TUNABLE X-RAY QUARTER-WAVE PLATES FOR X-RAY MAGNETIC CIRCULAR-DICHROISM EXPERIMENTS WITH THE ENERGY-DISPERSIVE ABSORPTION SPECTROMETER

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 nondispersive 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 L(II) (6400 eV) to Tm L(III) (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.