The complex refractive index of many materials is poorly known in the soft X-ray range across absorption edges. This is due to saturation effects that occur there in total-electron-yield and fluorescence-yield spectroscopy and that are strongest at resonance energies. Aiming to obtain reliable optical constants, a procedure that reconciles electron-yield measurements and reflection spectroscopy by correcting these saturation effects is presented. The procedure takes into account the energy- and polarization-dependence of the photon penetration depth as well as the creation efficiency for secondary electrons and their escape length. From corrected electron-yield spectra the absorption constants and the imaginary parts of the refractive index of the material are determined. The real parts of the index are subsequently obtained through a Kramers-Kronig transformation. These preliminary optical constants are refined by simulating reflection spectra and adapting them, so that measured reflection spectra are reproduced best. The efficacy of the new procedure is demonstrated for graphite. The optical constants that have been determined for linearly polarized synchrotron light incident with p- and s-geometry provide a detailed and reliable representation of the complex refractive index of the material near π- and σ-resonances. They are also suitable for allotropes of graphite such as graphene.

The complex refractive index has been determined for graphene for linearly polarised light in p- and s-geometry at energies across the carbon 1s edge. The imaginary part was measured with absorption spectroscopy. The real part was derived using Kramers-Kronig transformations. Results have been validated by X-ray reflectometry of substrate-supported graphene. Theoretical modelling demonstrates that reflection spectra are strongly affected by the relative difference between the reflectance of graphene and its substrate. Measured reflection spectra show that graphene is often sandwiched between a carbonaceous layer of surface-adsorbates and another carbonaceous layer between graphene and substrate. Reflectometry distinguishes adventitious layers from the graphene because of a different energy-dependence of the refractive index. The carbonaceous interface layers can be modelled well with the refractive index function measured for an adventitious surface layer on silicon. The carbon observed at the substrate-interface may originate from the transfer of a graphene sheet from a deposition-substrate. The optically-effective thickness and anisotropy of adventitious carbonaceous layers can be extracted from reflectance spectra. The bonds of adventitious carbon tend to be normal to the basal plane of graphene. Transferred graphene typically has a substrate-interface three times its own thickness and is covered by one layer of surface adsorbates.

Using polarization analysis of linearly polarized synchrotron radiation we demonstrate the existence of a giant magneto-optical Faraday effect at the carbon 1s edge of single-layer graphene on Co, reaching Faraday rotation angles of 2.9 x 10(5)deg/mm. This value is of the order of those observed at the Co 3p and 2p edges. Using element-selective magnetic hysteresis curves we find that graphene on Co exhibits ferromagnetic order. The magnetism in graphene is found to be carried by and be strongly enhanced by aligned n orbitals of carbon atoms. It is induced by hybridization with the Co 3d(z)2 orbitals while carbon a bonds show negligible magnetism due to insignificant hybridization with Co. From additional x-ray magnetic circular dichroism and transversal magneto-optical Kerr effect spectra a magnetic moment of 0.14 mu(B) is estimated for graphene. From Faraday spectra the complete set of x-ray magneto-optical constants of graphene has been deduced which allows for future modeling of magneto-optical devices based on graphene. The strong magnetism in graphene results from hybridization of carbon p(z) and metal 3d orbitals. Atoms of the graphene sublattice A, placed on top of Co, lead to strongest hybridization with Co 3d(z)( )(2)orbitals. Carbon atoms of sublattice B, and those of rotated graphene domains without Co atoms beneath, hybridize with each other and with 3d(xy) and 3d(yz) orbitals of neighboring Co atoms forming tilted p(z) bonds. We show that the related reduction of A-B symmetry leads to a splitting of the spin-polarized density of conduction-band states which is responsible for the strong magneto-optical Faraday effect.

High-quality metrology with synchrotron radiation requires in particular a very high spectral purity of the incident beam. This is usually achieved by a set of transmission filters with suitable absorption edges to suppress high-order radiation of the monochromator. The at-wavelength metrology station at a BESSY-II bending-magnet collimated plane-grating monochromator (c-PGM) beamline has recently commissioned a high-order suppression system (HiOS) based on four reflections from mirrors which can be inserted into the beam path. Two pairs of mirrors are aligned parallel so as not to disturb the original beam path and are rotated clockwise and counter-clockwise. Three sets of coatings are available for the different energy ranges and the incidence angle is freely tunable to find the optimum figure of merit for maximum suppression at maximum transmission for each photon energy required. Measured performance results of the HiOS for the EUV and XUV range are compared with simulations, and applications are discussed.

The existence of ferromagnetic ordering in graphene on cobalt is demonstrated by means of resonant magnetic reflection spectroscopy exploiting the transversal magneto-optical Kerr-effect (T-MOKE). Using linearly polarized synchrotron radiation in the soft x-ray range with energies spanning the carbon 1s edge, the π- and σ- bonds of graphene were excited individually, showing that magnetism in graphene is carried by the π – orbitals. Magnetic signals were detected over a wide energy range from 257 – 340 eV with a T-MOKE peak value of 1.1 % at the π – resonance energy near 285 eV. By comparison with corresponding spectra measured at the 2p edges of the Co substrate, a large induced magnetic moment of 0.14 μB was derived for graphene. Individual hysteresis curves monitored at the Co 2p and C 1s edges show that the carbon magnetism is induced by the Co substrate.

