Strong K-edge magnetic circular dichroism observed in photon-in-photon-out spectroscopy

A compact von Hámos spectrometer for parallel X-ray Raman scattering and X-ray emission spectroscopy at ID20 of the European Synchrotron Radiation Facility

A compact spectrometer for medium-resolution resonant and non-resonant X-ray emission spectroscopy in von Hámos geometry is described. The main motivation for the design and construction of the spectrometer is to allow for acquisition of non-resonant X-ray emission spectra while measuring non-resonant X-ray Raman scattering spectra at beamline ID20 of the European Synchrotron Radiation Facility. Technical details are provided and the performance and possible use of the spectrometer are demonstrated by presenting results of several X-ray spectroscopic methods on various compounds.

X-ray-Based Techniques to Study the Nano-Bio Interface

X-ray-based analytics are routinely applied in many fields, including physics, chemistry, materials science, and engineering. The full potential of such techniques in the life sciences and medicine, however, has not yet been fully exploited. We highlight current and upcoming advances in this direction. We describe different X-ray-based methodologies (including those performed at synchrotron light sources and X-ray free-electron lasers) and their potentials for application to investigate the nano-bio interface. The discussion is predominantly guided by asking how such methods could better help to understand and to improve nanoparticle-based drug delivery, though the concepts also apply to nano-bio interactions in general. We discuss current limitations and how they might be overcome, particularly for future use in vivo.

New reflections on hard X-ray photon-in/photon-out spectroscopy

Analysis of the electronic structure and local coordination of an element is an important aspect in the study of the chemical and physical properties of materials. This is particularly relevant at the nanoscale where new phases of matter may emerge below a critical size. X-ray emission spectroscopy (XES) at synchrotron radiation sources and free electron lasers has enriched the field of X-ray spectroscopy. The spectroscopic techniques derived from the combination of X-ray absorption and emission spectroscopy (XAS-XES), such as resonant inelastic X-ray scattering (RIXS) and high energy resolution fluorescence detected (HERFD) XAS, are an ideal tool for the study of nanomaterials. New installations and beamline upgrades now often include wavelength dispersive instruments for the analysis of the emitted X-rays. With the growing use of XAS-XES, scientists are learning about the possibilities and pitfalls. We discuss some experimental aspects, assess the feasibility of measuring weak fluorescence lines in dilute, radiation sensitive samples, and present new experimental approaches for studying magnetic properties of colloidal nanoparticles directly in the liquid phase.

Selective magnetometry of superparamagnetic iron oxide nanoparticles in liquids

We show that the properties of superparamagnetic iron oxide nanoparticles suspended in liquids can be effectively studied using Magnetic Circular Dichroism in Resonant Inelastic X-ray Scattering. Analysis of the spectral shape and magnetic contrast produced by this experiment enables an assessment of the site distribution and magnetic state of metal ions in the spinel phase. The selective magnetization profile of particles as derived from the field dependence of dichroism empowers an estimation of particle size distribution. Furthermore, the new proposed methodology discriminates sizes that are below the detection limits of X-ray and light scattering probes and that are difficult to spot in TEM.

Bad neighbour, good neighbour: how magnetic dipole interactions between soft and hard ferrimagnetic nanoparticles affect macroscopic magnetic properties in ferrofluids

Fluids responding to magnetic fields (ferrofluids) offer a scene with no equivalent in nature to explore long-range magnetic dipole interactions. Here, we studied the very original class of binary ferrofluids, embedding soft and hard ferrimagnetic nanoparticles. We used a combination of X-ray magnetic spectroscopy measurements supported by multi-scale experimental techniques and Monte-Carlo simulations to unveil the origin of the emergent macroscopic magnetic properties of the binary mixture. We found that the association of soft and hard magnetic nanoparticles in the fluid has a considerable influence on their inherent magnetic properties. While the ferrofluid remains in a single phase, magnetic interactions at the nanoscale between both types of particles induce a modification of their respective coercive fields. By connecting the microscopic properties of binary ferrofluids containing small particles, our findings lay the groundwork for the manipulation of magnetic interactions between particles at the nanometer scale in magnetic liquids.

Imaging three-dimensional magnetic systems with x-rays

Recent progress in nanofabrication and additive manufacturing have facilitated the building of nanometer-scale three-dimensional (3D) structures, that promise to lead to an emergence of new functionalities within a number of fields, compared to state-of-the-art two dimensional systems. In magnetism, the move to 3D systems offers the possibility for novel magnetic properties not available in planar systems, as well as enhanced performance, both of which are key for the development of new technological applications. In this review paper we will focus our attention on 3D magnetic systems and how their magnetic configuration can be retrieved using x-ray magnetic nanotomography. We will start with an introduction to magnetic materials, and their relevance to our everyday life, along with the growing impact that they will have in the coming years in, for example, reducing energy consumption. We will then briefly introduce common methods used to study magnetic materials, such as electron holography, neutron and x-ray imaging. In particular, we will focus on x-ray magnetic circular dichroism (XMCD) and how it can be used to image magnetic moment configurations. As a next step we will introduce tomography for 3D imaging, and how it can be adapted to study magnetic materials. Particular attention will be given to explaining the reconstruction algorithms that can be used to retrieve the magnetic moment configuration from the experimental data, as these represent one of the main challenges so far, as well as the different experimental geometries that are available. Recent experimental results will be used as specific examples to guide the reader through each step in order to make sure that the paper will be accessible for those interested in the topic that do not have a specialized background on magnetic imaging. Finally, we will describe the future prospects of such studies, identifying the current challenges facing the field, and how these can be tackled. In particular we will highlight the exciting possibilities offered by the next generation of synchrotron sources which will deliver diffraction limited beams, as well as with the extension of well-established methodologies currently implemented for the study of two-dimensional magnetic materials to achieve higher dimensional investigations.

