C. Giles

Energy-Dispersive Phase Plate for Magnetic Circular Dichroism Experiments in the X-ray Range

A 220 diamond phase plate was combined with an energy-dispersive absorption spectrometer to convert linearly polarized X-rays into circularly polarized photons and to detect circular magnetic X-ray dichroism (CMXD) from ferromagnetic samples. In these experiments, carried out at LURE, the energy-dispersive spectrometer was equipped with a bent Si (311) polychromator and vertically collimating slits accepting essentially a linearly polarized incident beam. The quarter-wave plate was operated in the Bragg geometry but well outside the range of quasi total reflection, with the consequence that the forward-diffracted beam was circularly polarized with a polarization rate approaching 80% over the whole energy bandpass of the polychromator. CMXD spectra of GdFe2 and GdCo2 intermetallic compounds were recorded at ca 7.2u2005keV near the Gd LIII absorption edge: they are essentially identical to the spectra commonly recorded with elliptically polarized X-ray photons collected out of the orbit plane of the storage ring. It is suggested that the energy-dispersive phase plate will be very useful to detect CMXD spectra with energy-dispersive spectrometers exploiting the well collimated linearly polarized emission of standard undulators installed on the storage rings of the third generation.

X‐ray polarimetry with phase plates

Diamond crystals were used as quarter‐wave plates (QWPs) and half‐wave plates (HWPs) to produce and completely analyze circular and vertical linear polarization of monochromatic x rays from an undulator source (ID10) at the European Synchrotron Radiation Facility. The horizontal linear polarization state of the forward diffracted beam in the Bragg or Laue geometry was efficiently converted into circular (99%) and vertical polarization (97%) by tuning the crystal conveniently away from the Bragg peak. Also, the combination of two QWPs was used to obtain linear polarization at any desired azimuth (e.g., 95% at 45°). Further, the second QWP and the linear polarimeter were used to completely characterize the polarization state of the circularly polarized beam after the first QWP. High efficiencies at QWPs and HWPs and high transmitted intensity (40%) were achieved with the diamond crystal in the asymmetric Laue geometry.

Perfect crystal and mosaic crystal quarter‐wave plates for circular magnetic x‐ray dichroism experiments

Circular magnetic x‐ray dichroism experiments require circularly polarized x rays usually provided by bending magnet synchrotron radiation below or above the orbit plane. It is shown here that a very promising alternative consists of using linearly polarized x rays converted to circularly polarized ones by quarter‐wave plates made of nonabsorbing crystals like good diamond crystals and also mosaic beryllium crystals. Convincing experimental results obtained on the energy‐dispersive absorption spectrometer at LURE (Orsay‐France) are presented.

Energy and polarization-tunable X-ray quarter-wave plates for energy dispersive absorption spectrometer

Abstract Diamond crystals inserted in an energy dispersive setup are shown to be efficient quarter-wave plates at transforming a linearly polarized X-ray beam into a circularly polarized one. Circular magnetic X-ray dichroism (CMXD) spectra have been recorded at different absorption edges demonstrating the energy tunability of the device. CMXD spectra have also been obtained by switching the helicity of the circular polarization, i.e., flipping the quarter-wave plate between two opposite angular offsets from the Bragg peak. This development opens new classes of experiments.

Design of an X-ray Phase-Plate Analyzer to Measure the Circular Polarization Rate of a Helical Undulator Source

A quarter-wave plate made of a ca 16 mum-thick silicon single-crystal was used at energies as low as 2.8 keV to convert circularly polarized photons into linearly polarized photons. Coupled to a linear polarimeter, this quarter-wave plate enabled the characterization of the circular polarization rate of the radiation emitted by one of the ESRF helical undulators, Helios-I. The measured value (ca 97%) is in good agreement with theoretical predictions. Special attention was paid to the alignment procedures of all relevant optical components of the beamline.

