X-ray Dichroism Research Articles

Probing nanoscale behavior of magnetic materials with soft X-ray spectromicroscopy

AbstractThe magnetic properties of matter continue to be a vibrant research area driven both by scientific curiosity to unravel the basic physical processes which govern magnetism and the vast and diverse utilization of magnetic materials in current and future devices, e.g., in information and sensor technologies. Relevant length and time scales approach fundamental limits of magnetism and with state-of-the-art synthesis approaches we are able to create and tailor unprecedented properties. Novel analytical tools are required to match these advances and soft X-ray probes are among the most promising ones. Strong and element-specific magnetic X-ray dichroism effects as well as the nanometer wavelength of photons and the availability of fsec short and intense X-ray pulses at upcoming X-ray sources enable unique experimental opportunities for the study of magnetic behavior. This article provides an overview of recent achievements and future perspectives in magnetic soft X-ray spectromicroscopies which permit us to gain spatially resolved insight into the ultrafast spin dynamics and the magnetic properties of buried interfaces of advanced magnetic nanostructures.

The role of hydrogen in room-temperature ferromagnetism at graphite surfaces

We present an x-ray dichroism study of graphite surfaces that addresses the origin and magnitude of ferromagnetism in metal-free carbon. We find that, in addition to carbon π-states, hydrogen-mediated electronic states also exhibit a net spin polarization with significant magnetic remanence at room temperature. The observed magnetism is restricted to the top ≈10 nm of the irradiated sample where the average magnetization reaches ≃15 emu g− 1 at room temperature. We prove that the ferromagnetism found in metal-free untreated graphite is intrinsic and has a similar origin to that found in proton-bombarded graphite. Our findings also show that the magnetic properties of graphite surfaces, thin films or two-dimensional graphene samples can be reliably studied using soft x-ray dichroism. Fundamental new insights into the magnetic properties of carbon-based systems can thus be obtained.

High Specificity in Protein Recognition by Hydrogen‐Bond‐Surrogate α‐Helices: Selective Inhibition of the p53/MDM2 Complex

Stabilized α-helices and nonpeptidic helix mimetics have emerged as powerful molecular scaffolds for the discovery of protein-protein interaction inhibitors.[1–8] Protein-protein interactions often involve large contact areas, which are often difficult for small molecules to target with high specificity.[9–10] The hypothesis behind the design of stabilized helices and helix mimetics is that these medium-sized molecules may pursue their targets with higher specificity because of a larger number of contacts. We recently introduced a new strategy for the preparation of stabilized α-helices, termed hydrogen bond surrogate (HBS) helices, which involves replacement of one of the main chain hydrogen bonds with a covalent linkage (Figure 1A).[11] The salient feature of the HBS approach is its ability to constrain very short peptides into highly stable α-helical conformation without blocking any molecular recognition surfaces. We have extensively analyzed the conformation adopted by HBS α-helices with 2D NMR, X-ray, and circular dichroism spectroscopies.[12–14] In addition, HBS helices have been shown to target their expected protein partners with high affinity in cell-free and cell culture assays.[15–17]

ChemInform Abstract: Absolute Stereochemistry of 1,2′-Linked Bi(naphthoquinone)s.

Asymmetric syntheses of the enantiomers of the naturally occurring atropisomerically chiral 1,2′-linked bi(naphthoquinone)s 8′-hydroxyisodiospyrin and isodiospyrin have allowed their absolute configurations to be assigned by X-ray and circular dichroism spectral studies of synthetic intermediates.

Measuring the magnetization of three monolayer thick Co islands and films by x-ray dichroism

Co islands and films are characterized by x-ray magnetic circular dichroism photoemission electron microscopy. The spatial resolution capabilities of the technique together with atomic growth control permit obtaining perfectly flat triangular islands with a given thickness (3 ML), very close to an abrupt spin-reorientation transition. The magnetic domain configurations are found to depend on island size: while small islands can be magnetized in a single-domain state, larger islands show more complex patterns. Furthermore, the magnetization pattern of the larger islands presents a common chirality. By means of dichroic spectromicroscopy at the $\text{Co}\text{ }L$ absorption edges, an experimental estimate of the ratio of the spin and orbital magnetic moment for three monolayer thick films is obtained.

