Laurence Bouchenoire

The XMaS beamline at ESRF: instrumental developments and high-resolution diffraction studies.

The beamline, which is situated on a bending magnet at ESRF, comprises a unique combination of instrumentation for high-resolution and magnetic single-crystal diffraction. White-beam operation is possible, as well as focused and unfocused monochromatic modes. In addition to an eleven-axis Huber diffractometer, which facilitates simple operation in both vertical and horizontal scattering geometries, there is an in-vacuum polarization analyser and slit system, mirrors for harmonic rejection, sub 4.2 K and 1 Tesla magnetic field sample environment, plus a diamond phase plate for polarization conditioning. The instrumentation developed specifically for this beamline is described, and its use illustrated by recent scientific results.

Quiescent bilayers at the mica–water interface

Despite extensive studies with many experimental techniques, the morphology and structure of the self-assembled aggregates of quaternary alkyl ammonium bromides (CnTABs; where n denotes the number of hydrocarbons in the surfactant tail) at the solid–liquid interface remains controversial, with results from atomic force microscopy (AFM) imaging pointing to a variety of surface aggregates such as cylinders and surface micelles, whilst surface force measurements and neutron reflectivity (NR) measurements reporting bilayer structures. Using a home-built liquid cell that employs the “bending mica” method, we have performed unprecedented synchrotron X-ray reflectometry (XRR) measurements to study the adsorption behaviour of a CnTAB series (n = 10, 12, 14, 16 and 18) at the mica–water interface at different surfactant concentrations. We find that our XRR data cannot be described by surface aggregates such as cylindrical and spherical structures reported by AFM studies. In addition we have observed that the bilayer thickness, surface coverage and the tilt angle all depend on the surfactant concentration and surfactant hydrocarbon chain length n, and that the bilayer thickness reaches a maximum value at approximately the critical micellisation concentration (∼1 cmc) for all the CnTABs investigated. We propose that CnTABs form disordered bilayer structures on mica at concentrations below cmc, whilst at ∼1 cmc they form more densely packed bilayers with the tails possibly tilted at an angle θt ranging from ∼40 to 60° with respect to the surface normal in order to satisfy the packing constraints due to the mica lattice charge, i.e. so that the cross-section area of the tilted chain would match that of the area of the lattice charge (As ≅ 46.8 A2). As the surfactant concentration further increases, we find that the bilayer thickness decreases, and we ascribe this to the desorption of surfactant molecules, which recovers certain disorder and fluidity in the chain and thus leads to interdigitated bilayers again. In light of our XRR results, previously unattainable at the mica–water interface, we suggest that the surface aggregates observed by AFM could be induced by the interaction between the scanning probe and the surfactant layer, thus representing transient surface aggregation morphologies; whereas the CnTAB bilayers we observe with XRR are intrinsic structures under quiescent conditions. The suggestion of such quiescent bilayers will have fundamental implications to processes such as lubrication, self-assembly under confinement, detergency and wetting, where the morphology and structure of surfactant layers at the solid–liquid interface is an important consideration.

The microscopic structure of charge density waves in underdoped YBa2Cu3O6.54 revealed by X-ray diffraction.

Charge density wave (CDW) order appears throughout the underdoped high-temperature cuprate superconductors, but the underlying symmetry breaking and the origin of the CDW remain unclear. We use X-ray diffraction to determine the microscopic structure of the CDWs in an archetypical cuprate YBa2Cu3O6.54 at its superconducting transition temperature ∼60 K. We find that the CDWs in this material break the mirror symmetry of the CuO2 bilayers. The ionic displacements in the CDWs have two components, which are perpendicular and parallel to the CuO2 planes, and are out of phase with each other. The planar oxygen atoms have the largest displacements, perpendicular to the CuO2 planes. Our results allow many electronic properties of the underdoped cuprates to be understood. For instance, the CDWs will lead to local variations in the electronic structure, giving an explicit explanation of density-wave states with broken symmetry observed in scanning tunnelling microscopy and soft X-ray measurements.