Evolution of electronic and magnetic properties of nominal magnetite nanoparticles at high pressure probed by x-ray absorption and emission techniques

We present a study of electronic and magnetic properties of nominal magnetite nanoparticles (NPs) (~6 nm) at high pressure in the presence of silicon pressure medium using x-ray absorption near edge structure (XANES), x-ray magnetic circular dichroism (XMCD) and non-resonant x-ray emission spectroscopy (XES). XANES data show a reduction of Fe charge state, a change in local environment around Fe at tetrahedral sites, and a reduced occupation of Fe 4p  orbitals, not seen in previous pressure studies of bulk magnetite. XMCD data show a continuous magnetic moment reduction of ~50% between ambient pressure and 20 GPa, similar to what was observed in previous bulk magnetite studies. XES spectra of NPs indicate a gradual change in spin configuration away from the high-spin state consistent with a postulated charge transfer from Fe 4p  to 3d states and the observed reduction in XMCD signal. Taken together, the results point to substantial differences in the response of electronic and magnetic properties of the nano-counterparts of bulk magnetite at high pressure.

The GALAXIES inelastic hard X-ray scattering end-station at Synchrotron SOLEIL

GALAXIES is an in-vacuum undulator hard X-ray micro-focused beamline dedicated to the study of the electronic structure of materials with high energy resolution using both photoelectron spectroscopy and inelastic X-ray scattering and under both non-resonant (NR-IXS) and resonant (RIXS) conditions. Due to the penetrating power of hard X-rays and the `photon-in/photon-out' technique, the sample environment is not a limitation. Materials under extreme conditions, for example in diamond anvil cells or catalysis chambers, thus constitute a major research direction. Here, the design and performance of the inelastic X-ray scattering end-station that operates in the energy range from ∼4 keV up to 12 keV is reported, and its capabilities are highlighted using a selection of data taken from recently performed experiments. The ability to scan `on the fly' the incident and scattered/emitted X-ray energies, and the sample position enables fast data collection and high experimental throughput. A diamond X-ray transmission phase retarder, which can be used to generate circularly polarized light, will also be discussed in the light of the recent RIXS-MCD approach.

Magnetic Circular Dichroism in X-Ray Emission from Ferromagnets

The existence of novel magnetic circular dichroism in core-level x-ray emission is reported. By means of circular polarization analysis, the dichroic effect of the Fe Kα_{1} emission spectrum is measured on an Fe single crystal. The observed dichroic effect (12%) is remarkably large, if one takes into account the small dichroic effect (about 0.5%) in the conventional K-edge absorption spectroscopy of 3d transition metal elements. The mechanism is ascribed to exchange splitting of the 2p level possessing large spin-orbit coupling. This new magnetooptical effect enables us to explore a variety of new research subjects in the magnetism of 3d transition metals and their compounds by fully utilizing its large dichroic effect, the true bulk sensitivity of hard x rays, and the element selectivity of core-level spectroscopy.

Determination of Conduction and Valence Band Electronic Structure of LaTiOx Ny Thin Film

The nitrogen substitution into the oxygen sites of several oxide materials leads to a reduction of the band gap to the visible-light energy range, which makes these oxynitride semiconductors potential photocatalysts for efficient solar water splitting. Oxynitrides typically show a different crystal structure compared to the pristine oxide material. As the band gap is correlated to both the chemical composition and the crystal structure, it is not trivial to distinguish which modifications of the electronic structure induced by the nitrogen substitution are related to compositional and/or structural effects. Here, X-ray emission and absorption spectroscopy are used to investigate the electronic structures of orthorhombic perovskite LaTiO_x N_y thin films in comparison with films of the pristine oxide LaTiO_x with similar orthorhombic structure and cationic oxidation state. Experiment and theory show the expected upward shift in energy of the valence band maximum that reduces the band gap as a consequence of the nitrogen incorporation. This study also shows that the conduction band minimum, typically considered almost unaffected by nitrogen substitution, undergoes a significant downward shift in energy. For a rational design of oxynitride photocatalysts, the observed changes of both the unoccupied and occupied electronic states have to be taken into account to justify the total band-gap narrowing induced by the nitrogen incorporation.