Linear dichroism in molecular resonant inelastic x-ray scattering

Polarization-dependent resonant inelastic x-ray scattering (RIXS) is shown to be a new probe of molecular-field effects on molecular electronic structure. Combining experiment and theory, linear dichroism in Cl 2p RIXS following Cl 1s excitation in HCl, Cl2, and CF3Cl is ascribed to molecular-field effects, indicating polarized-RIXS provides a direct probe of spin-orbit-state populations generally applicable to all molecules.

X-ray spectroscopy of lanthanum manganites: Nature of doping holes, correlation effects, and orbital ordering

The Mn3s X-ray photoelectron spectra of manganites were studied. It was shown that for the formal valence of manganese from 3+ to 3.3+, the doping holes are O2p in character; as the valence of manganese increases further, the Mn3d states acquire holes. For La0.7Sr0.3MnO3, the Mn3p-3d resonance spectra provided information about the occupied and unoccupied Mn3d states, and the correlation energy U = 6.7 eV was determined experimentally. An analysis of X-ray dichroism on the L absorption spectra of three-dimensional La7/8Sr1/8MnO3 showed that the cooperative Jahn Teller distortion of the orthorhombic phase at 240 K was related to (x 2 − z 2)/(y 2 − z 2) type orbital ordering.

Crystal-Field Level Inversion in Lightly Mn-DopedSr3Ru2O7

Sr3(Ru(1-x)Mnx)2O7, in which 4d-Ru is substituted by the more localized 3d-Mn, is studied by x-ray dichroism and spin-resolved density functional theory. We find that Mn impurities do not exhibit the same 4+ valence of Ru, but act as 3+ acceptors; the extra eg electron occupies the in-plane 3d(x2-y2) orbital instead of the expected out-of-plane 3d(3z2-r2). We propose that the 3d-4d interplay, via the ligand oxygen orbitals, is responsible for this crystal-field level inversion and the material's transition to an antiferromagnetic, possibly orbitally ordered, low-temperature state.

Relationship between C-reactive protein concentration, bone mineral density and cardiovascular disturbances in patients with rheumatoid arthritis

Objective. To study relationship between serum level of hs-CRP, bone mineral density (BMD) and cardiovascular disturbances in patients with rheumatoid arthritis (RA). Material and methods. 132 pts with RA with mean age 50 years (45-53 years) and mean disease duration 132 months (48-216 months) were examined. BMD was evaluated by dichroic X-ray densitometry in femur neck with Gologic apparatus. CRP concentration was assessed by high sensitivity nephelometric immunoassay with latex amplification with BN 100 analyzer (Dade Behring, Germany). Results. Mean BMD value in pts with RA was lower than in control group —1,4 SD and -0,45 SD respectively (p=0,00001). Normal BMD, osteoporosis and osteopenia were revealed in 38%, 47% and 15% of pts respectively. Clinical and subclinical signs of atherosclerosis in RA were more frequent than in control: coronary heart disease (CHD) and stroke (ST) in 25% and in 6% respectively (p=0,004), plaques (P) and intima-media complex (IMC) thickening in 65% and 35% respectively (p=0,003). In groups with osteopenia and osteoporosis ST and CHD revealed after RA development were more frequent, (p 22,l mg/l. Age, duration of disease, frequency of clinical and subclinical signs of atherosclerosis did not differ in these groups. Increase of hs-CRP concentration was associated with decrease of BMD and IMC (p<0,05) Conclusion. Decrease of BMD in RA pts is accompanied by increase of clinical signs of atherosclerosis and hs-CRP level. Increase of CRP may reflect inflammatory activity of the disease and may be a marker of atherosclerosis.

X-ray detected optical activity

Optical Activity (OA) was only measured quite recently in the X-ray range using electric dipole–electric quadrupole interference terms that mix multipoles of opposite parity but are only present in systems with broken inversion symmetry. Natural OA refers to effects that are even with respect to time-reversal symmetry, whereas non-reciprocal OA is concerned with time-reversal odd contributions. Various types of X-ray dichroism related to either natural or non-reciprocal OA have been detected and are reviewed in the present paper. To cite this article: A. Rogalev et al., C. R. Physique 9 (2008).