A new Optics Beamline coupled to a versatile UHV reflectometer is successfully operating at BESSY-II. It is used to carry out at-wavelength characterization and calibration of in-house produced gratings and novel nano-optical devices as well as mirrors and multilayer systems in the UV and XUV spectral region. This paper presents most recent commissioning data of the beamline and shows their correlation with initial beamline design calculations. Special attention is paid to beamline key parameters which determine the quality of the measurements such as high-order suppression and stray light behavior. The facility is open to user operation.

A technology center for the production of high-precision reflection gratings has been established. Within this project a new optics beamline and a versatile reflectometer for at-wavelength characterization of UV- and XUV-reflection gratings and other (nano-) optical elements has been set up at BESSY-II. The Plane Grating Monochromator beamline operated in collimated light (c-PGM) is equipped with an SX700 monochromator, of which the blazed gratings (600 and 1200 lines mm−1) have been recently exchanged for new ones of improved performance produced in-house. Over the operating range from 10 to 2000 eV this beamline has very high spectral purity achieved by (i) a four-mirror arrangement of different coatings which can be inserted into the beam at different angles and (ii) by absorber filters for high-order suppression. Stray light and scattered radiation is removed efficiently by double sets ofin situexchangeable apertures and slits. By use of in- and off-plane bending-magnet radiation the beamline can be adjusted to either linear or elliptical polarization. One of the main features of a novel 11-axes reflectometer is the possibility to incorporate real life-sized gratings. The samples are adjustable within six degrees of freedom by a newly developed UHV-tripod system carrying a load up to 4 kg, and the reflectivity can be measured between 0 and 90° incidence angle for boths- andp-polarization geometry. This novel powerful metrology facility has gone into operation recently and is now open for external users. First results on optical performance and measurements on multilayer gratings will be presented here.

The existence of natural birefringence in x-ray reflection on graphene is demonstrated at energies spanning the carbon 1s absorption edge. This new x-ray effect has been discovered with precision measurements of the polarization-plane rotation and the polarization-ellipticity changes that occur upon reflection of linearly polarized synchrotron radiation on monolayer graphene. Extraordinarily large polarization-plane rotations of up to 30 degrees, accompanied by a change from linearly to circularly polarized radiation have been measured for graphene on copper. Graphene on single crystalline cobalt, grown on tungsten, exhibits rotation values of up to 17 degrees. Both graphene systems show resonantly enhanced effects at the pi* and sigma* energies. The results are referenced against those obtained for polycrystalline carbon and highly oriented pyrolytic graphite (HOPG), respectively. As expected, polycrystalline carbon shows negligible rotation, whereas a huge maximum rotation of 140 degrees has been observed for HOPG that may be considered a graphene multilayer system. HOPG is found to exhibit such large rotation values over a broad energy range, even well beyond the pi* resonance energy due to the contributions of numerous graphene layers. To explain the origin of the observed natural birefringence of graphene, the Stokes parameters as well as the x-ray natural linear dichroism in reflection have been determined. It is shown that the birefringence directly results from the optical anisotropy related to the orthogonal alignment of pi* and sigma* bonds in the graphene layer. Our polarization analysis reveals a strong bonding of graphene on Co with a reduced sigma* excitation energy and a strong tilt of 50% of the p(z) orbitals towards diagonal orientation. In contrast, graphene on Cu is weakly bound with an orthogonal orientation of the p(z) orbitals. Exhibiting such a large natural birefringence that can be controlled through substrate choice, and because of excellent heat conductivity, graphene materials have a potential to be used as tunable x-ray phase shifting lambda/4 or lambda/2 plates in the design of future high-intensity light sources.

Reflectivity measurements on graphitic materials such as graphene at energies across the carbon K-edge are frustrated by significant intensity loss due to adventitious carbon on beamline mirrors. Such intensity reduction enhances effects due to perturbing high-order harmonics in the beam. These effects distort the actual structure of the reflectance curve. In order to overcome this limitation, a correction technique has been developed and demonstrated first with measurements for highly ordered pyrolytic graphite. The same approach may be applied to other graphitic materials such as graphene and it may be used with other synchrotron beamlines. The fraction of high-order harmonics was determined by passing the incident beam through a 87nm thin silicon nitride absorber that can be well modeled. Using the corrected measurements the x-ray natural linear dichroism of the sample has been determined.

We present combined first-principle calculations and experimental results of the transversal magneto-optical Kerr effect (T-MOKE) of thin Fe films across the 3p edges using linearly polarized synchrotron radiation. We show that the experimental T-MOKE spectra at the 3p edges of Fe exhibit clear signals that are strongly influenced by interference effects. Ab initio calculated T-MOKE asymmetry spectra confirm the importance of interference effects. The comparison of experimental with calculated spectra reveals some differences that we attribute to metal/metal interface roughness that is not taken into account in the calculations.

