have attracted attention due to their doubly-chiral magnetic ground state within an enantiomorphic crystal. We report on a detailed resonant soft X-ray diffraction study of the multiferroic Ba$_3$TaFe$_3$Si$_2$O$_{14}$ at the Fe $L_{2,3}$ and oxygen $K$ edges. Below $T_N$ ($\approx27K$) we observe the satellite reflections $(0,0,\tau)$, $(0,0,2\tau)$, $(0,0,3\tau)$ and $(0,0,1-3\tau)$ where $\tau \approx 0.140 \pm 0.001$. The dependence of the scattering intensity on X-ray polarization and azimuthal angle indicate that the odd harmonics are dominated by the out-of-plane ($\mathbf{\hat{c}}$-axis) magnetic dipole while the $(0,0,2\tau)$ originates from the electron density distortions accompanying magnetic order. We observe dissimilar energy dependences of the diffraction intensity of the purely magnetic odd-harmonic satellites at the Fe $L_3$ edge. Utilizing first-principles calculations, we show that this is a consequence of the loss of threefold crystal symmetry in the multiferroic phase.

We study the spin-dependent electronic structure of UTe and UT_{2}Si_{2} (T=Cu and Mn) compounds with a combination of x-ray magnetic circular dichroism measurements and first principle calculations. By exploiting the presence of sizable quadrupolar and dipolar contributions to the U L_{2,3}-edge x-ray absorption cross section we are able to provide unique information on the extent of hybridization between 5f and 6d/3d electronic states, a key parameter regulating the physical properties of all actinide materials. Since this information is hardly accessible to other probes, the new methodology opens up new venues for investigating this important class of materials., Comment: 5 pages, 3 figures

Contains fulltext : 34915.pdf (Publisher’s version ) (Closed access) 3 p.

The local environment of nickel atoms in Y NiO3 across the pressure- induced insulator to metal (IM) transition was studied using X-ray absorption spectroscopy (XAS) supported by ab initio calculations. The monotonic contraction of the NiO6 units under applied pressure observed up to 13 GPa, stops in a limited pressure domain around 14 GPa, before resuming above 16 GPa. In this narrow pressure range, crystallographic modifications basically occur in the medium/long range, not in the NiO6 octahedron, whereas the evolution of the near-edge XAS features can be associated to metallization. Ab initio calculations show that these features are related to medium range order, provided that the Ni-O-Ni angle enables a proper overlap of the Ni eg and O 2p orbitals. Metallization is then not directly related to modifications in the average local geometry of the NiO6 units but more likely to an inter-octahedra rearrangement. These outcomes provides evidences of the bandwidth driven nature of the IM transition., Comment: 6 pages with figures

Resonant-magnetic x-ray scattering (RMXS) near the vanadium $L_{2,3}$-absorption edges has been used to investigate the low temperature magnetic structure of high quality \bvs\ single crystals. Below $T_N$ = 31 K, the strong resonance revealed a triple-incommensurate magnetic ordering at wave vector (0.226 0.226 $\xi$) in the hexagonal notation, with $\xi$ = 0.033. The simulations of the experimental RMXS spectra with a time-dependent density functional theory indicate an antiferromagnetic order with the spins polarized along $a$ in the monoclinic structure., Comment: 4 pages, 4 figures, submitted to Phys. Rev. Lett

The development of smart materials for use in transducer devices has driven considerable research in recent decades. In this field, phenomena such as magnetostriction underpin the operation of sensors and actuators, and energy harvesting devices. Thus, knowledge of their origin and mechanics, and how they manifest themselves in different materials, enables the development of new devices and optimisation of existing technologies. Yet this research is not without its limitations. Theoretical studies naturally describe material properties at a fundamental, atomic-scale. But experimental work struggles to verify such models at a similarly microscopic scale. Saturation strains from typical magnetostrictive materials induce displacements between neighbouring atoms of only a few femtometres. On such a scale, even commonly employed local probes, such as x-ray absorption spectroscopy (XAS), lack the resolution to observe this motion by some two orders of magnitude. As a result, experiments typically employ strain gauges on large, macroscopic samples, where the strain is easier to detect, but where atomic information is lost. However, the recent development of differential XAS (DiffXAS) at the ESRF presents a unique opportunity to bridge this gap between macro-scale experimental work, and fundamental theoretical models [1]. This talk will study the development of DiffXAS via the most significant results obtained to date. These include studies of the important Fe-Ga and Fe-Pt systems, where DiffXAS analysis procedures have allowed chemically-selective, atomic magnetostriction coefficients to be quantified [2][3]; and an investigation of the magneto-elastic coupling of FeCo through measurements under applied hydrostatic pressure [4]. [1] R.F. Pettifer et al., Nature 435, 79 (2005) [2] M. P. Ruffoni et al., Phys. Rev. Lett. 101, 147202 (2008) [3] S. Pascarelli et al., Phys. Rev. B 77, 184406 (2008) [4] S. Pascarelli et al., Phys. Rev. Lett. 99, 237204 (2007) Submitted version