X-ray dichroism and the pseudogap phase of cuprates

A recent polarized x-ray absorption experiment on the high-temperature cuprate superconductor ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}\mathrm{Ca}{\mathrm{Cu}}_{2}{\mathrm{O}}_{8+x}$ indicates the presence of broken parity symmetry below the temperature ${T}^{*}$, where a pseudogap appears in photoemission. We critically analyze the x-ray data and conclude that a parity-breaking signal of the kind suggested is unlikely based on the crystal structures reported in the literature. Possible other origins of the observed dichroism signal are discussed. We propose x-ray scattering experiments that can be done in order to determine whether such alternative interpretations are valid or not.

Prediction of Strong Dichroism Induced by X Rays Carrying Orbital Momentum

We predict the presence of strong dichroic effects induced by x-ray beams carrying orbital angular momentum (OAM). Taking the difference between spectra obtained with positive and negative OAM states allows the separation of quadrupolar from dipolar transitions at, e.g., the transition-metal K edges, enabling the study of the unoccupied states in the absence of strong core-hole effects. We study the dependence of OAM-induced x-ray dichroism on different polarization vectors and derive sum rules relating the integrated intensity to ground-state hole densities. Calculations of spectral line shapes for cuprates, manganites, and ruthenates confirm the strong OAM-induced dichroism and indicate the potential of this new spectroscopy in the fields of orbital physics and magnetism.

Theoretical justification of ground-state moment analysis of magnetic dichroic x-ray absorption spectra for3dtransition metals

A theoretical justification is given for the phenomenologically introduced method of ground-state moment analysis of magnetic dichroic x-ray absorption spectra of $3d$-transition metals. This method allows us to determine in an element-specific manner ground-state moments such as, e.g., spin and orbital moments of atoms in solids, from the line shape of the dichroic spectra rather than from the use of sum rules. It may be used for early transition metals for which the spin-orbit coupling for the core states is so small that the ${\mathrm{L}}_{2}$- and ${\mathrm{L}}_{3}$-absorption spectra overlap and the sum rule analysis thus cannot be used.

Epitaxial films of the magnetic shape memory material [formula omitted

By DC-sputtering from a stoichiometric target onto sapphire substrates we prepared epitaxial films of Ni 2 MnGa . X-ray diffraction in two circle geometry shows only the Ni 2 MnGa peaks corresponding to a (1 1 0) oriented growth on the (1 1 0) oriented sapphire substrate. Rocking curve scans shows that the out of plane orientation has a width of only 1 . 1 ∘ . By four circle X-ray diffraction we find an in-plane aligned growth. The Curie temperature as determined by SQUID magnetometry is 368 K. On cooling down the low field magnetization drops at around 260–280 K indicating the austenite to martensite transition. The presence of two different phases at high and low temperature also shows up in temperature-dependent X-ray measurements. The element specific magnetic moments for thin film samples have been determined by magnetic circular X-ray dichroism (XMCD) measurements.

Magnetic domains and magnetization reversal of ion-induced magnetically patterned Ruderman-Kittel-Kasuya-Yoshida-coupled Ni81Fe19∕Ru∕Co90Fe10 films

Pure magnetic patterning by means of ion-beam irradiation of magnetic thin films and multilayers often results from a postdeposition local modification of the interface structure with only minor effects on the film topography. In the study presented here a 60keV fine-focused Co ion beam was used to change the coupling in a Ni81Fe19∕Ru∕Co90Fe10 structure from antiferromagnetic to ferromagnetic on a micron scale. Thereby an artificial structure with locally varying interlayer exchange coupling and therefore magnetization alignment is produced. High-resolution full-field x-ray microscopy is used to determine the magnetic domain configuration during the magnetization reversal process locally and layer resolved due to the element-specific contrast in circular x-ray dichroism. In the magnetically patterned structure there is, in addition to the locally varying interlayer exchange coupling across the Ru layer, also the direct exchange coupling within each ferromagnetic layer present. Therefore the magnetization reversal behavior of the irradiated stripes is largely influenced by the surrounding magnetic film.