Further development of synchrotron light sources and new concepts for free electron lasers require undulators with short periods and high magnetic fields. A promising approach is the cryogenic permanent magnet undulator concept based on an advanced magnet material. This new rare earth alloy (Pr, N)(2) Fe-14 B, shows an increasing remanent field of up to 1.7 T without the limits of spin reorientation transition. This work presents first spectral measurements of a prototype cryogenic permanent magnet undulator, consisting of 20 periods of 9 mm in length, cooled by a closed cycle cryo-cooler to temperatures below 30 K. The K parameter of 0.837 at RT is increased by more than 15% to 0.966, and an increase of the third harmonics photon flux of up to 66% was achieved. A possible degradation of the on-axis field quality due to thermally induced magnetic field errors, deduced from the measured bandwidth of the spectrum, is below the limits of the detector resolution of 2%.

We report an investigation of the influence of the crystal structure of Co thin films on the X-ray magnetic linear dichroism (XMLD) spectrum. We compare XMLD spectra measured in reflection at the 3p-edges for two distinct orientations of the magnetization in the crystalline Co film with ab initio calculated spectra. The latter was computed for the face-centered cubic as well as the hexagonal-close packed crystal structures of Co. We find that the XMLD signal is strongly dependent on the magnetization direction with respect to the crystal axes as well as strongly influenced by the crystal structure.

X-ray magnetic linear dichroism spectra measured in reflection (XMLD-R) on crystalline bcc Fe thin films across the 3 p absorption edges are reported. A series of measurements with varying orientation of the electric field vector of the linear polarized synchrotron radiation with respect to the crystal axes reveals a strong magnetocrystalline anisotropy in the XMLD-R spectra. The spectra agree well with theoretical spectra calculated within the framework of the density-functional theory accounting for the spin-orbital and exchange splitting of the 3 p semicore states on an equal footing.

Short period undulators and in particular in-vacuum cryogenic permanent magnet undulators are the upcoming technique for FEL radiators, because they permit a significant reduction of linac and undulator length. For achieving high photon energies with low electron energies (short period lengths, e.g. below 10mm) permanent magnet structures are superior, due to their high surface current density of 16 kA/cm as compared to electromagnetic or even superconducting devices. The geometrical tolerances scale with the period length. This requires new fabrication techniques and structure designs, particularly for sub-cm period lengths. Solutions for these demands will be presented and results from a first prototype using various new technologies such as compound poles will be discussed and compared with common approaches.

Within our technology center for production of highly efficient precision gratings a versatile 4-circle UHV-reflectometer for synchrotron radiation based at-wavelength characterization has been fabricated. The main feature is the possibility to incorporate real live-sized gratings. The samples are adjustable within six degrees of freedom by a novel UHV-tripod system, and the reflectivity can be measured at all incidence angles for both s-and p-polarization geometry. The reflectometer has been setup in a clean room hutch and it is coupled permanently to the optics beamline PM-1 for the UV and XUV range with the polarization adjustable to either linear or elliptical. The setup will be open to users by the end of 2014.

The magnetocrystalline anisotropy of X-ray magnetic linear dichroism (XMLD) reflection spectra measured on single-crystalline bcc Fe films across the 3p and 2p edges are presented. The XMLD spectra were obtained from a series of reflection spectra by aligning the electric field vector of linearly polarized undulator radiation with respect to the crystal axes. Our results show the presence of a huge magnetocrystalline anisotropy in the XMLD reflection spectra. The XMLD signal is further investigated as a function of the Fe film thickness in Au/Fe/Ag/GaAs layered systems. Simulations of the reflection spectra reveal the influences of interference effects, which can enhance or diminish the XMLD signals. The measured spectra are in good agreement with ab initio calculated spectra.

The Helmholtz-Zentrum Berlin (HZB) builds a 2m long in-vacuum-hall-probe-bench for the characterization of several cryogenic undulators currently under development. Short period lengths and small gaps require an accurate correlation between Hall probe position / orientation and the 3D-magnetic field. The geometric tolerances of an in-vacuum bench in the presence of strong temperature gradients do not permit a Hall probe movement along a straight line without corrections. The HZB-bench employs a system of laser interferometers and position sensitive detectors, which is used in a feed-back loop for the Hall probe position / orientation. First measurements on the accuracy and reproducibility of the new device are presented.

We investigate polarisation properties of a reflective Mo/Si multilayer system in the EUV range using polarized synchrotron radiation at BESSY-II. The characterization involves reflectivity measurements with s- and p-polarized light as a function of the wavelength for three different angles near normal incidence. The phase retardance is determined near normal incidence for one fixed angle of incidence as a function of the wavelength. As an additional spin-off of the polarimetry measurement the Stokes parameters of the beamline could be determined. With the 8-axis UHV-polarimeter we have measured the complex reflection coefficients for the first time and establish this ellipsometry technique as an additional sensitive probe to characterize and model multilayer optical elements.