The structural changes exhibited by materials in response to their surroundings is of fundamental importance across countless disciplines in science and engineering. In the field of transducer technologies, strain-inducing phenomena (such as magnetostriction) are utilised in their own right, and form the physical foundation for sensor and actuator devices. Knowledge of their origin, and how they manifest themselves in different materials, underpins the development of new transducers and the optimisation of existing technologies. Yet in spite of this, direct measurement of a material’s intrinsic magnetostriction – the strain induced between individual atoms at a microscopic level – has proven elusive. This has essentially been due to the difficulty in measuring atomic motion with sufficient precision. In most materials, magnetostrictive atomic displacements saturate at just a few femtometres. Here, even sensitive atomic probes, such as x-ray absorption spectroscopy (XAS), lack the resolution to observe this motion by some two orders of magnitude. As a result, common experiments employ strain gauges on macroscopic samples, where the strain is easier to detect, but where atomic information is lost. However, with the recent development of differential XAS (DiffXAS) at the European Synchrotron Radiation Facility (ESRF), such atomic-scale measurements have become possible [1,2]. Initial studies into the magnetostriction of FeCo thin films demonstrated a sensitivity to femtometre scale motion [1]. Subsequent work extended this study to investigate the magnetoelastic coupling of FeCo under applied hydrostatic pressure [3], and new experiments looked into the behaviour of FePt, and rare-earth iron alloys such as Fe2Tb. But the most significant study conducted to date has been that of the Fe-Ga system [4,5]. In recent years, binary metal alloys such as Fe-Al or Fe-Ga, have attracted considerable interest [6]. It is well known that pure Fe exhibits only an extremely small magnetostriction, but when alloyed with certain nonmagnetic metallic elements, it can be enhanced by over an order of magnitude. Compositions of Fe-Ga with around 19at% Ga have reported strains of up to 400 ppm [7]. Although this doesn’t constitute a truly ‘giant’ magnetostriction, it is of interest for device applications since Fe-Ga is devoid of expensive rare-earth components, saturates in fields of only several hundred Oersteds, and possesses more desirable mechanical properties than, say, the much studied Terfenol-D alloy. In order to identify the origin of this enhancement, we took a splat-cooled foil of Fe81Ga19 and measured its Joule saturation magnetostriction with DiffXAS [4]. Unlike conventional macroscopic measurements that describe the sample as a whole, magnetostriction coefficients provided by DiffXAS describe the strain of just the first two or three atomic coordination shells surrounding a photo-excited atom. Furthermore, since it is possible to tune the x-ray energy, and so select which atom in the material is excited, it is possible to look at the strain in the local environment of each atomic species separately. Contributions from different types of bond within the structure may then be decoupled and analysed. Such fundamental information has immense value when attempting to verify theoretical models. In 2002, Wu [8] proposed a model for the magnetostriction of FexGa(1-x) that suggested a tetragonal “B2-like” structure in the vicinity of the Ga atoms was responsible for the observed enhancement. More recently, Cullen et al. [9] reached a similar conclusion, and stated that such a structure could be formed by Ga pairs randomly arranged throughout the material. Conventional XAS studies have confirmed the presence of these Ga pairs [5], but it is DiffXAS that describes how they influence the magnetostriction. From our DiffXAS spectra, we extracted the strain present in different types of bond, and with a subsequent analysis, solved the magnetostriction tensor for the material. This provided two sets of magnetostriction coefficients. In the environment around Fe, (3/2)100 = 40ppm and (3/2)111 = -32ppm, and in the environment around Ga, (3/2)100 = 390ppm and (3/2)111 ~0ppm. This demonstrates that the observed enhancement is dominated by strain in the vicinity of the Ga atoms. Furthermore, our analysis revealed that the strain in the Ga pairs was negligible, indicating that they do not contribute directly to the enhanced magnetostriction, but rather mediate the enhancement in the surrounding Ga-Fe bonds. Further experiments are planned to examine the full range of compositions over which magnetostriction enhancement is observed in this system, and, with the use of single crystal samples, investigate how the microscopic magnetostriction scales up to that seen macroscopically. [1] R.F. Pettifer, O. Mathon, S. Pascarelli, M.D. Cooke, M.R. J. Gibbs, Nature 435, 79 (2005) [2] M. P. Ruffoni, R.F. Pettifer, S. Pascarelli et al., AIP Conf. Proc. No. 882, 838 (2007) [3] S. Pascarelli, M. P. Ruffoni, A. Trapananti et al., Phys. Rev. Lett. 99, 237204 (2007) [4] M. P. Ruffoni, S. Pascarelli, R. Grössinger et al., Phys. Rev. Lett. 101, 147202 (2008) [5] S. Pascarelli, M. P. Ruffoni, R. Sato-Turtelli et al., Phys. Rev. B 77, 184406 (2008) [6] E. M. Summers, T. A. Lograsso, M. Wun-Fogle, J. Mater. Sci. 42, 9582 (2007) [7] A. E. Clark, A. B. Hathaway, M. Wun-Fogle et al., J. Appl. Phys. 93, 8621 (2003) [8] R. Wu, J. Appl. Phys. 91, 7358 (2002) [9] J. Cullen, P. Zhao, M. Wuttig, J. Appl. Phys. 101, 123922 (2007) Submitted version