Resonant inelastic X-ray scattering and its magnetic circular dichroism

Abstract Magnetic circular dichroism in resonant inelastic X-ray scattering (MCD-RIXS) of ferromagnetic systems is discussed for the longitudinal geometry (LG), where the directions of incident X-ray and magnetization are parallel, and the transverse geometry (TG), where they are perpendicular. MCD-RIXS in LG is represented by two successive real processes, X-ray absorption and X-ray emission, while MCD-RIXS in TG is given by the interference process. We describe an application of MCD-RIXS in LG to the detection of Ce 4 f 2 contribution in Ce, L 3 absorption edge of CeFe 2 , and theoretical and experimental studies of MCD-RIXS in TG for Gd and Sm systems.

Antiferromagnetic-to-ferromagnetic transition induced by diluted Co inSrFe1−xCoxO3: Magnetic circular x-ray dichroism study

The antiferromagnetic-to-ferromagnetic transition in $\mathrm{Sr}{\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}{\mathrm{O}}_{3}$ $({x}_{c}\ensuremath{\simeq}0.15)$ induced by Co doping has been studied by magnetic circular x-ray dichroism. The orbital and spin magnetic moments of the Fe and Co $3d$ states under the magenetic field of $2\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ are found to show different $x$ dependences: The spin polarization of Fe $3d$ gradually increases with Co concentration; on the other hand, a large spin polarization of Co $3d$ is induced already in the antiferromagnetic phase, indicating that the Co moment is nearly fully alligned already in the antiferromagnetic phase. This suggests that the alignment of the Fe magnetic moment in $\mathrm{Sr}{\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}{\mathrm{O}}_{3}$ is induced by interaction with doped Co magnetic moment. Possible formation of ferromagnetic Co rich region is discussed.

Identification of extrinsic Mn contributions in Ga1−xMnxAs by field-dependent magnetic circular X-ray dichroism

We combine sensitivity to atomic number, chemical shifts, probing depth, and magnetic order in a field-dependent magnetic circular X-ray dichroism study at the Mn L-edge of the diluted ferromagnetic semiconductor Ga1−xMnxAs and observe different Mn constituents: ferromagnetic Mn with an nd>5 lineshape and paramagnetic Mn with distinct nd=5 lineshape. The paramagnetic Mn is assigned to interstitials with surface segregation tendency.

X-ray natural linear dichroism (XNLD) applied to the determination of relaxations around transition metal impurities in α-Al2O3

The local environments around substitutional Cr3+ or Fe3+ ions in doped α-Al2O3 have been studied experimentally by dichroic x-ray absorption at the Cr and Fe K-edges in single crystals. A reliable method has been applied to remove diffraction peaks. This paper shows that the dichroic EXAFS analysis leads to the precise determination of the environment around impurity ions in α-Al2O3. The Cr or Fe coordination shell in doped α-Al2O3 is similar to the one of Cr in α-Cr2O3 or of Fe in α-Fe2O . Aluminum or oxygen atoms at a farther distance are weakly affected by the presence of the impurity.

High speed parallel synthesis of banana-shaped molecules and phase transition behaviour of 4-bromo-substituted derivatives

Novel derivative series of the well known bent-shaped P-n-O-PIMB liquid crystal mesogens, referred to as ‘4Br-P-n-O-PIMB’, ‘4Cl-P-n-O-PIMB’ and ‘5Cl-P-n-O-PIMB’, having halogen atoms substituted on the phenyl ring in the central core, were synthesized by solution phase parallel synthesis based on a combinatorial approach. The mesomorphic behaviour and physical properties of all the new compounds were studied by means of optical microscopy, differential scanning calorimetry, X-ray and circular dichroism spectroscopy. We found interesting transitional behaviour for the 4Br-P-n-O-PIMB homologous series. The homologues with alkyl tails having carbon numbers of n=3–10, 12, 14 exhibit rather complicated mesomorphic behaviour, which is strongly sensitive to n. The chiral fluid smectic B2 phase with SmCAPA structure and unidentified smectic Bx phase were observed in the homologues with n=9, 10, 12, 14 and n=3–5, respectively. Interestingly, every member exhibits frustrated and/or helical ordered phases at low temperatures, designated as X1, X2, and X3 phases, which result from a spontaneous escape from a macroscopic polarization. The mesomorphic behaviour and mesophase structures differ remarkably from those of the parent P-n-O-PIMB homologues, suggesting that substitution of the halogen atoms at the central core essentially creates a particular interaction between molecules.