X-ray phase plate for energy-dispersive and mononchromatic experiments

Diamond crystals are excellent phase plates for the forward diffracted X-rays in the Bragg geometry. The phase-shift between the (sigma) and (pi) components of the transmitted wave varies with the incident angular offset from the center of the reflection profile and can be adjusted to any desired value. Due to the low absorption coefficient of diamond, rather thick crystals (about 1 mm thick) can be used and operated in a region where the variation of the phase-shift with the angle of incidence is not too strong. This property was exploited to obtain a high degree of circular polarization in an energy dispersive absorption spectrometer at LURE (Orsay) where the source is rather broad.

Probing the localized to itinerant behavior of the 4f electron in CeIn 3-xSn x by Gd3 + electron spin resonance

The CeIn 3-xSn x cubic heavy fermion system presents an antiferromagnetic transition at T N=10 K, for x=0, that decreases continuously down to 0 K upon Sn substitution at a critical concentration of x c0.65. In the vicinity of T N→0 the system shows non-Fermi liquid behavior due to antiferromagnetic critical fluctuations. For a high Sn content, x2.2, intermediate valence effects are present. In this work we show that Gd3 +-doped electron spin resonance (ESR) probes a change in the character of the Ce 4f electron, as a function of Sn substitution. The Gd3 + ESR results indicate a transition of the Ce 4f spin behavior from localized to itinerant. Near the quantum critical point, on the antiferromagnetic side of the magnetic phase diagram, both localized and itinerant behaviors coexist.

Tunable X-ray quarter-wave plates for X-ray magnetic circular dichroism experiments with the energy dispersive absorption spectrometer☆

Abstract Perfect and mosaic crystals (respectively diamond and beryllium) have been used as X-ray quarter-wave plates to convert linearly polarised X-rays into circularly polarised photons, and to measure X-ray magnetic circular dichroism (XMCD) from ferro-(ferri-) magnetic samples with an energy dispersive absorption spectrometer. Bragg reflections from both the bent Si polychromator and the quarter-wave plate (QWP) have been matched in order to obtain a non-dispersive geometry. This results in a high circular polarisation rate in an energy band pass which is presently QWP size-limited, about 180 eV with the used diamond crystal and 280 eV with the beryllium crystal. The energy tunability of the diamond QWP was demonstrated by recording XMCD at different L-edges from Pr LII (6400 eV) to Tm LIII (8648 eV). XMCD spectra were also obtained by switching the helicity of the photons, flipping the QWP between two opposite angular offsets over the diamond Bragg peak. This optical device, delivering polarisation-tunable X-rays sources, is by far better than the classical off-orbit plane source from a bending magnet. It opens up new classes of experiments.

Cd doping effects in the heavy-fermion compounds Ce2MIn8 (M = Rh and Ir)

C. Adriano, ∗ C. Giles, E. M. Bittar, L. N. Coelho, F. de Bergevin, C. Mazzoli, L. Paolasini, W. Ratcliff, R. Bindel, J. W. Lynn, Z. Fisk, and P. G. Pagliuso Instituto de F́ısica “Gleb Wataghin”, Universidade Estadual de Campinas, UNICAMP,13083-970, Campinas, São Paulo, Brazil. European Synchrotron Radiation Facility, BP 220, F-38043 Grenoble Cedex, France. NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA University of California, Irvine, CA 92697-4574, USA (Dated: December 22, 2009)

Soft to firm x-ray spectroscopy at a hard x-ray facility: polarization-sensitive experiments

The ESRF beamline #6 is dedicated to polarization dependent spectroscopies at excitation energies ranging from 0.4 up to 20 keV. This broad energy range will be covered with two helical undulators (Helios-I and Helios II) providing the user with a full control of the photon polarization. Two kinds of optics will be made available: (1) low energy experiments (0.4 keV <EQ E <EQ 2.0 keV) will be carried out on a side-branch beamline equipped with a Dragon-type grating spectrometer; (2) single crystal Bragg optics will be used above 2 keV on the straight branch part of the beamline. We report on the present status and optical arrangement of both branches. Special emphasis is put onto the polarization transfer in Bragg optics with elliptically or circularly polarized light. Various experimental configurations have been evaluated with respect to the figure of merit I.(tau) 2, where I is the transmitted intensity and (tau) is the circular polarization rate. Operation of the beamline will start with a simplified, conservative optical configuration, while more challenging options require specific technical developments.