A thorough tensor analysis of the Bragg-forbidden reflection (00.3)$_h$ in corundum systems having a global center of inversion, like V$_2$O$_3$ and $\alpha$-Fe$_2$O$_3$, shows that anomalous x-ray resonant diffraction can access chiral properties related to the dipole-quadrupole (E1-E2) channel via an interference with the pure quadrupole-quadrupole (E2-E2) process. This is also confirmed by independent {\it ab initio} numerical simulations. In such a way it becomes possible, in this particular case, to estimate the intensity of the ``twisted'' trigonal crystal field ($C_3$ symmetry) and, in general, to detect chiral quantities in systems where dichroic absorption techniques are ineffective., Comment: 4 pages, 4 figures

Anomalous x-ray diffraction studies show that the charge ordering in $��^\prime$-NaV$_2$O$_5$ is of zig-zag type in all vanadium ladders. We have found that there are two models of the stacking of layers along \emph{c-}direction, each of them consisting of 2 degenerated patterns, and that the experimental data is well reproduced if the 2 patterns appears simultaneously. We believe that the low temperature structure contains stacking faults separating regions corresponding to the four possible patterns., Submitted to Phys. Rev. Lett., 4 pages, 4 eps figures inserted in the text

The paper has been withdrawn, Comment: 5 pages, 5 figures

High-energy-resolution fluorescence detection-X-ray absorption near-edge structure (HERFD-XANES) at the Au L3 edge was used to study gold catalysts supported on ceria in order to unravel the role of gold 5d orbital modifications during the activation of molecular oxygen. The variations in the HERFD-XANES resonance peak, which are directly correlated with the d-band occupancy, were monitored in situ during the redox process of the CO oxidation at room temperature under both aerobic and anaerobic conditions. Interestingly, upon the oxidation treatment and also during the aerobic CO oxidation treatment, the gold d-band fluctuates around an average value, proving that the gold clusters are partially charged. The dynamism of the gold electronic configuration was evidenced by a temporary variation of the gold d-orbital population during the oxidation reaction. The interpretation of X-ray spectroscopy data is supported by ab initio simulations performed using the FDMNES code.

The incidence of cancer is continuing to rise and risk-tailored early diagnostic and/or primary prevention strategies are urgently required. The ideal risk-predictive test should: integrate the effects of both genetic and nongenetic factors and aim to capture these effects using an approach that is both biologically stable and technically reproducible; derive a score from easily accessible biological samples that acts as a surrogate for the organ in question; and enable the effectiveness of risk-reducing measures to be monitored. Substantial evidence has accumulated suggesting that the epigenome and, in particular, DNA methylation-based tests meet all of these requirements. However, the development and implementation of DNA methylation-based risk-prediction tests poses considerable challenges. In particular, the cell type specificity of DNA methylation and the extensive cellular heterogeneity of the easily accessible surrogate cells that might contain information relevant to less accessible tissues necessitates the use of novel methods in order to account for these confounding issues. Furthermore, the engagement of the scientific community with health-care professionals, policymakers and the public is required in order to identify and address the organizational, ethical, legal, social and economic challenges associated with the routine use of epigenetic testing.

Encuadernada con otras obras., Publicado en la colección del Domingo, el documento está datado en Génova el 15 de octubre de 1858., 1 p. ; 29 cm.

Mal encuadernada la página 53 del texto se encuentra detrás de la p. 494, E.W. Thebussem and M. Droap son pseudonimos de Mariano Pardo de Figueroa., 56 p.