Temperature dependence of the spin and orbital magnetization density in Sm 0.982 Gd 0.018 Al 2 around the spin-orbital compensation point

Nonresonant ferromagnetic x-ray diffraction has been used to separate the spin and orbital contribution to the magnetization density of the proposed zero-moment ferromagnet

Synchrotron XRR study of soft nanofilms at the mica–water interface

{\mathrm{Sm}}_{0.982}{\mathrm{Gd}}_{0.018}{\mathrm{Al}}_{2}.

Antiferromagnetic order and domains in Sr3Ir2O7 probed by x-ray resonant scattering

The alignment of the spin and orbital moments relative to the net magnetization shows a sign reversal at 84 K, the compensation temperature. Below this temperature the orbital moment is larger than the spin moment, and vice versa above it. This result implies that the compensation mechanism is driven by the different temperature dependencies of the

The use of synchrotron X-rays to observe copper corrosion in real time

4f

Dipolar Excitations at the LIII X-Ray Absorption Edges of the Heavy Rare-Earth Metals

spin and orbital moments. Specific heat data indicate that the system remains ferromagnetically ordered throughout.

Performance of phase plates on the XMaS beamline at the ESRF

We describe here the design of a liquid cell specific for synchrotron X-ray reflectometry (XRR) characterisation of soft matter nanofilms at the mica–water interface. The feature of the cell is a “bending mica” method: by slightly bending the mica substrate over an underling cylinder the rigidity of the mica sheet along the bending axis is enhanced, providing sufficient flatness along the apex of the cylinder as required by XRR measurements. Using this cell, we have performed XRR measurements for a number of systems and in this article we show example results: (1) a cationic surfactant, C16TAB; (2) a zwitterionic surfactant, C12H25PC; (3) a semi-fluorinated surfactant, F4H11(d)TAB; and (4) surface complex of an anionic fluorinated surfactant, CsPFN, and a positively charged polymer, PEI. For the data analysis we have taken into account the mica crystal truncation rod, i.e. the reflectivity from the mica substrate, and fitted the data with a custom Java™ based software package. Our results unravel detailed structural information of these soft nanofilms, indicating that this method is suitable for XRR measurements of a wide range of soft matter structures at the mica–water interface.

Static Magnetic Proximity Effect in Pt Layers on Sputter-Deposited NiFe 2 O 4 and on Fe of Various Thicknesses Investigated by XRMR

This article reports a detailed x-ray resonant scattering study of the bilayer iridate compound, Sr3Ir2O7, at the Ir L2 and L3 edges. Resonant scattering at the Ir L3 edge has been used to determine that Sr3Ir2O7 is a long-range ordered antiferromagnet below TN 230K with an ordering wavevector, q=(1/2,1/2,0). The energy resonance at the L3 edge was found to be a factor of ~30 times larger than that at the L2. This remarkable effect has been seen in the single layer compound Sr2IrO4 and has been linked to the observation of a Jeff=1/2 spin-orbit insulator. Our result shows that despite the modified electronic structure of the bilayer compound, caused by the larger bandwidth, the effect of strong spin-orbit coupling on the resonant magnetic scattering persists. Using the programme SARAh, we have determined that the magnetic order consists of two domains with propagation vectors k1=(1/2,1/2,0) and k2=(1/2,-1/2,0), respectively. A raster measurement of a focussed x-ray beam across the surface of the sample yielded images of domains of the order of 100 microns size, with odd and even L components, respectively. Fully relativistic, monoelectronic calculations (FDMNES), using the Greens function technique for a muffin-tin potential have been employed to calculate the relative intensities of the L2,3 edge resonances, comparing the effects of including spin-orbit coupling and the Hubbard, U, term. A large L3 to L2 edge intensity ratio (~5) was found for calculations including spin-orbit coupling. Adding the Hubbard, U, term resulted in changes to the intensity ratio <5%.