X-ray magnetic circular dichroism measured at the Fe K-edge with a reduced intrinsic broadening: x-ray absorption spectroscopy versus resonant inelastic x-ray scattering measurements

X-ray magnetic circular dichroism is measured at the Fe K pre-edge in yttrium iron garnet using two different procedures that allow reducing the intrinsic broadening due to the 1s corehole lifetime. First, deconvolution of XMCD data measured in total fluorescence yield (TFY) with an extremely high signal-to-noise ratio enables a factor of 2.4 to be gained in the XMCD intensity. Ligand field multiplet calculations performed with different values of intrinsic broadening show that deconvolving such high quality XMCD data is similar to reducing the lifetime broadening from a 1s corehole to a 2p corehole. Second, MCD is measured by resonant inelastic x-ray scattering spectroscopy as a function of incident energy and emission energy. Selection of a fixed emission energy, instead of using the TFY, allows enhancing the MCD intensity up to a factor of  ∼4.7. However, this significantly changes the spectral shape of the XMCD signal, which cannot be interpreted any more as an absorption spectrum.

Direct evidence for an interdiffused intermediate layer in bi-magnetic core-shell nanoparticles

Core-shell nanoparticles attract continuously growing interest due to their numerous applications, which are driven by the possibility of tuning their functionalities by adjusting structural and morphological parameters. However, despite the critical role interdiffused interfaces may have in the properties, these are usually only estimated in indirect ways. Here we directly evidence the existence of a 1.1 nm thick (Fe,Mn)3O4 interdiffused intermediate shell in nominally γ-Fe2O3-Mn3O4 core-shell nanoparticles using resonant inelastic X-ray scattering spectroscopy combined with magnetic circular dichroism (RIXS-MCD). This recently developed magneto-spectroscopic probe exploits the unique advantages of hard X-rays (i.e., chemical selectivity, bulk sensitivity, and low self-absorption at the K pre-edge) and can be advantageously combined with transmission electron microscopy and electron energy loss spectroscopy to quantitatively elucidate the buried internal structure of complex objects. The detailed information on the structure of the nanoparticles allows understanding the influence of the interface quality on the magnetic properties.

Magnetic manipulation of molecules on a non-magnetic catalytic surface

Non-magnetic Pt catalysts, supported on carbon coated magnetic Co nanoparticles, changed catalytic performance in the presence of an external magnetic field. This behavior relates to an electronic change of Pt induced by a localized magnetic field, which modifies the CO adsorption geometry. In situ resonant inelastic X-ray scattering (RIXS) experiments and theory reveal the change of atop CO adsorption geometry on the Pt catalyst to bridged geometry under an external magnetic field. This observation opens the possibility of catalytic control by means of an external magnetic field.

Visualization of custom-tailored iron oxide nanoparticles chemistry, uptake, and toxicity

Nanoparticles of iron oxide generated by wearing of vehicles have been modelled with a tailored solution of size-uniform engineered magnetite particles produced by the Bradley reaction, a solvothermal metal-organic approach rendering hydrophilic particles. The latter does not bear any pronounced surface charge in analogy with that originating from anthropogenic sources in the environment. Physicochemical properties of the nanoparticles were thoroughly characterized by a wide range of methods, including XPD, TEM, SEM, DLS and spectroscopic techniques. The magnetite nanoparticles were found to be sensitive for transformation into maghemite under ambient conditions. This process was clearly revealed by Raman spectroscopy for high surface energy magnetite particles containing minor impurities of the hydromaghemite phase and was followed by quantitative measurements with EXAFS spectroscopy. In order to assess the toxicological effects of the produced nanoparticles in humans, with and without surface modification with ATP (a model of bio-corona formed in alveolar liquid), a pathway of potential uptake and clearance was modelled with a sequence of in vitro studies using A549 lung epithelial cells, lymphocyte 221-B cells, and 293T embryonal kidney cells, respectively. Raman microscopy unambiguously showed that magnetite nanoparticles are internalized within the A549 cells after 24 h co-incubation, and that the ATP ligand is retained on the nanoparticles throughout the uptake process. The toxicity of the nanoparticles was estimated using confocal fluorescence microscopy and indicated no principal difference for unmodified and modified particles, but revealed considerably different biochemical responses. The IL-8 cytokine response was found to be significantly lower for the magnetite nanoparticles compared to TiO(2), while an enhancement of ROS was observed, which was further increased for the ATP-modified nanoparticles, implicating involvement of the ATP signalling pathway in the epithelium.

A von Hamos x-ray spectrometer based on a segmented-type diffraction crystal for single-shot x-ray emission spectroscopy and time-resolved resonant inelastic x-ray scattering studies

We report on the design and performance of a wavelength-dispersive type spectrometer based on the von Hamos geometry. The spectrometer is equipped with a segmented-type crystal for x-ray diffraction and provides an energy resolution in the order of 0.25 eV and 1 eV over an energy range of 8000 eV-9600 eV. The use of a segmented crystal results in a simple and straightforward crystal preparation that allows to preserve the spectrometer resolution and spectrometer efficiency. Application of the spectrometer for time-resolved resonant inelastic x-ray scattering and single-shot x-ray emission spectroscopy is demonstrated.